X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/ff51d159dd8b08acb373f02f7f005c4d7ac8b9a2..e3c52a63624c102a41f60bc741294cd3f23ceb89:/doc/bison.texinfo?ds=inline diff --git a/doc/bison.texinfo b/doc/bison.texinfo index 6b26ef85..302bc4a1 100644 --- a/doc/bison.texinfo +++ b/doc/bison.texinfo @@ -1,30 +1,26 @@ \input texinfo @c -*-texinfo-*- @comment %**start of header @setfilename bison.info -@settitle Bison 1.20 +@include version.texi +@settitle Bison @value{VERSION} @setchapternewpage odd -@c SMALL BOOK version +@finalout + +@c SMALL BOOK version @c This edition has been formatted so that you can format and print it in -@c the smallbook format. +@c the smallbook format. @c @smallbook -@c next time, consider using @set for edition number, etc... - -@c Set following if you have the new `shorttitlepage' command -@c @clear shorttitlepage-enabled -@c @set shorttitlepage-enabled - -@c ISPELL CHECK: done, 14 Jan 1993 --bob - -@c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo -@c titlepage; should NOT be changed in the GPL. --mew +@c Set following if you want to document %default-prec and %no-default-prec. +@c This feature is experimental and may change in future Bison versions. +@c @set defaultprec -@iftex +@ifnotinfo @syncodeindex fn cp @syncodeindex vr cp @syncodeindex tp cp -@end iftex +@end ifnotinfo @ifinfo @synindex fn cp @synindex vr cp @@ -32,165 +28,155 @@ @end ifinfo @comment %**end of header -@ifinfo -This file documents the Bison parser generator. +@copying -Copyright (C) 1988, 1989, 1990, 1991, 1992 Free Software Foundation, Inc. +This manual (@value{UPDATED}) is for @acronym{GNU} Bison (version +@value{VERSION}), the @acronym{GNU} parser generator. -Permission is granted to make and distribute verbatim copies of -this manual provided the copyright notice and this permission notice -are preserved on all copies. +Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, +1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software +Foundation, Inc. -@ignore -Permission is granted to process this file through Tex and print the -results, provided the printed document carries copying permission -notice identical to this one except for the removal of this paragraph -(this paragraph not being relevant to the printed manual). +@quotation +Permission is granted to copy, distribute and/or modify this document +under the terms of the @acronym{GNU} Free Documentation License, +Version 1.2 or any later version published by the Free Software +Foundation; with no Invariant Sections, with the Front-Cover texts +being ``A @acronym{GNU} Manual,'' and with the Back-Cover Texts as in +(a) below. A copy of the license is included in the section entitled +``@acronym{GNU} Free Documentation License.'' + +(a) The FSF's Back-Cover Text is: ``You have the freedom to copy and +modify this @acronym{GNU} manual. Buying copies from the @acronym{FSF} +supports it in developing @acronym{GNU} and promoting software +freedom.'' +@end quotation +@end copying -@end ignore -Permission is granted to copy and distribute modified versions of this -manual under the conditions for verbatim copying, provided also that the -sections entitled ``GNU General Public License'' and ``Conditions for -Using Bison'' are included exactly as in the original, and provided that -the entire resulting derived work is distributed under the terms of a -permission notice identical to this one. - -Permission is granted to copy and distribute translations of this manual -into another language, under the above conditions for modified versions, -except that the sections entitled ``GNU General Public License'', -``Conditions for Using Bison'' and this permission notice may be -included in translations approved by the Free Software Foundation -instead of in the original English. -@end ifinfo +@dircategory Software development +@direntry +* bison: (bison). @acronym{GNU} parser generator (Yacc replacement). +@end direntry -@ifset shorttitlepage-enabled -@shorttitlepage Bison -@end ifset @titlepage @title Bison -@subtitle The YACC-compatible Parser Generator -@subtitle December 1992, Bison Version 1.20 +@subtitle The Yacc-compatible Parser Generator +@subtitle @value{UPDATED}, Bison Version @value{VERSION} @author by Charles Donnelly and Richard Stallman @page @vskip 0pt plus 1filll -Copyright @copyright{} 1988, 1989, 1990, 1991, 1992 Free Software -Foundation - +@insertcopying @sp 2 Published by the Free Software Foundation @* -675 Massachusetts Avenue @* -Cambridge, MA 02139 USA @* -Printed copies are available for $15 each.@* -ISBN-1-882114-30-2 - -Permission is granted to make and distribute verbatim copies of -this manual provided the copyright notice and this permission notice -are preserved on all copies. - -@ignore -Permission is granted to process this file through TeX and print the -results, provided the printed document carries copying permission -notice identical to this one except for the removal of this paragraph -(this paragraph not being relevant to the printed manual). - -@end ignore -Permission is granted to copy and distribute modified versions of this -manual under the conditions for verbatim copying, provided also that the -sections entitled ``GNU General Public License'' and ``Conditions for -Using Bison'' are included exactly as in the original, and provided that -the entire resulting derived work is distributed under the terms of a -permission notice identical to this one. - -Permission is granted to copy and distribute translations of this manual -into another language, under the above conditions for modified versions, -except that the sections entitled ``GNU General Public License'', -``Conditions for Using Bison'' and this permission notice may be -included in translations approved by the Free Software Foundation -instead of in the original English. +51 Franklin Street, Fifth Floor @* +Boston, MA 02110-1301 USA @* +Printed copies are available from the Free Software Foundation.@* +@acronym{ISBN} 1-882114-44-2 @sp 2 Cover art by Etienne Suvasa. @end titlepage -@page -@node Top, Introduction, (dir), (dir) +@contents -@ifinfo -This manual documents version 1.20 of Bison. -@end ifinfo +@ifnottex +@node Top +@top Bison +@insertcopying +@end ifnottex @menu -* Introduction:: -* Conditions:: -* Copying:: The GNU General Public License says - how you can copy and share Bison +* Introduction:: +* Conditions:: +* Copying:: The @acronym{GNU} General Public License says + how you can copy and share Bison. Tutorial sections: -* Concepts:: Basic concepts for understanding Bison. -* Examples:: Three simple explained examples of using Bison. +* Concepts:: Basic concepts for understanding Bison. +* Examples:: Three simple explained examples of using Bison. Reference sections: -* Grammar File:: Writing Bison declarations and rules. -* Interface:: C-language interface to the parser function @code{yyparse}. -* Algorithm:: How the Bison parser works at run-time. -* Error Recovery:: Writing rules for error recovery. +* Grammar File:: Writing Bison declarations and rules. +* Interface:: C-language interface to the parser function @code{yyparse}. +* Algorithm:: How the Bison parser works at run-time. +* Error Recovery:: Writing rules for error recovery. * Context Dependency:: What to do if your language syntax is too - messy for Bison to handle straightforwardly. -* Debugging:: Debugging Bison parsers that parse wrong. -* Invocation:: How to run Bison (to produce the parser source file). -* Table of Symbols:: All the keywords of the Bison language are explained. -* Glossary:: Basic concepts are explained. -* Index:: Cross-references to the text. - + messy for Bison to handle straightforwardly. +* Debugging:: Understanding or debugging Bison parsers. +* Invocation:: How to run Bison (to produce the parser source file). +* Other Languages:: Creating C++ and Java parsers. +* FAQ:: Frequently Asked Questions +* Table of Symbols:: All the keywords of the Bison language are explained. +* Glossary:: Basic concepts are explained. +* Copying This Manual:: License for copying this manual. +* Index:: Cross-references to the text. + +@detailmenu --- The Detailed Node Listing --- The Concepts of Bison -* Language and Grammar:: Languages and context-free grammars, - as mathematical ideas. -* Grammar in Bison:: How we represent grammars for Bison's sake. -* Semantic Values:: Each token or syntactic grouping can have - a semantic value (the value of an integer, - the name of an identifier, etc.). -* Semantic Actions:: Each rule can have an action containing C code. -* Bison Parser:: What are Bison's input and output, - how is the output used? -* Stages:: Stages in writing and running Bison grammars. -* Grammar Layout:: Overall structure of a Bison grammar file. +* Language and Grammar:: Languages and context-free grammars, + as mathematical ideas. +* Grammar in Bison:: How we represent grammars for Bison's sake. +* Semantic Values:: Each token or syntactic grouping can have + a semantic value (the value of an integer, + the name of an identifier, etc.). +* Semantic Actions:: Each rule can have an action containing C code. +* GLR Parsers:: Writing parsers for general context-free languages. +* Locations Overview:: Tracking Locations. +* Bison Parser:: What are Bison's input and output, + how is the output used? +* Stages:: Stages in writing and running Bison grammars. +* Grammar Layout:: Overall structure of a Bison grammar file. + +Writing @acronym{GLR} Parsers + +* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars. +* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities. +* GLR Semantic Actions:: Deferred semantic actions have special concerns. +* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler. Examples -* RPN Calc:: Reverse polish notation calculator; - a first example with no operator precedence. -* Infix Calc:: Infix (algebraic) notation calculator. - Operator precedence is introduced. +* RPN Calc:: Reverse polish notation calculator; + a first example with no operator precedence. +* Infix Calc:: Infix (algebraic) notation calculator. + Operator precedence is introduced. * Simple Error Recovery:: Continuing after syntax errors. +* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$. * Multi-function Calc:: Calculator with memory and trig functions. - It uses multiple data-types for semantic values. -* Exercises:: Ideas for improving the multi-function calculator. + It uses multiple data-types for semantic values. +* Exercises:: Ideas for improving the multi-function calculator. Reverse Polish Notation Calculator -* Decls: Rpcalc Decls. Bison and C declarations for rpcalc. -* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation. -* Lexer: Rpcalc Lexer. The lexical analyzer. -* Main: Rpcalc Main. The controlling function. -* Error: Rpcalc Error. The error reporting function. -* Gen: Rpcalc Gen. Running Bison on the grammar file. -* Comp: Rpcalc Compile. Run the C compiler on the output code. +* Rpcalc Declarations:: Prologue (declarations) for rpcalc. +* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation. +* Rpcalc Lexer:: The lexical analyzer. +* Rpcalc Main:: The controlling function. +* Rpcalc Error:: The error reporting function. +* Rpcalc Generate:: Running Bison on the grammar file. +* Rpcalc Compile:: Run the C compiler on the output code. Grammar Rules for @code{rpcalc} -* Rpcalc Input:: -* Rpcalc Line:: -* Rpcalc Expr:: +* Rpcalc Input:: +* Rpcalc Line:: +* Rpcalc Expr:: + +Location Tracking Calculator: @code{ltcalc} + +* Ltcalc Declarations:: Bison and C declarations for ltcalc. +* Ltcalc Rules:: Grammar rules for ltcalc, with explanations. +* Ltcalc Lexer:: The lexical analyzer. Multi-Function Calculator: @code{mfcalc} -* Decl: Mfcalc Decl. Bison declarations for multi-function calculator. -* Rules: Mfcalc Rules. Grammar rules for the calculator. -* Symtab: Mfcalc Symtab. Symbol table management subroutines. +* Mfcalc Declarations:: Bison declarations for multi-function calculator. +* Mfcalc Rules:: Grammar rules for the calculator. +* Mfcalc Symbol Table:: Symbol table management subroutines. Bison Grammar Files @@ -199,15 +185,17 @@ Bison Grammar Files * Rules:: How to write grammar rules. * Recursion:: Writing recursive rules. * Semantics:: Semantic values and actions. +* Locations:: Locations and actions. * Declarations:: All kinds of Bison declarations are described here. * Multiple Parsers:: Putting more than one Bison parser in one program. Outline of a Bison Grammar -* C Declarations:: Syntax and usage of the C declarations section. -* Bison Declarations:: Syntax and usage of the Bison declarations section. -* Grammar Rules:: Syntax and usage of the grammar rules section. -* C Code:: Syntax and usage of the additional C code section. +* Prologue:: Syntax and usage of the prologue. +* Prologue Alternatives:: Syntax and usage of alternatives to the prologue. +* Bison Declarations:: Syntax and usage of the Bison declarations section. +* Grammar Rules:: Syntax and usage of the grammar rules section. +* Epilogue:: Syntax and usage of the epilogue. Defining Language Semantics @@ -219,51 +207,69 @@ Defining Language Semantics This says when, why and how to use the exceptional action in the middle of a rule. +Tracking Locations + +* Location Type:: Specifying a data type for locations. +* Actions and Locations:: Using locations in actions. +* Location Default Action:: Defining a general way to compute locations. + Bison Declarations +* Require Decl:: Requiring a Bison version. * Token Decl:: Declaring terminal symbols. * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. -* Expect Decl:: Suppressing warnings about shift/reduce conflicts. +* Initial Action Decl:: Code run before parsing starts. +* Destructor Decl:: Declaring how symbols are freed. +* Expect Decl:: Suppressing warnings about parsing conflicts. * Start Decl:: Specifying the start symbol. * Pure Decl:: Requesting a reentrant parser. +* Push Decl:: Requesting a push parser. * Decl Summary:: Table of all Bison declarations. Parser C-Language Interface -* Parser Function:: How to call @code{yyparse} and what it returns. -* Lexical:: You must supply a function @code{yylex} - which reads tokens. -* Error Reporting:: You must supply a function @code{yyerror}. -* Action Features:: Special features for use in actions. +* Parser Function:: How to call @code{yyparse} and what it returns. +* Push Parser Function:: How to call @code{yypush_parse} and what it returns. +* Pull Parser Function:: How to call @code{yypull_parse} and what it returns. +* Parser Create Function:: How to call @code{yypstate_new} and what it returns. +* Parser Delete Function:: How to call @code{yypstate_delete} and what it returns. +* Lexical:: You must supply a function @code{yylex} + which reads tokens. +* Error Reporting:: You must supply a function @code{yyerror}. +* Action Features:: Special features for use in actions. +* Internationalization:: How to let the parser speak in the user's + native language. The Lexical Analyzer Function @code{yylex} * Calling Convention:: How @code{yyparse} calls @code{yylex}. -* Token Values:: How @code{yylex} must return the semantic value - of the token it has read. -* Token Positions:: How @code{yylex} must return the text position - (line number, etc.) of the token, if the - actions want that. -* Pure Calling:: How the calling convention differs - in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). +* Token Values:: How @code{yylex} must return the semantic value + of the token it has read. +* Token Locations:: How @code{yylex} must return the text location + (line number, etc.) of the token, if the + actions want that. +* Pure Calling:: How the calling convention differs in a pure parser + (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). -The Bison Parser Algorithm +The Bison Parser Algorithm -* Look-Ahead:: Parser looks one token ahead when deciding what to do. +* Lookahead:: Parser looks one token ahead when deciding what to do. * Shift/Reduce:: Conflicts: when either shifting or reduction is valid. * Precedence:: Operator precedence works by resolving conflicts. * Contextual Precedence:: When an operator's precedence depends on context. * Parser States:: The parser is a finite-state-machine with stack. * Reduce/Reduce:: When two rules are applicable in the same situation. -* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. -* Stack Overflow:: What happens when stack gets full. How to avoid it. +* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. +* Generalized LR Parsing:: Parsing arbitrary context-free grammars. +* Memory Management:: What happens when memory is exhausted. How to avoid it. Operator Precedence * Why Precedence:: An example showing why precedence is needed. -* Using Precedence:: How to specify precedence in Bison grammars. +* Using Precedence:: How to specify precedence and associativity. +* Precedence Only:: How to specify precedence only. * Precedence Examples:: How these features are used in the previous example. * How Precedence:: How they work. @@ -274,468 +280,141 @@ Handling Context Dependencies * Tie-in Recovery:: Lexical tie-ins have implications for how error recovery rules must be written. -Invoking Bison - -* Bison Options:: All the options described in detail, - in alphabetical order by short options. -* Option Cross Key:: Alphabetical list of long options. -* VMS Invocation:: Bison command syntax on VMS. -@end menu - -@node Introduction, Conditions, Top, Top -@unnumbered Introduction -@cindex introduction - -@dfn{Bison} is a general-purpose parser generator that converts a -grammar description for an LALR(1) context-free grammar into a C -program to parse that grammar. Once you are proficient with Bison, -you may use it to develop a wide range of language parsers, from those -used in simple desk calculators to complex programming languages. - -Bison is upward compatible with Yacc: all properly-written Yacc grammars -ought to work with Bison with no change. Anyone familiar with Yacc -should be able to use Bison with little trouble. You need to be fluent in -C programming in order to use Bison or to understand this manual. - -We begin with tutorial chapters that explain the basic concepts of using -Bison and show three explained examples, each building on the last. If you -don't know Bison or Yacc, start by reading these chapters. Reference -chapters follow which describe specific aspects of Bison in detail. - -Bison was written primarily by Robert Corbett; Richard Stallman made -it Yacc-compatible. This edition corresponds to version 1.20 of Bison. - -@node Conditions, Copying, Introduction, Top -@unnumbered Conditions for Using Bison - -Bison grammars can be used only in programs that are free software. This -is in contrast to what happens with the GNU C compiler and the other -GNU programming tools. - -The reason Bison is special is that the output of the Bison utility---the -Bison parser file---contains a verbatim copy of a sizable piece of Bison, -which is the code for the @code{yyparse} function. (The actions from your -grammar are inserted into this function at one point, but the rest of the -function is not changed.) - -As a result, the Bison parser file is covered by the same copying -conditions that cover Bison itself and the rest of the GNU system: any -program containing it has to be distributed under the standard GNU copying -conditions. - -Occasionally people who would like to use Bison to develop proprietary -programs complain about this. - -We don't particularly sympathize with their complaints. The purpose of the -GNU project is to promote the right to share software and the practice of -sharing software; it is a means of changing society. The people who -complain are planning to be uncooperative toward the rest of the world; why -should they deserve our help in doing so? - -However, it's possible that a change in these conditions might encourage -computer companies to use and distribute the GNU system. If so, then we -might decide to change the terms on @code{yyparse} as a matter of the -strategy of promoting the right to share. Such a change would be -irrevocable. Since we stand by the copying permissions we have announced, -we cannot withdraw them once given. - -We mustn't make an irrevocable change hastily. We have to wait until there -is a complete GNU system and there has been time to learn how this issue -affects its reception. - -@node Copying, Concepts, Conditions, Top -@unnumbered GNU GENERAL PUBLIC LICENSE -@center Version 2, June 1991 - -@display -Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc. -675 Mass Ave, Cambridge, MA 02139, USA - -Everyone is permitted to copy and distribute verbatim copies -of this license document, but changing it is not allowed. -@end display - -@unnumberedsec Preamble - - The licenses for most software are designed to take away your -freedom to share and change it. By contrast, the GNU General Public -License is intended to guarantee your freedom to share and change free -software---to make sure the software is free for all its users. This -General Public License applies to most of the Free Software -Foundation's software and to any other program whose authors commit to -using it. (Some other Free Software Foundation software is covered by -the GNU Library General Public License instead.) You can apply it to -your programs, too. - - When we speak of free software, we are referring to freedom, not -price. Our General Public Licenses are designed to make sure that you -have the freedom to distribute copies of free software (and charge for -this service if you wish), that you receive source code or can get it -if you want it, that you can change the software or use pieces of it -in new free programs; and that you know you can do these things. - - To protect your rights, we need to make restrictions that forbid -anyone to deny you these rights or to ask you to surrender the rights. -These restrictions translate to certain responsibilities for you if you -distribute copies of the software, or if you modify it. - - For example, if you distribute copies of such a program, whether -gratis or for a fee, you must give the recipients all the rights that -you have. You must make sure that they, too, receive or can get the -source code. And you must show them these terms so they know their -rights. - - We protect your rights with two steps: (1) copyright the software, and -(2) offer you this license which gives you legal permission to copy, -distribute and/or modify the software. - - Also, for each author's protection and ours, we want to make certain -that everyone understands that there is no warranty for this free -software. If the software is modified by someone else and passed on, we -want its recipients to know that what they have is not the original, so -that any problems introduced by others will not reflect on the original -authors' reputations. - - Finally, any free program is threatened constantly by software -patents. We wish to avoid the danger that redistributors of a free -program will individually obtain patent licenses, in effect making the -program proprietary. To prevent this, we have made it clear that any -patent must be licensed for everyone's free use or not licensed at all. - - The precise terms and conditions for copying, distribution and -modification follow. - -@iftex -@unnumberedsec TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION -@end iftex -@ifinfo -@center TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION -@end ifinfo - -@enumerate 0 -@item -This License applies to any program or other work which contains -a notice placed by the copyright holder saying it may be distributed -under the terms of this General Public License. The ``Program'', below, -refers to any such program or work, and a ``work based on the Program'' -means either the Program or any derivative work under copyright law: -that is to say, a work containing the Program or a portion of it, -either verbatim or with modifications and/or translated into another -language. (Hereinafter, translation is included without limitation in -the term ``modification''.) Each licensee is addressed as ``you''. - -Activities other than copying, distribution and modification are not -covered by this License; they are outside its scope. The act of -running the Program is not restricted, and the output from the Program -is covered only if its contents constitute a work based on the -Program (independent of having been made by running the Program). -Whether that is true depends on what the Program does. - -@item -You may copy and distribute verbatim copies of the Program's -source code as you receive it, in any medium, provided that you -conspicuously and appropriately publish on each copy an appropriate -copyright notice and disclaimer of warranty; keep intact all the -notices that refer to this License and to the absence of any warranty; -and give any other recipients of the Program a copy of this License -along with the Program. - -You may charge a fee for the physical act of transferring a copy, and -you may at your option offer warranty protection in exchange for a fee. - -@item -You may modify your copy or copies of the Program or any portion -of it, thus forming a work based on the Program, and copy and -distribute such modifications or work under the terms of Section 1 -above, provided that you also meet all of these conditions: - -@enumerate a -@item -You must cause the modified files to carry prominent notices -stating that you changed the files and the date of any change. - -@item -You must cause any work that you distribute or publish, that in -whole or in part contains or is derived from the Program or any -part thereof, to be licensed as a whole at no charge to all third -parties under the terms of this License. - -@item -If the modified program normally reads commands interactively -when run, you must cause it, when started running for such -interactive use in the most ordinary way, to print or display an -announcement including an appropriate copyright notice and a -notice that there is no warranty (or else, saying that you provide -a warranty) and that users may redistribute the program under -these conditions, and telling the user how to view a copy of this -License. (Exception: if the Program itself is interactive but -does not normally print such an announcement, your work based on -the Program is not required to print an announcement.) -@end enumerate +Debugging Your Parser -These requirements apply to the modified work as a whole. If -identifiable sections of that work are not derived from the Program, -and can be reasonably considered independent and separate works in -themselves, then this License, and its terms, do not apply to those -sections when you distribute them as separate works. But when you -distribute the same sections as part of a whole which is a work based -on the Program, the distribution of the whole must be on the terms of -this License, whose permissions for other licensees extend to the -entire whole, and thus to each and every part regardless of who wrote it. - -Thus, it is not the intent of this section to claim rights or contest -your rights to work written entirely by you; rather, the intent is to -exercise the right to control the distribution of derivative or -collective works based on the Program. - -In addition, mere aggregation of another work not based on the Program -with the Program (or with a work based on the Program) on a volume of -a storage or distribution medium does not bring the other work under -the scope of this License. - -@item -You may copy and distribute the Program (or a work based on it, -under Section 2) in object code or executable form under the terms of -Sections 1 and 2 above provided that you also do one of the following: - -@enumerate a -@item -Accompany it with the complete corresponding machine-readable -source code, which must be distributed under the terms of Sections -1 and 2 above on a medium customarily used for software interchange; or, - -@item -Accompany it with a written offer, valid for at least three -years, to give any third party, for a charge no more than your -cost of physically performing source distribution, a complete -machine-readable copy of the corresponding source code, to be -distributed under the terms of Sections 1 and 2 above on a medium -customarily used for software interchange; or, - -@item -Accompany it with the information you received as to the offer -to distribute corresponding source code. (This alternative is -allowed only for noncommercial distribution and only if you -received the program in object code or executable form with such -an offer, in accord with Subsection b above.) -@end enumerate +* Understanding:: Understanding the structure of your parser. +* Tracing:: Tracing the execution of your parser. -The source code for a work means the preferred form of the work for -making modifications to it. For an executable work, complete source -code means all the source code for all modules it contains, plus any -associated interface definition files, plus the scripts used to -control compilation and installation of the executable. However, as a -special exception, the source code distributed need not include -anything that is normally distributed (in either source or binary -form) with the major components (compiler, kernel, and so on) of the -operating system on which the executable runs, unless that component -itself accompanies the executable. - -If distribution of executable or object code is made by offering -access to copy from a designated place, then offering equivalent -access to copy the source code from the same place counts as -distribution of the source code, even though third parties are not -compelled to copy the source along with the object code. +Invoking Bison -@item -You may not copy, modify, sublicense, or distribute the Program -except as expressly provided under this License. Any attempt -otherwise to copy, modify, sublicense or distribute the Program is -void, and will automatically terminate your rights under this License. -However, parties who have received copies, or rights, from you under -this License will not have their licenses terminated so long as such -parties remain in full compliance. +* Bison Options:: All the options described in detail, + in alphabetical order by short options. +* Option Cross Key:: Alphabetical list of long options. +* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}. -@item -You are not required to accept this License, since you have not -signed it. However, nothing else grants you permission to modify or -distribute the Program or its derivative works. These actions are -prohibited by law if you do not accept this License. Therefore, by -modifying or distributing the Program (or any work based on the -Program), you indicate your acceptance of this License to do so, and -all its terms and conditions for copying, distributing or modifying -the Program or works based on it. +Parsers Written In Other Languages -@item -Each time you redistribute the Program (or any work based on the -Program), the recipient automatically receives a license from the -original licensor to copy, distribute or modify the Program subject to -these terms and conditions. You may not impose any further -restrictions on the recipients' exercise of the rights granted herein. -You are not responsible for enforcing compliance by third parties to -this License. +* C++ Parsers:: The interface to generate C++ parser classes +* Java Parsers:: The interface to generate Java parser classes -@item -If, as a consequence of a court judgment or allegation of patent -infringement or for any other reason (not limited to patent issues), -conditions are imposed on you (whether by court order, agreement or -otherwise) that contradict the conditions of this License, they do not -excuse you from the conditions of this License. If you cannot -distribute so as to satisfy simultaneously your obligations under this -License and any other pertinent obligations, then as a consequence you -may not distribute the Program at all. For example, if a patent -license would not permit royalty-free redistribution of the Program by -all those who receive copies directly or indirectly through you, then -the only way you could satisfy both it and this License would be to -refrain entirely from distribution of the Program. - -If any portion of this section is held invalid or unenforceable under -any particular circumstance, the balance of the section is intended to -apply and the section as a whole is intended to apply in other -circumstances. - -It is not the purpose of this section to induce you to infringe any -patents or other property right claims or to contest validity of any -such claims; this section has the sole purpose of protecting the -integrity of the free software distribution system, which is -implemented by public license practices. Many people have made -generous contributions to the wide range of software distributed -through that system in reliance on consistent application of that -system; it is up to the author/donor to decide if he or she is willing -to distribute software through any other system and a licensee cannot -impose that choice. - -This section is intended to make thoroughly clear what is believed to -be a consequence of the rest of this License. +C++ Parsers -@item -If the distribution and/or use of the Program is restricted in -certain countries either by patents or by copyrighted interfaces, the -original copyright holder who places the Program under this License -may add an explicit geographical distribution limitation excluding -those countries, so that distribution is permitted only in or among -countries not thus excluded. In such case, this License incorporates -the limitation as if written in the body of this License. +* C++ Bison Interface:: Asking for C++ parser generation +* C++ Semantic Values:: %union vs. C++ +* C++ Location Values:: The position and location classes +* C++ Parser Interface:: Instantiating and running the parser +* C++ Scanner Interface:: Exchanges between yylex and parse +* A Complete C++ Example:: Demonstrating their use -@item -The Free Software Foundation may publish revised and/or new versions -of the General Public License from time to time. Such new versions will -be similar in spirit to the present version, but may differ in detail to -address new problems or concerns. - -Each version is given a distinguishing version number. If the Program -specifies a version number of this License which applies to it and ``any -later version'', you have the option of following the terms and conditions -either of that version or of any later version published by the Free -Software Foundation. If the Program does not specify a version number of -this License, you may choose any version ever published by the Free Software -Foundation. +A Complete C++ Example -@item -If you wish to incorporate parts of the Program into other free -programs whose distribution conditions are different, write to the author -to ask for permission. For software which is copyrighted by the Free -Software Foundation, write to the Free Software Foundation; we sometimes -make exceptions for this. Our decision will be guided by the two goals -of preserving the free status of all derivatives of our free software and -of promoting the sharing and reuse of software generally. - -@iftex -@heading NO WARRANTY -@end iftex -@ifinfo -@center NO WARRANTY -@end ifinfo +* Calc++ --- C++ Calculator:: The specifications +* Calc++ Parsing Driver:: An active parsing context +* Calc++ Parser:: A parser class +* Calc++ Scanner:: A pure C++ Flex scanner +* Calc++ Top Level:: Conducting the band -@item -BECAUSE THE PROGRAM IS LICENSED FREE OF CHARGE, THERE IS NO WARRANTY -FOR THE PROGRAM, TO THE EXTENT PERMITTED BY APPLICABLE LAW. EXCEPT WHEN -OTHERWISE STATED IN WRITING THE COPYRIGHT HOLDERS AND/OR OTHER PARTIES -PROVIDE THE PROGRAM ``AS IS'' WITHOUT WARRANTY OF ANY KIND, EITHER EXPRESSED -OR IMPLIED, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF -MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. THE ENTIRE RISK AS -TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU. SHOULD THE -PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, -REPAIR OR CORRECTION. +Java Parsers -@item -IN NO EVENT UNLESS REQUIRED BY APPLICABLE LAW OR AGREED TO IN WRITING -WILL ANY COPYRIGHT HOLDER, OR ANY OTHER PARTY WHO MAY MODIFY AND/OR -REDISTRIBUTE THE PROGRAM AS PERMITTED ABOVE, BE LIABLE TO YOU FOR DAMAGES, -INCLUDING ANY GENERAL, SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES ARISING -OUT OF THE USE OR INABILITY TO USE THE PROGRAM (INCLUDING BUT NOT LIMITED -TO LOSS OF DATA OR DATA BEING RENDERED INACCURATE OR LOSSES SUSTAINED BY -YOU OR THIRD PARTIES OR A FAILURE OF THE PROGRAM TO OPERATE WITH ANY OTHER -PROGRAMS), EVEN IF SUCH HOLDER OR OTHER PARTY HAS BEEN ADVISED OF THE -POSSIBILITY OF SUCH DAMAGES. -@end enumerate +* Java Bison Interface:: Asking for Java parser generation +* Java Semantic Values:: %type and %token vs. Java +* Java Location Values:: The position and location classes +* Java Parser Interface:: Instantiating and running the parser +* Java Scanner Interface:: Specifying the scanner for the parser +* Java Action Features:: Special features for use in actions +* Java Differences:: Differences between C/C++ and Java Grammars +* Java Declarations Summary:: List of Bison declarations used with Java -@iftex -@heading END OF TERMS AND CONDITIONS -@end iftex -@ifinfo -@center END OF TERMS AND CONDITIONS -@end ifinfo +Frequently Asked Questions -@page -@unnumberedsec How to Apply These Terms to Your New Programs +* Memory Exhausted:: Breaking the Stack Limits +* How Can I Reset the Parser:: @code{yyparse} Keeps some State +* Strings are Destroyed:: @code{yylval} Loses Track of Strings +* Implementing Gotos/Loops:: Control Flow in the Calculator +* Multiple start-symbols:: Factoring closely related grammars +* Secure? Conform?:: Is Bison @acronym{POSIX} safe? +* I can't build Bison:: Troubleshooting +* Where can I find help?:: Troubleshouting +* Bug Reports:: Troublereporting +* More Languages:: Parsers in C++, Java, and so on +* Beta Testing:: Experimenting development versions +* Mailing Lists:: Meeting other Bison users - If you develop a new program, and you want it to be of the greatest -possible use to the public, the best way to achieve this is to make it -free software which everyone can redistribute and change under these terms. +Copying This Manual - To do so, attach the following notices to the program. It is safest -to attach them to the start of each source file to most effectively -convey the exclusion of warranty; and each file should have at least -the ``copyright'' line and a pointer to where the full notice is found. +* Copying This Manual:: License for copying this manual. -@smallexample -@var{one line to give the program's name and a brief idea of what it does.} -Copyright (C) 19@var{yy} @var{name of author} - -This program is free software; you can redistribute it and/or modify -it under the terms of the GNU General Public License as published by -the Free Software Foundation; either version 2 of the License, or -(at your option) any later version. - -This program is distributed in the hope that it will be useful, -but WITHOUT ANY WARRANTY; without even the implied warranty of -MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the -GNU General Public License for more details. - -You should have received a copy of the GNU General Public License -along with this program; if not, write to the Free Software -Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. -@end smallexample +@end detailmenu +@end menu -Also add information on how to contact you by electronic and paper mail. +@node Introduction +@unnumbered Introduction +@cindex introduction -If the program is interactive, make it output a short notice like this -when it starts in an interactive mode: +@dfn{Bison} is a general-purpose parser generator that converts an +annotated context-free grammar into an @acronym{LALR}(1) or +@acronym{GLR} parser for that grammar. Once you are proficient with +Bison, you can use it to develop a wide range of language parsers, from those +used in simple desk calculators to complex programming languages. -@smallexample -Gnomovision version 69, Copyright (C) 19@var{yy} @var{name of author} -Gnomovision comes with ABSOLUTELY NO WARRANTY; for details -type `show w'. -This is free software, and you are welcome to redistribute it -under certain conditions; type `show c' for details. -@end smallexample +Bison is upward compatible with Yacc: all properly-written Yacc grammars +ought to work with Bison with no change. Anyone familiar with Yacc +should be able to use Bison with little trouble. You need to be fluent in +C or C++ programming in order to use Bison or to understand this manual. -The hypothetical commands @samp{show w} and @samp{show c} should show -the appropriate parts of the General Public License. Of course, the -commands you use may be called something other than @samp{show w} and -@samp{show c}; they could even be mouse-clicks or menu items---whatever -suits your program. +We begin with tutorial chapters that explain the basic concepts of using +Bison and show three explained examples, each building on the last. If you +don't know Bison or Yacc, start by reading these chapters. Reference +chapters follow which describe specific aspects of Bison in detail. -You should also get your employer (if you work as a programmer) or your -school, if any, to sign a ``copyright disclaimer'' for the program, if -necessary. Here is a sample; alter the names: +Bison was written primarily by Robert Corbett; Richard Stallman made it +Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added +multi-character string literals and other features. -@smallexample -Yoyodyne, Inc., hereby disclaims all copyright interest in the program -`Gnomovision' (which makes passes at compilers) written by James Hacker. +This edition corresponds to version @value{VERSION} of Bison. -@var{signature of Ty Coon}, 1 April 1989 -Ty Coon, President of Vice -@end smallexample +@node Conditions +@unnumbered Conditions for Using Bison -This General Public License does not permit incorporating your program into -proprietary programs. If your program is a subroutine library, you may -consider it more useful to permit linking proprietary applications with the -library. If this is what you want to do, use the GNU Library General -Public License instead of this License. +The distribution terms for Bison-generated parsers permit using the +parsers in nonfree programs. Before Bison version 2.2, these extra +permissions applied only when Bison was generating @acronym{LALR}(1) +parsers in C@. And before Bison version 1.24, Bison-generated +parsers could be used only in programs that were free software. + +The other @acronym{GNU} programming tools, such as the @acronym{GNU} C +compiler, have never +had such a requirement. They could always be used for nonfree +software. The reason Bison was different was not due to a special +policy decision; it resulted from applying the usual General Public +License to all of the Bison source code. + +The output of the Bison utility---the Bison parser file---contains a +verbatim copy of a sizable piece of Bison, which is the code for the +parser's implementation. (The actions from your grammar are inserted +into this implementation at one point, but most of the rest of the +implementation is not changed.) When we applied the @acronym{GPL} +terms to the skeleton code for the parser's implementation, +the effect was to restrict the use of Bison output to free software. + +We didn't change the terms because of sympathy for people who want to +make software proprietary. @strong{Software should be free.} But we +concluded that limiting Bison's use to free software was doing little to +encourage people to make other software free. So we decided to make the +practical conditions for using Bison match the practical conditions for +using the other @acronym{GNU} tools. + +This exception applies when Bison is generating code for a parser. +You can tell whether the exception applies to a Bison output file by +inspecting the file for text beginning with ``As a special +exception@dots{}''. The text spells out the exact terms of the +exception. + +@node Copying +@unnumbered GNU GENERAL PUBLIC LICENSE +@include gpl-3.0.texi -@node Concepts, Examples, Copying, Top +@node Concepts @chapter The Concepts of Bison This chapter introduces many of the basic concepts without which the @@ -743,24 +422,24 @@ details of Bison will not make sense. If you do not already know how to use Bison or Yacc, we suggest you start by reading this chapter carefully. @menu -* Language and Grammar:: Languages and context-free grammars, - as mathematical ideas. -* Grammar in Bison:: How we represent grammars for Bison's sake. -* Semantic Values:: Each token or syntactic grouping can have - a semantic value (the value of an integer, - the name of an identifier, etc.). -* Semantic Actions:: Each rule can have an action containing C code. -* Bison Parser:: What are Bison's input and output, - how is the output used? -* Stages:: Stages in writing and running Bison grammars. -* Grammar Layout:: Overall structure of a Bison grammar file. +* Language and Grammar:: Languages and context-free grammars, + as mathematical ideas. +* Grammar in Bison:: How we represent grammars for Bison's sake. +* Semantic Values:: Each token or syntactic grouping can have + a semantic value (the value of an integer, + the name of an identifier, etc.). +* Semantic Actions:: Each rule can have an action containing C code. +* GLR Parsers:: Writing parsers for general context-free languages. +* Locations Overview:: Tracking Locations. +* Bison Parser:: What are Bison's input and output, + how is the output used? +* Stages:: Stages in writing and running Bison grammars. +* Grammar Layout:: Overall structure of a Bison grammar file. @end menu -@node Language and Grammar, Grammar in Bison, , Concepts +@node Language and Grammar @section Languages and Context-Free Grammars -@c !!! ``An expression can be an integer'' is not a valid Bison -@c expression---Bison cannot read English! --rjc 6 Feb 1992 @cindex context-free grammar @cindex grammar, context-free In order for Bison to parse a language, it must be described by a @@ -773,57 +452,92 @@ can be made of a minus sign and another expression''. Another would be, recursive, but there must be at least one rule which leads out of the recursion. -@cindex BNF +@cindex @acronym{BNF} @cindex Backus-Naur form The most common formal system for presenting such rules for humans to read -is @dfn{Backus-Naur Form} or ``BNF'', which was developed in order to -specify the language Algol 60. Any grammar expressed in BNF is a -context-free grammar. The input to Bison is essentially machine-readable -BNF. - -Not all context-free languages can be handled by Bison, only those -that are LALR(1). In brief, this means that it must be possible to +is @dfn{Backus-Naur Form} or ``@acronym{BNF}'', which was developed in +order to specify the language Algol 60. Any grammar expressed in +@acronym{BNF} is a context-free grammar. The input to Bison is +essentially machine-readable @acronym{BNF}. + +@cindex @acronym{LALR}(1) grammars +@cindex @acronym{LR}(1) grammars +There are various important subclasses of context-free grammar. Although it +can handle almost all context-free grammars, Bison is optimized for what +are called @acronym{LALR}(1) grammars. +In brief, in these grammars, it must be possible to tell how to parse any portion of an input string with just a single -token of look-ahead. Strictly speaking, that is a description of an -LR(1) grammar, and LALR(1) involves additional restrictions that are +token of lookahead. Strictly speaking, that is a description of an +@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional +restrictions that are hard to explain simply; but it is rare in actual practice to find an -LR(1) grammar that fails to be LALR(1). @xref{Mystery Conflicts, , -Mysterious Reduce/Reduce Conflicts}, for more information on this. +@acronym{LR}(1) grammar that fails to be @acronym{LALR}(1). +@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for +more information on this. + +@cindex @acronym{GLR} parsing +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing +@cindex ambiguous grammars +@cindex nondeterministic parsing + +Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning +roughly that the next grammar rule to apply at any point in the input is +uniquely determined by the preceding input and a fixed, finite portion +(called a @dfn{lookahead}) of the remaining input. A context-free +grammar can be @dfn{ambiguous}, meaning that there are multiple ways to +apply the grammar rules to get the same inputs. Even unambiguous +grammars can be @dfn{nondeterministic}, meaning that no fixed +lookahead always suffices to determine the next grammar rule to apply. +With the proper declarations, Bison is also able to parse these more +general context-free grammars, using a technique known as @acronym{GLR} +parsing (for Generalized @acronym{LR}). Bison's @acronym{GLR} parsers +are able to handle any context-free grammar for which the number of +possible parses of any given string is finite. @cindex symbols (abstract) @cindex token @cindex syntactic grouping @cindex grouping, syntactic -In the formal grammatical rules for a language, each kind of syntactic unit -or grouping is named by a @dfn{symbol}. Those which are built by grouping -smaller constructs according to grammatical rules are called +In the formal grammatical rules for a language, each kind of syntactic +unit or grouping is named by a @dfn{symbol}. Those which are built by +grouping smaller constructs according to grammatical rules are called @dfn{nonterminal symbols}; those which can't be subdivided are called @dfn{terminal symbols} or @dfn{token types}. We call a piece of input corresponding to a single terminal symbol a @dfn{token}, and a piece -corresponding to a single nonterminal symbol a @dfn{grouping}.@refill +corresponding to a single nonterminal symbol a @dfn{grouping}. We can use the C language as an example of what symbols, terminal and -nonterminal, mean. The tokens of C are identifiers, constants (numeric and -string), and the various keywords, arithmetic operators and punctuation -marks. So the terminal symbols of a grammar for C include `identifier', -`number', `string', plus one symbol for each keyword, operator or -punctuation mark: `if', `return', `const', `static', `int', `char', -`plus-sign', `open-brace', `close-brace', `comma' and many more. (These -tokens can be subdivided into characters, but that is a matter of +nonterminal, mean. The tokens of C are identifiers, constants (numeric +and string), and the various keywords, arithmetic operators and +punctuation marks. So the terminal symbols of a grammar for C include +`identifier', `number', `string', plus one symbol for each keyword, +operator or punctuation mark: `if', `return', `const', `static', `int', +`char', `plus-sign', `open-brace', `close-brace', `comma' and many more. +(These tokens can be subdivided into characters, but that is a matter of lexicography, not grammar.) Here is a simple C function subdivided into tokens: +@ifinfo +@example +int /* @r{keyword `int'} */ +square (int x) /* @r{identifier, open-paren, keyword `int',} + @r{identifier, close-paren} */ +@{ /* @r{open-brace} */ + return x * x; /* @r{keyword `return', identifier, asterisk,} + @r{identifier, semicolon} */ +@} /* @r{close-brace} */ +@end example +@end ifinfo +@ifnotinfo @example int /* @r{keyword `int'} */ -square (x) /* @r{identifier, open-paren,} */ - /* @r{identifier, close-paren} */ - int x; /* @r{keyword `int', identifier, semicolon} */ +square (int x) /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */ @{ /* @r{open-brace} */ - return x * x; /* @r{keyword `return', identifier,} */ - /* @r{asterisk, identifier, semicolon} */ + return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */ @} /* @r{close-brace} */ @end example +@end ifnotinfo The syntactic groupings of C include the expression, the statement, the declaration, and the function definition. These are represented in the @@ -867,7 +581,7 @@ the grammar's start symbol. If we use a grammar for C, the entire input must be a `sequence of definitions and declarations'. If not, the parser reports a syntax error. -@node Grammar in Bison, Semantic Values, Language and Grammar, Concepts +@node Grammar in Bison @section From Formal Rules to Bison Input @cindex Bison grammar @cindex grammar, Bison @@ -878,7 +592,7 @@ for Bison, you must write a file expressing the grammar in Bison syntax: a @dfn{Bison grammar} file. @xref{Grammar File, ,Bison Grammar Files}. A nonterminal symbol in the formal grammar is represented in Bison input -as an identifier, like an identifier in C. By convention, it should be +as an identifier, like an identifier in C@. By convention, it should be in lower case, such as @code{expr}, @code{stmt} or @code{declaration}. The Bison representation for a terminal symbol is also called a @dfn{token @@ -888,13 +602,16 @@ nonterminals: for example, @code{INTEGER}, @code{IDENTIFIER}, @code{IF} or @code{RETURN}. A terminal symbol that stands for a particular keyword in the language should be named after that keyword converted to upper case. The terminal symbol @code{error} is reserved for error recovery. -@xref{Symbols}.@refill +@xref{Symbols}. A terminal symbol can also be represented as a character literal, just like a C character constant. You should do this whenever a token is just a single character (parenthesis, plus-sign, etc.): use that same character in a literal as the terminal symbol for that token. +A third way to represent a terminal symbol is with a C string constant +containing several characters. @xref{Symbols}, for more information. + The grammar rules also have an expression in Bison syntax. For example, here is the Bison rule for a C @code{return} statement. The semicolon in quotes is a literal character token, representing part of the C syntax for @@ -909,7 +626,7 @@ stmt: RETURN expr ';' @noindent @xref{Rules, ,Syntax of Grammar Rules}. -@node Semantic Values, Semantic Actions, Grammar in Bison, Concepts +@node Semantic Values @section Semantic Values @cindex semantic value @cindex value, semantic @@ -919,19 +636,20 @@ if a rule mentions the terminal symbol `integer constant', it means that @emph{any} integer constant is grammatically valid in that position. The precise value of the constant is irrelevant to how to parse the input: if @samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally -grammatical.@refill +grammatical. But the precise value is very important for what the input means once it is parsed. A compiler is useless if it fails to distinguish between 4, 1 and 3989 as constants in the program! Therefore, each token in a Bison grammar -has both a token type and a @dfn{semantic value}. @xref{Semantics, ,Defining Language Semantics}, +has both a token type and a @dfn{semantic value}. @xref{Semantics, +,Defining Language Semantics}, for details. The token type is a terminal symbol defined in the grammar, such as @code{INTEGER}, @code{IDENTIFIER} or @code{','}. It tells everything you need to know to decide where the token may validly appear and how to group it with other tokens. The grammar rules know nothing about tokens -except their types.@refill +except their types. The semantic value has all the rest of the information about the meaning of the token, such as the value of an integer, or the name of an @@ -951,7 +669,7 @@ semantic value that is a number. In a compiler for a programming language, an expression typically has a semantic value that is a tree structure describing the meaning of the expression. -@node Semantic Actions, Bison Parser, Semantic Values, Concepts +@node Semantic Actions @section Semantic Actions @cindex semantic actions @cindex actions, semantic @@ -961,7 +679,7 @@ also produce some output based on the input. In a Bison grammar, a grammar rule can have an @dfn{action} made up of C statements. Each time the parser recognizes a match for that rule, the action is executed. @xref{Actions}. - + Most of the time, the purpose of an action is to compute the semantic value of the whole construct from the semantic values of its parts. For example, suppose we have a rule which says an expression can be the sum of two @@ -982,140 +700,675 @@ expr: expr '+' expr @{ $$ = $1 + $3; @} The action says how to produce the semantic value of the sum expression from the values of the two subexpressions. -@node Bison Parser, Stages, Semantic Actions, Concepts -@section Bison Output: the Parser File -@cindex Bison parser -@cindex Bison utility -@cindex lexical analyzer, purpose -@cindex parser +@node GLR Parsers +@section Writing @acronym{GLR} Parsers +@cindex @acronym{GLR} parsing +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing +@findex %glr-parser +@cindex conflicts +@cindex shift/reduce conflicts +@cindex reduce/reduce conflicts + +In some grammars, Bison's standard +@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a +certain grammar rule at a given point. That is, it may not be able to +decide (on the basis of the input read so far) which of two possible +reductions (applications of a grammar rule) applies, or whether to apply +a reduction or read more of the input and apply a reduction later in the +input. These are known respectively as @dfn{reduce/reduce} conflicts +(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts +(@pxref{Shift/Reduce}). + +To use a grammar that is not easily modified to be @acronym{LALR}(1), a +more general parsing algorithm is sometimes necessary. If you include +@code{%glr-parser} among the Bison declarations in your file +(@pxref{Grammar Outline}), the result is a Generalized @acronym{LR} +(@acronym{GLR}) parser. These parsers handle Bison grammars that +contain no unresolved conflicts (i.e., after applying precedence +declarations) identically to @acronym{LALR}(1) parsers. However, when +faced with unresolved shift/reduce and reduce/reduce conflicts, +@acronym{GLR} parsers use the simple expedient of doing both, +effectively cloning the parser to follow both possibilities. Each of +the resulting parsers can again split, so that at any given time, there +can be any number of possible parses being explored. The parsers +proceed in lockstep; that is, all of them consume (shift) a given input +symbol before any of them proceed to the next. Each of the cloned +parsers eventually meets one of two possible fates: either it runs into +a parsing error, in which case it simply vanishes, or it merges with +another parser, because the two of them have reduced the input to an +identical set of symbols. + +During the time that there are multiple parsers, semantic actions are +recorded, but not performed. When a parser disappears, its recorded +semantic actions disappear as well, and are never performed. When a +reduction makes two parsers identical, causing them to merge, Bison +records both sets of semantic actions. Whenever the last two parsers +merge, reverting to the single-parser case, Bison resolves all the +outstanding actions either by precedences given to the grammar rules +involved, or by performing both actions, and then calling a designated +user-defined function on the resulting values to produce an arbitrary +merged result. -When you run Bison, you give it a Bison grammar file as input. The output -is a C source file that parses the language described by the grammar. -This file is called a @dfn{Bison parser}. Keep in mind that the Bison -utility and the Bison parser are two distinct programs: the Bison utility -is a program whose output is the Bison parser that becomes part of your -program. +@menu +* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars. +* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities. +* GLR Semantic Actions:: Deferred semantic actions have special concerns. +* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler. +@end menu -The job of the Bison parser is to group tokens into groupings according to -the grammar rules---for example, to build identifiers and operators into -expressions. As it does this, it runs the actions for the grammar rules it -uses. +@node Simple GLR Parsers +@subsection Using @acronym{GLR} on Unambiguous Grammars +@cindex @acronym{GLR} parsing, unambiguous grammars +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars +@findex %glr-parser +@findex %expect-rr +@cindex conflicts +@cindex reduce/reduce conflicts +@cindex shift/reduce conflicts -The tokens come from a function called the @dfn{lexical analyzer} that you -must supply in some fashion (such as by writing it in C). The Bison parser -calls the lexical analyzer each time it wants a new token. It doesn't know -what is ``inside'' the tokens (though their semantic values may reflect -this). Typically the lexical analyzer makes the tokens by parsing -characters of text, but Bison does not depend on this. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}. +In the simplest cases, you can use the @acronym{GLR} algorithm +to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1). +Such grammars typically require more than one symbol of lookahead, +or (in rare cases) fall into the category of grammars in which the +@acronym{LALR}(1) algorithm throws away too much information (they are in +@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}). -The Bison parser file is C code which defines a function named -@code{yyparse} which implements that grammar. This function does not make -a complete C program: you must supply some additional functions. One is -the lexical analyzer. Another is an error-reporting function which the -parser calls to report an error. In addition, a complete C program must -start with a function called @code{main}; you have to provide this, and -arrange for it to call @code{yyparse} or the parser will never run. -@xref{Interface, ,Parser C-Language Interface}. +Consider a problem that +arises in the declaration of enumerated and subrange types in the +programming language Pascal. Here are some examples: -Aside from the token type names and the symbols in the actions you -write, all variable and function names used in the Bison parser file -begin with @samp{yy} or @samp{YY}. This includes interface functions -such as the lexical analyzer function @code{yylex}, the error reporting -function @code{yyerror} and the parser function @code{yyparse} itself. -This also includes numerous identifiers used for internal purposes. -Therefore, you should avoid using C identifiers starting with @samp{yy} -or @samp{YY} in the Bison grammar file except for the ones defined in -this manual. +@example +type subrange = lo .. hi; +type enum = (a, b, c); +@end example -@node Stages, Grammar Layout, Bison Parser, Concepts -@section Stages in Using Bison -@cindex stages in using Bison -@cindex using Bison +@noindent +The original language standard allows only numeric +literals and constant identifiers for the subrange bounds (@samp{lo} +and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC} +10206) and many other +Pascal implementations allow arbitrary expressions there. This gives +rise to the following situation, containing a superfluous pair of +parentheses: -The actual language-design process using Bison, from grammar specification -to a working compiler or interpreter, has these parts: +@example +type subrange = (a) .. b; +@end example -@enumerate -@item -Formally specify the grammar in a form recognized by Bison -(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule in the language, -describe the action that is to be taken when an instance of that rule -is recognized. The action is described by a sequence of C statements. +@noindent +Compare this to the following declaration of an enumerated +type with only one value: -@item -Write a lexical analyzer to process input and pass tokens to the -parser. The lexical analyzer may be written by hand in C -(@pxref{Lexical, ,The Lexical Analyzer Function @code{yylex}}). It could also be produced using Lex, but the use -of Lex is not discussed in this manual. +@example +type enum = (a); +@end example -@item -Write a controlling function that calls the Bison-produced parser. +@noindent +(These declarations are contrived, but they are syntactically +valid, and more-complicated cases can come up in practical programs.) + +These two declarations look identical until the @samp{..} token. +With normal @acronym{LALR}(1) one-token lookahead it is not +possible to decide between the two forms when the identifier +@samp{a} is parsed. It is, however, desirable +for a parser to decide this, since in the latter case +@samp{a} must become a new identifier to represent the enumeration +value, while in the former case @samp{a} must be evaluated with its +current meaning, which may be a constant or even a function call. + +You could parse @samp{(a)} as an ``unspecified identifier in parentheses'', +to be resolved later, but this typically requires substantial +contortions in both semantic actions and large parts of the +grammar, where the parentheses are nested in the recursive rules for +expressions. + +You might think of using the lexer to distinguish between the two +forms by returning different tokens for currently defined and +undefined identifiers. But if these declarations occur in a local +scope, and @samp{a} is defined in an outer scope, then both forms +are possible---either locally redefining @samp{a}, or using the +value of @samp{a} from the outer scope. So this approach cannot +work. + +A simple solution to this problem is to declare the parser to +use the @acronym{GLR} algorithm. +When the @acronym{GLR} parser reaches the critical state, it +merely splits into two branches and pursues both syntax rules +simultaneously. Sooner or later, one of them runs into a parsing +error. If there is a @samp{..} token before the next +@samp{;}, the rule for enumerated types fails since it cannot +accept @samp{..} anywhere; otherwise, the subrange type rule +fails since it requires a @samp{..} token. So one of the branches +fails silently, and the other one continues normally, performing +all the intermediate actions that were postponed during the split. + +If the input is syntactically incorrect, both branches fail and the parser +reports a syntax error as usual. + +The effect of all this is that the parser seems to ``guess'' the +correct branch to take, or in other words, it seems to use more +lookahead than the underlying @acronym{LALR}(1) algorithm actually allows +for. In this example, @acronym{LALR}(2) would suffice, but also some cases +that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way. + +In general, a @acronym{GLR} parser can take quadratic or cubic worst-case time, +and the current Bison parser even takes exponential time and space +for some grammars. In practice, this rarely happens, and for many +grammars it is possible to prove that it cannot happen. +The present example contains only one conflict between two +rules, and the type-declaration context containing the conflict +cannot be nested. So the number of +branches that can exist at any time is limited by the constant 2, +and the parsing time is still linear. + +Here is a Bison grammar corresponding to the example above. It +parses a vastly simplified form of Pascal type declarations. -@item -Write error-reporting routines. -@end enumerate +@example +%token TYPE DOTDOT ID -To turn this source code as written into a runnable program, you -must follow these steps: +@group +%left '+' '-' +%left '*' '/' +@end group -@enumerate -@item -Run Bison on the grammar to produce the parser. +%% -@item -Compile the code output by Bison, as well as any other source files. +@group +type_decl : TYPE ID '=' type ';' + ; +@end group -@item -Link the object files to produce the finished product. -@end enumerate +@group +type : '(' id_list ')' + | expr DOTDOT expr + ; +@end group -@node Grammar Layout, , Stages, Concepts -@section The Overall Layout of a Bison Grammar -@cindex grammar file -@cindex file format -@cindex format of grammar file -@cindex layout of Bison grammar +@group +id_list : ID + | id_list ',' ID + ; +@end group -The input file for the Bison utility is a @dfn{Bison grammar file}. The -general form of a Bison grammar file is as follows: +@group +expr : '(' expr ')' + | expr '+' expr + | expr '-' expr + | expr '*' expr + | expr '/' expr + | ID + ; +@end group +@end example + +When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains +about one reduce/reduce conflict. In the conflicting situation the +parser chooses one of the alternatives, arbitrarily the one +declared first. Therefore the following correct input is not +recognized: + +@example +type t = (a) .. b; +@end example + +The parser can be turned into a @acronym{GLR} parser, while also telling Bison +to be silent about the one known reduce/reduce conflict, by +adding these two declarations to the Bison input file (before the first +@samp{%%}): + +@example +%glr-parser +%expect-rr 1 +@end example + +@noindent +No change in the grammar itself is required. Now the +parser recognizes all valid declarations, according to the +limited syntax above, transparently. In fact, the user does not even +notice when the parser splits. + +So here we have a case where we can use the benefits of @acronym{GLR}, +almost without disadvantages. Even in simple cases like this, however, +there are at least two potential problems to beware. First, always +analyze the conflicts reported by Bison to make sure that @acronym{GLR} +splitting is only done where it is intended. A @acronym{GLR} parser +splitting inadvertently may cause problems less obvious than an +@acronym{LALR} parser statically choosing the wrong alternative in a +conflict. Second, consider interactions with the lexer (@pxref{Semantic +Tokens}) with great care. Since a split parser consumes tokens without +performing any actions during the split, the lexer cannot obtain +information via parser actions. Some cases of lexer interactions can be +eliminated by using @acronym{GLR} to shift the complications from the +lexer to the parser. You must check the remaining cases for +correctness. + +In our example, it would be safe for the lexer to return tokens based on +their current meanings in some symbol table, because no new symbols are +defined in the middle of a type declaration. Though it is possible for +a parser to define the enumeration constants as they are parsed, before +the type declaration is completed, it actually makes no difference since +they cannot be used within the same enumerated type declaration. + +@node Merging GLR Parses +@subsection Using @acronym{GLR} to Resolve Ambiguities +@cindex @acronym{GLR} parsing, ambiguous grammars +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars +@findex %dprec +@findex %merge +@cindex conflicts +@cindex reduce/reduce conflicts + +Let's consider an example, vastly simplified from a C++ grammar. @example %@{ -@var{C declarations} + #include + #define YYSTYPE char const * + int yylex (void); + void yyerror (char const *); %@} -@var{Bison declarations} +%token TYPENAME ID + +%right '=' +%left '+' + +%glr-parser %% -@var{Grammar rules} -%% -@var{Additional C code} + +prog : + | prog stmt @{ printf ("\n"); @} + ; + +stmt : expr ';' %dprec 1 + | decl %dprec 2 + ; + +expr : ID @{ printf ("%s ", $$); @} + | TYPENAME '(' expr ')' + @{ printf ("%s ", $1); @} + | expr '+' expr @{ printf ("+ "); @} + | expr '=' expr @{ printf ("= "); @} + ; + +decl : TYPENAME declarator ';' + @{ printf ("%s ", $1); @} + | TYPENAME declarator '=' expr ';' + @{ printf ("%s ", $1); @} + ; + +declarator : ID @{ printf ("\"%s\" ", $1); @} + | '(' declarator ')' + ; @end example @noindent -The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears -in every Bison grammar file to separate the sections. +This models a problematic part of the C++ grammar---the ambiguity between +certain declarations and statements. For example, -The C declarations may define types and variables used in the actions. -You can also use preprocessor commands to define macros used there, and use -@code{#include} to include header files that do any of these things. +@example +T (x) = y+z; +@end example -The Bison declarations declare the names of the terminal and nonterminal -symbols, and may also describe operator precedence and the data types of -semantic values of various symbols. +@noindent +parses as either an @code{expr} or a @code{stmt} +(assuming that @samp{T} is recognized as a @code{TYPENAME} and +@samp{x} as an @code{ID}). +Bison detects this as a reduce/reduce conflict between the rules +@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the +time it encounters @code{x} in the example above. Since this is a +@acronym{GLR} parser, it therefore splits the problem into two parses, one for +each choice of resolving the reduce/reduce conflict. +Unlike the example from the previous section (@pxref{Simple GLR Parsers}), +however, neither of these parses ``dies,'' because the grammar as it stands is +ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and +the other reduces @code{stmt : decl}, after which both parsers are in an +identical state: they've seen @samp{prog stmt} and have the same unprocessed +input remaining. We say that these parses have @dfn{merged.} + +At this point, the @acronym{GLR} parser requires a specification in the +grammar of how to choose between the competing parses. +In the example above, the two @code{%dprec} +declarations specify that Bison is to give precedence +to the parse that interprets the example as a +@code{decl}, which implies that @code{x} is a declarator. +The parser therefore prints -The grammar rules define how to construct each nonterminal symbol from its -parts. +@example +"x" y z + T +@end example -The additional C code can contain any C code you want to use. Often the -definition of the lexical analyzer @code{yylex} goes here, plus subroutines -called by the actions in the grammar rules. In a simple program, all the -rest of the program can go here. +The @code{%dprec} declarations only come into play when more than one +parse survives. Consider a different input string for this parser: -@node Examples, Grammar File, Concepts, Top -@chapter Examples -@cindex simple examples -@cindex examples, simple +@example +T (x) + y; +@end example + +@noindent +This is another example of using @acronym{GLR} to parse an unambiguous +construct, as shown in the previous section (@pxref{Simple GLR Parsers}). +Here, there is no ambiguity (this cannot be parsed as a declaration). +However, at the time the Bison parser encounters @code{x}, it does not +have enough information to resolve the reduce/reduce conflict (again, +between @code{x} as an @code{expr} or a @code{declarator}). In this +case, no precedence declaration is used. Again, the parser splits +into two, one assuming that @code{x} is an @code{expr}, and the other +assuming @code{x} is a @code{declarator}. The second of these parsers +then vanishes when it sees @code{+}, and the parser prints + +@example +x T y + +@end example + +Suppose that instead of resolving the ambiguity, you wanted to see all +the possibilities. For this purpose, you must merge the semantic +actions of the two possible parsers, rather than choosing one over the +other. To do so, you could change the declaration of @code{stmt} as +follows: + +@example +stmt : expr ';' %merge + | decl %merge + ; +@end example + +@noindent +and define the @code{stmtMerge} function as: + +@example +static YYSTYPE +stmtMerge (YYSTYPE x0, YYSTYPE x1) +@{ + printf (" "); + return ""; +@} +@end example + +@noindent +with an accompanying forward declaration +in the C declarations at the beginning of the file: + +@example +%@{ + #define YYSTYPE char const * + static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1); +%@} +@end example + +@noindent +With these declarations, the resulting parser parses the first example +as both an @code{expr} and a @code{decl}, and prints + +@example +"x" y z + T x T y z + = +@end example + +Bison requires that all of the +productions that participate in any particular merge have identical +@samp{%merge} clauses. Otherwise, the ambiguity would be unresolvable, +and the parser will report an error during any parse that results in +the offending merge. + +@node GLR Semantic Actions +@subsection GLR Semantic Actions + +@cindex deferred semantic actions +By definition, a deferred semantic action is not performed at the same time as +the associated reduction. +This raises caveats for several Bison features you might use in a semantic +action in a @acronym{GLR} parser. + +@vindex yychar +@cindex @acronym{GLR} parsers and @code{yychar} +@vindex yylval +@cindex @acronym{GLR} parsers and @code{yylval} +@vindex yylloc +@cindex @acronym{GLR} parsers and @code{yylloc} +In any semantic action, you can examine @code{yychar} to determine the type of +the lookahead token present at the time of the associated reduction. +After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF}, +you can then examine @code{yylval} and @code{yylloc} to determine the +lookahead token's semantic value and location, if any. +In a nondeferred semantic action, you can also modify any of these variables to +influence syntax analysis. +@xref{Lookahead, ,Lookahead Tokens}. + +@findex yyclearin +@cindex @acronym{GLR} parsers and @code{yyclearin} +In a deferred semantic action, it's too late to influence syntax analysis. +In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to +shallow copies of the values they had at the time of the associated reduction. +For this reason alone, modifying them is dangerous. +Moreover, the result of modifying them is undefined and subject to change with +future versions of Bison. +For example, if a semantic action might be deferred, you should never write it +to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free +memory referenced by @code{yylval}. + +@findex YYERROR +@cindex @acronym{GLR} parsers and @code{YYERROR} +Another Bison feature requiring special consideration is @code{YYERROR} +(@pxref{Action Features}), which you can invoke in a semantic action to +initiate error recovery. +During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is +the same as its effect in an @acronym{LALR}(1) parser. +In a deferred semantic action, its effect is undefined. +@c The effect is probably a syntax error at the split point. + +Also, see @ref{Location Default Action, ,Default Action for Locations}, which +describes a special usage of @code{YYLLOC_DEFAULT} in @acronym{GLR} parsers. + +@node Compiler Requirements +@subsection Considerations when Compiling @acronym{GLR} Parsers +@cindex @code{inline} +@cindex @acronym{GLR} parsers and @code{inline} + +The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or +later. In addition, they use the @code{inline} keyword, which is not +C89, but is C99 and is a common extension in pre-C99 compilers. It is +up to the user of these parsers to handle +portability issues. For instance, if using Autoconf and the Autoconf +macro @code{AC_C_INLINE}, a mere + +@example +%@{ + #include +%@} +@end example + +@noindent +will suffice. Otherwise, we suggest + +@example +%@{ + #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline + #define inline + #endif +%@} +@end example + +@node Locations Overview +@section Locations +@cindex location +@cindex textual location +@cindex location, textual + +Many applications, like interpreters or compilers, have to produce verbose +and useful error messages. To achieve this, one must be able to keep track of +the @dfn{textual location}, or @dfn{location}, of each syntactic construct. +Bison provides a mechanism for handling these locations. + +Each token has a semantic value. In a similar fashion, each token has an +associated location, but the type of locations is the same for all tokens and +groupings. Moreover, the output parser is equipped with a default data +structure for storing locations (@pxref{Locations}, for more details). + +Like semantic values, locations can be reached in actions using a dedicated +set of constructs. In the example above, the location of the whole grouping +is @code{@@$}, while the locations of the subexpressions are @code{@@1} and +@code{@@3}. + +When a rule is matched, a default action is used to compute the semantic value +of its left hand side (@pxref{Actions}). In the same way, another default +action is used for locations. However, the action for locations is general +enough for most cases, meaning there is usually no need to describe for each +rule how @code{@@$} should be formed. When building a new location for a given +grouping, the default behavior of the output parser is to take the beginning +of the first symbol, and the end of the last symbol. + +@node Bison Parser +@section Bison Output: the Parser File +@cindex Bison parser +@cindex Bison utility +@cindex lexical analyzer, purpose +@cindex parser + +When you run Bison, you give it a Bison grammar file as input. The output +is a C source file that parses the language described by the grammar. +This file is called a @dfn{Bison parser}. Keep in mind that the Bison +utility and the Bison parser are two distinct programs: the Bison utility +is a program whose output is the Bison parser that becomes part of your +program. + +The job of the Bison parser is to group tokens into groupings according to +the grammar rules---for example, to build identifiers and operators into +expressions. As it does this, it runs the actions for the grammar rules it +uses. + +The tokens come from a function called the @dfn{lexical analyzer} that +you must supply in some fashion (such as by writing it in C). The Bison +parser calls the lexical analyzer each time it wants a new token. It +doesn't know what is ``inside'' the tokens (though their semantic values +may reflect this). Typically the lexical analyzer makes the tokens by +parsing characters of text, but Bison does not depend on this. +@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}. + +The Bison parser file is C code which defines a function named +@code{yyparse} which implements that grammar. This function does not make +a complete C program: you must supply some additional functions. One is +the lexical analyzer. Another is an error-reporting function which the +parser calls to report an error. In addition, a complete C program must +start with a function called @code{main}; you have to provide this, and +arrange for it to call @code{yyparse} or the parser will never run. +@xref{Interface, ,Parser C-Language Interface}. + +Aside from the token type names and the symbols in the actions you +write, all symbols defined in the Bison parser file itself +begin with @samp{yy} or @samp{YY}. This includes interface functions +such as the lexical analyzer function @code{yylex}, the error reporting +function @code{yyerror} and the parser function @code{yyparse} itself. +This also includes numerous identifiers used for internal purposes. +Therefore, you should avoid using C identifiers starting with @samp{yy} +or @samp{YY} in the Bison grammar file except for the ones defined in +this manual. Also, you should avoid using the C identifiers +@samp{malloc} and @samp{free} for anything other than their usual +meanings. + +In some cases the Bison parser file includes system headers, and in +those cases your code should respect the identifiers reserved by those +headers. On some non-@acronym{GNU} hosts, @code{}, @code{}, +@code{}, and @code{} are included as needed to +declare memory allocators and related types. @code{} is +included if message translation is in use +(@pxref{Internationalization}). Other system headers may +be included if you define @code{YYDEBUG} to a nonzero value +(@pxref{Tracing, ,Tracing Your Parser}). + +@node Stages +@section Stages in Using Bison +@cindex stages in using Bison +@cindex using Bison + +The actual language-design process using Bison, from grammar specification +to a working compiler or interpreter, has these parts: + +@enumerate +@item +Formally specify the grammar in a form recognized by Bison +(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule +in the language, describe the action that is to be taken when an +instance of that rule is recognized. The action is described by a +sequence of C statements. + +@item +Write a lexical analyzer to process input and pass tokens to the parser. +The lexical analyzer may be written by hand in C (@pxref{Lexical, ,The +Lexical Analyzer Function @code{yylex}}). It could also be produced +using Lex, but the use of Lex is not discussed in this manual. + +@item +Write a controlling function that calls the Bison-produced parser. + +@item +Write error-reporting routines. +@end enumerate + +To turn this source code as written into a runnable program, you +must follow these steps: + +@enumerate +@item +Run Bison on the grammar to produce the parser. + +@item +Compile the code output by Bison, as well as any other source files. + +@item +Link the object files to produce the finished product. +@end enumerate + +@node Grammar Layout +@section The Overall Layout of a Bison Grammar +@cindex grammar file +@cindex file format +@cindex format of grammar file +@cindex layout of Bison grammar + +The input file for the Bison utility is a @dfn{Bison grammar file}. The +general form of a Bison grammar file is as follows: + +@example +%@{ +@var{Prologue} +%@} + +@var{Bison declarations} + +%% +@var{Grammar rules} +%% +@var{Epilogue} +@end example + +@noindent +The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears +in every Bison grammar file to separate the sections. + +The prologue may define types and variables used in the actions. You can +also use preprocessor commands to define macros used there, and use +@code{#include} to include header files that do any of these things. +You need to declare the lexical analyzer @code{yylex} and the error +printer @code{yyerror} here, along with any other global identifiers +used by the actions in the grammar rules. + +The Bison declarations declare the names of the terminal and nonterminal +symbols, and may also describe operator precedence and the data types of +semantic values of various symbols. + +The grammar rules define how to construct each nonterminal symbol from its +parts. + +The epilogue can contain any code you want to use. Often the +definitions of functions declared in the prologue go here. In a +simple program, all the rest of the program can go here. + +@node Examples +@chapter Examples +@cindex simple examples +@cindex examples, simple Now we show and explain three sample programs written using Bison: a reverse polish notation calculator, an algebraic (infix) notation @@ -1124,24 +1377,22 @@ under BSD Unix 4.3; each produces a usable, though limited, interactive desk-top calculator. These examples are simple, but Bison grammars for real programming -languages are written the same way. -@ifinfo -You can copy these examples out of the Info file and into a source file -to try them. -@end ifinfo +languages are written the same way. You can copy these examples into a +source file to try them. @menu -* RPN Calc:: Reverse polish notation calculator; - a first example with no operator precedence. -* Infix Calc:: Infix (algebraic) notation calculator. - Operator precedence is introduced. +* RPN Calc:: Reverse polish notation calculator; + a first example with no operator precedence. +* Infix Calc:: Infix (algebraic) notation calculator. + Operator precedence is introduced. * Simple Error Recovery:: Continuing after syntax errors. -* Multi-function Calc:: Calculator with memory and trig functions. - It uses multiple data-types for semantic values. -* Exercises:: Ideas for improving the multi-function calculator. +* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$. +* Multi-function Calc:: Calculator with memory and trig functions. + It uses multiple data-types for semantic values. +* Exercises:: Ideas for improving the multi-function calculator. @end menu -@node RPN Calc, Infix Calc, , Examples +@node RPN Calc @section Reverse Polish Notation Calculator @cindex reverse polish notation @cindex polish notation calculator @@ -1157,56 +1408,66 @@ The source code for this calculator is named @file{rpcalc.y}. The @samp{.y} extension is a convention used for Bison input files. @menu -* Decls: Rpcalc Decls. Bison and C declarations for rpcalc. -* Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation. -* Lexer: Rpcalc Lexer. The lexical analyzer. -* Main: Rpcalc Main. The controlling function. -* Error: Rpcalc Error. The error reporting function. -* Gen: Rpcalc Gen. Running Bison on the grammar file. -* Comp: Rpcalc Compile. Run the C compiler on the output code. +* Rpcalc Declarations:: Prologue (declarations) for rpcalc. +* Rpcalc Rules:: Grammar Rules for rpcalc, with explanation. +* Rpcalc Lexer:: The lexical analyzer. +* Rpcalc Main:: The controlling function. +* Rpcalc Error:: The error reporting function. +* Rpcalc Generate:: Running Bison on the grammar file. +* Rpcalc Compile:: Run the C compiler on the output code. @end menu -@node Rpcalc Decls, Rpcalc Rules, , RPN Calc +@node Rpcalc Declarations @subsection Declarations for @code{rpcalc} Here are the C and Bison declarations for the reverse polish notation calculator. As in C, comments are placed between @samp{/*@dots{}*/}. @example -/* Reverse polish notation calculator. */ +/* Reverse polish notation calculator. */ %@{ -#define YYSTYPE double -#include + #define YYSTYPE double + #include + int yylex (void); + void yyerror (char const *); %@} %token NUM -%% /* Grammar rules and actions follow */ +%% /* Grammar rules and actions follow. */ @end example -The C declarations section (@pxref{C Declarations, ,The C Declarations Section}) contains two -preprocessor directives. +The declarations section (@pxref{Prologue, , The prologue}) contains two +preprocessor directives and two forward declarations. The @code{#define} directive defines the macro @code{YYSTYPE}, thus -specifying the C data type for semantic values of both tokens and groupings -(@pxref{Value Type, ,Data Types of Semantic Values}). The Bison parser will use whatever type -@code{YYSTYPE} is defined as; if you don't define it, @code{int} is the -default. Because we specify @code{double}, each token and each expression -has an associated value, which is a floating point number. +specifying the C data type for semantic values of both tokens and +groupings (@pxref{Value Type, ,Data Types of Semantic Values}). The +Bison parser will use whatever type @code{YYSTYPE} is defined as; if you +don't define it, @code{int} is the default. Because we specify +@code{double}, each token and each expression has an associated value, +which is a floating point number. The @code{#include} directive is used to declare the exponentiation function @code{pow}. -The second section, Bison declarations, provides information to Bison about -the token types (@pxref{Bison Declarations, ,The Bison Declarations Section}). Each terminal symbol that is -not a single-character literal must be declared here. (Single-character +The forward declarations for @code{yylex} and @code{yyerror} are +needed because the C language requires that functions be declared +before they are used. These functions will be defined in the +epilogue, but the parser calls them so they must be declared in the +prologue. + +The second section, Bison declarations, provides information to Bison +about the token types (@pxref{Bison Declarations, ,The Bison +Declarations Section}). Each terminal symbol that is not a +single-character literal must be declared here. (Single-character literals normally don't need to be declared.) In this example, all the arithmetic operators are designated by single-character literals, so the only terminal symbol that needs to be declared is @code{NUM}, the token type for numeric constants. -@node Rpcalc Rules, Rpcalc Lexer, Rpcalc Decls, RPN Calc +@node Rpcalc Rules @subsection Grammar Rules for @code{rpcalc} Here are the grammar rules for the reverse polish notation calculator. @@ -1217,18 +1478,18 @@ input: /* empty */ ; line: '\n' - | exp '\n' @{ printf ("\t%.10g\n", $1); @} + | exp '\n' @{ printf ("\t%.10g\n", $1); @} ; -exp: NUM @{ $$ = $1; @} - | exp exp '+' @{ $$ = $1 + $2; @} - | exp exp '-' @{ $$ = $1 - $2; @} - | exp exp '*' @{ $$ = $1 * $2; @} - | exp exp '/' @{ $$ = $1 / $2; @} - /* Exponentiation */ - | exp exp '^' @{ $$ = pow ($1, $2); @} - /* Unary minus */ - | exp 'n' @{ $$ = -$1; @} +exp: NUM @{ $$ = $1; @} + | exp exp '+' @{ $$ = $1 + $2; @} + | exp exp '-' @{ $$ = $1 - $2; @} + | exp exp '*' @{ $$ = $1 * $2; @} + | exp exp '/' @{ $$ = $1 / $2; @} + /* Exponentiation */ + | exp exp '^' @{ $$ = pow ($1, $2); @} + /* Unary minus */ + | exp 'n' @{ $$ = -$1; @} ; %% @end example @@ -1236,7 +1497,7 @@ exp: NUM @{ $$ = $1; @} The groupings of the rpcalc ``language'' defined here are the expression (given the name @code{exp}), the line of input (@code{line}), and the complete input transcript (@code{input}). Each of these nonterminal -symbols has several alternate rules, joined by the @samp{|} punctuator +symbols has several alternate rules, joined by the vertical bar @samp{|} which is read as ``or''. The following sections explain what these rules mean. @@ -1252,12 +1513,12 @@ main job of most actions. The semantic values of the components of the rule are referred to as @code{$1}, @code{$2}, and so on. @menu -* Rpcalc Input:: -* Rpcalc Line:: -* Rpcalc Expr:: +* Rpcalc Input:: +* Rpcalc Line:: +* Rpcalc Expr:: @end menu -@node Rpcalc Input, Rpcalc Line, , Rpcalc Rules +@node Rpcalc Input @subsubsection Explanation of @code{input} Consider the definition of @code{input}: @@ -1289,9 +1550,9 @@ more times. The parser function @code{yyparse} continues to process input until a grammatical error is seen or the lexical analyzer says there are no more -input tokens; we will arrange for the latter to happen at end of file. +input tokens; we will arrange for the latter to happen at end-of-input. -@node Rpcalc Line, Rpcalc Expr, Rpcalc Input, Rpcalc Rules +@node Rpcalc Line @subsubsection Explanation of @code{line} Now consider the definition of @code{line}: @@ -1316,7 +1577,7 @@ uninitialized (its value will be unpredictable). This would be a bug if that value were ever used, but we don't use it: once rpcalc has printed the value of the user's input line, that value is no longer needed. -@node Rpcalc Expr, , Rpcalc Line, Rpcalc Rules +@node Rpcalc Expr @subsubsection Explanation of @code{expr} The @code{exp} grouping has several rules, one for each kind of expression. @@ -1356,11 +1617,11 @@ action, Bison by default copies the value of @code{$1} into @code{$$}. This is what happens in the first rule (the one that uses @code{NUM}). The formatting shown here is the recommended convention, but Bison does -not require it. You can add or change whitespace as much as you wish. +not require it. You can add or change white space as much as you wish. For example, this: @example -exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} +exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ; @end example @noindent @@ -1370,21 +1631,24 @@ means the same thing as this: exp: NUM | exp exp '+' @{ $$ = $1 + $2; @} | @dots{} +; @end example @noindent The latter, however, is much more readable. -@node Rpcalc Lexer, Rpcalc Main, Rpcalc Rules, RPN Calc +@node Rpcalc Lexer @subsection The @code{rpcalc} Lexical Analyzer @cindex writing a lexical analyzer @cindex lexical analyzer, writing -The lexical analyzer's job is low-level parsing: converting characters or -sequences of characters into tokens. The Bison parser gets its tokens by -calling the lexical analyzer. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}. +The lexical analyzer's job is low-level parsing: converting characters +or sequences of characters into tokens. The Bison parser gets its +tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical +Analyzer Function @code{yylex}}. -Only a simple lexical analyzer is needed for the RPN calculator. This +Only a simple lexical analyzer is needed for the @acronym{RPN} +calculator. This lexical analyzer skips blanks and tabs, then reads in numbers as @code{double} and returns them as @code{NUM} tokens. Any other character that isn't part of a number is a separate token. Note that the token-code @@ -1394,45 +1658,46 @@ The return value of the lexical analyzer function is a numeric code which represents a token type. The same text used in Bison rules to stand for this token type is also a C expression for the numeric code for the type. This works in two ways. If the token type is a character literal, then its -numeric code is the ASCII code for that character; you can use the same +numeric code is that of the character; you can use the same character literal in the lexical analyzer to express the number. If the token type is an identifier, that identifier is defined by Bison as a C macro whose definition is the appropriate number. In this example, therefore, @code{NUM} becomes a macro for @code{yylex} to use. -The semantic value of the token (if it has one) is stored into the global -variable @code{yylval}, which is where the Bison parser will look for it. -(The C data type of @code{yylval} is @code{YYSTYPE}, which was defined -at the beginning of the grammar; @pxref{Rpcalc Decls, ,Declarations for @code{rpcalc}}.) +The semantic value of the token (if it has one) is stored into the +global variable @code{yylval}, which is where the Bison parser will look +for it. (The C data type of @code{yylval} is @code{YYSTYPE}, which was +defined at the beginning of the grammar; @pxref{Rpcalc Declarations, +,Declarations for @code{rpcalc}}.) -A token type code of zero is returned if the end-of-file is encountered. -(Bison recognizes any nonpositive value as indicating the end of the -input.) +A token type code of zero is returned if the end-of-input is encountered. +(Bison recognizes any nonpositive value as indicating end-of-input.) Here is the code for the lexical analyzer: @example @group -/* Lexical analyzer returns a double floating point - number on the stack and the token NUM, or the ASCII - character read if not a number. Skips all blanks - and tabs, returns 0 for EOF. */ +/* The lexical analyzer returns a double floating point + number on the stack and the token NUM, or the numeric code + of the character read if not a number. It skips all blanks + and tabs, and returns 0 for end-of-input. */ #include @end group @group -yylex () +int +yylex (void) @{ int c; - /* skip white space */ - while ((c = getchar ()) == ' ' || c == '\t') + /* Skip white space. */ + while ((c = getchar ()) == ' ' || c == '\t') ; @end group @group - /* process numbers */ - if (c == '.' || isdigit (c)) + /* Process numbers. */ + if (c == '.' || isdigit (c)) @{ ungetc (c, stdin); scanf ("%lf", &yylval); @@ -1440,16 +1705,16 @@ yylex () @} @end group @group - /* return end-of-file */ - if (c == EOF) + /* Return end-of-input. */ + if (c == EOF) return 0; - /* return single chars */ - return c; + /* Return a single char. */ + return c; @} @end group @end example -@node Rpcalc Main, Rpcalc Error, Rpcalc Lexer, RPN Calc +@node Rpcalc Main @subsection The Controlling Function @cindex controlling function @cindex main function in simple example @@ -1460,30 +1725,33 @@ kept to the bare minimum. The only requirement is that it call @example @group -main () +int +main (void) @{ - yyparse (); + return yyparse (); @} @end group @end example -@node Rpcalc Error, Rpcalc Gen, Rpcalc Main, RPN Calc +@node Rpcalc Error @subsection The Error Reporting Routine @cindex error reporting routine When @code{yyparse} detects a syntax error, it calls the error reporting -function @code{yyerror} to print an error message (usually but not always -@code{"parse error"}). It is up to the programmer to supply @code{yyerror} -(@pxref{Interface, ,Parser C-Language Interface}), so here is the definition we will use: +function @code{yyerror} to print an error message (usually but not +always @code{"syntax error"}). It is up to the programmer to supply +@code{yyerror} (@pxref{Interface, ,Parser C-Language Interface}), so +here is the definition we will use: @example @group #include -yyerror (s) /* Called by yyparse on error */ - char *s; +/* Called by yyparse on error. */ +void +yyerror (char const *s) @{ - printf ("%s\n", s); + fprintf (stderr, "%s\n", s); @} @end group @end example @@ -1493,17 +1761,18 @@ and continue parsing if the grammar contains a suitable error rule (@pxref{Error Recovery}). Otherwise, @code{yyparse} returns nonzero. We have not written any error rules in this example, so any invalid input will cause the calculator program to exit. This is not clean behavior for a -real calculator, but it is adequate in the first example. +real calculator, but it is adequate for the first example. -@node Rpcalc Gen, Rpcalc Compile, Rpcalc Error, RPN Calc +@node Rpcalc Generate @subsection Running Bison to Make the Parser @cindex running Bison (introduction) -Before running Bison to produce a parser, we need to decide how to arrange -all the source code in one or more source files. For such a simple example, -the easiest thing is to put everything in one file. The definitions of -@code{yylex}, @code{yyerror} and @code{main} go at the end, in the -``additional C code'' section of the file (@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}). +Before running Bison to produce a parser, we need to decide how to +arrange all the source code in one or more source files. For such a +simple example, the easiest thing is to put everything in one file. The +definitions of @code{yylex}, @code{yyerror} and @code{main} go at the +end, in the epilogue of the file +(@pxref{Grammar Layout, ,The Overall Layout of a Bison Grammar}). For a large project, you would probably have several source files, and use @code{make} to arrange to recompile them. @@ -1512,18 +1781,18 @@ With all the source in a single file, you use the following command to convert it into a parser file: @example -bison @var{file_name}.y +bison @var{file}.y @end example @noindent In this example the file was called @file{rpcalc.y} (for ``Reverse Polish -CALCulator''). Bison produces a file named @file{@var{file_name}.tab.c}, -removing the @samp{.y} from the original file name. The file output by +@sc{calc}ulator''). Bison produces a file named @file{@var{file}.tab.c}, +removing the @samp{.y} from the original file name. The file output by Bison contains the source code for @code{yyparse}. The additional functions in the input file (@code{yylex}, @code{yyerror} and @code{main}) are copied verbatim to the output. -@node Rpcalc Compile, , Rpcalc Gen, RPN Calc +@node Rpcalc Compile @subsection Compiling the Parser File @cindex compiling the parser @@ -1532,19 +1801,19 @@ Here is how to compile and run the parser file: @example @group # @r{List files in current directory.} -% ls +$ @kbd{ls} rpcalc.tab.c rpcalc.y @end group @group # @r{Compile the Bison parser.} # @r{@samp{-lm} tells compiler to search math library for @code{pow}.} -% cc rpcalc.tab.c -lm -o rpcalc +$ @kbd{cc -lm -o rpcalc rpcalc.tab.c} @end group @group # @r{List files again.} -% ls +$ @kbd{ls} rpcalc rpcalc.tab.c rpcalc.y @end group @end example @@ -1553,22 +1822,22 @@ The file @file{rpcalc} now contains the executable code. Here is an example session using @code{rpcalc}. @example -% rpcalc -4 9 + +$ @kbd{rpcalc} +@kbd{4 9 +} 13 -3 7 + 3 4 5 *+- +@kbd{3 7 + 3 4 5 *+-} -13 -3 7 + 3 4 5 * + - n @r{Note the unary minus, @samp{n}} +@kbd{3 7 + 3 4 5 * + - n} @r{Note the unary minus, @samp{n}} 13 -5 6 / 4 n + +@kbd{5 6 / 4 n +} -3.166666667 -3 4 ^ @r{Exponentiation} +@kbd{3 4 ^} @r{Exponentiation} 81 -^D @r{End-of-file indicator} -% +@kbd{^D} @r{End-of-file indicator} +$ @end example -@node Infix Calc, Simple Error Recovery, RPN Calc, Examples +@node Infix Calc @section Infix Notation Calculator: @code{calc} @cindex infix notation calculator @cindex @code{calc} @@ -1580,23 +1849,25 @@ parentheses nested to arbitrary depth. Here is the Bison code for @file{calc.y}, an infix desk-top calculator. @example -/* Infix notation calculator--calc */ +/* Infix notation calculator. */ %@{ -#define YYSTYPE double -#include + #define YYSTYPE double + #include + #include + int yylex (void); + void yyerror (char const *); %@} -/* BISON Declarations */ +/* Bison declarations. */ %token NUM %left '-' '+' %left '*' '/' -%left NEG /* negation--unary minus */ -%right '^' /* exponentiation */ +%precedence NEG /* negation--unary minus */ +%right '^' /* exponentiation */ -/* Grammar follows */ -%% -input: /* empty string */ +%% /* The grammar follows. */ +input: /* empty */ | input line ; @@ -1617,8 +1888,8 @@ exp: NUM @{ $$ = $1; @} @end example @noindent -The functions @code{yylex}, @code{yyerror} and @code{main} can be the same -as before. +The functions @code{yylex}, @code{yyerror} and @code{main} can be the +same as before. There are two important new features shown in this code. @@ -1626,44 +1897,47 @@ In the second section (Bison declarations), @code{%left} declares token types and says they are left-associative operators. The declarations @code{%left} and @code{%right} (right associativity) take the place of @code{%token} which is used to declare a token type name without -associativity. (These tokens are single-character literals, which +associativity/precedence. (These tokens are single-character literals, which ordinarily don't need to be declared. We declare them here to specify -the associativity.) +the associativity/precedence.) Operator precedence is determined by the line ordering of the declarations; the higher the line number of the declaration (lower on the page or screen), the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (@code{NEG}) is next, followed -by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator Precedence}. +by @samp{*} and @samp{/}, and so on. Unary minus is not associative, +only precedence matters (@code{%precedence}. @xref{Precedence, ,Operator +Precedence}. -The other important new feature is the @code{%prec} in the grammar section -for the unary minus operator. The @code{%prec} simply instructs Bison that -the rule @samp{| '-' exp} has the same precedence as @code{NEG}---in this -case the next-to-highest. @xref{Contextual Precedence, ,Context-Dependent Precedence}. +The other important new feature is the @code{%prec} in the grammar +section for the unary minus operator. The @code{%prec} simply instructs +Bison that the rule @samp{| '-' exp} has the same precedence as +@code{NEG}---in this case the next-to-highest. @xref{Contextual +Precedence, ,Context-Dependent Precedence}. Here is a sample run of @file{calc.y}: @need 500 @example -% calc -4 + 4.5 - (34/(8*3+-3)) +$ @kbd{calc} +@kbd{4 + 4.5 - (34/(8*3+-3))} 6.880952381 --56 + 2 +@kbd{-56 + 2} -54 -3 ^ 2 +@kbd{3 ^ 2} 9 @end example -@node Simple Error Recovery, Multi-function Calc, Infix Calc, Examples +@node Simple Error Recovery @section Simple Error Recovery @cindex error recovery, simple Up to this point, this manual has not addressed the issue of @dfn{error recovery}---how to continue parsing after the parser detects a syntax -error. All we have handled is error reporting with @code{yyerror}. Recall -that by default @code{yyparse} returns after calling @code{yyerror}. This -means that an erroneous input line causes the calculator program to exit. -Now we show how to rectify this deficiency. +error. All we have handled is error reporting with @code{yyerror}. +Recall that by default @code{yyparse} returns after calling +@code{yyerror}. This means that an erroneous input line causes the +calculator program to exit. Now we show how to rectify this deficiency. The Bison language itself includes the reserved word @code{error}, which may be included in the grammar rules. In the example below it has @@ -1678,14 +1952,15 @@ line: '\n' @end group @end example -This addition to the grammar allows for simple error recovery in the event -of a parse error. If an expression that cannot be evaluated is read, the -error will be recognized by the third rule for @code{line}, and parsing -will continue. (The @code{yyerror} function is still called upon to print -its message as well.) The action executes the statement @code{yyerrok}, a -macro defined automatically by Bison; its meaning is that error recovery is -complete (@pxref{Error Recovery}). Note the difference between -@code{yyerrok} and @code{yyerror}; neither one is a misprint.@refill +This addition to the grammar allows for simple error recovery in the +event of a syntax error. If an expression that cannot be evaluated is +read, the error will be recognized by the third rule for @code{line}, +and parsing will continue. (The @code{yyerror} function is still called +upon to print its message as well.) The action executes the statement +@code{yyerrok}, a macro defined automatically by Bison; its meaning is +that error recovery is complete (@pxref{Error Recovery}). Note the +difference between @code{yyerrok} and @code{yyerror}; neither one is a +misprint. This form of error recovery deals with syntax errors. There are other kinds of errors; for example, division by zero, which raises an exception @@ -1695,127 +1970,344 @@ input lines; it would also have to discard the rest of the current line of input. We won't discuss this issue further because it is not specific to Bison programs. -@node Multi-function Calc, Exercises, Simple Error Recovery, Examples -@section Multi-Function Calculator: @code{mfcalc} -@cindex multi-function calculator -@cindex @code{mfcalc} -@cindex calculator, multi-function - -Now that the basics of Bison have been discussed, it is time to move on to -a more advanced problem. The above calculators provided only five -functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}. It would -be nice to have a calculator that provides other mathematical functions such -as @code{sin}, @code{cos}, etc. - -It is easy to add new operators to the infix calculator as long as they are -only single-character literals. The lexical analyzer @code{yylex} passes -back all non-number characters as tokens, so new grammar rules suffice for -adding a new operator. But we want something more flexible: built-in -functions whose syntax has this form: - -@example -@var{function_name} (@var{argument}) -@end example - -@noindent -At the same time, we will add memory to the calculator, by allowing you -to create named variables, store values in them, and use them later. -Here is a sample session with the multi-function calculator: - -@example -% acalc -pi = 3.141592653589 -3.1415926536 -sin(pi) -0.0000000000 -alpha = beta1 = 2.3 -2.3000000000 -alpha -2.3000000000 -ln(alpha) -0.8329091229 -exp(ln(beta1)) -2.3000000000 -% -@end example +@node Location Tracking Calc +@section Location Tracking Calculator: @code{ltcalc} +@cindex location tracking calculator +@cindex @code{ltcalc} +@cindex calculator, location tracking -Note that multiple assignment and nested function calls are permitted. +This example extends the infix notation calculator with location +tracking. This feature will be used to improve the error messages. For +the sake of clarity, this example is a simple integer calculator, since +most of the work needed to use locations will be done in the lexical +analyzer. @menu -* Decl: Mfcalc Decl. Bison declarations for multi-function calculator. -* Rules: Mfcalc Rules. Grammar rules for the calculator. -* Symtab: Mfcalc Symtab. Symbol table management subroutines. +* Ltcalc Declarations:: Bison and C declarations for ltcalc. +* Ltcalc Rules:: Grammar rules for ltcalc, with explanations. +* Ltcalc Lexer:: The lexical analyzer. @end menu -@node Mfcalc Decl, Mfcalc Rules, , Multi-function Calc -@subsection Declarations for @code{mfcalc} +@node Ltcalc Declarations +@subsection Declarations for @code{ltcalc} -Here are the C and Bison declarations for the multi-function calculator. +The C and Bison declarations for the location tracking calculator are +the same as the declarations for the infix notation calculator. + +@example +/* Location tracking calculator. */ -@smallexample %@{ -#include /* For math functions, cos(), sin(), etc. */ -#include "calc.h" /* Contains definition of `symrec' */ + #define YYSTYPE int + #include + int yylex (void); + void yyerror (char const *); %@} -%union @{ -double val; /* For returning numbers. */ -symrec *tptr; /* For returning symbol-table pointers */ -@} -%token NUM /* Simple double precision number */ -%token VAR FNCT /* Variable and Function */ -%type exp +/* Bison declarations. */ +%token NUM -%right '=' %left '-' '+' %left '*' '/' -%left NEG /* Negation--unary minus */ -%right '^' /* Exponentiation */ +%precedence NEG +%right '^' -/* Grammar follows */ +%% /* The grammar follows. */ +@end example -%% -@end smallexample +@noindent +Note there are no declarations specific to locations. Defining a data +type for storing locations is not needed: we will use the type provided +by default (@pxref{Location Type, ,Data Types of Locations}), which is a +four member structure with the following integer fields: +@code{first_line}, @code{first_column}, @code{last_line} and +@code{last_column}. By conventions, and in accordance with the GNU +Coding Standards and common practice, the line and column count both +start at 1. + +@node Ltcalc Rules +@subsection Grammar Rules for @code{ltcalc} + +Whether handling locations or not has no effect on the syntax of your +language. Therefore, grammar rules for this example will be very close +to those of the previous example: we will only modify them to benefit +from the new information. + +Here, we will use locations to report divisions by zero, and locate the +wrong expressions or subexpressions. -The above grammar introduces only two new features of the Bison language. -These features allow semantic values to have various data types -(@pxref{Multiple Types, ,More Than One Value Type}). +@example +@group +input : /* empty */ + | input line +; +@end group -The @code{%union} declaration specifies the entire list of possible types; -this is instead of defining @code{YYSTYPE}. The allowable types are now -double-floats (for @code{exp} and @code{NUM}) and pointers to entries in -the symbol table. @xref{Union Decl, ,The Collection of Value Types}. +@group +line : '\n' + | exp '\n' @{ printf ("%d\n", $1); @} +; +@end group -Since values can now have various types, it is necessary to associate a -type with each grammar symbol whose semantic value is used. These symbols -are @code{NUM}, @code{VAR}, @code{FNCT}, and @code{exp}. Their -declarations are augmented with information about their data type (placed -between angle brackets). +@group +exp : NUM @{ $$ = $1; @} + | exp '+' exp @{ $$ = $1 + $3; @} + | exp '-' exp @{ $$ = $1 - $3; @} + | exp '*' exp @{ $$ = $1 * $3; @} +@end group +@group + | exp '/' exp + @{ + if ($3) + $$ = $1 / $3; + else + @{ + $$ = 1; + fprintf (stderr, "%d.%d-%d.%d: division by zero", + @@3.first_line, @@3.first_column, + @@3.last_line, @@3.last_column); + @} + @} +@end group +@group + | '-' exp %prec NEG @{ $$ = -$2; @} + | exp '^' exp @{ $$ = pow ($1, $3); @} + | '(' exp ')' @{ $$ = $2; @} +@end group +@end example -The Bison construct @code{%type} is used for declaring nonterminal symbols, -just as @code{%token} is used for declaring token types. We have not used -@code{%type} before because nonterminal symbols are normally declared -implicitly by the rules that define them. But @code{exp} must be declared -explicitly so we can specify its value type. @xref{Type Decl, ,Nonterminal Symbols}. +This code shows how to reach locations inside of semantic actions, by +using the pseudo-variables @code{@@@var{n}} for rule components, and the +pseudo-variable @code{@@$} for groupings. -@node Mfcalc Rules, Mfcalc Symtab, Mfcalc Decl, Multi-function Calc -@subsection Grammar Rules for @code{mfcalc} +We don't need to assign a value to @code{@@$}: the output parser does it +automatically. By default, before executing the C code of each action, +@code{@@$} is set to range from the beginning of @code{@@1} to the end +of @code{@@@var{n}}, for a rule with @var{n} components. This behavior +can be redefined (@pxref{Location Default Action, , Default Action for +Locations}), and for very specific rules, @code{@@$} can be computed by +hand. + +@node Ltcalc Lexer +@subsection The @code{ltcalc} Lexical Analyzer. + +Until now, we relied on Bison's defaults to enable location +tracking. The next step is to rewrite the lexical analyzer, and make it +able to feed the parser with the token locations, as it already does for +semantic values. + +To this end, we must take into account every single character of the +input text, to avoid the computed locations of being fuzzy or wrong: + +@example +@group +int +yylex (void) +@{ + int c; +@end group + +@group + /* Skip white space. */ + while ((c = getchar ()) == ' ' || c == '\t') + ++yylloc.last_column; +@end group + +@group + /* Step. */ + yylloc.first_line = yylloc.last_line; + yylloc.first_column = yylloc.last_column; +@end group + +@group + /* Process numbers. */ + if (isdigit (c)) + @{ + yylval = c - '0'; + ++yylloc.last_column; + while (isdigit (c = getchar ())) + @{ + ++yylloc.last_column; + yylval = yylval * 10 + c - '0'; + @} + ungetc (c, stdin); + return NUM; + @} +@end group + + /* Return end-of-input. */ + if (c == EOF) + return 0; + + /* Return a single char, and update location. */ + if (c == '\n') + @{ + ++yylloc.last_line; + yylloc.last_column = 0; + @} + else + ++yylloc.last_column; + return c; +@} +@end example + +Basically, the lexical analyzer performs the same processing as before: +it skips blanks and tabs, and reads numbers or single-character tokens. +In addition, it updates @code{yylloc}, the global variable (of type +@code{YYLTYPE}) containing the token's location. + +Now, each time this function returns a token, the parser has its number +as well as its semantic value, and its location in the text. The last +needed change is to initialize @code{yylloc}, for example in the +controlling function: + +@example +@group +int +main (void) +@{ + yylloc.first_line = yylloc.last_line = 1; + yylloc.first_column = yylloc.last_column = 0; + return yyparse (); +@} +@end group +@end example + +Remember that computing locations is not a matter of syntax. Every +character must be associated to a location update, whether it is in +valid input, in comments, in literal strings, and so on. + +@node Multi-function Calc +@section Multi-Function Calculator: @code{mfcalc} +@cindex multi-function calculator +@cindex @code{mfcalc} +@cindex calculator, multi-function + +Now that the basics of Bison have been discussed, it is time to move on to +a more advanced problem. The above calculators provided only five +functions, @samp{+}, @samp{-}, @samp{*}, @samp{/} and @samp{^}. It would +be nice to have a calculator that provides other mathematical functions such +as @code{sin}, @code{cos}, etc. + +It is easy to add new operators to the infix calculator as long as they are +only single-character literals. The lexical analyzer @code{yylex} passes +back all nonnumeric characters as tokens, so new grammar rules suffice for +adding a new operator. But we want something more flexible: built-in +functions whose syntax has this form: + +@example +@var{function_name} (@var{argument}) +@end example + +@noindent +At the same time, we will add memory to the calculator, by allowing you +to create named variables, store values in them, and use them later. +Here is a sample session with the multi-function calculator: + +@example +$ @kbd{mfcalc} +@kbd{pi = 3.141592653589} +3.1415926536 +@kbd{sin(pi)} +0.0000000000 +@kbd{alpha = beta1 = 2.3} +2.3000000000 +@kbd{alpha} +2.3000000000 +@kbd{ln(alpha)} +0.8329091229 +@kbd{exp(ln(beta1))} +2.3000000000 +$ +@end example + +Note that multiple assignment and nested function calls are permitted. + +@menu +* Mfcalc Declarations:: Bison declarations for multi-function calculator. +* Mfcalc Rules:: Grammar rules for the calculator. +* Mfcalc Symbol Table:: Symbol table management subroutines. +@end menu + +@node Mfcalc Declarations +@subsection Declarations for @code{mfcalc} + +Here are the C and Bison declarations for the multi-function calculator. + +@smallexample +@group +%@{ + #include /* For math functions, cos(), sin(), etc. */ + #include "calc.h" /* Contains definition of `symrec'. */ + int yylex (void); + void yyerror (char const *); +%@} +@end group +@group +%union @{ + double val; /* For returning numbers. */ + symrec *tptr; /* For returning symbol-table pointers. */ +@} +@end group +%token NUM /* Simple double precision number. */ +%token VAR FNCT /* Variable and Function. */ +%type exp + +@group +%right '=' +%left '-' '+' +%left '*' '/' +%precedence NEG /* negation--unary minus */ +%right '^' /* exponentiation */ +@end group +%% /* The grammar follows. */ +@end smallexample + +The above grammar introduces only two new features of the Bison language. +These features allow semantic values to have various data types +(@pxref{Multiple Types, ,More Than One Value Type}). + +The @code{%union} declaration specifies the entire list of possible types; +this is instead of defining @code{YYSTYPE}. The allowable types are now +double-floats (for @code{exp} and @code{NUM}) and pointers to entries in +the symbol table. @xref{Union Decl, ,The Collection of Value Types}. + +Since values can now have various types, it is necessary to associate a +type with each grammar symbol whose semantic value is used. These symbols +are @code{NUM}, @code{VAR}, @code{FNCT}, and @code{exp}. Their +declarations are augmented with information about their data type (placed +between angle brackets). + +The Bison construct @code{%type} is used for declaring nonterminal +symbols, just as @code{%token} is used for declaring token types. We +have not used @code{%type} before because nonterminal symbols are +normally declared implicitly by the rules that define them. But +@code{exp} must be declared explicitly so we can specify its value type. +@xref{Type Decl, ,Nonterminal Symbols}. + +@node Mfcalc Rules +@subsection Grammar Rules for @code{mfcalc} Here are the grammar rules for the multi-function calculator. Most of them are copied directly from @code{calc}; three rules, those which mention @code{VAR} or @code{FNCT}, are new. @smallexample +@group input: /* empty */ | input line ; +@end group +@group line: '\n' | exp '\n' @{ printf ("\t%.10g\n", $1); @} | error '\n' @{ yyerrok; @} ; +@end group +@group exp: NUM @{ $$ = $1; @} | VAR @{ $$ = $1->value.var; @} | VAR '=' exp @{ $$ = $3; $1->value.var = $3; @} @@ -1828,11 +2320,12 @@ exp: NUM @{ $$ = $1; @} | exp '^' exp @{ $$ = pow ($1, $3); @} | '(' exp ')' @{ $$ = $2; @} ; -/* End of grammar */ +@end group +/* End of grammar. */ %% @end smallexample -@node Mfcalc Symtab, , Mfcalc Rules, Multi-function Calc +@node Mfcalc Symbol Table @subsection The @code{mfcalc} Symbol Table @cindex symbol table example @@ -1847,27 +2340,33 @@ provides for either functions or variables to be placed in the table. @smallexample @group -/* Data type for links in the chain of symbols. */ +/* Function type. */ +typedef double (*func_t) (double); +@end group + +@group +/* Data type for links in the chain of symbols. */ struct symrec @{ - char *name; /* name of symbol */ + char *name; /* name of symbol */ int type; /* type of symbol: either VAR or FNCT */ - union @{ - double var; /* value of a VAR */ - double (*fnctptr)(); /* value of a FNCT */ + union + @{ + double var; /* value of a VAR */ + func_t fnctptr; /* value of a FNCT */ @} value; - struct symrec *next; /* link field */ + struct symrec *next; /* link field */ @}; @end group @group typedef struct symrec symrec; -/* The symbol table: a chain of `struct symrec'. */ +/* The symbol table: a chain of `struct symrec'. */ extern symrec *sym_table; -symrec *putsym (); -symrec *getsym (); +symrec *putsym (char const *, int); +symrec *getsym (char const *); @end group @end smallexample @@ -1876,48 +2375,47 @@ function that initializes the symbol table. Here it is, and @code{init_table} as well: @smallexample -@group #include -main () -@{ - init_table (); - yyparse (); -@} -@end group - @group -yyerror (s) /* Called by yyparse on error */ - char *s; +/* Called by yyparse on error. */ +void +yyerror (char const *s) @{ printf ("%s\n", s); @} +@end group +@group struct init @{ - char *fname; - double (*fnct)(); + char const *fname; + double (*fnct) (double); @}; @end group @group -struct init arith_fncts[] - = @{ - "sin", sin, - "cos", cos, - "atan", atan, - "ln", log, - "exp", exp, - "sqrt", sqrt, - 0, 0 - @}; +struct init const arith_fncts[] = +@{ + "sin", sin, + "cos", cos, + "atan", atan, + "ln", log, + "exp", exp, + "sqrt", sqrt, + 0, 0 +@}; +@end group +@group /* The symbol table: a chain of `struct symrec'. */ -symrec *sym_table = (symrec *)0; +symrec *sym_table; @end group @group -init_table () /* puts arithmetic functions in table. */ +/* Put arithmetic functions in table. */ +void +init_table (void) @{ int i; symrec *ptr; @@ -1928,6 +2426,15 @@ init_table () /* puts arithmetic functions in table. */ @} @} @end group + +@group +int +main (void) +@{ + init_table (); + return yyparse (); +@} +@end group @end smallexample By simply editing the initialization list and adding the necessary include @@ -1942,24 +2449,21 @@ found, a pointer to that symbol is returned; otherwise zero is returned. @smallexample symrec * -putsym (sym_name,sym_type) - char *sym_name; - int sym_type; +putsym (char const *sym_name, int sym_type) @{ symrec *ptr; ptr = (symrec *) malloc (sizeof (symrec)); ptr->name = (char *) malloc (strlen (sym_name) + 1); strcpy (ptr->name,sym_name); ptr->type = sym_type; - ptr->value.var = 0; /* set value to 0 even if fctn. */ + ptr->value.var = 0; /* Set value to 0 even if fctn. */ ptr->next = (struct symrec *)sym_table; sym_table = ptr; return ptr; @} symrec * -getsym (sym_name) - char *sym_name; +getsym (char const *sym_name) @{ symrec *ptr; for (ptr = sym_table; ptr != (symrec *) 0; @@ -1972,7 +2476,7 @@ getsym (sym_name) The function @code{yylex} must now recognize variables, numeric values, and the single-character arithmetic operators. Strings of alphanumeric -characters with a leading nondigit are recognized as either variables or +characters with a leading letter are recognized as either variables or functions depending on what the symbol table says about them. The string is passed to @code{getsym} for look up in the symbol table. If @@ -1980,7 +2484,7 @@ the name appears in the table, a pointer to its location and its type (@code{VAR} or @code{FNCT}) is returned to @code{yyparse}. If it is not already in the table, then it is installed as a @code{VAR} using @code{putsym}. Again, a pointer and its type (which must be @code{VAR}) is -returned to @code{yyparse}.@refill +returned to @code{yyparse}. No change is needed in the handling of numeric values and arithmetic operators in @code{yylex}. @@ -1988,11 +2492,15 @@ operators in @code{yylex}. @smallexample @group #include -yylex () +@end group + +@group +int +yylex (void) @{ int c; - /* Ignore whitespace, get first nonwhite character. */ + /* Ignore white space, get first nonwhite character. */ while ((c = getchar ()) == ' ' || c == '\t'); if (c == EOF) @@ -2034,7 +2542,7 @@ yylex () if (i == length) @{ length *= 2; - symbuf = (char *)realloc (symbuf, length + 1); + symbuf = (char *) realloc (symbuf, length + 1); @} /* Add this character to the buffer. */ symbuf[i++] = c; @@ -2043,7 +2551,7 @@ yylex () @} @end group @group - while (c != EOF && isalnum (c)); + while (isalnum (c)); ungetc (c, stdin); symbuf[i] = '\0'; @@ -2063,11 +2571,11 @@ yylex () @end group @end smallexample -This program is both powerful and flexible. You may easily add new -functions, and it is a simple job to modify this code to install predefined -variables such as @code{pi} or @code{e} as well. +This program is both powerful and flexible. You may easily add new +functions, and it is a simple job to modify this code to install +predefined variables such as @code{pi} or @code{e} as well. -@node Exercises, , Multi-function Calc, Examples +@node Exercises @section Exercises @cindex exercises @@ -2085,13 +2593,14 @@ Make the program report an error if the user refers to an uninitialized variable in any way except to store a value in it. @end enumerate -@node Grammar File, Interface, Examples, Top +@node Grammar File @chapter Bison Grammar Files Bison takes as input a context-free grammar specification and produces a C-language function that recognizes correct instances of the grammar. The Bison grammar input file conventionally has a name ending in @samp{.y}. +@xref{Invocation, ,Invoking Bison}. @menu * Grammar Outline:: Overall layout of the grammar file. @@ -2099,11 +2608,12 @@ The Bison grammar input file conventionally has a name ending in @samp{.y}. * Rules:: How to write grammar rules. * Recursion:: Writing recursive rules. * Semantics:: Semantic values and actions. +* Locations:: Locations and actions. * Declarations:: All kinds of Bison declarations are described here. * Multiple Parsers:: Putting more than one Bison parser in one program. @end menu -@node Grammar Outline, Symbols, , Grammar File +@node Grammar Outline @section Outline of a Bison Grammar A Bison grammar file has four main sections, shown here with the @@ -2111,7 +2621,7 @@ appropriate delimiters: @example %@{ -@var{C declarations} + @var{Prologue} %@} @var{Bison declarations} @@ -2120,32 +2630,362 @@ appropriate delimiters: @var{Grammar rules} %% -@var{Additional C code} +@var{Epilogue} @end example Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections. +As a @acronym{GNU} extension, @samp{//} introduces a comment that +continues until end of line. @menu -* C Declarations:: Syntax and usage of the C declarations section. -* Bison Declarations:: Syntax and usage of the Bison declarations section. -* Grammar Rules:: Syntax and usage of the grammar rules section. -* C Code:: Syntax and usage of the additional C code section. +* Prologue:: Syntax and usage of the prologue. +* Prologue Alternatives:: Syntax and usage of alternatives to the prologue. +* Bison Declarations:: Syntax and usage of the Bison declarations section. +* Grammar Rules:: Syntax and usage of the grammar rules section. +* Epilogue:: Syntax and usage of the epilogue. @end menu -@node C Declarations, Bison Declarations, , Grammar Outline -@subsection The C Declarations Section -@cindex C declarations section -@cindex declarations, C +@node Prologue +@subsection The prologue +@cindex declarations section +@cindex Prologue +@cindex declarations + +The @var{Prologue} section contains macro definitions and declarations +of functions and variables that are used in the actions in the grammar +rules. These are copied to the beginning of the parser file so that +they precede the definition of @code{yyparse}. You can use +@samp{#include} to get the declarations from a header file. If you +don't need any C declarations, you may omit the @samp{%@{} and +@samp{%@}} delimiters that bracket this section. + +The @var{Prologue} section is terminated by the first occurrence +of @samp{%@}} that is outside a comment, a string literal, or a +character constant. + +You may have more than one @var{Prologue} section, intermixed with the +@var{Bison declarations}. This allows you to have C and Bison +declarations that refer to each other. For example, the @code{%union} +declaration may use types defined in a header file, and you may wish to +prototype functions that take arguments of type @code{YYSTYPE}. This +can be done with two @var{Prologue} blocks, one before and one after the +@code{%union} declaration. + +@smallexample +%@{ + #define _GNU_SOURCE + #include + #include "ptypes.h" +%@} + +%union @{ + long int n; + tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */ +@} + +%@{ + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(F, N, L) print_token_value (F, N, L) +%@} + +@dots{} +@end smallexample + +When in doubt, it is usually safer to put prologue code before all +Bison declarations, rather than after. For example, any definitions +of feature test macros like @code{_GNU_SOURCE} or +@code{_POSIX_C_SOURCE} should appear before all Bison declarations, as +feature test macros can affect the behavior of Bison-generated +@code{#include} directives. + +@node Prologue Alternatives +@subsection Prologue Alternatives +@cindex Prologue Alternatives + +@findex %code +@findex %code requires +@findex %code provides +@findex %code top +(The prologue alternatives described here are experimental. +More user feedback will help to determine whether they should become permanent +features.) + +The functionality of @var{Prologue} sections can often be subtle and +inflexible. +As an alternative, Bison provides a %code directive with an explicit qualifier +field, which identifies the purpose of the code and thus the location(s) where +Bison should generate it. +For C/C++, the qualifier can be omitted for the default location, or it can be +one of @code{requires}, @code{provides}, @code{top}. +@xref{Decl Summary,,%code}. + +Look again at the example of the previous section: + +@smallexample +%@{ + #define _GNU_SOURCE + #include + #include "ptypes.h" +%@} + +%union @{ + long int n; + tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */ +@} + +%@{ + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(F, N, L) print_token_value (F, N, L) +%@} + +@dots{} +@end smallexample + +@noindent +Notice that there are two @var{Prologue} sections here, but there's a subtle +distinction between their functionality. +For example, if you decide to override Bison's default definition for +@code{YYLTYPE}, in which @var{Prologue} section should you write your new +definition? +You should write it in the first since Bison will insert that code into the +parser source code file @emph{before} the default @code{YYLTYPE} definition. +In which @var{Prologue} section should you prototype an internal function, +@code{trace_token}, that accepts @code{YYLTYPE} and @code{yytokentype} as +arguments? +You should prototype it in the second since Bison will insert that code +@emph{after} the @code{YYLTYPE} and @code{yytokentype} definitions. + +This distinction in functionality between the two @var{Prologue} sections is +established by the appearance of the @code{%union} between them. +This behavior raises a few questions. +First, why should the position of a @code{%union} affect definitions related to +@code{YYLTYPE} and @code{yytokentype}? +Second, what if there is no @code{%union}? +In that case, the second kind of @var{Prologue} section is not available. +This behavior is not intuitive. + +To avoid this subtle @code{%union} dependency, rewrite the example using a +@code{%code top} and an unqualified @code{%code}. +Let's go ahead and add the new @code{YYLTYPE} definition and the +@code{trace_token} prototype at the same time: + +@smallexample +%code top @{ + #define _GNU_SOURCE + #include + + /* WARNING: The following code really belongs + * in a `%code requires'; see below. */ + + #include "ptypes.h" + #define YYLTYPE YYLTYPE + typedef struct YYLTYPE + @{ + int first_line; + int first_column; + int last_line; + int last_column; + char *filename; + @} YYLTYPE; +@} + +%union @{ + long int n; + tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */ +@} + +%code @{ + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(F, N, L) print_token_value (F, N, L) + static void trace_token (enum yytokentype token, YYLTYPE loc); +@} + +@dots{} +@end smallexample + +@noindent +In this way, @code{%code top} and the unqualified @code{%code} achieve the same +functionality as the two kinds of @var{Prologue} sections, but it's always +explicit which kind you intend. +Moreover, both kinds are always available even in the absence of @code{%union}. + +The @code{%code top} block above logically contains two parts. +The first two lines before the warning need to appear near the top of the +parser source code file. +The first line after the warning is required by @code{YYSTYPE} and thus also +needs to appear in the parser source code file. +However, if you've instructed Bison to generate a parser header file +(@pxref{Decl Summary, ,%defines}), you probably want that line to appear before +the @code{YYSTYPE} definition in that header file as well. +The @code{YYLTYPE} definition should also appear in the parser header file to +override the default @code{YYLTYPE} definition there. + +In other words, in the @code{%code top} block above, all but the first two +lines are dependency code required by the @code{YYSTYPE} and @code{YYLTYPE} +definitions. +Thus, they belong in one or more @code{%code requires}: + +@smallexample +%code top @{ + #define _GNU_SOURCE + #include +@} + +%code requires @{ + #include "ptypes.h" +@} +%union @{ + long int n; + tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */ +@} + +%code requires @{ + #define YYLTYPE YYLTYPE + typedef struct YYLTYPE + @{ + int first_line; + int first_column; + int last_line; + int last_column; + char *filename; + @} YYLTYPE; +@} + +%code @{ + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(F, N, L) print_token_value (F, N, L) + static void trace_token (enum yytokentype token, YYLTYPE loc); +@} + +@dots{} +@end smallexample + +@noindent +Now Bison will insert @code{#include "ptypes.h"} and the new @code{YYLTYPE} +definition before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE} +definitions in both the parser source code file and the parser header file. +(By the same reasoning, @code{%code requires} would also be the appropriate +place to write your own definition for @code{YYSTYPE}.) + +When you are writing dependency code for @code{YYSTYPE} and @code{YYLTYPE}, you +should prefer @code{%code requires} over @code{%code top} regardless of whether +you instruct Bison to generate a parser header file. +When you are writing code that you need Bison to insert only into the parser +source code file and that has no special need to appear at the top of that +file, you should prefer the unqualified @code{%code} over @code{%code top}. +These practices will make the purpose of each block of your code explicit to +Bison and to other developers reading your grammar file. +Following these practices, we expect the unqualified @code{%code} and +@code{%code requires} to be the most important of the four @var{Prologue} +alternatives. + +At some point while developing your parser, you might decide to provide +@code{trace_token} to modules that are external to your parser. +Thus, you might wish for Bison to insert the prototype into both the parser +header file and the parser source code file. +Since this function is not a dependency required by @code{YYSTYPE} or +@code{YYLTYPE}, it doesn't make sense to move its prototype to a +@code{%code requires}. +More importantly, since it depends upon @code{YYLTYPE} and @code{yytokentype}, +@code{%code requires} is not sufficient. +Instead, move its prototype from the unqualified @code{%code} to a +@code{%code provides}: + +@smallexample +%code top @{ + #define _GNU_SOURCE + #include +@} + +%code requires @{ + #include "ptypes.h" +@} +%union @{ + long int n; + tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */ +@} + +%code requires @{ + #define YYLTYPE YYLTYPE + typedef struct YYLTYPE + @{ + int first_line; + int first_column; + int last_line; + int last_column; + char *filename; + @} YYLTYPE; +@} + +%code provides @{ + void trace_token (enum yytokentype token, YYLTYPE loc); +@} + +%code @{ + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(F, N, L) print_token_value (F, N, L) +@} + +@dots{} +@end smallexample -The @var{C declarations} section contains macro definitions and -declarations of functions and variables that are used in the actions in the -grammar rules. These are copied to the beginning of the parser file so -that they precede the definition of @code{yyparse}. You can use -@samp{#include} to get the declarations from a header file. If you don't -need any C declarations, you may omit the @samp{%@{} and @samp{%@}} -delimiters that bracket this section. +@noindent +Bison will insert the @code{trace_token} prototype into both the parser header +file and the parser source code file after the definitions for +@code{yytokentype}, @code{YYLTYPE}, and @code{YYSTYPE}. + +The above examples are careful to write directives in an order that reflects +the layout of the generated parser source code and header files: +@code{%code top}, @code{%code requires}, @code{%code provides}, and then +@code{%code}. +While your grammar files may generally be easier to read if you also follow +this order, Bison does not require it. +Instead, Bison lets you choose an organization that makes sense to you. + +You may declare any of these directives multiple times in the grammar file. +In that case, Bison concatenates the contained code in declaration order. +This is the only way in which the position of one of these directives within +the grammar file affects its functionality. + +The result of the previous two properties is greater flexibility in how you may +organize your grammar file. +For example, you may organize semantic-type-related directives by semantic +type: + +@smallexample +%code requires @{ #include "type1.h" @} +%union @{ type1 field1; @} +%destructor @{ type1_free ($$); @} +%printer @{ type1_print ($$); @} + +%code requires @{ #include "type2.h" @} +%union @{ type2 field2; @} +%destructor @{ type2_free ($$); @} +%printer @{ type2_print ($$); @} +@end smallexample -@node Bison Declarations, Grammar Rules, C Declarations, Grammar Outline +@noindent +You could even place each of the above directive groups in the rules section of +the grammar file next to the set of rules that uses the associated semantic +type. +(In the rules section, you must terminate each of those directives with a +semicolon.) +And you don't have to worry that some directive (like a @code{%union}) in the +definitions section is going to adversely affect their functionality in some +counter-intuitive manner just because it comes first. +Such an organization is not possible using @var{Prologue} sections. + +This section has been concerned with explaining the advantages of the four +@var{Prologue} alternatives over the original Yacc @var{Prologue}. +However, in most cases when using these directives, you shouldn't need to +think about all the low-level ordering issues discussed here. +Instead, you should simply use these directives to label each block of your +code according to its purpose and let Bison handle the ordering. +@code{%code} is the most generic label. +Move code to @code{%code requires}, @code{%code provides}, or @code{%code top} +as needed. + +@node Bison Declarations @subsection The Bison Declarations Section @cindex Bison declarations (introduction) @cindex declarations, Bison (introduction) @@ -2155,7 +2995,7 @@ terminal and nonterminal symbols, specify precedence, and so on. In some simple grammars you may not need any declarations. @xref{Declarations, ,Bison Declarations}. -@node Grammar Rules, C Code, Bison Declarations, Grammar Outline +@node Grammar Rules @subsection The Grammar Rules Section @cindex grammar rules section @cindex rules section for grammar @@ -2167,27 +3007,31 @@ There must always be at least one grammar rule, and the first @samp{%%} (which precedes the grammar rules) may never be omitted even if it is the first thing in the file. -@node C Code, , Grammar Rules, Grammar Outline -@subsection The Additional C Code Section +@node Epilogue +@subsection The epilogue @cindex additional C code section +@cindex epilogue @cindex C code, section for additional -The @var{additional C code} section is copied verbatim to the end of -the parser file, just as the @var{C declarations} section is copied to -the beginning. This is the most convenient place to put anything -that you want to have in the parser file but which need not come before -the definition of @code{yyparse}. For example, the definitions of -@code{yylex} and @code{yyerror} often go here. @xref{Interface, ,Parser C-Language Interface}. +The @var{Epilogue} is copied verbatim to the end of the parser file, just as +the @var{Prologue} is copied to the beginning. This is the most convenient +place to put anything that you want to have in the parser file but which need +not come before the definition of @code{yyparse}. For example, the +definitions of @code{yylex} and @code{yyerror} often go here. Because +C requires functions to be declared before being used, you often need +to declare functions like @code{yylex} and @code{yyerror} in the Prologue, +even if you define them in the Epilogue. +@xref{Interface, ,Parser C-Language Interface}. If the last section is empty, you may omit the @samp{%%} that separates it from the grammar rules. -The Bison parser itself contains many static variables whose names start -with @samp{yy} and many macros whose names start with @samp{YY}. It is a -good idea to avoid using any such names (except those documented in this -manual) in the additional C code section of the grammar file. +The Bison parser itself contains many macros and identifiers whose names +start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using +any such names (except those documented in this manual) in the epilogue +of the grammar file. -@node Symbols, Rules, Grammar Outline, Grammar File +@node Symbols @section Symbols, Terminal and Nonterminal @cindex nonterminal symbol @cindex terminal symbol @@ -2201,23 +3045,23 @@ A @dfn{terminal symbol} (also known as a @dfn{token type}) represents a class of syntactically equivalent tokens. You use the symbol in grammar rules to mean that a token in that class is allowed. The symbol is represented in the Bison parser by a numeric code, and the @code{yylex} -function returns a token type code to indicate what kind of token has been -read. You don't need to know what the code value is; you can use the -symbol to stand for it. +function returns a token type code to indicate what kind of token has +been read. You don't need to know what the code value is; you can use +the symbol to stand for it. -A @dfn{nonterminal symbol} stands for a class of syntactically equivalent -groupings. The symbol name is used in writing grammar rules. By convention, -it should be all lower case. +A @dfn{nonterminal symbol} stands for a class of syntactically +equivalent groupings. The symbol name is used in writing grammar rules. +By convention, it should be all lower case. Symbol names can contain letters, digits (not at the beginning), underscores and periods. Periods make sense only in nonterminals. -There are two ways of writing terminal symbols in the grammar: +There are three ways of writing terminal symbols in the grammar: @itemize @bullet @item A @dfn{named token type} is written with an identifier, like an -identifier in C. By convention, it should be all upper case. Each +identifier in C@. By convention, it should be all upper case. Each such name must be defined with a Bison declaration such as @code{%token}. @xref{Token Decl, ,Token Type Names}. @@ -2225,12 +3069,13 @@ such name must be defined with a Bison declaration such as @cindex character token @cindex literal token @cindex single-character literal -A @dfn{character token type} (or @dfn{literal token}) is written in -the grammar using the same syntax used in C for character constants; -for example, @code{'+'} is a character token type. A character token -type doesn't need to be declared unless you need to specify its -semantic value data type (@pxref{Value Type, ,Data Types of Semantic Values}), associativity, or -precedence (@pxref{Precedence, ,Operator Precedence}). +A @dfn{character token type} (or @dfn{literal character token}) is +written in the grammar using the same syntax used in C for character +constants; for example, @code{'+'} is a character token type. A +character token type doesn't need to be declared unless you need to +specify its semantic value data type (@pxref{Value Type, ,Data Types of +Semantic Values}), associativity, or precedence (@pxref{Precedence, +,Operator Precedence}). By convention, a character token type is used only to represent a token that consists of that particular character. Thus, the token @@ -2240,22 +3085,59 @@ your program will confuse other readers. All the usual escape sequences used in character literals in C can be used in Bison as well, but you must not use the null character as a -character literal because its ASCII code, zero, is the code -@code{yylex} returns for end-of-input (@pxref{Calling Convention, ,Calling Convention for @code{yylex}}). +character literal because its numeric code, zero, signifies +end-of-input (@pxref{Calling Convention, ,Calling Convention +for @code{yylex}}). Also, unlike standard C, trigraphs have no +special meaning in Bison character literals, nor is backslash-newline +allowed. + +@item +@cindex string token +@cindex literal string token +@cindex multicharacter literal +A @dfn{literal string token} is written like a C string constant; for +example, @code{"<="} is a literal string token. A literal string token +doesn't need to be declared unless you need to specify its semantic +value data type (@pxref{Value Type}), associativity, or precedence +(@pxref{Precedence}). + +You can associate the literal string token with a symbolic name as an +alias, using the @code{%token} declaration (@pxref{Token Decl, ,Token +Declarations}). If you don't do that, the lexical analyzer has to +retrieve the token number for the literal string token from the +@code{yytname} table (@pxref{Calling Convention}). + +@strong{Warning}: literal string tokens do not work in Yacc. + +By convention, a literal string token is used only to represent a token +that consists of that particular string. Thus, you should use the token +type @code{"<="} to represent the string @samp{<=} as a token. Bison +does not enforce this convention, but if you depart from it, people who +read your program will be confused. + +All the escape sequences used in string literals in C can be used in +Bison as well, except that you must not use a null character within a +string literal. Also, unlike Standard C, trigraphs have no special +meaning in Bison string literals, nor is backslash-newline allowed. A +literal string token must contain two or more characters; for a token +containing just one character, use a character token (see above). @end itemize How you choose to write a terminal symbol has no effect on its grammatical meaning. That depends only on where it appears in rules and on when the parser function returns that symbol. -The value returned by @code{yylex} is always one of the terminal symbols -(or 0 for end-of-input). Whichever way you write the token type in the -grammar rules, you write it the same way in the definition of @code{yylex}. -The numeric code for a character token type is simply the ASCII code for -the character, so @code{yylex} can use the identical character constant to -generate the requisite code. Each named token type becomes a C macro in +The value returned by @code{yylex} is always one of the terminal +symbols, except that a zero or negative value signifies end-of-input. +Whichever way you write the token type in the grammar rules, you write +it the same way in the definition of @code{yylex}. The numeric code +for a character token type is simply the positive numeric code of the +character, so @code{yylex} can use the identical value to generate the +requisite code, though you may need to convert it to @code{unsigned +char} to avoid sign-extension on hosts where @code{char} is signed. +Each named token type becomes a C macro in the parser file, so @code{yylex} can use the name to stand for the code. -(This is why periods don't make sense in terminal symbols.) +(This is why periods don't make sense in terminal symbols.) @xref{Calling Convention, ,Calling Convention for @code{yylex}}. If @code{yylex} is defined in a separate file, you need to arrange for the @@ -2264,11 +3146,35 @@ option when you run Bison, so that it will write these macro definitions into a separate header file @file{@var{name}.tab.h} which you can include in the other source files that need it. @xref{Invocation, ,Invoking Bison}. +If you want to write a grammar that is portable to any Standard C +host, you must use only nonnull character tokens taken from the basic +execution character set of Standard C@. This set consists of the ten +digits, the 52 lower- and upper-case English letters, and the +characters in the following C-language string: + +@example +"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~" +@end example + +The @code{yylex} function and Bison must use a consistent character set +and encoding for character tokens. For example, if you run Bison in an +@acronym{ASCII} environment, but then compile and run the resulting +program in an environment that uses an incompatible character set like +@acronym{EBCDIC}, the resulting program may not work because the tables +generated by Bison will assume @acronym{ASCII} numeric values for +character tokens. It is standard practice for software distributions to +contain C source files that were generated by Bison in an +@acronym{ASCII} environment, so installers on platforms that are +incompatible with @acronym{ASCII} must rebuild those files before +compiling them. + The symbol @code{error} is a terminal symbol reserved for error recovery (@pxref{Error Recovery}); you shouldn't use it for any other purpose. -In particular, @code{yylex} should never return this value. +In particular, @code{yylex} should never return this value. The default +value of the error token is 256, unless you explicitly assigned 256 to +one of your tokens with a @code{%token} declaration. -@node Rules, Recursion, Symbols, Grammar File +@node Rules @section Syntax of Grammar Rules @cindex rule syntax @cindex grammar rule syntax @@ -2277,14 +3183,16 @@ In particular, @code{yylex} should never return this value. A Bison grammar rule has the following general form: @example +@group @var{result}: @var{components}@dots{} ; +@end group @end example @noindent -where @var{result} is the nonterminal symbol that this rule describes +where @var{result} is the nonterminal symbol that this rule describes, and @var{components} are various terminal and nonterminal symbols that -are put together by this rule (@pxref{Symbols}). +are put together by this rule (@pxref{Symbols}). For example, @@ -2299,8 +3207,8 @@ exp: exp '+' exp says that two groupings of type @code{exp}, with a @samp{+} token in between, can be combined into a larger grouping of type @code{exp}. -Whitespace in rules is significant only to separate symbols. You can add -extra whitespace as you wish. +White space in rules is significant only to separate symbols. You can add +extra white space as you wish. Scattered among the components can be @var{actions} that determine the semantics of the rule. An action looks like this: @@ -2310,6 +3218,22 @@ the semantics of the rule. An action looks like this: @end example @noindent +@cindex braced code +This is an example of @dfn{braced code}, that is, C code surrounded by +braces, much like a compound statement in C@. Braced code can contain +any sequence of C tokens, so long as its braces are balanced. Bison +does not check the braced code for correctness directly; it merely +copies the code to the output file, where the C compiler can check it. + +Within braced code, the balanced-brace count is not affected by braces +within comments, string literals, or character constants, but it is +affected by the C digraphs @samp{<%} and @samp{%>} that represent +braces. At the top level braced code must be terminated by @samp{@}} +and not by a digraph. Bison does not look for trigraphs, so if braced +code uses trigraphs you should ensure that they do not affect the +nesting of braces or the boundaries of comments, string literals, or +character constants. + Usually there is only one action and it follows the components. @xref{Actions}. @@ -2317,15 +3241,6 @@ Usually there is only one action and it follows the components. Multiple rules for the same @var{result} can be written separately or can be joined with the vertical-bar character @samp{|} as follows: -@ifinfo -@example -@var{result}: @var{rule1-components}@dots{} - | @var{rule2-components}@dots{} - @dots{} - ; -@end example -@end ifinfo -@iftex @example @group @var{result}: @var{rule1-components}@dots{} @@ -2334,7 +3249,6 @@ be joined with the vertical-bar character @samp{|} as follows: ; @end group @end example -@end iftex @noindent They are still considered distinct rules even when joined in this way. @@ -2361,15 +3275,15 @@ expseq1: exp It is customary to write a comment @samp{/* empty */} in each rule with no components. -@node Recursion, Semantics, Rules, Grammar File +@node Recursion @section Recursive Rules @cindex recursive rule -A rule is called @dfn{recursive} when its @var{result} nonterminal appears -also on its right hand side. Nearly all Bison grammars need to use -recursion, because that is the only way to define a sequence of any number -of somethings. Consider this recursive definition of a comma-separated -sequence of one or more expressions: +A rule is called @dfn{recursive} when its @var{result} nonterminal +appears also on its right hand side. Nearly all Bison grammars need to +use recursion, because that is the only way to define a sequence of any +number of a particular thing. Consider this recursive definition of a +comma-separated sequence of one or more expressions: @example @group @@ -2395,20 +3309,20 @@ expseq1: exp @end example @noindent -Any kind of sequence can be defined using either left recursion or -right recursion, but you should always use left recursion, because it -can parse a sequence of any number of elements with bounded stack -space. Right recursion uses up space on the Bison stack in proportion -to the number of elements in the sequence, because all the elements -must be shifted onto the stack before the rule can be applied even -once. @xref{Algorithm, ,The Bison Parser Algorithm }, for -further explanation of this. +Any kind of sequence can be defined using either left recursion or right +recursion, but you should always use left recursion, because it can +parse a sequence of any number of elements with bounded stack space. +Right recursion uses up space on the Bison stack in proportion to the +number of elements in the sequence, because all the elements must be +shifted onto the stack before the rule can be applied even once. +@xref{Algorithm, ,The Bison Parser Algorithm}, for further explanation +of this. @cindex mutual recursion @dfn{Indirect} or @dfn{mutual} recursion occurs when the result of the rule does not appear directly on its right hand side, but does appear in rules for other nonterminals which do appear on its right hand -side. +side. For example: @@ -2430,10 +3344,10 @@ primary: constant defines two mutually-recursive nonterminals, since each refers to the other. -@node Semantics, Declarations, Recursion, Grammar File +@node Semantics @section Defining Language Semantics @cindex defining language semantics -@cindex language semantics, defining +@cindex language semantics, defining The grammar rules for a language determine only the syntax. The semantics are determined by the semantic values associated with various tokens and @@ -2454,7 +3368,7 @@ the numbers associated with @var{x} and @var{y}. action in the middle of a rule. @end menu -@node Value Type, Multiple Types, , Semantics +@node Value Type @subsection Data Types of Semantic Values @cindex semantic value type @cindex value type, semantic @@ -2463,9 +3377,11 @@ the numbers associated with @var{x} and @var{y}. In a simple program it may be sufficient to use the same data type for the semantic values of all language constructs. This was true in the -RPN and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish Notation Calculator}). +@acronym{RPN} and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish +Notation Calculator}). -Bison's default is to use type @code{int} for all semantic values. To +Bison normally uses the type @code{int} for semantic values if your +program uses the same data type for all language constructs. To specify some other type, define @code{YYSTYPE} as a macro, like this: @example @@ -2473,33 +3389,40 @@ specify some other type, define @code{YYSTYPE} as a macro, like this: @end example @noindent -This macro definition must go in the C declarations section of the grammar -file (@pxref{Grammar Outline, ,Outline of a Bison Grammar}). +@code{YYSTYPE}'s replacement list should be a type name +that does not contain parentheses or square brackets. +This macro definition must go in the prologue of the grammar file +(@pxref{Grammar Outline, ,Outline of a Bison Grammar}). -@node Multiple Types, Actions, Value Type, Semantics +@node Multiple Types @subsection More Than One Value Type In most programs, you will need different data types for different kinds of tokens and groupings. For example, a numeric constant may need type -@code{int} or @code{long}, while a string constant needs type @code{char *}, -and an identifier might need a pointer to an entry in the symbol table. +@code{int} or @code{long int}, while a string constant needs type +@code{char *}, and an identifier might need a pointer to an entry in the +symbol table. To use more than one data type for semantic values in one parser, Bison requires you to do two things: @itemize @bullet @item -Specify the entire collection of possible data types, with the -@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of Value Types}). +Specify the entire collection of possible data types, either by using the +@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of +Value Types}), or by using a @code{typedef} or a @code{#define} to +define @code{YYSTYPE} to be a union type whose member names are +the type tags. @item -Choose one of those types for each symbol (terminal or nonterminal) -for which semantic values are used. This is done for tokens with the -@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names}) and for groupings -with the @code{%type} Bison declaration (@pxref{Type Decl, ,Nonterminal Symbols}). +Choose one of those types for each symbol (terminal or nonterminal) for +which semantic values are used. This is done for tokens with the +@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names}) +and for groupings with the @code{%type} Bison declaration (@pxref{Type +Decl, ,Nonterminal Symbols}). @end itemize -@node Actions, Action Types, Multiple Types, Semantics +@node Actions @subsection Actions @cindex action @vindex $$ @@ -2510,18 +3433,20 @@ each time an instance of that rule is recognized. The task of most actions is to compute a semantic value for the grouping built by the rule from the semantic values associated with tokens or smaller groupings. -An action consists of C statements surrounded by braces, much like a -compound statement in C. It can be placed at any position in the rule; it -is executed at that position. Most rules have just one action at the end -of the rule, following all the components. Actions in the middle of a rule -are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}). +An action consists of braced code containing C statements, and can be +placed at any position in the rule; +it is executed at that position. Most rules have just one action at the +end of the rule, following all the components. Actions in the middle of +a rule are tricky and used only for special purposes (@pxref{Mid-Rule +Actions, ,Actions in Mid-Rule}). The C code in an action can refer to the semantic values of the components matched by the rule with the construct @code{$@var{n}}, which stands for the value of the @var{n}th component. The semantic value for the grouping -being constructed is @code{$$}. (Bison translates both of these constructs -into array element references when it copies the actions into the parser -file.) +being constructed is @code{$$}. Bison translates both of these +constructs into expressions of the appropriate type when it copies the +actions into the parser file. @code{$$} is translated to a modifiable +lvalue, so it can be assigned to. Here is a typical example: @@ -2541,19 +3466,31 @@ which are the first and third symbols on the right hand side of the rule. The sum is stored into @code{$$} so that it becomes the semantic value of the addition-expression just recognized by the rule. If there were a useful semantic value associated with the @samp{+} token, it could be -referred to as @code{$2}.@refill +referred to as @code{$2}. -@cindex default action -If you don't specify an action for a rule, Bison supplies a default: -@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule becomes -the value of the whole rule. Of course, the default rule is valid only -if the two data types match. There is no meaningful default action for -an empty rule; every empty rule must have an explicit action unless the -rule's value does not matter. +Note that the vertical-bar character @samp{|} is really a rule +separator, and actions are attached to a single rule. This is a +difference with tools like Flex, for which @samp{|} stands for either +``or'', or ``the same action as that of the next rule''. In the +following example, the action is triggered only when @samp{b} is found: -@code{$@var{n}} with @var{n} zero or negative is allowed for reference -to tokens and groupings on the stack @emph{before} those that match the -current rule. This is a very risky practice, and to use it reliably +@example +@group +a-or-b: 'a'|'b' @{ a_or_b_found = 1; @}; +@end group +@end example + +@cindex default action +If you don't specify an action for a rule, Bison supplies a default: +@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule +becomes the value of the whole rule. Of course, the default action is +valid only if the two data types match. There is no meaningful default +action for an empty rule; every empty rule must have an explicit action +unless the rule's value does not matter. + +@code{$@var{n}} with @var{n} zero or negative is allowed for reference +to tokens and groupings on the stack @emph{before} those that match the +current rule. This is a very risky practice, and to use it reliably you must be certain of the context in which the rule is applied. Here is a case in which you can use this reliably: @@ -2575,7 +3512,13 @@ As long as @code{bar} is used only in the fashion shown here, @code{$0} always refers to the @code{expr} which precedes @code{bar} in the definition of @code{foo}. -@node Action Types, Mid-Rule Actions, Actions, Semantics +@vindex yylval +It is also possible to access the semantic value of the lookahead token, if +any, from a semantic action. +This semantic value is stored in @code{yylval}. +@xref{Action Features, ,Special Features for Use in Actions}. + +@node Action Types @subsection Data Types of Values in Actions @cindex action data types @cindex data types in actions @@ -2587,7 +3530,7 @@ If you have used @code{%union} to specify a variety of data types, then you must declare a choice among these types for each terminal or nonterminal symbol that can have a semantic value. Then each time you use @code{$$} or @code{$@var{n}}, its data type is determined by which symbol it refers to -in the rule. In this example,@refill +in the rule. In this example, @example @group @@ -2601,7 +3544,7 @@ exp: @dots{} @code{$1} and @code{$3} refer to instances of @code{exp}, so they all have the data type declared for the nonterminal symbol @code{exp}. If @code{$2} were used, it would have the data type declared for the -terminal symbol @code{'+'}, whatever that might be.@refill +terminal symbol @code{'+'}, whatever that might be. Alternatively, you can specify the data type when you refer to the value, by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the @@ -2620,7 +3563,7 @@ reference. For example, if you have defined types as shown here: then you can write @code{$1} to refer to the first subunit of the rule as an integer, or @code{$1} to refer to it as a double. -@node Mid-Rule Actions, , Action Types, Semantics +@node Mid-Rule Actions @subsection Actions in Mid-Rule @cindex actions in mid-rule @cindex mid-rule actions @@ -2643,8 +3586,8 @@ The mid-rule action can also have a semantic value. The action can set its value with an assignment to @code{$$}, and actions later in the rule can refer to the value using @code{$@var{n}}. Since there is no symbol to name the action, there is no way to declare a data type for the value -in advance, so you must use the @samp{$<@dots{}>} construct to specify a -data type each time you refer to this value. +in advance, so you must use the @samp{$<@dots{}>@var{n}} construct to +specify a data type each time you refer to this value. There is no way to set the value of the entire rule with a mid-rule action, because assignments to @code{$$} do not have that effect. The @@ -2684,6 +3627,46 @@ earlier action is used to restore the prior list of variables. This removes the temporary @code{let}-variable from the list so that it won't appear to exist while the rest of the program is parsed. +@findex %destructor +@cindex discarded symbols, mid-rule actions +@cindex error recovery, mid-rule actions +In the above example, if the parser initiates error recovery (@pxref{Error +Recovery}) while parsing the tokens in the embedded statement @code{stmt}, +it might discard the previous semantic context @code{$5} without +restoring it. +Thus, @code{$5} needs a destructor (@pxref{Destructor Decl, , Freeing +Discarded Symbols}). +However, Bison currently provides no means to declare a destructor specific to +a particular mid-rule action's semantic value. + +One solution is to bury the mid-rule action inside a nonterminal symbol and to +declare a destructor for that symbol: + +@example +@group +%type let +%destructor @{ pop_context ($$); @} let + +%% + +stmt: let stmt + @{ $$ = $2; + pop_context ($1); @} + ; + +let: LET '(' var ')' + @{ $$ = push_context (); + declare_variable ($3); @} + ; + +@end group +@end example + +@noindent +Note that the action is now at the end of its rule. +Any mid-rule action can be converted to an end-of-rule action in this way, and +this is what Bison actually does to implement mid-rule actions. + Taking action before a rule is completely recognized often leads to conflicts since the parser must commit to a parse in order to execute the action. For example, the following two rules, without mid-rule actions, @@ -2718,8 +3701,8 @@ Now the parser is forced to decide whether to run the mid-rule action when it has read no farther than the open-brace. In other words, it must commit to using one rule or the other, without sufficient information to do it correctly. (The open-brace token is what is called -the @dfn{look-ahead} token at this time, since the parser is still -deciding what to do about it. @xref{Look-Ahead, ,Look-Ahead Tokens}.) +the @dfn{lookahead} token at this time, since the parser is still +deciding what to do about it. @xref{Lookahead, ,Lookahead Tokens}.) You might think that you could correct the problem by putting identical actions into the two rules, like this: @@ -2777,12 +3760,207 @@ compound: subroutine @noindent Now Bison can execute the action in the rule for @code{subroutine} without -deciding which rule for @code{compound} it will eventually use. Note that -the action is now at the end of its rule. Any mid-rule action can be -converted to an end-of-rule action in this way, and this is what Bison -actually does to implement mid-rule actions. +deciding which rule for @code{compound} it will eventually use. + +@node Locations +@section Tracking Locations +@cindex location +@cindex textual location +@cindex location, textual + +Though grammar rules and semantic actions are enough to write a fully +functional parser, it can be useful to process some additional information, +especially symbol locations. + +The way locations are handled is defined by providing a data type, and +actions to take when rules are matched. + +@menu +* Location Type:: Specifying a data type for locations. +* Actions and Locations:: Using locations in actions. +* Location Default Action:: Defining a general way to compute locations. +@end menu + +@node Location Type +@subsection Data Type of Locations +@cindex data type of locations +@cindex default location type + +Defining a data type for locations is much simpler than for semantic values, +since all tokens and groupings always use the same type. + +You can specify the type of locations by defining a macro called +@code{YYLTYPE}, just as you can specify the semantic value type by +defining a @code{YYSTYPE} macro (@pxref{Value Type}). +When @code{YYLTYPE} is not defined, Bison uses a default structure type with +four members: + +@example +typedef struct YYLTYPE +@{ + int first_line; + int first_column; + int last_line; + int last_column; +@} YYLTYPE; +@end example + +At the beginning of the parsing, Bison initializes all these fields to 1 +for @code{yylloc}. + +@node Actions and Locations +@subsection Actions and Locations +@cindex location actions +@cindex actions, location +@vindex @@$ +@vindex @@@var{n} + +Actions are not only useful for defining language semantics, but also for +describing the behavior of the output parser with locations. + +The most obvious way for building locations of syntactic groupings is very +similar to the way semantic values are computed. In a given rule, several +constructs can be used to access the locations of the elements being matched. +The location of the @var{n}th component of the right hand side is +@code{@@@var{n}}, while the location of the left hand side grouping is +@code{@@$}. + +Here is a basic example using the default data type for locations: + +@example +@group +exp: @dots{} + | exp '/' exp + @{ + @@$.first_column = @@1.first_column; + @@$.first_line = @@1.first_line; + @@$.last_column = @@3.last_column; + @@$.last_line = @@3.last_line; + if ($3) + $$ = $1 / $3; + else + @{ + $$ = 1; + fprintf (stderr, + "Division by zero, l%d,c%d-l%d,c%d", + @@3.first_line, @@3.first_column, + @@3.last_line, @@3.last_column); + @} + @} +@end group +@end example + +As for semantic values, there is a default action for locations that is +run each time a rule is matched. It sets the beginning of @code{@@$} to the +beginning of the first symbol, and the end of @code{@@$} to the end of the +last symbol. + +With this default action, the location tracking can be fully automatic. The +example above simply rewrites this way: + +@example +@group +exp: @dots{} + | exp '/' exp + @{ + if ($3) + $$ = $1 / $3; + else + @{ + $$ = 1; + fprintf (stderr, + "Division by zero, l%d,c%d-l%d,c%d", + @@3.first_line, @@3.first_column, + @@3.last_line, @@3.last_column); + @} + @} +@end group +@end example + +@vindex yylloc +It is also possible to access the location of the lookahead token, if any, +from a semantic action. +This location is stored in @code{yylloc}. +@xref{Action Features, ,Special Features for Use in Actions}. + +@node Location Default Action +@subsection Default Action for Locations +@vindex YYLLOC_DEFAULT +@cindex @acronym{GLR} parsers and @code{YYLLOC_DEFAULT} + +Actually, actions are not the best place to compute locations. Since +locations are much more general than semantic values, there is room in +the output parser to redefine the default action to take for each +rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is +matched, before the associated action is run. It is also invoked +while processing a syntax error, to compute the error's location. +Before reporting an unresolvable syntactic ambiguity, a @acronym{GLR} +parser invokes @code{YYLLOC_DEFAULT} recursively to compute the location +of that ambiguity. + +Most of the time, this macro is general enough to suppress location +dedicated code from semantic actions. + +The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is +the location of the grouping (the result of the computation). When a +rule is matched, the second parameter identifies locations of +all right hand side elements of the rule being matched, and the third +parameter is the size of the rule's right hand side. +When a @acronym{GLR} parser reports an ambiguity, which of multiple candidate +right hand sides it passes to @code{YYLLOC_DEFAULT} is undefined. +When processing a syntax error, the second parameter identifies locations +of the symbols that were discarded during error processing, and the third +parameter is the number of discarded symbols. + +By default, @code{YYLLOC_DEFAULT} is defined this way: + +@smallexample +@group +# define YYLLOC_DEFAULT(Current, Rhs, N) \ + do \ + if (N) \ + @{ \ + (Current).first_line = YYRHSLOC(Rhs, 1).first_line; \ + (Current).first_column = YYRHSLOC(Rhs, 1).first_column; \ + (Current).last_line = YYRHSLOC(Rhs, N).last_line; \ + (Current).last_column = YYRHSLOC(Rhs, N).last_column; \ + @} \ + else \ + @{ \ + (Current).first_line = (Current).last_line = \ + YYRHSLOC(Rhs, 0).last_line; \ + (Current).first_column = (Current).last_column = \ + YYRHSLOC(Rhs, 0).last_column; \ + @} \ + while (0) +@end group +@end smallexample + +where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol +in @var{rhs} when @var{k} is positive, and the location of the symbol +just before the reduction when @var{k} and @var{n} are both zero. + +When defining @code{YYLLOC_DEFAULT}, you should consider that: + +@itemize @bullet +@item +All arguments are free of side-effects. However, only the first one (the +result) should be modified by @code{YYLLOC_DEFAULT}. + +@item +For consistency with semantic actions, valid indexes within the +right hand side range from 1 to @var{n}. When @var{n} is zero, only 0 is a +valid index, and it refers to the symbol just before the reduction. +During error processing @var{n} is always positive. + +@item +Your macro should parenthesize its arguments, if need be, since the +actual arguments may not be surrounded by parentheses. Also, your +macro should expand to something that can be used as a single +statement when it is followed by a semicolon. +@end itemize -@node Declarations, Multiple Parsers, Semantics, Grammar File +@node Declarations @section Bison Declarations @cindex declarations, Bison @cindex Bison declarations @@ -2798,23 +3976,43 @@ value (@pxref{Multiple Types, ,More Than One Value Type}). The first rule in the file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare -it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars}). +it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free +Grammars}). @menu +* Require Decl:: Requiring a Bison version. * Token Decl:: Declaring terminal symbols. * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. -* Expect Decl:: Suppressing warnings about shift/reduce conflicts. +* Initial Action Decl:: Code run before parsing starts. +* Destructor Decl:: Declaring how symbols are freed. +* Expect Decl:: Suppressing warnings about parsing conflicts. * Start Decl:: Specifying the start symbol. * Pure Decl:: Requesting a reentrant parser. +* Push Decl:: Requesting a push parser. * Decl Summary:: Table of all Bison declarations. @end menu -@node Token Decl, Precedence Decl, , Declarations +@node Require Decl +@subsection Require a Version of Bison +@cindex version requirement +@cindex requiring a version of Bison +@findex %require + +You may require the minimum version of Bison to process the grammar. If +the requirement is not met, @command{bison} exits with an error (exit +status 63). + +@example +%require "@var{version}" +@end example + +@node Token Decl @subsection Token Type Names @cindex declaring token type names @cindex token type names, declaring +@cindex declaring literal string tokens @findex %token The basic way to declare a token type name (terminal symbol) is as follows: @@ -2827,25 +4025,30 @@ Bison will convert this into a @code{#define} directive in the parser, so that the function @code{yylex} (if it is in this file) can use the name @var{name} to stand for this token type's code. -Alternatively, you can use @code{%left}, @code{%right}, or @code{%nonassoc} -instead of @code{%token}, if you wish to specify precedence. -@xref{Precedence Decl, ,Operator Precedence}. +Alternatively, you can use @code{%left}, @code{%right}, +@code{%precedence}, or +@code{%nonassoc} instead of @code{%token}, if you wish to specify +associativity and precedence. @xref{Precedence Decl, ,Operator +Precedence}. You can explicitly specify the numeric code for a token type by appending -an integer value in the field immediately following the token name: +a nonnegative decimal or hexadecimal integer value in the field immediately +following the token name: @example %token NUM 300 +%token XNUM 0x12d // a GNU extension @end example @noindent It is generally best, however, to let Bison choose the numeric codes for all token types. Bison will automatically select codes that don't conflict -with each other or with ASCII characters. +with each other or with normal characters. In the event that the stack type is a union, you must augment the @code{%token} or other token declaration to include the data type -alternative delimited by angle-brackets (@pxref{Multiple Types, ,More Than One Value Type}). +alternative delimited by angle-brackets (@pxref{Multiple Types, ,More +Than One Value Type}). For example: @@ -2859,18 +4062,54 @@ For example: @end group @end example -@node Precedence Decl, Union Decl, Token Decl, Declarations +You can associate a literal string token with a token type name by +writing the literal string at the end of a @code{%token} +declaration which declares the name. For example: + +@example +%token arrow "=>" +@end example + +@noindent +For example, a grammar for the C language might specify these names with +equivalent literal string tokens: + +@example +%token OR "||" +%token LE 134 "<=" +%left OR "<=" +@end example + +@noindent +Once you equate the literal string and the token name, you can use them +interchangeably in further declarations or the grammar rules. The +@code{yylex} function can use the token name or the literal string to +obtain the token type code number (@pxref{Calling Convention}). +Syntax error messages passed to @code{yyerror} from the parser will reference +the literal string instead of the token name. + +The token numbered as 0 corresponds to end of file; the following line +allows for nicer error messages referring to ``end of file'' instead +of ``$end'': + +@example +%token END 0 "end of file" +@end example + +@node Precedence Decl @subsection Operator Precedence @cindex precedence declarations @cindex declaring operator precedence @cindex operator precedence, declaring -Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to +Use the @code{%left}, @code{%right}, @code{%nonassoc}, or +@code{%precedence} declaration to declare a token and specify its precedence and associativity, all at once. These are called @dfn{precedence declarations}. -@xref{Precedence, ,Operator Precedence}, for general information on operator precedence. +@xref{Precedence, ,Operator Precedence}, for general information on +operator precedence. -The syntax of a precedence declaration is the same as that of +The syntax of a precedence declaration is nearly the same as that of @code{%token}: either @example @@ -2900,6 +4139,10 @@ left-associativity (grouping @var{x} with @var{y} first) and means that @samp{@var{x} @var{op} @var{y} @var{op} @var{z}} is considered a syntax error. +@code{%precedence} gives only precedence to the @var{symbols}, and +defines no associativity at all. Use this to define precedence only, +and leave any potential conflict due to associativity enabled. + @item The precedence of an operator determines how it nests with other operators. All the tokens declared in a single precedence declaration have equal @@ -2908,16 +4151,28 @@ When two tokens declared in different precedence declarations associate, the one declared later has the higher precedence and is grouped first. @end itemize -@node Union Decl, Type Decl, Precedence Decl, Declarations +For backward compatibility, there is a confusing difference between the +argument lists of @code{%token} and precedence declarations. +Only a @code{%token} can associate a literal string with a token type name. +A precedence declaration always interprets a literal string as a reference to a +separate token. +For example: + +@example +%left OR "<=" // Does not declare an alias. +%left OR 134 "<=" 135 // Declares 134 for OR and 135 for "<=". +@end example + +@node Union Decl @subsection The Collection of Value Types @cindex declaring value types @cindex value types, declaring @findex %union -The @code{%union} declaration specifies the entire collection of possible -data types for semantic values. The keyword @code{%union} is followed by a -pair of braces containing the same thing that goes inside a @code{union} in -C. +The @code{%union} declaration specifies the entire collection of +possible data types for semantic values. The keyword @code{%union} is +followed by braced code containing the same thing that goes inside a +@code{union} in C@. For example: @@ -2936,10 +4191,60 @@ This says that the two alternative types are @code{double} and @code{symrec in the @code{%token} and @code{%type} declarations to pick one of the types for a terminal or nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}). -Note that, unlike making a @code{union} declaration in C, you do not write +As an extension to @acronym{POSIX}, a tag is allowed after the +@code{union}. For example: + +@example +@group +%union value @{ + double val; + symrec *tptr; +@} +@end group +@end example + +@noindent +specifies the union tag @code{value}, so the corresponding C type is +@code{union value}. If you do not specify a tag, it defaults to +@code{YYSTYPE}. + +As another extension to @acronym{POSIX}, you may specify multiple +@code{%union} declarations; their contents are concatenated. However, +only the first @code{%union} declaration can specify a tag. + +Note that, unlike making a @code{union} declaration in C, you need not write a semicolon after the closing brace. -@node Type Decl, Expect Decl, Union Decl, Declarations +Instead of @code{%union}, you can define and use your own union type +@code{YYSTYPE} if your grammar contains at least one +@samp{<@var{type}>} tag. For example, you can put the following into +a header file @file{parser.h}: + +@example +@group +union YYSTYPE @{ + double val; + symrec *tptr; +@}; +typedef union YYSTYPE YYSTYPE; +@end group +@end example + +@noindent +and then your grammar can use the following +instead of @code{%union}: + +@example +@group +%@{ +#include "parser.h" +%@} +%type expr +%token ID +@end group +@end example + +@node Type Decl @subsection Nonterminal Symbols @cindex declaring value types, nonterminals @cindex value types, nonterminals, declaring @@ -2955,26 +4260,205 @@ used. This is done with a @code{%type} declaration, like this: @end example @noindent -Here @var{nonterminal} is the name of a nonterminal symbol, and @var{type} -is the name given in the @code{%union} to the alternative that you want -(@pxref{Union Decl, ,The Collection of Value Types}). You can give any number of nonterminal symbols in -the same @code{%type} declaration, if they have the same value type. Use -spaces to separate the symbol names. +Here @var{nonterminal} is the name of a nonterminal symbol, and +@var{type} is the name given in the @code{%union} to the alternative +that you want (@pxref{Union Decl, ,The Collection of Value Types}). You +can give any number of nonterminal symbols in the same @code{%type} +declaration, if they have the same value type. Use spaces to separate +the symbol names. + +You can also declare the value type of a terminal symbol. To do this, +use the same @code{<@var{type}>} construction in a declaration for the +terminal symbol. All kinds of token declarations allow +@code{<@var{type}>}. + +@node Initial Action Decl +@subsection Performing Actions before Parsing +@findex %initial-action + +Sometimes your parser needs to perform some initializations before +parsing. The @code{%initial-action} directive allows for such arbitrary +code. + +@deffn {Directive} %initial-action @{ @var{code} @} +@findex %initial-action +Declare that the braced @var{code} must be invoked before parsing each time +@code{yyparse} is called. The @var{code} may use @code{$$} and +@code{@@$} --- initial value and location of the lookahead --- and the +@code{%parse-param}. +@end deffn + +For instance, if your locations use a file name, you may use + +@example +%parse-param @{ char const *file_name @}; +%initial-action +@{ + @@$.initialize (file_name); +@}; +@end example + + +@node Destructor Decl +@subsection Freeing Discarded Symbols +@cindex freeing discarded symbols +@findex %destructor +@findex <*> +@findex <> +During error recovery (@pxref{Error Recovery}), symbols already pushed +on the stack and tokens coming from the rest of the file are discarded +until the parser falls on its feet. If the parser runs out of memory, +or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the +symbols on the stack must be discarded. Even if the parser succeeds, it +must discard the start symbol. + +When discarded symbols convey heap based information, this memory is +lost. While this behavior can be tolerable for batch parsers, such as +in traditional compilers, it is unacceptable for programs like shells or +protocol implementations that may parse and execute indefinitely. + +The @code{%destructor} directive defines code that is called when a +symbol is automatically discarded. + +@deffn {Directive} %destructor @{ @var{code} @} @var{symbols} +@findex %destructor +Invoke the braced @var{code} whenever the parser discards one of the +@var{symbols}. +Within @var{code}, @code{$$} designates the semantic value associated +with the discarded symbol, and @code{@@$} designates its location. +The additional parser parameters are also available (@pxref{Parser Function, , +The Parser Function @code{yyparse}}). + +When a symbol is listed among @var{symbols}, its @code{%destructor} is called a +per-symbol @code{%destructor}. +You may also define a per-type @code{%destructor} by listing a semantic type +tag among @var{symbols}. +In that case, the parser will invoke this @var{code} whenever it discards any +grammar symbol that has that semantic type tag unless that symbol has its own +per-symbol @code{%destructor}. + +Finally, you can define two different kinds of default @code{%destructor}s. +(These default forms are experimental. +More user feedback will help to determine whether they should become permanent +features.) +You can place each of @code{<*>} and @code{<>} in the @var{symbols} list of +exactly one @code{%destructor} declaration in your grammar file. +The parser will invoke the @var{code} associated with one of these whenever it +discards any user-defined grammar symbol that has no per-symbol and no per-type +@code{%destructor}. +The parser uses the @var{code} for @code{<*>} in the case of such a grammar +symbol for which you have formally declared a semantic type tag (@code{%type} +counts as such a declaration, but @code{$$} does not). +The parser uses the @var{code} for @code{<>} in the case of such a grammar +symbol that has no declared semantic type tag. +@end deffn + +@noindent +For example: + +@smallexample +%union @{ char *string; @} +%token STRING1 +%token STRING2 +%type string1 +%type string2 +%union @{ char character; @} +%token CHR +%type chr +%token TAGLESS + +%destructor @{ @} +%destructor @{ free ($$); @} <*> +%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1 +%destructor @{ printf ("Discarding tagless symbol.\n"); @} <> +@end smallexample + +@noindent +guarantees that, when the parser discards any user-defined symbol that has a +semantic type tag other than @code{}, it passes its semantic value +to @code{free} by default. +However, when the parser discards a @code{STRING1} or a @code{string1}, it also +prints its line number to @code{stdout}. +It performs only the second @code{%destructor} in this case, so it invokes +@code{free} only once. +Finally, the parser merely prints a message whenever it discards any symbol, +such as @code{TAGLESS}, that has no semantic type tag. + +A Bison-generated parser invokes the default @code{%destructor}s only for +user-defined as opposed to Bison-defined symbols. +For example, the parser will not invoke either kind of default +@code{%destructor} for the special Bison-defined symbols @code{$accept}, +@code{$undefined}, or @code{$end} (@pxref{Table of Symbols, ,Bison Symbols}), +none of which you can reference in your grammar. +It also will not invoke either for the @code{error} token (@pxref{Table of +Symbols, ,error}), which is always defined by Bison regardless of whether you +reference it in your grammar. +However, it may invoke one of them for the end token (token 0) if you +redefine it from @code{$end} to, for example, @code{END}: + +@smallexample +%token END 0 +@end smallexample + +@cindex actions in mid-rule +@cindex mid-rule actions +Finally, Bison will never invoke a @code{%destructor} for an unreferenced +mid-rule semantic value (@pxref{Mid-Rule Actions,,Actions in Mid-Rule}). +That is, Bison does not consider a mid-rule to have a semantic value if you do +not reference @code{$$} in the mid-rule's action or @code{$@var{n}} (where +@var{n} is the RHS symbol position of the mid-rule) in any later action in that +rule. +However, if you do reference either, the Bison-generated parser will invoke the +@code{<>} @code{%destructor} whenever it discards the mid-rule symbol. + +@ignore +@noindent +In the future, it may be possible to redefine the @code{error} token as a +nonterminal that captures the discarded symbols. +In that case, the parser will invoke the default destructor for it as well. +@end ignore + +@sp 1 + +@cindex discarded symbols +@dfn{Discarded symbols} are the following: + +@itemize +@item +stacked symbols popped during the first phase of error recovery, +@item +incoming terminals during the second phase of error recovery, +@item +the current lookahead and the entire stack (except the current +right-hand side symbols) when the parser returns immediately, and +@item +the start symbol, when the parser succeeds. +@end itemize + +The parser can @dfn{return immediately} because of an explicit call to +@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory +exhaustion. -@node Expect Decl, Start Decl, Type Decl, Declarations +Right-hand side symbols of a rule that explicitly triggers a syntax +error via @code{YYERROR} are not discarded automatically. As a rule +of thumb, destructors are invoked only when user actions cannot manage +the memory. + +@node Expect Decl @subsection Suppressing Conflict Warnings @cindex suppressing conflict warnings @cindex preventing warnings about conflicts @cindex warnings, preventing @cindex conflicts, suppressing warnings of @findex %expect +@findex %expect-rr Bison normally warns if there are any conflicts in the grammar -(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars have harmless shift/reduce -conflicts which are resolved in a predictable way and would be difficult to -eliminate. It is desirable to suppress the warning about these conflicts -unless the number of conflicts changes. You can do this with the -@code{%expect} declaration. +(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars +have harmless shift/reduce conflicts which are resolved in a predictable +way and would be difficult to eliminate. It is desirable to suppress +the warning about these conflicts unless the number of conflicts +changes. You can do this with the @code{%expect} declaration. The declaration looks like this: @@ -2982,10 +4466,22 @@ The declaration looks like this: %expect @var{n} @end example -Here @var{n} is a decimal integer. The declaration says there should be no -warning if there are @var{n} shift/reduce conflicts and no reduce/reduce -conflicts. The usual warning is given if there are either more or fewer -conflicts, or if there are any reduce/reduce conflicts. +Here @var{n} is a decimal integer. The declaration says there should +be @var{n} shift/reduce conflicts and no reduce/reduce conflicts. +Bison reports an error if the number of shift/reduce conflicts differs +from @var{n}, or if there are any reduce/reduce conflicts. + +For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more +serious, and should be eliminated entirely. Bison will always report +reduce/reduce conflicts for these parsers. With @acronym{GLR} +parsers, however, both kinds of conflicts are routine; otherwise, +there would be no need to use @acronym{GLR} parsing. Therefore, it is +also possible to specify an expected number of reduce/reduce conflicts +in @acronym{GLR} parsers, using the declaration: + +@example +%expect-rr @var{n} +@end example In general, using @code{%expect} involves these steps: @@ -3002,14 +4498,14 @@ go back to the beginning. @item Add an @code{%expect} declaration, copying the number @var{n} from the -number which Bison printed. +number which Bison printed. With @acronym{GLR} parsers, add an +@code{%expect-rr} declaration as well. @end itemize -Now Bison will stop annoying you about the conflicts you have checked, but -it will warn you again if changes in the grammar result in additional -conflicts. +Now Bison will warn you if you introduce an unexpected conflict, but +will keep silent otherwise. -@node Start Decl, Pure Decl, Expect Decl, Declarations +@node Start Decl @subsection The Start-Symbol @cindex declaring the start symbol @cindex start symbol, declaring @@ -3024,11 +4520,11 @@ may override this restriction with the @code{%start} declaration as follows: %start @var{symbol} @end example -@node Pure Decl, Decl Summary, Start Decl, Declarations +@node Pure Decl @subsection A Pure (Reentrant) Parser @cindex reentrant parser @cindex pure parser -@findex %pure_parser +@findex %define api.pure A @dfn{reentrant} program is one which does not alter in the course of execution; in other words, it consists entirely of @dfn{pure} (read-only) @@ -3037,150 +4533,864 @@ for example, a nonreentrant program may not be safe to call from a signal handler. In systems with multiple threads of control, a nonreentrant program must be called only within interlocks. -The Bison parser is not normally a reentrant program, because it uses -statically allocated variables for communication with @code{yylex}. These -variables include @code{yylval} and @code{yylloc}. +Normally, Bison generates a parser which is not reentrant. This is +suitable for most uses, and it permits compatibility with Yacc. (The +standard Yacc interfaces are inherently nonreentrant, because they use +statically allocated variables for communication with @code{yylex}, +including @code{yylval} and @code{yylloc}.) -The Bison declaration @code{%pure_parser} says that you want the parser -to be reentrant. It looks like this: +Alternatively, you can generate a pure, reentrant parser. The Bison +declaration @code{%define api.pure} says that you want the parser to be +reentrant. It looks like this: @example -%pure_parser +%define api.pure @end example -The effect is that the two communication variables become local -variables in @code{yyparse}, and a different calling convention is used for -the lexical analyzer function @code{yylex}. @xref{Pure Calling, ,Calling for Pure Parsers}, for the -details of this. The variable @code{yynerrs} also becomes local in -@code{yyparse} (@pxref{Error Reporting, ,The Error Reporting Function @code{yyerror}}). The convention for calling +The result is that the communication variables @code{yylval} and +@code{yylloc} become local variables in @code{yyparse}, and a different +calling convention is used for the lexical analyzer function +@code{yylex}. @xref{Pure Calling, ,Calling Conventions for Pure +Parsers}, for the details of this. The variable @code{yynerrs} +becomes local in @code{yyparse} in pull mode but it becomes a member +of yypstate in push mode. (@pxref{Error Reporting, ,The Error +Reporting Function @code{yyerror}}). The convention for calling @code{yyparse} itself is unchanged. -@node Decl Summary, , Pure Decl, Declarations +Whether the parser is pure has nothing to do with the grammar rules. +You can generate either a pure parser or a nonreentrant parser from any +valid grammar. + +@node Push Decl +@subsection A Push Parser +@cindex push parser +@cindex push parser +@findex %define api.push_pull + +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) + +A pull parser is called once and it takes control until all its input +is completely parsed. A push parser, on the other hand, is called +each time a new token is made available. + +A push parser is typically useful when the parser is part of a +main event loop in the client's application. This is typically +a requirement of a GUI, when the main event loop needs to be triggered +within a certain time period. + +Normally, Bison generates a pull parser. +The following Bison declaration says that you want the parser to be a push +parser (@pxref{Decl Summary,,%define api.push_pull}): + +@example +%define api.push_pull "push" +@end example + +In almost all cases, you want to ensure that your push parser is also +a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). The only +time you should create an impure push parser is to have backwards +compatibility with the impure Yacc pull mode interface. Unless you know +what you are doing, your declarations should look like this: + +@example +%define api.pure +%define api.push_pull "push" +@end example + +There is a major notable functional difference between the pure push parser +and the impure push parser. It is acceptable for a pure push parser to have +many parser instances, of the same type of parser, in memory at the same time. +An impure push parser should only use one parser at a time. + +When a push parser is selected, Bison will generate some new symbols in +the generated parser. @code{yypstate} is a structure that the generated +parser uses to store the parser's state. @code{yypstate_new} is the +function that will create a new parser instance. @code{yypstate_delete} +will free the resources associated with the corresponding parser instance. +Finally, @code{yypush_parse} is the function that should be called whenever a +token is available to provide the parser. A trivial example +of using a pure push parser would look like this: + +@example +int status; +yypstate *ps = yypstate_new (); +do @{ + status = yypush_parse (ps, yylex (), NULL); +@} while (status == YYPUSH_MORE); +yypstate_delete (ps); +@end example + +If the user decided to use an impure push parser, a few things about +the generated parser will change. The @code{yychar} variable becomes +a global variable instead of a variable in the @code{yypush_parse} function. +For this reason, the signature of the @code{yypush_parse} function is +changed to remove the token as a parameter. A nonreentrant push parser +example would thus look like this: + +@example +extern int yychar; +int status; +yypstate *ps = yypstate_new (); +do @{ + yychar = yylex (); + status = yypush_parse (ps); +@} while (status == YYPUSH_MORE); +yypstate_delete (ps); +@end example + +That's it. Notice the next token is put into the global variable @code{yychar} +for use by the next invocation of the @code{yypush_parse} function. + +Bison also supports both the push parser interface along with the pull parser +interface in the same generated parser. In order to get this functionality, +you should replace the @code{%define api.push_pull "push"} declaration with the +@code{%define api.push_pull "both"} declaration. Doing this will create all of +the symbols mentioned earlier along with the two extra symbols, @code{yyparse} +and @code{yypull_parse}. @code{yyparse} can be used exactly as it normally +would be used. However, the user should note that it is implemented in the +generated parser by calling @code{yypull_parse}. +This makes the @code{yyparse} function that is generated with the +@code{%define api.push_pull "both"} declaration slower than the normal +@code{yyparse} function. If the user +calls the @code{yypull_parse} function it will parse the rest of the input +stream. It is possible to @code{yypush_parse} tokens to select a subgrammar +and then @code{yypull_parse} the rest of the input stream. If you would like +to switch back and forth between between parsing styles, you would have to +write your own @code{yypull_parse} function that knows when to quit looking +for input. An example of using the @code{yypull_parse} function would look +like this: + +@example +yypstate *ps = yypstate_new (); +yypull_parse (ps); /* Will call the lexer */ +yypstate_delete (ps); +@end example + +Adding the @code{%define api.pure} declaration does exactly the same thing to +the generated parser with @code{%define api.push_pull "both"} as it did for +@code{%define api.push_pull "push"}. + +@node Decl Summary @subsection Bison Declaration Summary @cindex Bison declaration summary @cindex declaration summary @cindex summary, Bison declaration -Here is a summary of all Bison declarations: +Here is a summary of the declarations used to define a grammar: -@table @code -@item %union +@deffn {Directive} %union Declare the collection of data types that semantic values may have (@pxref{Union Decl, ,The Collection of Value Types}). +@end deffn -@item %token +@deffn {Directive} %token Declare a terminal symbol (token type name) with no precedence or associativity specified (@pxref{Token Decl, ,Token Type Names}). +@end deffn -@item %right +@deffn {Directive} %right Declare a terminal symbol (token type name) that is right-associative (@pxref{Precedence Decl, ,Operator Precedence}). +@end deffn -@item %left +@deffn {Directive} %left Declare a terminal symbol (token type name) that is left-associative (@pxref{Precedence Decl, ,Operator Precedence}). +@end deffn -@item %nonassoc +@deffn {Directive} %nonassoc Declare a terminal symbol (token type name) that is nonassociative -(using it in a way that would be associative is a syntax error) (@pxref{Precedence Decl, ,Operator Precedence}). +Using it in a way that would be associative is a syntax error. +@end deffn + +@ifset defaultprec +@deffn {Directive} %default-prec +Assign a precedence to rules lacking an explicit @code{%prec} modifier +(@pxref{Contextual Precedence, ,Context-Dependent Precedence}). +@end deffn +@end ifset -@item %type +@deffn {Directive} %type Declare the type of semantic values for a nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}). +@end deffn -@item %start -Specify the grammar's start symbol (@pxref{Start Decl, ,The Start-Symbol}). +@deffn {Directive} %start +Specify the grammar's start symbol (@pxref{Start Decl, ,The +Start-Symbol}). +@end deffn -@item %expect +@deffn {Directive} %expect Declare the expected number of shift-reduce conflicts (@pxref{Expect Decl, ,Suppressing Conflict Warnings}). +@end deffn -@item %pure_parser -Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). -@end table -@node Multiple Parsers, , Declarations, Grammar File -@section Multiple Parsers in the Same Program +@sp 1 +@noindent +In order to change the behavior of @command{bison}, use the following +directives: + +@deffn {Directive} %code @{@var{code}@} +@findex %code +This is the unqualified form of the @code{%code} directive. +It inserts @var{code} verbatim at a language-dependent default location in the +output@footnote{The default location is actually skeleton-dependent; + writers of non-standard skeletons however should choose the default location + consistently with the behavior of the standard Bison skeletons.}. + +@cindex Prologue +For C/C++, the default location is the parser source code +file after the usual contents of the parser header file. +Thus, @code{%code} replaces the traditional Yacc prologue, +@code{%@{@var{code}%@}}, for most purposes. +For a detailed discussion, see @ref{Prologue Alternatives}. + +For Java, the default location is inside the parser class. + +(Like all the Yacc prologue alternatives, this directive is experimental. +More user feedback will help to determine whether it should become a permanent +feature.) +@end deffn + +@deffn {Directive} %code @var{qualifier} @{@var{code}@} +This is the qualified form of the @code{%code} directive. +If you need to specify location-sensitive verbatim @var{code} that does not +belong at the default location selected by the unqualified @code{%code} form, +use this form instead. + +@var{qualifier} identifies the purpose of @var{code} and thus the location(s) +where Bison should generate it. +Not all values of @var{qualifier} are available for all target languages: -Most programs that use Bison parse only one language and therefore contain -only one Bison parser. But what if you want to parse more than one -language with the same program? Then you need to avoid a name conflict -between different definitions of @code{yyparse}, @code{yylval}, and so on. +@itemize @bullet +@item requires +@findex %code requires -The easy way to do this is to use the option @samp{-p @var{prefix}} -(@pxref{Invocation, ,Invoking Bison}). This renames the interface functions and -variables of the Bison parser to start with @var{prefix} instead of -@samp{yy}. You can use this to give each parser distinct names that do -not conflict. +@itemize @bullet +@item Language(s): C, C++ -The precise list of symbols renamed is @code{yyparse}, @code{yylex}, -@code{yyerror}, @code{yylval}, @code{yychar} and @code{yydebug}. For -example, if you use @samp{-p c}, the names become @code{cparse}, -@code{clex}, and so on. +@item Purpose: This is the best place to write dependency code required for +@code{YYSTYPE} and @code{YYLTYPE}. +In other words, it's the best place to define types referenced in @code{%union} +directives, and it's the best place to override Bison's default @code{YYSTYPE} +and @code{YYLTYPE} definitions. -@strong{All the other variables and macros associated with Bison are not -renamed.} These others are not global; there is no conflict if the same -name is used in different parsers. For example, @code{YYSTYPE} is not -renamed, but defining this in different ways in different parsers causes -no trouble (@pxref{Value Type, ,Data Types of Semantic Values}). +@item Location(s): The parser header file and the parser source code file +before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE} definitions. +@end itemize -The @samp{-p} option works by adding macro definitions to the beginning -of the parser source file, defining @code{yyparse} as -@code{@var{prefix}parse}, and so on. This effectively substitutes one -name for the other in the entire parser file. +@item provides +@findex %code provides -@node Interface, Algorithm, Grammar File, Top -@chapter Parser C-Language Interface -@cindex C-language interface -@cindex interface +@itemize @bullet +@item Language(s): C, C++ -The Bison parser is actually a C function named @code{yyparse}. Here we -describe the interface conventions of @code{yyparse} and the other -functions that it needs to use. +@item Purpose: This is the best place to write additional definitions and +declarations that should be provided to other modules. -Keep in mind that the parser uses many C identifiers starting with -@samp{yy} and @samp{YY} for internal purposes. If you use such an -identifier (aside from those in this manual) in an action or in additional -C code in the grammar file, you are likely to run into trouble. +@item Location(s): The parser header file and the parser source code file after +the Bison-generated @code{YYSTYPE}, @code{YYLTYPE}, and token definitions. +@end itemize -@menu -* Parser Function:: How to call @code{yyparse} and what it returns. -* Lexical:: You must supply a function @code{yylex} - which reads tokens. -* Error Reporting:: You must supply a function @code{yyerror}. -* Action Features:: Special features for use in actions. -@end menu +@item top +@findex %code top -@node Parser Function, Lexical, , Interface -@section The Parser Function @code{yyparse} -@findex yyparse +@itemize @bullet +@item Language(s): C, C++ -You call the function @code{yyparse} to cause parsing to occur. This -function reads tokens, executes actions, and ultimately returns when it -encounters end-of-input or an unrecoverable syntax error. You can also -write an action which directs @code{yyparse} to return immediately without -reading further. +@item Purpose: The unqualified @code{%code} or @code{%code requires} should +usually be more appropriate than @code{%code top}. +However, occasionally it is necessary to insert code much nearer the top of the +parser source code file. +For example: -The value returned by @code{yyparse} is 0 if parsing was successful (return -is due to end-of-input). +@smallexample +%code top @{ + #define _GNU_SOURCE + #include +@} +@end smallexample -The value is 1 if parsing failed (return is due to a syntax error). +@item Location(s): Near the top of the parser source code file. +@end itemize -In an action, you can cause immediate return from @code{yyparse} by using -these macros: +@item imports +@findex %code imports + +@itemize @bullet +@item Language(s): Java + +@item Purpose: This is the best place to write Java import directives. + +@item Location(s): The parser Java file after any Java package directive and +before any class definitions. +@end itemize +@end itemize + +(Like all the Yacc prologue alternatives, this directive is experimental. +More user feedback will help to determine whether it should become a permanent +feature.) + +@cindex Prologue +For a detailed discussion of how to use @code{%code} in place of the +traditional Yacc prologue for C/C++, see @ref{Prologue Alternatives}. +@end deffn + +@deffn {Directive} %debug +In the parser file, define the macro @code{YYDEBUG} to 1 if it is not +already defined, so that the debugging facilities are compiled. +@end deffn +@xref{Tracing, ,Tracing Your Parser}. + +@deffn {Directive} %define @var{variable} +@deffnx {Directive} %define @var{variable} "@var{value}" +Define a variable to adjust Bison's behavior. +The possible choices for @var{variable}, as well as their meanings, depend on +the selected target language and/or the parser skeleton (@pxref{Decl +Summary,,%language}, @pxref{Decl Summary,,%skeleton}). + +Bison will warn if a @var{variable} is defined multiple times. + +Omitting @code{"@var{value}"} is always equivalent to specifying it as +@code{""}. + +Some @var{variable}s may be used as Booleans. +In this case, Bison will complain if the variable definition does not meet one +of the following four conditions: + +@enumerate +@item @code{"@var{value}"} is @code{"true"} + +@item @code{"@var{value}"} is omitted (or is @code{""}). +This is equivalent to @code{"true"}. + +@item @code{"@var{value}"} is @code{"false"}. + +@item @var{variable} is never defined. +In this case, Bison selects a default value, which may depend on the selected +target language and/or parser skeleton. +@end enumerate + +Some of the accepted @var{variable}s are: + +@itemize @bullet +@item api.pure +@findex %define api.pure + +@itemize @bullet +@item Language(s): C + +@item Purpose: Request a pure (reentrant) parser program. +@xref{Pure Decl, ,A Pure (Reentrant) Parser}. + +@item Accepted Values: Boolean + +@item Default Value: @code{"false"} +@end itemize + +@item api.push_pull +@findex %define api.push_pull + +@itemize @bullet +@item Language(s): C (LALR(1) only) + +@item Purpose: Requests a pull parser, a push parser, or both. +@xref{Push Decl, ,A Push Parser}. +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) + +@item Accepted Values: @code{"pull"}, @code{"push"}, @code{"both"} + +@item Default Value: @code{"pull"} +@end itemize + +@item lr.keep_unreachable_states +@findex %define lr.keep_unreachable_states + +@itemize @bullet +@item Language(s): all + +@item Purpose: Requests that Bison allow unreachable parser states to remain in +the parser tables. +Bison considers a state to be unreachable if there exists no sequence of +transitions from the start state to that state. +A state can become unreachable during conflict resolution if Bison disables a +shift action leading to it from a predecessor state. +Keeping unreachable states is sometimes useful for analysis purposes, but they +are useless in the generated parser. + +@item Accepted Values: Boolean + +@item Default Value: @code{"false"} + +@item Caveats: + +@itemize @bullet + +@item Unreachable states may contain conflicts and may use rules not used in +any other state. +Thus, keeping unreachable states may induce warnings that are irrelevant to +your parser's behavior, and it may eliminate warnings that are relevant. +Of course, the change in warnings may actually be relevant to a parser table +analysis that wants to keep unreachable states, so this behavior will likely +remain in future Bison releases. + +@item While Bison is able to remove unreachable states, it is not guaranteed to +remove other kinds of useless states. +Specifically, when Bison disables reduce actions during conflict resolution, +some goto actions may become useless, and thus some additional states may +become useless. +If Bison were to compute which goto actions were useless and then disable those +actions, it could identify such states as unreachable and then remove those +states. +However, Bison does not compute which goto actions are useless. +@end itemize +@end itemize + +@item namespace +@findex %define namespace + +@itemize +@item Languages(s): C++ + +@item Purpose: Specifies the namespace for the parser class. +For example, if you specify: + +@smallexample +%define namespace "foo::bar" +@end smallexample + +Bison uses @code{foo::bar} verbatim in references such as: + +@smallexample +foo::bar::parser::semantic_type +@end smallexample + +However, to open a namespace, Bison removes any leading @code{::} and then +splits on any remaining occurrences: + +@smallexample +namespace foo @{ namespace bar @{ + class position; + class location; +@} @} +@end smallexample + +@item Accepted Values: Any absolute or relative C++ namespace reference without +a trailing @code{"::"}. +For example, @code{"foo"} or @code{"::foo::bar"}. + +@item Default Value: The value specified by @code{%name-prefix}, which defaults +to @code{yy}. +This usage of @code{%name-prefix} is for backward compatibility and can be +confusing since @code{%name-prefix} also specifies the textual prefix for the +lexical analyzer function. +Thus, if you specify @code{%name-prefix}, it is best to also specify +@code{%define namespace} so that @code{%name-prefix} @emph{only} affects the +lexical analyzer function. +For example, if you specify: + +@smallexample +%define namespace "foo" +%name-prefix "bar::" +@end smallexample + +The parser namespace is @code{foo} and @code{yylex} is referenced as +@code{bar::lex}. +@end itemize +@end itemize + +@end deffn + +@deffn {Directive} %defines +Write a header file containing macro definitions for the token type +names defined in the grammar as well as a few other declarations. +If the parser output file is named @file{@var{name}.c} then this file +is named @file{@var{name}.h}. + +For C parsers, the output header declares @code{YYSTYPE} unless +@code{YYSTYPE} is already defined as a macro or you have used a +@code{<@var{type}>} tag without using @code{%union}. +Therefore, if you are using a @code{%union} +(@pxref{Multiple Types, ,More Than One Value Type}) with components that +require other definitions, or if you have defined a @code{YYSTYPE} macro +or type definition +(@pxref{Value Type, ,Data Types of Semantic Values}), you need to +arrange for these definitions to be propagated to all modules, e.g., by +putting them in a prerequisite header that is included both by your +parser and by any other module that needs @code{YYSTYPE}. + +Unless your parser is pure, the output header declares @code{yylval} +as an external variable. @xref{Pure Decl, ,A Pure (Reentrant) +Parser}. + +If you have also used locations, the output header declares +@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of +the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations, ,Tracking +Locations}. + +This output file is normally essential if you wish to put the definition +of @code{yylex} in a separate source file, because @code{yylex} +typically needs to be able to refer to the above-mentioned declarations +and to the token type codes. @xref{Token Values, ,Semantic Values of +Tokens}. + +@findex %code requires +@findex %code provides +If you have declared @code{%code requires} or @code{%code provides}, the output +header also contains their code. +@xref{Decl Summary, ,%code}. +@end deffn + +@deffn {Directive} %defines @var{defines-file} +Same as above, but save in the file @var{defines-file}. +@end deffn + +@deffn {Directive} %destructor +Specify how the parser should reclaim the memory associated to +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +@end deffn + +@deffn {Directive} %file-prefix "@var{prefix}" +Specify a prefix to use for all Bison output file names. The names are +chosen as if the input file were named @file{@var{prefix}.y}. +@end deffn + +@deffn {Directive} %language "@var{language}" +Specify the programming language for the generated parser. Currently +supported languages include C, C++, and Java. +@var{language} is case-insensitive. + +This directive is experimental and its effect may be modified in future +releases. +@end deffn + +@deffn {Directive} %locations +Generate the code processing the locations (@pxref{Action Features, +,Special Features for Use in Actions}). This mode is enabled as soon as +the grammar uses the special @samp{@@@var{n}} tokens, but if your +grammar does not use it, using @samp{%locations} allows for more +accurate syntax error messages. +@end deffn + +@deffn {Directive} %name-prefix "@var{prefix}" +Rename the external symbols used in the parser so that they start with +@var{prefix} instead of @samp{yy}. The precise list of symbols renamed +in C parsers +is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs}, +@code{yylval}, @code{yychar}, @code{yydebug}, and +(if locations are used) @code{yylloc}. If you use a push parser, +@code{yypush_parse}, @code{yypull_parse}, @code{yypstate}, +@code{yypstate_new} and @code{yypstate_delete} will +also be renamed. For example, if you use @samp{%name-prefix "c_"}, the +names become @code{c_parse}, @code{c_lex}, and so on. +For C++ parsers, see the @code{%define namespace} documentation in this +section. +@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}. +@end deffn + +@ifset defaultprec +@deffn {Directive} %no-default-prec +Do not assign a precedence to rules lacking an explicit @code{%prec} +modifier (@pxref{Contextual Precedence, ,Context-Dependent +Precedence}). +@end deffn +@end ifset + +@deffn {Directive} %no-lines +Don't generate any @code{#line} preprocessor commands in the parser +file. Ordinarily Bison writes these commands in the parser file so that +the C compiler and debuggers will associate errors and object code with +your source file (the grammar file). This directive causes them to +associate errors with the parser file, treating it an independent source +file in its own right. +@end deffn + +@deffn {Directive} %output "@var{file}" +Specify @var{file} for the parser file. +@end deffn + +@deffn {Directive} %pure-parser +Deprecated version of @code{%define api.pure} (@pxref{Decl Summary, ,%define}), +for which Bison is more careful to warn about unreasonable usage. +@end deffn + +@deffn {Directive} %require "@var{version}" +Require version @var{version} or higher of Bison. @xref{Require Decl, , +Require a Version of Bison}. +@end deffn + +@deffn {Directive} %skeleton "@var{file}" +Specify the skeleton to use. + +@c You probably don't need this option unless you are developing Bison. +@c You should use @code{%language} if you want to specify the skeleton for a +@c different language, because it is clearer and because it will always choose the +@c correct skeleton for non-deterministic or push parsers. + +If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton +file in the Bison installation directory. +If it does, @var{file} is an absolute file name or a file name relative to the +directory of the grammar file. +This is similar to how most shells resolve commands. +@end deffn + +@deffn {Directive} %token-table +Generate an array of token names in the parser file. The name of the +array is @code{yytname}; @code{yytname[@var{i}]} is the name of the +token whose internal Bison token code number is @var{i}. The first +three elements of @code{yytname} correspond to the predefined tokens +@code{"$end"}, +@code{"error"}, and @code{"$undefined"}; after these come the symbols +defined in the grammar file. + +The name in the table includes all the characters needed to represent +the token in Bison. For single-character literals and literal +strings, this includes the surrounding quoting characters and any +escape sequences. For example, the Bison single-character literal +@code{'+'} corresponds to a three-character name, represented in C as +@code{"'+'"}; and the Bison two-character literal string @code{"\\/"} +corresponds to a five-character name, represented in C as +@code{"\"\\\\/\""}. + +When you specify @code{%token-table}, Bison also generates macro +definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and +@code{YYNRULES}, and @code{YYNSTATES}: @table @code -@item YYACCEPT +@item YYNTOKENS +The highest token number, plus one. +@item YYNNTS +The number of nonterminal symbols. +@item YYNRULES +The number of grammar rules, +@item YYNSTATES +The number of parser states (@pxref{Parser States}). +@end table +@end deffn + +@deffn {Directive} %verbose +Write an extra output file containing verbose descriptions of the +parser states and what is done for each type of lookahead token in +that state. @xref{Understanding, , Understanding Your Parser}, for more +information. +@end deffn + +@deffn {Directive} %yacc +Pretend the option @option{--yacc} was given, i.e., imitate Yacc, +including its naming conventions. @xref{Bison Options}, for more. +@end deffn + + +@node Multiple Parsers +@section Multiple Parsers in the Same Program + +Most programs that use Bison parse only one language and therefore contain +only one Bison parser. But what if you want to parse more than one +language with the same program? Then you need to avoid a name conflict +between different definitions of @code{yyparse}, @code{yylval}, and so on. + +The easy way to do this is to use the option @samp{-p @var{prefix}} +(@pxref{Invocation, ,Invoking Bison}). This renames the interface +functions and variables of the Bison parser to start with @var{prefix} +instead of @samp{yy}. You can use this to give each parser distinct +names that do not conflict. + +The precise list of symbols renamed is @code{yyparse}, @code{yylex}, +@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yylloc}, +@code{yychar} and @code{yydebug}. If you use a push parser, +@code{yypush_parse}, @code{yypull_parse}, @code{yypstate}, +@code{yypstate_new} and @code{yypstate_delete} will also be renamed. +For example, if you use @samp{-p c}, the names become @code{cparse}, +@code{clex}, and so on. + +@strong{All the other variables and macros associated with Bison are not +renamed.} These others are not global; there is no conflict if the same +name is used in different parsers. For example, @code{YYSTYPE} is not +renamed, but defining this in different ways in different parsers causes +no trouble (@pxref{Value Type, ,Data Types of Semantic Values}). + +The @samp{-p} option works by adding macro definitions to the beginning +of the parser source file, defining @code{yyparse} as +@code{@var{prefix}parse}, and so on. This effectively substitutes one +name for the other in the entire parser file. + +@node Interface +@chapter Parser C-Language Interface +@cindex C-language interface +@cindex interface + +The Bison parser is actually a C function named @code{yyparse}. Here we +describe the interface conventions of @code{yyparse} and the other +functions that it needs to use. + +Keep in mind that the parser uses many C identifiers starting with +@samp{yy} and @samp{YY} for internal purposes. If you use such an +identifier (aside from those in this manual) in an action or in epilogue +in the grammar file, you are likely to run into trouble. + +@menu +* Parser Function:: How to call @code{yyparse} and what it returns. +* Push Parser Function:: How to call @code{yypush_parse} and what it returns. +* Pull Parser Function:: How to call @code{yypull_parse} and what it returns. +* Parser Create Function:: How to call @code{yypstate_new} and what it returns. +* Parser Delete Function:: How to call @code{yypstate_delete} and what it returns. +* Lexical:: You must supply a function @code{yylex} + which reads tokens. +* Error Reporting:: You must supply a function @code{yyerror}. +* Action Features:: Special features for use in actions. +* Internationalization:: How to let the parser speak in the user's + native language. +@end menu + +@node Parser Function +@section The Parser Function @code{yyparse} +@findex yyparse + +You call the function @code{yyparse} to cause parsing to occur. This +function reads tokens, executes actions, and ultimately returns when it +encounters end-of-input or an unrecoverable syntax error. You can also +write an action which directs @code{yyparse} to return immediately +without reading further. + + +@deftypefun int yyparse (void) +The value returned by @code{yyparse} is 0 if parsing was successful (return +is due to end-of-input). + +The value is 1 if parsing failed because of invalid input, i.e., input +that contains a syntax error or that causes @code{YYABORT} to be +invoked. + +The value is 2 if parsing failed due to memory exhaustion. +@end deftypefun + +In an action, you can cause immediate return from @code{yyparse} by using +these macros: + +@defmac YYACCEPT @findex YYACCEPT Return immediately with value 0 (to report success). +@end defmac -@item YYABORT +@defmac YYABORT @findex YYABORT Return immediately with value 1 (to report failure). -@end table +@end defmac + +If you use a reentrant parser, you can optionally pass additional +parameter information to it in a reentrant way. To do so, use the +declaration @code{%parse-param}: + +@deffn {Directive} %parse-param @{@var{argument-declaration}@} +@findex %parse-param +Declare that an argument declared by the braced-code +@var{argument-declaration} is an additional @code{yyparse} argument. +The @var{argument-declaration} is used when declaring +functions or prototypes. The last identifier in +@var{argument-declaration} must be the argument name. +@end deffn + +Here's an example. Write this in the parser: + +@example +%parse-param @{int *nastiness@} +%parse-param @{int *randomness@} +@end example + +@noindent +Then call the parser like this: + +@example +@{ + int nastiness, randomness; + @dots{} /* @r{Store proper data in @code{nastiness} and @code{randomness}.} */ + value = yyparse (&nastiness, &randomness); + @dots{} +@} +@end example + +@noindent +In the grammar actions, use expressions like this to refer to the data: + +@example +exp: @dots{} @{ @dots{}; *randomness += 1; @dots{} @} +@end example -@node Lexical, Error Reporting, Parser Function, Interface +@node Push Parser Function +@section The Push Parser Function @code{yypush_parse} +@findex yypush_parse + +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) + +You call the function @code{yypush_parse} to parse a single token. This +function is available if either the @code{%define api.push_pull "push"} or +@code{%define api.push_pull "both"} declaration is used. +@xref{Push Decl, ,A Push Parser}. + +@deftypefun int yypush_parse (yypstate *yyps) +The value returned by @code{yypush_parse} is the same as for yyparse with the +following exception. @code{yypush_parse} will return YYPUSH_MORE if more input +is required to finish parsing the grammar. +@end deftypefun + +@node Pull Parser Function +@section The Pull Parser Function @code{yypull_parse} +@findex yypull_parse + +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) + +You call the function @code{yypull_parse} to parse the rest of the input +stream. This function is available if the @code{%define api.push_pull "both"} +declaration is used. +@xref{Push Decl, ,A Push Parser}. + +@deftypefun int yypull_parse (yypstate *yyps) +The value returned by @code{yypull_parse} is the same as for @code{yyparse}. +@end deftypefun + +@node Parser Create Function +@section The Parser Create Function @code{yystate_new} +@findex yypstate_new + +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) + +You call the function @code{yypstate_new} to create a new parser instance. +This function is available if either the @code{%define api.push_pull "push"} or +@code{%define api.push_pull "both"} declaration is used. +@xref{Push Decl, ,A Push Parser}. + +@deftypefun yypstate *yypstate_new (void) +The fuction will return a valid parser instance if there was memory available +or 0 if no memory was available. +In impure mode, it will also return 0 if a parser instance is currently +allocated. +@end deftypefun + +@node Parser Delete Function +@section The Parser Delete Function @code{yystate_delete} +@findex yypstate_delete + +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) + +You call the function @code{yypstate_delete} to delete a parser instance. +function is available if either the @code{%define api.push_pull "push"} or +@code{%define api.push_pull "both"} declaration is used. +@xref{Push Decl, ,A Push Parser}. + +@deftypefun void yypstate_delete (yypstate *yyps) +This function will reclaim the memory associated with a parser instance. +After this call, you should no longer attempt to use the parser instance. +@end deftypefun + +@node Lexical @section The Lexical Analyzer Function @code{yylex} @findex yylex @cindex lexical analyzer @@ -3196,24 +5406,25 @@ need to arrange for the token-type macro definitions to be available there. To do this, use the @samp{-d} option when you run Bison, so that it will write these macro definitions into a separate header file @file{@var{name}.tab.h} which you can include in the other source files -that need it. @xref{Invocation, ,Invoking Bison}.@refill +that need it. @xref{Invocation, ,Invoking Bison}. @menu * Calling Convention:: How @code{yyparse} calls @code{yylex}. -* Token Values:: How @code{yylex} must return the semantic value - of the token it has read. -* Token Positions:: How @code{yylex} must return the text position - (line number, etc.) of the token, if the - actions want that. -* Pure Calling:: How the calling convention differs - in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). +* Token Values:: How @code{yylex} must return the semantic value + of the token it has read. +* Token Locations:: How @code{yylex} must return the text location + (line number, etc.) of the token, if the + actions want that. +* Pure Calling:: How the calling convention differs in a pure parser + (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). @end menu -@node Calling Convention, Token Values, , Lexical +@node Calling Convention @subsection Calling Convention for @code{yylex} -The value that @code{yylex} returns must be the numeric code for the type -of token it has just found, or 0 for end-of-input. +The value that @code{yylex} returns must be the positive numeric code +for the type of token it has just found; a zero or negative value +signifies end-of-input. When a token is referred to in the grammar rules by a name, that name in the parser file becomes a C macro whose definition is the proper @@ -3222,23 +5433,25 @@ to indicate that type. @xref{Symbols}. When a token is referred to in the grammar rules by a character literal, the numeric code for that character is also the code for the token type. -So @code{yylex} can simply return that character code. The null character -must not be used this way, because its code is zero and that is what +So @code{yylex} can simply return that character code, possibly converted +to @code{unsigned char} to avoid sign-extension. The null character +must not be used this way, because its code is zero and that signifies end-of-input. Here is an example showing these things: @example -yylex () +int +yylex (void) @{ @dots{} - if (c == EOF) /* Detect end of file. */ + if (c == EOF) /* Detect end-of-input. */ return 0; @dots{} if (c == '+' || c == '-') - return c; /* Assume token type for `+' is '+'. */ + return c; /* Assume token type for `+' is '+'. */ @dots{} - return INT; /* Return the type of the token. */ + return INT; /* Return the type of the token. */ @dots{} @} @end example @@ -3247,7 +5460,47 @@ yylex () This interface has been designed so that the output from the @code{lex} utility can be used without change as the definition of @code{yylex}. -@node Token Values, Token Positions, Calling Convention, Lexical +If the grammar uses literal string tokens, there are two ways that +@code{yylex} can determine the token type codes for them: + +@itemize @bullet +@item +If the grammar defines symbolic token names as aliases for the +literal string tokens, @code{yylex} can use these symbolic names like +all others. In this case, the use of the literal string tokens in +the grammar file has no effect on @code{yylex}. + +@item +@code{yylex} can find the multicharacter token in the @code{yytname} +table. The index of the token in the table is the token type's code. +The name of a multicharacter token is recorded in @code{yytname} with a +double-quote, the token's characters, and another double-quote. The +token's characters are escaped as necessary to be suitable as input +to Bison. + +Here's code for looking up a multicharacter token in @code{yytname}, +assuming that the characters of the token are stored in +@code{token_buffer}, and assuming that the token does not contain any +characters like @samp{"} that require escaping. + +@smallexample +for (i = 0; i < YYNTOKENS; i++) + @{ + if (yytname[i] != 0 + && yytname[i][0] == '"' + && ! strncmp (yytname[i] + 1, token_buffer, + strlen (token_buffer)) + && yytname[i][strlen (token_buffer) + 1] == '"' + && yytname[i][strlen (token_buffer) + 2] == 0) + break; + @} +@end smallexample + +The @code{yytname} table is generated only if you use the +@code{%token-table} declaration. @xref{Decl Summary}. +@end itemize + +@node Token Values @subsection Semantic Values of Tokens @vindex yylval @@ -3260,16 +5513,17 @@ Thus, if the type is @code{int} (the default), you might write this in @example @group @dots{} - yylval = value; /* Put value onto Bison stack. */ - return INT; /* Return the type of the token. */ + yylval = value; /* Put value onto Bison stack. */ + return INT; /* Return the type of the token. */ @dots{} @end group @end example When you are using multiple data types, @code{yylval}'s type is a union -made from the @code{%union} declaration (@pxref{Union Decl, ,The Collection of Value Types}). So when -you store a token's value, you must use the proper member of the union. -If the @code{%union} declaration looks like this: +made from the @code{%union} declaration (@pxref{Union Decl, ,The +Collection of Value Types}). So when you store a token's value, you +must use the proper member of the union. If the @code{%union} +declaration looks like this: @example @group @@ -3287,44 +5541,46 @@ then the code in @code{yylex} might look like this: @example @group @dots{} - yylval.intval = value; /* Put value onto Bison stack. */ - return INT; /* Return the type of the token. */ + yylval.intval = value; /* Put value onto Bison stack. */ + return INT; /* Return the type of the token. */ @dots{} @end group @end example -@node Token Positions, Pure Calling, Token Values, Lexical -@subsection Textual Positions of Tokens +@node Token Locations +@subsection Textual Locations of Tokens @vindex yylloc -If you are using the @samp{@@@var{n}}-feature (@pxref{Action Features, ,Special Features for Use in Actions}) in -actions to keep track of the textual locations of tokens and groupings, -then you must provide this information in @code{yylex}. The function -@code{yyparse} expects to find the textual location of a token just parsed -in the global variable @code{yylloc}. So @code{yylex} must store the -proper data in that variable. The value of @code{yylloc} is a structure -and you need only initialize the members that are going to be used by the -actions. The four members are called @code{first_line}, -@code{first_column}, @code{last_line} and @code{last_column}. Note that -the use of this feature makes the parser noticeably slower. +If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, , +Tracking Locations}) in actions to keep track of the textual locations +of tokens and groupings, then you must provide this information in +@code{yylex}. The function @code{yyparse} expects to find the textual +location of a token just parsed in the global variable @code{yylloc}. +So @code{yylex} must store the proper data in that variable. + +By default, the value of @code{yylloc} is a structure and you need only +initialize the members that are going to be used by the actions. The +four members are called @code{first_line}, @code{first_column}, +@code{last_line} and @code{last_column}. Note that the use of this +feature makes the parser noticeably slower. @tindex YYLTYPE The data type of @code{yylloc} has the name @code{YYLTYPE}. -@node Pure Calling, , Token Positions, Lexical -@subsection Calling for Pure Parsers +@node Pure Calling +@subsection Calling Conventions for Pure Parsers -When you use the Bison declaration @code{%pure_parser} to request a pure, -reentrant parser, the global communication variables @code{yylval} and -@code{yylloc} cannot be used. (@xref{Pure Decl, ,A Pure (Reentrant) Parser}.) In such parsers the -two global variables are replaced by pointers passed as arguments to -@code{yylex}. You must declare them as shown here, and pass the -information back by storing it through those pointers. +When you use the Bison declaration @code{%define api.pure} to request a +pure, reentrant parser, the global communication variables @code{yylval} +and @code{yylloc} cannot be used. (@xref{Pure Decl, ,A Pure (Reentrant) +Parser}.) In such parsers the two global variables are replaced by +pointers passed as arguments to @code{yylex}. You must declare them as +shown here, and pass the information back by storing it through those +pointers. @example -yylex (lvalp, llocp) - YYSTYPE *lvalp; - YYLTYPE *llocp; +int +yylex (YYSTYPE *lvalp, YYLTYPE *llocp) @{ @dots{} *lvalp = value; /* Put value onto Bison stack. */ @@ -3334,49 +5590,94 @@ yylex (lvalp, llocp) @end example If the grammar file does not use the @samp{@@} constructs to refer to -textual positions, then the type @code{YYLTYPE} will not be defined. In +textual locations, then the type @code{YYLTYPE} will not be defined. In this case, omit the second argument; @code{yylex} will be called with only one argument. -@node Error Reporting, Action Features, Lexical, Interface + +If you wish to pass the additional parameter data to @code{yylex}, use +@code{%lex-param} just like @code{%parse-param} (@pxref{Parser +Function}). + +@deffn {Directive} lex-param @{@var{argument-declaration}@} +@findex %lex-param +Declare that the braced-code @var{argument-declaration} is an +additional @code{yylex} argument declaration. +@end deffn + +For instance: + +@example +%parse-param @{int *nastiness@} +%lex-param @{int *nastiness@} +%parse-param @{int *randomness@} +@end example + +@noindent +results in the following signature: + +@example +int yylex (int *nastiness); +int yyparse (int *nastiness, int *randomness); +@end example + +If @code{%define api.pure} is added: + +@example +int yylex (YYSTYPE *lvalp, int *nastiness); +int yyparse (int *nastiness, int *randomness); +@end example + +@noindent +and finally, if both @code{%define api.pure} and @code{%locations} are used: + +@example +int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness); +int yyparse (int *nastiness, int *randomness); +@end example + +@node Error Reporting @section The Error Reporting Function @code{yyerror} @cindex error reporting function @findex yyerror @cindex parse error @cindex syntax error -The Bison parser detects a @dfn{parse error} or @dfn{syntax error} -whenever it reads a token which cannot satisfy any syntax rule. A +The Bison parser detects a @dfn{syntax error} or @dfn{parse error} +whenever it reads a token which cannot satisfy any syntax rule. An action in the grammar can also explicitly proclaim an error, using the -macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use in Actions}). +macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use +in Actions}). The Bison parser expects to report the error by calling an error reporting function named @code{yyerror}, which you must supply. It is called by @code{yyparse} whenever a syntax error is found, and it -receives one argument. For a parse error, the string is normally -@w{@code{"parse error"}}. - -@findex YYERROR_VERBOSE -If you define the macro @code{YYERROR_VERBOSE} in the Bison declarations -section (@pxref{Bison Declarations, ,The Bison Declarations Section}), then Bison provides a more verbose -and specific error message string instead of just plain @w{@code{"parse -error"}}. It doesn't matter what definition you use for -@code{YYERROR_VERBOSE}, just whether you define it. - -The parser can detect one other kind of error: stack overflow. This -happens when the input contains constructions that are very deeply +receives one argument. For a syntax error, the string is normally +@w{@code{"syntax error"}}. + +@findex %error-verbose +If you invoke the directive @code{%error-verbose} in the Bison +declarations section (@pxref{Bison Declarations, ,The Bison Declarations +Section}), then Bison provides a more verbose and specific error message +string instead of just plain @w{@code{"syntax error"}}. + +The parser can detect one other kind of error: memory exhaustion. This +can happen when the input contains constructions that are very deeply nested. It isn't likely you will encounter this, since the Bison -parser extends its stack automatically up to a very large limit. But -if overflow happens, @code{yyparse} calls @code{yyerror} in the usual -fashion, except that the argument string is @w{@code{"parser stack -overflow"}}. +parser normally extends its stack automatically up to a very large limit. But +if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual +fashion, except that the argument string is @w{@code{"memory exhausted"}}. + +In some cases diagnostics like @w{@code{"syntax error"}} are +translated automatically from English to some other language before +they are passed to @code{yyerror}. @xref{Internationalization}. The following definition suffices in simple programs: @example @group -yyerror (s) - char *s; +void +yyerror (char const *s) @{ @end group @group @@ -3390,13 +5691,72 @@ error recovery if you have written suitable error recovery grammar rules (@pxref{Error Recovery}). If recovery is impossible, @code{yyparse} will immediately return 1. +Obviously, in location tracking pure parsers, @code{yyerror} should have +an access to the current location. +This is indeed the case for the @acronym{GLR} +parsers, but not for the Yacc parser, for historical reasons. I.e., if +@samp{%locations %define api.pure} is passed then the prototypes for +@code{yyerror} are: + +@example +void yyerror (char const *msg); /* Yacc parsers. */ +void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */ +@end example + +If @samp{%parse-param @{int *nastiness@}} is used, then: + +@example +void yyerror (int *nastiness, char const *msg); /* Yacc parsers. */ +void yyerror (int *nastiness, char const *msg); /* GLR parsers. */ +@end example + +Finally, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling +convention for absolutely pure parsers, i.e., when the calling +convention of @code{yylex} @emph{and} the calling convention of +@code{%define api.pure} are pure. +I.e.: + +@example +/* Location tracking. */ +%locations +/* Pure yylex. */ +%define api.pure +%lex-param @{int *nastiness@} +/* Pure yyparse. */ +%parse-param @{int *nastiness@} +%parse-param @{int *randomness@} +@end example + +@noindent +results in the following signatures for all the parser kinds: + +@example +int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness); +int yyparse (int *nastiness, int *randomness); +void yyerror (YYLTYPE *locp, + int *nastiness, int *randomness, + char const *msg); +@end example + +@noindent +The prototypes are only indications of how the code produced by Bison +uses @code{yyerror}. Bison-generated code always ignores the returned +value, so @code{yyerror} can return any type, including @code{void}. +Also, @code{yyerror} can be a variadic function; that is why the +message is always passed last. + +Traditionally @code{yyerror} returns an @code{int} that is always +ignored, but this is purely for historical reasons, and @code{void} is +preferable since it more accurately describes the return type for +@code{yyerror}. + @vindex yynerrs The variable @code{yynerrs} contains the number of syntax errors -encountered so far. Normally this variable is global; but if you -request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}) then it is a local variable -which only the actions can access. +reported so far. Normally this variable is global; but if you +request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}) +then it is a local variable which only the actions can access. -@node Action Features, , Error Reporting, Interface +@node Action Features @section Special Features for Use in Actions @cindex summary, action features @cindex action features summary @@ -3404,106 +5764,241 @@ which only the actions can access. Here is a table of Bison constructs, variables and macros that are useful in actions. -@table @samp -@item $$ +@deffn {Variable} $$ Acts like a variable that contains the semantic value for the grouping made by the current rule. @xref{Actions}. +@end deffn -@item $@var{n} +@deffn {Variable} $@var{n} Acts like a variable that contains the semantic value for the @var{n}th component of the current rule. @xref{Actions}. +@end deffn -@item $<@var{typealt}>$ +@deffn {Variable} $<@var{typealt}>$ Like @code{$$} but specifies alternative @var{typealt} in the union -specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}. +specified by the @code{%union} declaration. @xref{Action Types, ,Data +Types of Values in Actions}. +@end deffn -@item $<@var{typealt}>@var{n} +@deffn {Variable} $<@var{typealt}>@var{n} Like @code{$@var{n}} but specifies alternative @var{typealt} in the -union specified by the @code{%union} declaration. -@xref{Action Types, ,Data Types of Values in Actions}.@refill +union specified by the @code{%union} declaration. +@xref{Action Types, ,Data Types of Values in Actions}. +@end deffn -@item YYABORT; +@deffn {Macro} YYABORT; Return immediately from @code{yyparse}, indicating failure. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@item YYACCEPT; +@deffn {Macro} YYACCEPT; Return immediately from @code{yyparse}, indicating success. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@item YYBACKUP (@var{token}, @var{value}); +@deffn {Macro} YYBACKUP (@var{token}, @var{value}); @findex YYBACKUP Unshift a token. This macro is allowed only for rules that reduce -a single value, and only when there is no look-ahead token. -It installs a look-ahead token with token type @var{token} and +a single value, and only when there is no lookahead token. +It is also disallowed in @acronym{GLR} parsers. +It installs a lookahead token with token type @var{token} and semantic value @var{value}; then it discards the value that was going to be reduced by this rule. If the macro is used when it is not valid, such as when there is -a look-ahead token already, then it reports a syntax error with +a lookahead token already, then it reports a syntax error with a message @samp{cannot back up} and performs ordinary error recovery. In either case, the rest of the action is not executed. +@end deffn -@item YYEMPTY +@deffn {Macro} YYEMPTY @vindex YYEMPTY -Value stored in @code{yychar} when there is no look-ahead token. +Value stored in @code{yychar} when there is no lookahead token. +@end deffn -@item YYERROR; +@deffn {Macro} YYEOF +@vindex YYEOF +Value stored in @code{yychar} when the lookahead is the end of the input +stream. +@end deffn + +@deffn {Macro} YYERROR; @findex YYERROR Cause an immediate syntax error. This statement initiates error recovery just as if the parser itself had detected an error; however, it does not call @code{yyerror}, and does not print any message. If you want to print an error message, call @code{yyerror} explicitly before the @samp{YYERROR;} statement. @xref{Error Recovery}. +@end deffn -@item YYRECOVERING -This macro stands for an expression that has the value 1 when the parser -is recovering from a syntax error, and 0 the rest of the time. +@deffn {Macro} YYRECOVERING +@findex YYRECOVERING +The expression @code{YYRECOVERING ()} yields 1 when the parser +is recovering from a syntax error, and 0 otherwise. @xref{Error Recovery}. +@end deffn + +@deffn {Variable} yychar +Variable containing either the lookahead token, or @code{YYEOF} when the +lookahead is the end of the input stream, or @code{YYEMPTY} when no lookahead +has been performed so the next token is not yet known. +Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic +Actions}). +@xref{Lookahead, ,Lookahead Tokens}. +@end deffn + +@deffn {Macro} yyclearin; +Discard the current lookahead token. This is useful primarily in +error rules. +Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR +Semantic Actions}). +@xref{Error Recovery}. +@end deffn -@item yychar -Variable containing the current look-ahead token. (In a pure parser, -this is actually a local variable within @code{yyparse}.) When there is -no look-ahead token, the value @code{YYEMPTY} is stored in the variable. -@xref{Look-Ahead, ,Look-Ahead Tokens}. - -@item yyclearin; -Discard the current look-ahead token. This is useful primarily in -error rules. @xref{Error Recovery}. - -@item yyerrok; +@deffn {Macro} yyerrok; Resume generating error messages immediately for subsequent syntax -errors. This is useful primarily in error rules. +errors. This is useful primarily in error rules. @xref{Error Recovery}. - -@item @@@var{n} +@end deffn + +@deffn {Variable} yylloc +Variable containing the lookahead token location when @code{yychar} is not set +to @code{YYEMPTY} or @code{YYEOF}. +Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic +Actions}). +@xref{Actions and Locations, ,Actions and Locations}. +@end deffn + +@deffn {Variable} yylval +Variable containing the lookahead token semantic value when @code{yychar} is +not set to @code{YYEMPTY} or @code{YYEOF}. +Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic +Actions}). +@xref{Actions, ,Actions}. +@end deffn + +@deffn {Value} @@$ +@findex @@$ +Acts like a structure variable containing information on the textual location +of the grouping made by the current rule. @xref{Locations, , +Tracking Locations}. + +@c Check if those paragraphs are still useful or not. + +@c @example +@c struct @{ +@c int first_line, last_line; +@c int first_column, last_column; +@c @}; +@c @end example + +@c Thus, to get the starting line number of the third component, you would +@c use @samp{@@3.first_line}. + +@c In order for the members of this structure to contain valid information, +@c you must make @code{yylex} supply this information about each token. +@c If you need only certain members, then @code{yylex} need only fill in +@c those members. + +@c The use of this feature makes the parser noticeably slower. +@end deffn + +@deffn {Value} @@@var{n} @findex @@@var{n} -Acts like a structure variable containing information on the line -numbers and column numbers of the @var{n}th component of the current -rule. The structure has four members, like this: +Acts like a structure variable containing information on the textual location +of the @var{n}th component of the current rule. @xref{Locations, , +Tracking Locations}. +@end deffn + +@node Internationalization +@section Parser Internationalization +@cindex internationalization +@cindex i18n +@cindex NLS +@cindex gettext +@cindex bison-po + +A Bison-generated parser can print diagnostics, including error and +tracing messages. By default, they appear in English. However, Bison +also supports outputting diagnostics in the user's native language. To +make this work, the user should set the usual environment variables. +@xref{Users, , The User's View, gettext, GNU @code{gettext} utilities}. +For example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might +set the user's locale to French Canadian using the @acronym{UTF}-8 +encoding. The exact set of available locales depends on the user's +installation. + +The maintainer of a package that uses a Bison-generated parser enables +the internationalization of the parser's output through the following +steps. Here we assume a package that uses @acronym{GNU} Autoconf and +@acronym{GNU} Automake. + +@enumerate +@item +@cindex bison-i18n.m4 +Into the directory containing the @acronym{GNU} Autoconf macros used +by the package---often called @file{m4}---copy the +@file{bison-i18n.m4} file installed by Bison under +@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory. +For example: @example -struct @{ - int first_line, last_line; - int first_column, last_column; -@}; +cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4 @end example -Thus, to get the starting line number of the third component, use -@samp{@@3.first_line}. +@item +@findex BISON_I18N +@vindex BISON_LOCALEDIR +@vindex YYENABLE_NLS +In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT} +invocation, add an invocation of @code{BISON_I18N}. This macro is +defined in the file @file{bison-i18n.m4} that you copied earlier. It +causes @samp{configure} to find the value of the +@code{BISON_LOCALEDIR} variable, and it defines the source-language +symbol @code{YYENABLE_NLS} to enable translations in the +Bison-generated parser. -In order for the members of this structure to contain valid information, -you must make @code{yylex} supply this information about each token. -If you need only certain members, then @code{yylex} need only fill in -those members. +@item +In the @code{main} function of your program, designate the directory +containing Bison's runtime message catalog, through a call to +@samp{bindtextdomain} with domain name @samp{bison-runtime}. +For example: -The use of this feature makes the parser noticeably slower. -@end table +@example +bindtextdomain ("bison-runtime", BISON_LOCALEDIR); +@end example + +Typically this appears after any other call @code{bindtextdomain +(PACKAGE, LOCALEDIR)} that your package already has. Here we rely on +@samp{BISON_LOCALEDIR} to be defined as a string through the +@file{Makefile}. + +@item +In the @file{Makefile.am} that controls the compilation of the @code{main} +function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro, +either in @samp{DEFS} or in @samp{AM_CPPFLAGS}. For example: + +@example +DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"' +@end example -@node Algorithm, Error Recovery, Interface, Top -@chapter The Bison Parser Algorithm -@cindex Bison parser algorithm +or: + +@example +AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"' +@end example + +@item +Finally, invoke the command @command{autoreconf} to generate the build +infrastructure. +@end enumerate + + +@node Algorithm +@chapter The Bison Parser Algorithm +@cindex Bison parser algorithm @cindex algorithm of parser @cindex shifting @cindex reduction @@ -3558,19 +6053,20 @@ to a single grouping whose symbol is the grammar's start-symbol This kind of parser is known in the literature as a bottom-up parser. @menu -* Look-Ahead:: Parser looks one token ahead when deciding what to do. +* Lookahead:: Parser looks one token ahead when deciding what to do. * Shift/Reduce:: Conflicts: when either shifting or reduction is valid. * Precedence:: Operator precedence works by resolving conflicts. * Contextual Precedence:: When an operator's precedence depends on context. * Parser States:: The parser is a finite-state-machine with stack. * Reduce/Reduce:: When two rules are applicable in the same situation. -* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. -* Stack Overflow:: What happens when stack gets full. How to avoid it. +* Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. +* Generalized LR Parsing:: Parsing arbitrary context-free grammars. +* Memory Management:: What happens when memory is exhausted. How to avoid it. @end menu -@node Look-Ahead, Shift/Reduce, , Algorithm -@section Look-Ahead Tokens -@cindex look-ahead token +@node Lookahead +@section Lookahead Tokens +@cindex lookahead token The Bison parser does @emph{not} always reduce immediately as soon as the last @var{n} tokens and groupings match a rule. This is because such a @@ -3579,15 +6075,15 @@ reduction is possible, the parser sometimes ``looks ahead'' at the next token in order to decide what to do. When a token is read, it is not immediately shifted; first it becomes the -@dfn{look-ahead token}, which is not on the stack. Now the parser can +@dfn{lookahead token}, which is not on the stack. Now the parser can perform one or more reductions of tokens and groupings on the stack, while -the look-ahead token remains off to the side. When no more reductions -should take place, the look-ahead token is shifted onto the stack. This +the lookahead token remains off to the side. When no more reductions +should take place, the lookahead token is shifted onto the stack. This does not mean that all possible reductions have been done; depending on the -token type of the look-ahead token, some rules may choose to delay their +token type of the lookahead token, some rules may choose to delay their application. -Here is a simple case where look-ahead is needed. These three rules define +Here is a simple case where lookahead is needed. These three rules define expressions which contain binary addition operators and postfix unary factorial operators (@samp{!}), and allow parentheses for grouping. @@ -3620,10 +6116,14 @@ doing so would produce on the stack the sequence of symbols @code{expr '!'}. No rule allows that sequence. @vindex yychar -The current look-ahead token is stored in the variable @code{yychar}. +@vindex yylval +@vindex yylloc +The lookahead token is stored in the variable @code{yychar}. +Its semantic value and location, if any, are stored in the variables +@code{yylval} and @code{yylloc}. @xref{Action Features, ,Special Features for Use in Actions}. -@node Shift/Reduce, Precedence, Look-Ahead, Algorithm +@node Shift/Reduce @section Shift/Reduce Conflicts @cindex conflicts @cindex shift/reduce conflicts @@ -3646,7 +6146,7 @@ if_stmt: Here we assume that @code{IF}, @code{THEN} and @code{ELSE} are terminal symbols for specific keyword tokens. -When the @code{ELSE} token is read and becomes the look-ahead token, the +When the @code{ELSE} token is read and becomes the lookahead token, the contents of the stack (assuming the input is valid) are just right for reduction by the first rule. But it is also legitimate to shift the @code{ELSE}, because that would lead to eventual reduction by the second @@ -3719,7 +6219,7 @@ expr: variable ; @end example -@node Precedence, Contextual Precedence, Shift/Reduce, Algorithm +@node Precedence @section Operator Precedence @cindex operator precedence @cindex precedence of operators @@ -3731,12 +6231,13 @@ shift and when to reduce. @menu * Why Precedence:: An example showing why precedence is needed. -* Using Precedence:: How to specify precedence in Bison grammars. +* Using Precedence:: How to specify precedence and associativity. +* Precedence Only:: How to specify precedence only. * Precedence Examples:: How these features are used in the previous example. * How Precedence:: How they work. @end menu -@node Why Precedence, Using Precedence, , Precedence +@node Why Precedence @subsection When Precedence is Needed Consider the following ambiguous grammar fragment (ambiguous because the @@ -3755,39 +6256,40 @@ expr: expr '-' expr @noindent Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2}; -should it reduce them via the rule for the addition operator? It depends -on the next token. Of course, if the next token is @samp{)}, we must -reduce; shifting is invalid because no single rule can reduce the token -sequence @w{@samp{- 2 )}} or anything starting with that. But if the next -token is @samp{*} or @samp{<}, we have a choice: either shifting or -reduction would allow the parse to complete, but with different -results. - -To decide which one Bison should do, we must consider the -results. If the next operator token @var{op} is shifted, then it -must be reduced first in order to permit another opportunity to -reduce the sum. The result is (in effect) @w{@samp{1 - (2 -@var{op} 3)}}. On the other hand, if the subtraction is reduced -before shifting @var{op}, the result is @w{@samp{(1 - 2) @var{op} -3}}. Clearly, then, the choice of shift or reduce should depend -on the relative precedence of the operators @samp{-} and -@var{op}: @samp{*} should be shifted first, but not @samp{<}. +should it reduce them via the rule for the subtraction operator? It +depends on the next token. Of course, if the next token is @samp{)}, we +must reduce; shifting is invalid because no single rule can reduce the +token sequence @w{@samp{- 2 )}} or anything starting with that. But if +the next token is @samp{*} or @samp{<}, we have a choice: either +shifting or reduction would allow the parse to complete, but with +different results. + +To decide which one Bison should do, we must consider the results. If +the next operator token @var{op} is shifted, then it must be reduced +first in order to permit another opportunity to reduce the difference. +The result is (in effect) @w{@samp{1 - (2 @var{op} 3)}}. On the other +hand, if the subtraction is reduced before shifting @var{op}, the result +is @w{@samp{(1 - 2) @var{op} 3}}. Clearly, then, the choice of shift or +reduce should depend on the relative precedence of the operators +@samp{-} and @var{op}: @samp{*} should be shifted first, but not +@samp{<}. @cindex associativity What about input such as @w{@samp{1 - 2 - 5}}; should this be -@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}? For -most operators we prefer the former, which is called @dfn{left -association}. The latter alternative, @dfn{right association}, is -desirable for assignment operators. The choice of left or right -association is a matter of whether the parser chooses to shift or -reduce when the stack contains @w{@samp{1 - 2}} and the look-ahead -token is @samp{-}: shifting makes right-associativity. - -@node Using Precedence, Precedence Examples, Why Precedence, Precedence +@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}? For most +operators we prefer the former, which is called @dfn{left association}. +The latter alternative, @dfn{right association}, is desirable for +assignment operators. The choice of left or right association is a +matter of whether the parser chooses to shift or reduce when the stack +contains @w{@samp{1 - 2}} and the lookahead token is @samp{-}: shifting +makes right-associativity. + +@node Using Precedence @subsection Specifying Operator Precedence @findex %left -@findex %right @findex %nonassoc +@findex %precedence +@findex %right Bison allows you to specify these choices with the operator precedence declarations @code{%left} and @code{%right}. Each such declaration @@ -3797,20 +6299,70 @@ those operators left-associative and the @code{%right} declaration makes them right-associative. A third alternative is @code{%nonassoc}, which declares that it is a syntax error to find the same operator twice ``in a row''. +The last alternative, @code{%precedence}, allows to define only +precedence and no associativity at all. As a result, any +associativity-related conflict that remains will be reported as an +compile-time error. The directive @code{%nonassoc} creates run-time +error: using the operator in a associative way is a syntax error. The +directive @code{%precedence} creates compile-time errors: an operator +@emph{can} be involved in an associativity-related conflict, contrary to +what expected the grammar author. The relative precedence of different operators is controlled by the -order in which they are declared. The first @code{%left} or -@code{%right} declaration in the file declares the operators whose +order in which they are declared. The first precedence/associativity +declaration in the file declares the operators whose precedence is lowest, the next such declaration declares the operators whose precedence is a little higher, and so on. -@node Precedence Examples, How Precedence, Using Precedence, Precedence -@subsection Precedence Examples +@node Precedence Only +@subsection Specifying Precedence Only +@findex %precedence -In our example, we would want the following declarations: +Since @acronym{POSIX} Yacc defines only @code{%left}, @code{%right}, and +@code{%nonassoc}, which all defines precedence and associativity, little +attention is paid to the fact that precedence cannot be defined without +defining associativity. Yet, sometimes, when trying to solve a +conflict, precedence suffices. In such a case, using @code{%left}, +@code{%right}, or @code{%nonassoc} might hide future (associativity +related) conflicts that would remain hidden. + +The dangling @code{else} ambiguity (@pxref{Shift/Reduce, , Shift/Reduce +Conflicts}) can be solved explictly. This shift/reduce conflicts occurs +in the following situation, where the period denotes the current parsing +state: @example -%left '<' +if @var{e1} then if @var{e2} then @var{s1} . else @var{s2} +@end example + +The conflict involves the reduction of the rule @samp{IF expr THEN +stmt}, which precedence is by default that of its last token +(@code{THEN}), and the shifting of the token @code{ELSE}. The usual +disambiguation (attach the @code{else} to the closest @code{if}), +shifting must be preferred, i.e., the precedence of @code{ELSE} must be +higher than that of @code{THEN}. But neither is expected to be involved +in an associativity related conflict, which can be specified as follows. + +@example +%precedence THEN +%precedence ELSE +@end example + +The unary-minus is another typical example where associativity is +usually over-specified, see @ref{Infix Calc, , Infix Notation +Calculator: @code{calc}}. The @code{%left} directive is traditionaly +used to declare the precedence of @code{NEG}, which is more than needed +since it also defines its associativity. While this is harmless in the +traditional example, who knows how @code{NEG} might be used in future +evolutions of the grammar@dots{} + +@node Precedence Examples +@subsection Precedence Examples + +In our example, we would want the following declarations: + +@example +%left '<' %left '-' %left '*' @end example @@ -3830,28 +6382,29 @@ declared with @code{'-'}: and so on. We assume that these tokens are more than one character long and therefore are represented by names, not character literals.) -@node How Precedence, , Precedence Examples, Precedence +@node How Precedence @subsection How Precedence Works The first effect of the precedence declarations is to assign precedence levels to the terminal symbols declared. The second effect is to assign -precedence levels to certain rules: each rule gets its precedence from the -last terminal symbol mentioned in the components. (You can also specify -explicitly the precedence of a rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}.) - -Finally, the resolution of conflicts works by comparing the -precedence of the rule being considered with that of the -look-ahead token. If the token's precedence is higher, the -choice is to shift. If the rule's precedence is higher, the -choice is to reduce. If they have equal precedence, the choice -is made based on the associativity of that precedence level. The -verbose output file made by @samp{-v} (@pxref{Invocation, ,Invoking Bison}) says -how each conflict was resolved. +precedence levels to certain rules: each rule gets its precedence from +the last terminal symbol mentioned in the components. (You can also +specify explicitly the precedence of a rule. @xref{Contextual +Precedence, ,Context-Dependent Precedence}.) + +Finally, the resolution of conflicts works by comparing the precedence +of the rule being considered with that of the lookahead token. If the +token's precedence is higher, the choice is to shift. If the rule's +precedence is higher, the choice is to reduce. If they have equal +precedence, the choice is made based on the associativity of that +precedence level. The verbose output file made by @samp{-v} +(@pxref{Invocation, ,Invoking Bison}) says how each conflict was +resolved. Not all rules and not all tokens have precedence. If either the rule or -the look-ahead token has no precedence, then the default is to shift. +the lookahead token has no precedence, then the default is to shift. -@node Contextual Precedence, Parser States, Precedence, Algorithm +@node Contextual Precedence @section Context-Dependent Precedence @cindex context-dependent precedence @cindex unary operator precedence @@ -3864,11 +6417,11 @@ outlandish at first, but it is really very common. For example, a minus sign typically has a very high precedence as a unary operator, and a somewhat lower precedence (lower than multiplication) as a binary operator. -The Bison precedence declarations, @code{%left}, @code{%right} and -@code{%nonassoc}, can only be used once for a given token; so a token has +The Bison precedence declarations +can only be used once for a given token; so a token has only one precedence declared in this way. For context-dependent precedence, you need to use an additional mechanism: the @code{%prec} -modifier for rules.@refill +modifier for rules. The @code{%prec} modifier declares the precedence of a particular rule by specifying a terminal symbol whose precedence should be used for that rule. @@ -3909,7 +6462,29 @@ exp: @dots{} @end group @end example -@node Parser States, Reduce/Reduce, Contextual Precedence, Algorithm +@ifset defaultprec +If you forget to append @code{%prec UMINUS} to the rule for unary +minus, Bison silently assumes that minus has its usual precedence. +This kind of problem can be tricky to debug, since one typically +discovers the mistake only by testing the code. + +The @code{%no-default-prec;} declaration makes it easier to discover +this kind of problem systematically. It causes rules that lack a +@code{%prec} modifier to have no precedence, even if the last terminal +symbol mentioned in their components has a declared precedence. + +If @code{%no-default-prec;} is in effect, you must specify @code{%prec} +for all rules that participate in precedence conflict resolution. +Then you will see any shift/reduce conflict until you tell Bison how +to resolve it, either by changing your grammar or by adding an +explicit precedence. This will probably add declarations to the +grammar, but it helps to protect against incorrect rule precedences. + +The effect of @code{%no-default-prec;} can be reversed by giving +@code{%default-prec;}, which is the default. +@end ifset + +@node Parser States @section Parser States @cindex finite-state machine @cindex parser state @@ -3921,9 +6496,9 @@ represent the entire sequence of terminal and nonterminal symbols at or near the top of the stack. The current state collects all the information about previous input which is relevant to deciding what to do next. -Each time a look-ahead token is read, the current parser state together -with the type of look-ahead token are looked up in a table. This table -entry can say, ``Shift the look-ahead token.'' In this case, it also +Each time a lookahead token is read, the current parser state together +with the type of lookahead token are looked up in a table. This table +entry can say, ``Shift the lookahead token.'' In this case, it also specifies the new parser state, which is pushed onto the top of the parser stack. Or it can say, ``Reduce using rule number @var{n}.'' This means that a certain number of tokens or groupings are taken off @@ -3931,11 +6506,11 @@ the top of the stack, and replaced by one grouping. In other words, that number of states are popped from the stack, and one new state is pushed. -There is one other alternative: the table can say that the look-ahead token +There is one other alternative: the table can say that the lookahead token is erroneous in the current state. This causes error processing to begin (@pxref{Error Recovery}). -@node Reduce/Reduce, Mystery Conflicts, Parser States, Algorithm +@node Reduce/Reduce @section Reduce/Reduce Conflicts @cindex reduce/reduce conflict @cindex conflicts, reduce/reduce @@ -4050,7 +6625,7 @@ redirects:redirect ; @end example -@node Mystery Conflicts, Stack Overflow, Reduce/Reduce, Algorithm +@node Mystery Conflicts @section Mysterious Reduce/Reduce Conflicts Sometimes reduce/reduce conflicts can occur that don't look warranted. @@ -4089,27 +6664,29 @@ name_list: @end example It would seem that this grammar can be parsed with only a single token -of look-ahead: when a @code{param_spec} is being read, an @code{ID} is +of lookahead: when a @code{param_spec} is being read, an @code{ID} is a @code{name} if a comma or colon follows, or a @code{type} if another -@code{ID} follows. In other words, this grammar is LR(1). +@code{ID} follows. In other words, this grammar is @acronym{LR}(1). -@cindex LR(1) -@cindex LALR(1) +@cindex @acronym{LR}(1) +@cindex @acronym{LALR}(1) However, Bison, like most parser generators, cannot actually handle all -LR(1) grammars. In this grammar, two contexts, that after an @code{ID} +@acronym{LR}(1) grammars. In this grammar, two contexts, that after +an @code{ID} at the beginning of a @code{param_spec} and likewise at the beginning of a @code{return_spec}, are similar enough that Bison assumes they are the same. They appear similar because the same set of rules would be active---the rule for reducing to a @code{name} and that for reducing to a @code{type}. Bison is unable to determine at that stage of processing -that the rules would require different look-ahead tokens in the two +that the rules would require different lookahead tokens in the two contexts, so it makes a single parser state for them both. Combining the two contexts causes a conflict later. In parser terminology, this -occurrence means that the grammar is not LALR(1). +occurrence means that the grammar is not @acronym{LALR}(1). In general, it is better to fix deficiencies than to document them. But this particular deficiency is intrinsically hard to fix; parser -generators that can handle LR(1) grammars are hard to write and tend to +generators that can handle @acronym{LR}(1) grammars are hard to write +and tend to produce parsers that are very large. In practice, Bison is more useful as it is now. @@ -4158,46 +6735,159 @@ return_spec: ; @end example -@node Stack Overflow, , Mystery Conflicts, Algorithm -@section Stack Overflow, and How to Avoid It +For a more detailed exposition of @acronym{LALR}(1) parsers and parser +generators, please see: +Frank DeRemer and Thomas Pennello, Efficient Computation of +@acronym{LALR}(1) Look-Ahead Sets, @cite{@acronym{ACM} Transactions on +Programming Languages and Systems}, Vol.@: 4, No.@: 4 (October 1982), +pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}. + +@node Generalized LR Parsing +@section Generalized @acronym{LR} (@acronym{GLR}) Parsing +@cindex @acronym{GLR} parsing +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing +@cindex ambiguous grammars +@cindex nondeterministic parsing + +Bison produces @emph{deterministic} parsers that choose uniquely +when to reduce and which reduction to apply +based on a summary of the preceding input and on one extra token of lookahead. +As a result, normal Bison handles a proper subset of the family of +context-free languages. +Ambiguous grammars, since they have strings with more than one possible +sequence of reductions cannot have deterministic parsers in this sense. +The same is true of languages that require more than one symbol of +lookahead, since the parser lacks the information necessary to make a +decision at the point it must be made in a shift-reduce parser. +Finally, as previously mentioned (@pxref{Mystery Conflicts}), +there are languages where Bison's particular choice of how to +summarize the input seen so far loses necessary information. + +When you use the @samp{%glr-parser} declaration in your grammar file, +Bison generates a parser that uses a different algorithm, called +Generalized @acronym{LR} (or @acronym{GLR}). A Bison @acronym{GLR} +parser uses the same basic +algorithm for parsing as an ordinary Bison parser, but behaves +differently in cases where there is a shift-reduce conflict that has not +been resolved by precedence rules (@pxref{Precedence}) or a +reduce-reduce conflict. When a @acronym{GLR} parser encounters such a +situation, it +effectively @emph{splits} into a several parsers, one for each possible +shift or reduction. These parsers then proceed as usual, consuming +tokens in lock-step. Some of the stacks may encounter other conflicts +and split further, with the result that instead of a sequence of states, +a Bison @acronym{GLR} parsing stack is what is in effect a tree of states. + +In effect, each stack represents a guess as to what the proper parse +is. Additional input may indicate that a guess was wrong, in which case +the appropriate stack silently disappears. Otherwise, the semantics +actions generated in each stack are saved, rather than being executed +immediately. When a stack disappears, its saved semantic actions never +get executed. When a reduction causes two stacks to become equivalent, +their sets of semantic actions are both saved with the state that +results from the reduction. We say that two stacks are equivalent +when they both represent the same sequence of states, +and each pair of corresponding states represents a +grammar symbol that produces the same segment of the input token +stream. + +Whenever the parser makes a transition from having multiple +states to having one, it reverts to the normal @acronym{LALR}(1) parsing +algorithm, after resolving and executing the saved-up actions. +At this transition, some of the states on the stack will have semantic +values that are sets (actually multisets) of possible actions. The +parser tries to pick one of the actions by first finding one whose rule +has the highest dynamic precedence, as set by the @samp{%dprec} +declaration. Otherwise, if the alternative actions are not ordered by +precedence, but there the same merging function is declared for both +rules by the @samp{%merge} declaration, +Bison resolves and evaluates both and then calls the merge function on +the result. Otherwise, it reports an ambiguity. + +It is possible to use a data structure for the @acronym{GLR} parsing tree that +permits the processing of any @acronym{LALR}(1) grammar in linear time (in the +size of the input), any unambiguous (not necessarily +@acronym{LALR}(1)) grammar in +quadratic worst-case time, and any general (possibly ambiguous) +context-free grammar in cubic worst-case time. However, Bison currently +uses a simpler data structure that requires time proportional to the +length of the input times the maximum number of stacks required for any +prefix of the input. Thus, really ambiguous or nondeterministic +grammars can require exponential time and space to process. Such badly +behaving examples, however, are not generally of practical interest. +Usually, nondeterminism in a grammar is local---the parser is ``in +doubt'' only for a few tokens at a time. Therefore, the current data +structure should generally be adequate. On @acronym{LALR}(1) portions of a +grammar, in particular, it is only slightly slower than with the default +Bison parser. + +For a more detailed exposition of @acronym{GLR} parsers, please see: Elizabeth +Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style +Generalised @acronym{LR} Parsers, Royal Holloway, University of +London, Department of Computer Science, TR-00-12, +@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps}, +(2000-12-24). + +@node Memory Management +@section Memory Management, and How to Avoid Memory Exhaustion +@cindex memory exhaustion +@cindex memory management @cindex stack overflow @cindex parser stack overflow @cindex overflow of parser stack -The Bison parser stack can overflow if too many tokens are shifted and +The Bison parser stack can run out of memory if too many tokens are shifted and not reduced. When this happens, the parser function @code{yyparse} -returns a nonzero value, pausing only to call @code{yyerror} to report -the overflow. +calls @code{yyerror} and then returns 2. + +Because Bison parsers have growing stacks, hitting the upper limit +usually results from using a right recursion instead of a left +recursion, @xref{Recursion, ,Recursive Rules}. @vindex YYMAXDEPTH By defining the macro @code{YYMAXDEPTH}, you can control how deep the -parser stack can become before a stack overflow occurs. Define the +parser stack can become before memory is exhausted. Define the macro with a value that is an integer. This value is the maximum number of tokens that can be shifted (and not reduced) before overflow. -It must be a constant expression whose value is known at compile time. The stack space allowed is not necessarily allocated. If you specify a -large value for @code{YYMAXDEPTH}, the parser actually allocates a small +large value for @code{YYMAXDEPTH}, the parser normally allocates a small stack at first, and then makes it bigger by stages as needed. This increasing allocation happens automatically and silently. Therefore, you do not need to make @code{YYMAXDEPTH} painfully small merely to save space for ordinary inputs that do not need much stack. +However, do not allow @code{YYMAXDEPTH} to be a value so large that +arithmetic overflow could occur when calculating the size of the stack +space. Also, do not allow @code{YYMAXDEPTH} to be less than +@code{YYINITDEPTH}. + @cindex default stack limit The default value of @code{YYMAXDEPTH}, if you do not define it, is 10000. @vindex YYINITDEPTH You can control how much stack is allocated initially by defining the -macro @code{YYINITDEPTH}. This value too must be a compile-time -constant integer. The default is 200. +macro @code{YYINITDEPTH} to a positive integer. For the C +@acronym{LALR}(1) parser, this value must be a compile-time constant +unless you are assuming C99 or some other target language or compiler +that allows variable-length arrays. The default is 200. + +Do not allow @code{YYINITDEPTH} to be greater than @code{YYMAXDEPTH}. -@node Error Recovery, Context Dependency, Algorithm, Top +@c FIXME: C++ output. +Because of semantical differences between C and C++, the +@acronym{LALR}(1) parsers in C produced by Bison cannot grow when compiled +by C++ compilers. In this precise case (compiling a C parser as C++) you are +suggested to grow @code{YYINITDEPTH}. The Bison maintainers hope to fix +this deficiency in a future release. + +@node Error Recovery @chapter Error Recovery @cindex error recovery @cindex recovery from errors -It is not usually acceptable to have a program terminate on a parse +It is not usually acceptable to have a program terminate on a syntax error. For example, a compiler should recover sufficiently to parse the rest of the input file and check it for errors; a calculator should accept another expression. @@ -4216,7 +6906,7 @@ recognize the special token @code{error}. This is a terminal symbol that is always defined (you need not declare it) and reserved for error handling. The Bison parser generates an @code{error} token whenever a syntax error happens; if you have provided a rule to recognize this token -in the current context, the parse can continue. +in the current context, the parse can continue. For example: @@ -4239,22 +6929,24 @@ will be tokens to read before the next newline. So the rule is not applicable in the ordinary way. But Bison can force the situation to fit the rule, by discarding part of -the semantic context and part of the input. First it discards states and -objects from the stack until it gets back to a state in which the +the semantic context and part of the input. First it discards states +and objects from the stack until it gets back to a state in which the @code{error} token is acceptable. (This means that the subexpressions -already parsed are discarded, back to the last complete @code{stmnts}.) At -this point the @code{error} token can be shifted. Then, if the old -look-ahead token is not acceptable to be shifted next, the parser reads +already parsed are discarded, back to the last complete @code{stmnts}.) +At this point the @code{error} token can be shifted. Then, if the old +lookahead token is not acceptable to be shifted next, the parser reads tokens and discards them until it finds a token which is acceptable. In -this example, Bison reads and discards input until the next newline -so that the fourth rule can apply. +this example, Bison reads and discards input until the next newline so +that the fourth rule can apply. Note that discarded symbols are +possible sources of memory leaks, see @ref{Destructor Decl, , Freeing +Discarded Symbols}, for a means to reclaim this memory. The choice of error rules in the grammar is a choice of strategies for error recovery. A simple and useful strategy is simply to skip the rest of the current input line or current statement if an error is detected: @example -stmnt: error ';' /* on error, skip until ';' is read */ +stmnt: error ';' /* On error, skip until ';' is read. */ @end example It is also useful to recover to the matching close-delimiter of an @@ -4293,24 +6985,25 @@ error messages will be suppressed. This macro requires no arguments; @samp{yyerrok;} is a valid C statement. @findex yyclearin -The previous look-ahead token is reanalyzed immediately after an error. If +The previous lookahead token is reanalyzed immediately after an error. If this is unacceptable, then the macro @code{yyclearin} may be used to clear this token. Write the statement @samp{yyclearin;} in the error rule's action. +@xref{Action Features, ,Special Features for Use in Actions}. -For example, suppose that on a parse error, an error handling routine is +For example, suppose that on a syntax error, an error handling routine is called that advances the input stream to some point where parsing should once again commence. The next symbol returned by the lexical scanner is -probably correct. The previous look-ahead token ought to be discarded +probably correct. The previous lookahead token ought to be discarded with @samp{yyclearin;}. @vindex YYRECOVERING -The macro @code{YYRECOVERING} stands for an expression that has the -value 1 when the parser is recovering from a syntax error, and 0 the -rest of the time. A value of 1 indicates that error messages are -currently suppressed for new syntax errors. +The expression @code{YYRECOVERING ()} yields 1 when the parser +is recovering from a syntax error, and 0 otherwise. +Syntax error diagnostics are suppressed while recovering from a syntax +error. -@node Context Dependency, Debugging, Error Recovery, Top +@node Context Dependency @chapter Handling Context Dependencies The Bison paradigm is to parse tokens first, then group them into larger @@ -4329,7 +7022,7 @@ languages. (Actually, ``kludge'' means any technique that gets its job done but is neither clean nor robust.) -@node Semantic Tokens, Lexical Tie-ins, , Context Dependency +@node Semantic Tokens @section Semantic Info in Token Types The C language has a context dependency: the way an identifier is used @@ -4343,7 +7036,7 @@ This looks like a function call statement, but if @code{foo} is a typedef name, then this is actually a declaration of @code{x}. How can a Bison parser for C decide how to parse this input? -The method used in GNU C is to have two different token types, +The method used in @acronym{GNU} C is to have two different token types, @code{IDENTIFIER} and @code{TYPENAME}. When @code{yylex} finds an identifier, it looks up the current declaration of the identifier in order to decide which token type to return: @code{TYPENAME} if the identifier is @@ -4364,19 +7057,23 @@ redeclare a typedef name provided an explicit type has been specified earlier: @example -typedef int foo, bar, lose; -static foo (bar); /* @r{redeclare @code{bar} as static variable} */ -static int foo (lose); /* @r{redeclare @code{foo} as function} */ +typedef int foo, bar; +int baz (void) +@{ + static bar (bar); /* @r{redeclare @code{bar} as static variable} */ + extern foo foo (foo); /* @r{redeclare @code{foo} as function} */ + return foo (bar); +@} @end example Unfortunately, the name being declared is separated from the declaration construct itself by a complicated syntactic structure---the ``declarator''. -As a result, the part of Bison parser for C needs to be duplicated, with -all the nonterminal names changed: once for parsing a declaration in which -a typedef name can be redefined, and once for parsing a declaration in -which that can't be done. Here is a part of the duplication, with actions -omitted for brevity: +As a result, part of the Bison parser for C needs to be duplicated, with +all the nonterminal names changed: once for parsing a declaration in +which a typedef name can be redefined, and once for parsing a +declaration in which that can't be done. Here is a part of the +duplication, with actions omitted for brevity: @example initdcl: @@ -4404,7 +7101,7 @@ here the information is global, and is used for other purposes in the program. A true lexical tie-in has a special-purpose flag controlled by the syntactic context. -@node Lexical Tie-ins, Tie-in Recovery, Semantic Tokens, Context Dependency +@node Lexical Tie-ins @section Lexical Tie-ins @cindex lexical tie-in @@ -4421,7 +7118,9 @@ as an identifier if it appears in that context. Here is how you can do it: @example @group %@{ -int hexflag; + int hexflag; + int yylex (void); + void yyerror (char const *); %@} %% @dots{} @@ -4453,12 +7152,11 @@ Here we assume that @code{yylex} looks at the value of @code{hexflag}; when it is nonzero, all integers are parsed in hexadecimal, and tokens starting with letters are parsed as integers if possible. -The declaration of @code{hexflag} shown in the C declarations section of -the parser file is needed to make it accessible to the actions -(@pxref{C Declarations, ,The C Declarations Section}). You must also write the code in @code{yylex} -to obey the flag. +The declaration of @code{hexflag} shown in the prologue of the parser file +is needed to make it accessible to the actions (@pxref{Prologue, ,The Prologue}). +You must also write the code in @code{yylex} to obey the flag. -@node Tie-in Recovery, , Lexical Tie-ins, Context Dependency +@node Tie-in Recovery @section Lexical Tie-ins and Error Recovery Lexical tie-ins make strict demands on any error recovery rules you have. @@ -4513,9 +7211,421 @@ make sure your error recovery rules are not of this kind. Each rule must be such that you can be sure that it always will, or always won't, have to clear the flag. -@node Debugging, Invocation, Context Dependency, Top +@c ================================================== Debugging Your Parser + +@node Debugging @chapter Debugging Your Parser -@findex YYDEBUG + +Developing a parser can be a challenge, especially if you don't +understand the algorithm (@pxref{Algorithm, ,The Bison Parser +Algorithm}). Even so, sometimes a detailed description of the automaton +can help (@pxref{Understanding, , Understanding Your Parser}), or +tracing the execution of the parser can give some insight on why it +behaves improperly (@pxref{Tracing, , Tracing Your Parser}). + +@menu +* Understanding:: Understanding the structure of your parser. +* Tracing:: Tracing the execution of your parser. +@end menu + +@node Understanding +@section Understanding Your Parser + +As documented elsewhere (@pxref{Algorithm, ,The Bison Parser Algorithm}) +Bison parsers are @dfn{shift/reduce automata}. In some cases (much more +frequent than one would hope), looking at this automaton is required to +tune or simply fix a parser. Bison provides two different +representation of it, either textually or graphically (as a DOT file). + +The textual file is generated when the options @option{--report} or +@option{--verbose} are specified, see @xref{Invocation, , Invoking +Bison}. Its name is made by removing @samp{.tab.c} or @samp{.c} from +the parser output file name, and adding @samp{.output} instead. +Therefore, if the input file is @file{foo.y}, then the parser file is +called @file{foo.tab.c} by default. As a consequence, the verbose +output file is called @file{foo.output}. + +The following grammar file, @file{calc.y}, will be used in the sequel: + +@example +%token NUM STR +%left '+' '-' +%left '*' +%% +exp: exp '+' exp + | exp '-' exp + | exp '*' exp + | exp '/' exp + | NUM + ; +useless: STR; +%% +@end example + +@command{bison} reports: + +@example +calc.y: warning: 1 nonterminal and 1 rule useless in grammar +calc.y:11.1-7: warning: nonterminal useless in grammar: useless +calc.y:11.10-12: warning: rule useless in grammar: useless: STR +calc.y: conflicts: 7 shift/reduce +@end example + +When given @option{--report=state}, in addition to @file{calc.tab.c}, it +creates a file @file{calc.output} with contents detailed below. The +order of the output and the exact presentation might vary, but the +interpretation is the same. + +The first section includes details on conflicts that were solved thanks +to precedence and/or associativity: + +@example +Conflict in state 8 between rule 2 and token '+' resolved as reduce. +Conflict in state 8 between rule 2 and token '-' resolved as reduce. +Conflict in state 8 between rule 2 and token '*' resolved as shift. +@exdent @dots{} +@end example + +@noindent +The next section lists states that still have conflicts. + +@example +State 8 conflicts: 1 shift/reduce +State 9 conflicts: 1 shift/reduce +State 10 conflicts: 1 shift/reduce +State 11 conflicts: 4 shift/reduce +@end example + +@noindent +@cindex token, useless +@cindex useless token +@cindex nonterminal, useless +@cindex useless nonterminal +@cindex rule, useless +@cindex useless rule +The next section reports useless tokens, nonterminal and rules. Useless +nonterminals and rules are removed in order to produce a smaller parser, +but useless tokens are preserved, since they might be used by the +scanner (note the difference between ``useless'' and ``unused'' +below): + +@example +Nonterminals useless in grammar: + useless + +Terminals unused in grammar: + STR + +Rules useless in grammar: +#6 useless: STR; +@end example + +@noindent +The next section reproduces the exact grammar that Bison used: + +@example +Grammar + + Number, Line, Rule + 0 5 $accept -> exp $end + 1 5 exp -> exp '+' exp + 2 6 exp -> exp '-' exp + 3 7 exp -> exp '*' exp + 4 8 exp -> exp '/' exp + 5 9 exp -> NUM +@end example + +@noindent +and reports the uses of the symbols: + +@example +Terminals, with rules where they appear + +$end (0) 0 +'*' (42) 3 +'+' (43) 1 +'-' (45) 2 +'/' (47) 4 +error (256) +NUM (258) 5 + +Nonterminals, with rules where they appear + +$accept (8) + on left: 0 +exp (9) + on left: 1 2 3 4 5, on right: 0 1 2 3 4 +@end example + +@noindent +@cindex item +@cindex pointed rule +@cindex rule, pointed +Bison then proceeds onto the automaton itself, describing each state +with it set of @dfn{items}, also known as @dfn{pointed rules}. Each +item is a production rule together with a point (marked by @samp{.}) +that the input cursor. + +@example +state 0 + + $accept -> . exp $ (rule 0) + + NUM shift, and go to state 1 + + exp go to state 2 +@end example + +This reads as follows: ``state 0 corresponds to being at the very +beginning of the parsing, in the initial rule, right before the start +symbol (here, @code{exp}). When the parser returns to this state right +after having reduced a rule that produced an @code{exp}, the control +flow jumps to state 2. If there is no such transition on a nonterminal +symbol, and the lookahead is a @code{NUM}, then this token is shifted on +the parse stack, and the control flow jumps to state 1. Any other +lookahead triggers a syntax error.'' + +@cindex core, item set +@cindex item set core +@cindex kernel, item set +@cindex item set core +Even though the only active rule in state 0 seems to be rule 0, the +report lists @code{NUM} as a lookahead token because @code{NUM} can be +at the beginning of any rule deriving an @code{exp}. By default Bison +reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if +you want to see more detail you can invoke @command{bison} with +@option{--report=itemset} to list all the items, include those that can +be derived: + +@example +state 0 + + $accept -> . exp $ (rule 0) + exp -> . exp '+' exp (rule 1) + exp -> . exp '-' exp (rule 2) + exp -> . exp '*' exp (rule 3) + exp -> . exp '/' exp (rule 4) + exp -> . NUM (rule 5) + + NUM shift, and go to state 1 + + exp go to state 2 +@end example + +@noindent +In the state 1... + +@example +state 1 + + exp -> NUM . (rule 5) + + $default reduce using rule 5 (exp) +@end example + +@noindent +the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead token +(@samp{$default}), the parser will reduce it. If it was coming from +state 0, then, after this reduction it will return to state 0, and will +jump to state 2 (@samp{exp: go to state 2}). + +@example +state 2 + + $accept -> exp . $ (rule 0) + exp -> exp . '+' exp (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp . '/' exp (rule 4) + + $ shift, and go to state 3 + '+' shift, and go to state 4 + '-' shift, and go to state 5 + '*' shift, and go to state 6 + '/' shift, and go to state 7 +@end example + +@noindent +In state 2, the automaton can only shift a symbol. For instance, +because of the item @samp{exp -> exp . '+' exp}, if the lookahead if +@samp{+}, it will be shifted on the parse stack, and the automaton +control will jump to state 4, corresponding to the item @samp{exp -> exp +'+' . exp}. Since there is no default action, any other token than +those listed above will trigger a syntax error. + +The state 3 is named the @dfn{final state}, or the @dfn{accepting +state}: + +@example +state 3 + + $accept -> exp $ . (rule 0) + + $default accept +@end example + +@noindent +the initial rule is completed (the start symbol and the end +of input were read), the parsing exits successfully. + +The interpretation of states 4 to 7 is straightforward, and is left to +the reader. + +@example +state 4 + + exp -> exp '+' . exp (rule 1) + + NUM shift, and go to state 1 + + exp go to state 8 + +state 5 + + exp -> exp '-' . exp (rule 2) + + NUM shift, and go to state 1 + + exp go to state 9 + +state 6 + + exp -> exp '*' . exp (rule 3) + + NUM shift, and go to state 1 + + exp go to state 10 + +state 7 + + exp -> exp '/' . exp (rule 4) + + NUM shift, and go to state 1 + + exp go to state 11 +@end example + +As was announced in beginning of the report, @samp{State 8 conflicts: +1 shift/reduce}: + +@example +state 8 + + exp -> exp . '+' exp (rule 1) + exp -> exp '+' exp . (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp . '/' exp (rule 4) + + '*' shift, and go to state 6 + '/' shift, and go to state 7 + + '/' [reduce using rule 1 (exp)] + $default reduce using rule 1 (exp) +@end example + +Indeed, there are two actions associated to the lookahead @samp{/}: +either shifting (and going to state 7), or reducing rule 1. The +conflict means that either the grammar is ambiguous, or the parser lacks +information to make the right decision. Indeed the grammar is +ambiguous, as, since we did not specify the precedence of @samp{/}, the +sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM / +NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) / +NUM}, which corresponds to reducing rule 1. + +Because in @acronym{LALR}(1) parsing a single decision can be made, Bison +arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, , +Shift/Reduce Conflicts}. Discarded actions are reported in between +square brackets. + +Note that all the previous states had a single possible action: either +shifting the next token and going to the corresponding state, or +reducing a single rule. In the other cases, i.e., when shifting +@emph{and} reducing is possible or when @emph{several} reductions are +possible, the lookahead is required to select the action. State 8 is +one such state: if the lookahead is @samp{*} or @samp{/} then the action +is shifting, otherwise the action is reducing rule 1. In other words, +the first two items, corresponding to rule 1, are not eligible when the +lookahead token is @samp{*}, since we specified that @samp{*} has higher +precedence than @samp{+}. More generally, some items are eligible only +with some set of possible lookahead tokens. When run with +@option{--report=lookahead}, Bison specifies these lookahead tokens: + +@example +state 8 + + exp -> exp . '+' exp (rule 1) + exp -> exp '+' exp . [$, '+', '-', '/'] (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp . '/' exp (rule 4) + + '*' shift, and go to state 6 + '/' shift, and go to state 7 + + '/' [reduce using rule 1 (exp)] + $default reduce using rule 1 (exp) +@end example + +The remaining states are similar: + +@example +state 9 + + exp -> exp . '+' exp (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp '-' exp . (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp . '/' exp (rule 4) + + '*' shift, and go to state 6 + '/' shift, and go to state 7 + + '/' [reduce using rule 2 (exp)] + $default reduce using rule 2 (exp) + +state 10 + + exp -> exp . '+' exp (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp '*' exp . (rule 3) + exp -> exp . '/' exp (rule 4) + + '/' shift, and go to state 7 + + '/' [reduce using rule 3 (exp)] + $default reduce using rule 3 (exp) + +state 11 + + exp -> exp . '+' exp (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp . '/' exp (rule 4) + exp -> exp '/' exp . (rule 4) + + '+' shift, and go to state 4 + '-' shift, and go to state 5 + '*' shift, and go to state 6 + '/' shift, and go to state 7 + + '+' [reduce using rule 4 (exp)] + '-' [reduce using rule 4 (exp)] + '*' [reduce using rule 4 (exp)] + '/' [reduce using rule 4 (exp)] + $default reduce using rule 4 (exp) +@end example + +@noindent +Observe that state 11 contains conflicts not only due to the lack of +precedence of @samp{/} with respect to @samp{+}, @samp{-}, and +@samp{*}, but also because the +associativity of @samp{/} is not specified. + + +@node Tracing +@section Tracing Your Parser @findex yydebug @cindex debugging @cindex tracing the parser @@ -4523,16 +7633,40 @@ clear the flag. If a Bison grammar compiles properly but doesn't do what you want when it runs, the @code{yydebug} parser-trace feature can help you figure out why. -To enable compilation of trace facilities, you must define the macro -@code{YYDEBUG} when you compile the parser. You could use +There are several means to enable compilation of trace facilities: + +@table @asis +@item the macro @code{YYDEBUG} +@findex YYDEBUG +Define the macro @code{YYDEBUG} to a nonzero value when you compile the +parser. This is compliant with @acronym{POSIX} Yacc. You could use @samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define -YYDEBUG 1} in the C declarations section of the grammar file -(@pxref{C Declarations, ,The C Declarations Section}). Alternatively, use the @samp{-t} option when -you run Bison (@pxref{Invocation, ,Invoking Bison}). We always define @code{YYDEBUG} so that -debugging is always possible. +YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The +Prologue}). + +@item the option @option{-t}, @option{--debug} +Use the @samp{-t} option when you run Bison (@pxref{Invocation, +,Invoking Bison}). This is @acronym{POSIX} compliant too. + +@item the directive @samp{%debug} +@findex %debug +Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison +Declaration Summary}). This is a Bison extension, which will prove +useful when Bison will output parsers for languages that don't use a +preprocessor. Unless @acronym{POSIX} and Yacc portability matter to +you, this is +the preferred solution. +@end table + +We suggest that you always enable the debug option so that debugging is +always possible. -The trace facility uses @code{stderr}, so you must add @w{@code{#include -}} to the C declarations section unless it is already there. +The trace facility outputs messages with macro calls of the form +@code{YYFPRINTF (stderr, @var{format}, @var{args})} where +@var{format} and @var{args} are the usual @code{printf} format and variadic +arguments. If you define @code{YYDEBUG} to a nonzero value but do not +define @code{YYFPRINTF}, @code{} is automatically included +and @code{YYFPRINTF} is defined to @code{fprintf}. Once you have compiled the program with trace facilities, the way to request a trace is to store a nonzero value in the variable @code{yydebug}. @@ -4557,13 +7691,14 @@ of the state stack afterward. @end itemize To make sense of this information, it helps to refer to the listing file -produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking Bison}). This file -shows the meaning of each state in terms of positions in various rules, and -also what each state will do with each possible input token. As you read -the successive trace messages, you can see that the parser is functioning -according to its specification in the listing file. Eventually you will -arrive at the place where something undesirable happens, and you will see -which parts of the grammar are to blame. +produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking +Bison}). This file shows the meaning of each state in terms of +positions in various rules, and also what each state will do with each +possible input token. As you read the successive trace messages, you +can see that the parser is functioning according to its specification in +the listing file. Eventually you will arrive at the place where +something undesirable happens, and you will see which parts of the +grammar are to blame. The parser file is a C program and you can use C debuggers on it, but it's not easy to interpret what it is doing. The parser function is a @@ -4580,25 +7715,29 @@ standard I/O stream, the numeric code for the token type, and the token value (from @code{yylval}). Here is an example of @code{YYPRINT} suitable for the multi-function -calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}): +calculator (@pxref{Mfcalc Declarations, ,Declarations for @code{mfcalc}}): @smallexample -#define YYPRINT(file, type, value) yyprint (file, type, value) +%@{ + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(file, type, value) print_token_value (file, type, value) +%@} + +@dots{} %% @dots{} %% @dots{} static void -yyprint (file, type, value) - FILE *file; - int type; - YYSTYPE value; +print_token_value (FILE *file, int type, YYSTYPE value) @{ if (type == VAR) - fprintf (file, " %s", value.tptr->name); + fprintf (file, "%s", value.tptr->name); else if (type == NUM) - fprintf (file, " %d", value.val); + fprintf (file, "%d", value.val); @} @end smallexample -@node Invocation, Table of Symbols, Debugging, Top +@c ================================================= Invoking Bison + +@node Invocation @chapter Invoking Bison @cindex invoking Bison @cindex Bison invocation @@ -4612,18 +7751,43 @@ bison @var{infile} Here @var{infile} is the grammar file name, which usually ends in @samp{.y}. The parser file's name is made by replacing the @samp{.y} -with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields -@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields -@file{hack/foo.tab.c}.@refill +with @samp{.tab.c} and removing any leading directory. Thus, the +@samp{bison foo.y} file name yields +@file{foo.tab.c}, and the @samp{bison hack/foo.y} file name yields +@file{foo.tab.c}. It's also possible, in case you are writing +C++ code instead of C in your grammar file, to name it @file{foo.ypp} +or @file{foo.y++}. Then, the output files will take an extension like +the given one as input (respectively @file{foo.tab.cpp} and +@file{foo.tab.c++}). +This feature takes effect with all options that manipulate file names like +@samp{-o} or @samp{-d}. + +For example : + +@example +bison -d @var{infile.yxx} +@end example +@noindent +will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and + +@example +bison -d -o @var{output.c++} @var{infile.y} +@end example +@noindent +will produce @file{output.c++} and @file{outfile.h++}. + +For compatibility with @acronym{POSIX}, the standard Bison +distribution also contains a shell script called @command{yacc} that +invokes Bison with the @option{-y} option. @menu -* Bison Options:: All the options described in detail, - in alphabetical order by short options. +* Bison Options:: All the options described in detail, + in alphabetical order by short options. * Option Cross Key:: Alphabetical list of long options. -* VMS Invocation:: Bison command syntax on VMS. +* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}. @end menu -@node Bison Options, Option Cross Key, , Invocation +@node Bison Options @section Bison Options Bison supports both traditional single-letter options and mnemonic long @@ -4637,351 +7801,2517 @@ Here is a list of options that can be used with Bison, alphabetized by short option. It is followed by a cross key alphabetized by long option. -@table @samp -@item -b @var{file-prefix} -@itemx --file-prefix=@var{prefix} -Specify a prefix to use for all Bison output file names. The names are -chosen as if the input file were named @file{@var{prefix}.c}. - -@item -d -@itemx --defines -Write an extra output file containing macro definitions for the token -type names defined in the grammar and the semantic value type -@code{YYSTYPE}, as well as a few @code{extern} variable declarations. - -If the parser output file is named @file{@var{name}.c} then this file -is named @file{@var{name}.h}.@refill - -This output file is essential if you wish to put the definition of -@code{yylex} in a separate source file, because @code{yylex} needs to -be able to refer to token type codes and the variable -@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill - -@item -l -@itemx --no-lines -Don't put any @code{#line} preprocessor commands in the parser file. -Ordinarily Bison puts them in the parser file so that the C compiler -and debuggers will associate errors with your source file, the -grammar file. This option causes them to associate errors with the -parser file, treating it an independent source file in its own right. - -@item -o @var{outfile} -@itemx --output-file=@var{outfile} -Specify the name @var{outfile} for the parser file. - -The other output files' names are constructed from @var{outfile} -as described under the @samp{-v} and @samp{-d} switches. - -@item -p @var{prefix} -@itemx --name-prefix=@var{prefix} -Rename the external symbols used in the parser so that they start with -@var{prefix} instead of @samp{yy}. The precise list of symbols renamed -is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yylval}, -@code{yychar} and @code{yydebug}. - -For example, if you use @samp{-p c}, the names become @code{cparse}, -@code{clex}, and so on. - -@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}. - -@item -t -@itemx --debug -Output a definition of the macro @code{YYDEBUG} into the parser file, -so that the debugging facilities are compiled. @xref{Debugging, ,Debugging Your Parser}. - -@item -v -@itemx --verbose -Write an extra output file containing verbose descriptions of the -parser states and what is done for each type of look-ahead token in -that state. - -This file also describes all the conflicts, both those resolved by -operator precedence and the unresolved ones. - -The file's name is made by removing @samp{.tab.c} or @samp{.c} from -the parser output file name, and adding @samp{.output} instead.@refill - -Therefore, if the input file is @file{foo.y}, then the parser file is -called @file{foo.tab.c} by default. As a consequence, the verbose -output file is called @file{foo.output}.@refill +@c Please, keep this ordered as in `bison --help'. +@noindent +Operations modes: +@table @option +@item -h +@itemx --help +Print a summary of the command-line options to Bison and exit. @item -V @itemx --version Print the version number of Bison and exit. -@item -h -@itemx --help -Print a summary of the command-line options to Bison and exit. +@item --print-localedir +Print the name of the directory containing locale-dependent data. + +@item --print-datadir +Print the name of the directory containing skeletons and XSLT. -@need 1750 @item -y @itemx --yacc -@itemx --fixed-output-files -Equivalent to @samp{-o y.tab.c}; the parser output file is called +Act more like the traditional Yacc command. This can cause +different diagnostics to be generated, and may change behavior in +other minor ways. Most importantly, imitate Yacc's output +file name conventions, so that the parser output file is called @file{y.tab.c}, and the other outputs are called @file{y.output} and -@file{y.tab.h}. The purpose of this switch is to imitate Yacc's output -file name conventions. Thus, the following shell script can substitute -for Yacc:@refill +@file{y.tab.h}. +Also, if generating an @acronym{LALR}(1) parser in C, generate @code{#define} +statements in addition to an @code{enum} to associate token numbers with token +names. +Thus, the following shell script can substitute for Yacc, and the Bison +distribution contains such a script for compatibility with @acronym{POSIX}: @example -bison -y $* +#! /bin/sh +bison -y "$@@" @end example -@end table - -@node Option Cross Key, VMS Invocation, Bison Options, Invocation -@section Option Cross Key -Here is a list of options, alphabetized by long option, to help you find -the corresponding short option. +The @option{-y}/@option{--yacc} option is intended for use with +traditional Yacc grammars. If your grammar uses a Bison extension +like @samp{%glr-parser}, Bison might not be Yacc-compatible even if +this option is specified. -@tex -\def\leaderfill{\leaders\hbox to 1em{\hss.\hss}\hfill} - -{\tt -\line{ --debug \leaderfill -t} -\line{ --defines \leaderfill -d} -\line{ --file-prefix \leaderfill -b} -\line{ --fixed-output-files \leaderfill -y} -\line{ --help \leaderfill -h} -\line{ --name-prefix \leaderfill -p} -\line{ --no-lines \leaderfill -l} -\line{ --output-file \leaderfill -o} -\line{ --verbose \leaderfill -v} -\line{ --version \leaderfill -V} -\line{ --yacc \leaderfill -y} -} -@end tex +@item -W +@itemx --warnings +Output warnings falling in @var{category}. @var{category} can be one +of: +@table @code +@item midrule-values +Warn about mid-rule values that are set but not used within any of the actions +of the parent rule. +For example, warn about unused @code{$2} in: -@ifinfo @example ---debug -t ---defines -d ---file-prefix=@var{prefix} -b @var{file-prefix} ---fixed-output-files --yacc -y ---help -h ---name-prefix -p ---no-lines -l ---output-file=@var{outfile} -o @var{outfile} ---verbose -v ---version -V +exp: '1' @{ $$ = 1; @} '+' exp @{ $$ = $1 + $4; @}; @end example -@end ifinfo - -@node VMS Invocation, , Option Cross Key, Invocation -@section Invoking Bison under VMS -@cindex invoking Bison under VMS -@cindex VMS -The command line syntax for Bison on VMS is a variant of the usual -Bison command syntax---adapted to fit VMS conventions. - -To find the VMS equivalent for any Bison option, start with the long -option, and substitute a @samp{/} for the leading @samp{--}, and -substitute a @samp{_} for each @samp{-} in the name of the long option. -For example, the following invocation under VMS: +Also warn about mid-rule values that are used but not set. +For example, warn about unset @code{$$} in the mid-rule action in: @example -bison /debug/name_prefix=bar foo.y + exp: '1' @{ $1 = 1; @} '+' exp @{ $$ = $2 + $4; @}; @end example -@noindent -is equivalent to the following command under POSIX. - -@example -bison --debug --name-prefix=bar foo.y -@end example +These warnings are not enabled by default since they sometimes prove to +be false alarms in existing grammars employing the Yacc constructs +@code{$0} or @code{$-@var{n}} (where @var{n} is some positive integer). -The VMS file system does not permit filenames such as -@file{foo.tab.c}. In the above example, the output file -would instead be named @file{foo_tab.c}. -@node Table of Symbols, Glossary, Invocation, Top -@appendix Bison Symbols -@cindex Bison symbols, table of -@cindex symbols in Bison, table of +@item yacc +Incompatibilities with @acronym{POSIX} Yacc. -@table @code +@item all +All the warnings. +@item none +Turn off all the warnings. @item error -A token name reserved for error recovery. This token may be used in -grammar rules so as to allow the Bison parser to recognize an error in -the grammar without halting the process. In effect, a sentence -containing an error may be recognized as valid. On a parse error, the -token @code{error} becomes the current look-ahead token. Actions -corresponding to @code{error} are then executed, and the look-ahead -token is reset to the token that originally caused the violation. -@xref{Error Recovery}. +Treat warnings as errors. +@end table -@item YYABORT -Macro to pretend that an unrecoverable syntax error has occurred, by -making @code{yyparse} return 1 immediately. The error reporting -function @code{yyerror} is not called. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +A category can be turned off by prefixing its name with @samp{no-}. For +instance, @option{-Wno-syntax} will hide the warnings about unused +variables. +@end table -@item YYACCEPT -Macro to pretend that a complete utterance of the language has been -read, by making @code{yyparse} return 0 immediately. -@xref{Parser Function, ,The Parser Function @code{yyparse}}. +@noindent +Tuning the parser: + +@table @option +@item -t +@itemx --debug +In the parser file, define the macro @code{YYDEBUG} to 1 if it is not +already defined, so that the debugging facilities are compiled. +@xref{Tracing, ,Tracing Your Parser}. + +@item -D @var{name}[=@var{value}] +@itemx --define=@var{name}[=@var{value}] +Same as running @samp{%define @var{name} "@var{value}"} (@pxref{Decl +Summary, ,%define}). + +@item -L @var{language} +@itemx --language=@var{language} +Specify the programming language for the generated parser, as if +@code{%language} was specified (@pxref{Decl Summary, , Bison Declaration +Summary}). Currently supported languages include C, C++, and Java. +@var{language} is case-insensitive. + +This option is experimental and its effect may be modified in future +releases. + +@item --locations +Pretend that @code{%locations} was specified. @xref{Decl Summary}. + +@item -p @var{prefix} +@itemx --name-prefix=@var{prefix} +Pretend that @code{%name-prefix "@var{prefix}"} was specified. +@xref{Decl Summary}. + +@item -l +@itemx --no-lines +Don't put any @code{#line} preprocessor commands in the parser file. +Ordinarily Bison puts them in the parser file so that the C compiler +and debuggers will associate errors with your source file, the +grammar file. This option causes them to associate errors with the +parser file, treating it as an independent source file in its own right. + +@item -S @var{file} +@itemx --skeleton=@var{file} +Specify the skeleton to use, similar to @code{%skeleton} +(@pxref{Decl Summary, , Bison Declaration Summary}). + +@c You probably don't need this option unless you are developing Bison. +@c You should use @option{--language} if you want to specify the skeleton for a +@c different language, because it is clearer and because it will always +@c choose the correct skeleton for non-deterministic or push parsers. + +If @var{file} does not contain a @code{/}, @var{file} is the name of a skeleton +file in the Bison installation directory. +If it does, @var{file} is an absolute file name or a file name relative to the +current working directory. +This is similar to how most shells resolve commands. + +@item -k +@itemx --token-table +Pretend that @code{%token-table} was specified. @xref{Decl Summary}. +@end table + +@noindent +Adjust the output: + +@table @option +@item --defines[=@var{file}] +Pretend that @code{%defines} was specified, i.e., write an extra output +file containing macro definitions for the token type names defined in +the grammar, as well as a few other declarations. @xref{Decl Summary}. + +@item -d +This is the same as @code{--defines} except @code{-d} does not accept a +@var{file} argument since POSIX Yacc requires that @code{-d} can be bundled +with other short options. + +@item -b @var{file-prefix} +@itemx --file-prefix=@var{prefix} +Pretend that @code{%file-prefix} was specified, i.e., specify prefix to use +for all Bison output file names. @xref{Decl Summary}. + +@item -r @var{things} +@itemx --report=@var{things} +Write an extra output file containing verbose description of the comma +separated list of @var{things} among: + +@table @code +@item state +Description of the grammar, conflicts (resolved and unresolved), and +@acronym{LALR} automaton. + +@item lookahead +Implies @code{state} and augments the description of the automaton with +each rule's lookahead set. + +@item itemset +Implies @code{state} and augments the description of the automaton with +the full set of items for each state, instead of its core only. +@end table + +@item --report-file=@var{file} +Specify the @var{file} for the verbose description. + +@item -v +@itemx --verbose +Pretend that @code{%verbose} was specified, i.e., write an extra output +file containing verbose descriptions of the grammar and +parser. @xref{Decl Summary}. + +@item -o @var{file} +@itemx --output=@var{file} +Specify the @var{file} for the parser file. + +The other output files' names are constructed from @var{file} as +described under the @samp{-v} and @samp{-d} options. + +@item -g [@var{file}] +@itemx --graph[=@var{file}] +Output a graphical representation of the @acronym{LALR}(1) grammar +automaton computed by Bison, in @uref{http://www.graphviz.org/, Graphviz} +@uref{http://www.graphviz.org/doc/info/lang.html, @acronym{DOT}} format. +@code{@var{file}} is optional. +If omitted and the grammar file is @file{foo.y}, the output file will be +@file{foo.dot}. + +@item -x [@var{file}] +@itemx --xml[=@var{file}] +Output an XML report of the @acronym{LALR}(1) automaton computed by Bison. +@code{@var{file}} is optional. +If omitted and the grammar file is @file{foo.y}, the output file will be +@file{foo.xml}. +(The current XML schema is experimental and may evolve. +More user feedback will help to stabilize it.) +@end table + +@node Option Cross Key +@section Option Cross Key + +Here is a list of options, alphabetized by long option, to help you find +the corresponding short option. + +@multitable {@option{--defines=@var{defines-file}}} {@option{-D @var{name}[=@var{value}]}} {@code{%nondeterministic-parser}} +@headitem Long Option @tab Short Option @tab Bison Directive +@include cross-options.texi +@end multitable + +@node Yacc Library +@section Yacc Library + +The Yacc library contains default implementations of the +@code{yyerror} and @code{main} functions. These default +implementations are normally not useful, but @acronym{POSIX} requires +them. To use the Yacc library, link your program with the +@option{-ly} option. Note that Bison's implementation of the Yacc +library is distributed under the terms of the @acronym{GNU} General +Public License (@pxref{Copying}). + +If you use the Yacc library's @code{yyerror} function, you should +declare @code{yyerror} as follows: + +@example +int yyerror (char const *); +@end example + +Bison ignores the @code{int} value returned by this @code{yyerror}. +If you use the Yacc library's @code{main} function, your +@code{yyparse} function should have the following type signature: + +@example +int yyparse (void); +@end example + +@c ================================================= C++ Bison + +@node Other Languages +@chapter Parsers Written In Other Languages + +@menu +* C++ Parsers:: The interface to generate C++ parser classes +* Java Parsers:: The interface to generate Java parser classes +@end menu + +@node C++ Parsers +@section C++ Parsers + +@menu +* C++ Bison Interface:: Asking for C++ parser generation +* C++ Semantic Values:: %union vs. C++ +* C++ Location Values:: The position and location classes +* C++ Parser Interface:: Instantiating and running the parser +* C++ Scanner Interface:: Exchanges between yylex and parse +* A Complete C++ Example:: Demonstrating their use +@end menu + +@node C++ Bison Interface +@subsection C++ Bison Interface +@c - %skeleton "lalr1.cc" +@c - Always pure +@c - initial action + +The C++ @acronym{LALR}(1) parser is selected using the skeleton directive, +@samp{%skeleton "lalr1.c"}, or the synonymous command-line option +@option{--skeleton=lalr1.c}. +@xref{Decl Summary}. + +When run, @command{bison} will create several entities in the @samp{yy} +namespace. +@findex %define namespace +Use the @samp{%define namespace} directive to change the namespace name, see +@ref{Decl Summary}. +The various classes are generated in the following files: + +@table @file +@item position.hh +@itemx location.hh +The definition of the classes @code{position} and @code{location}, +used for location tracking. @xref{C++ Location Values}. + +@item stack.hh +An auxiliary class @code{stack} used by the parser. + +@item @var{file}.hh +@itemx @var{file}.cc +(Assuming the extension of the input file was @samp{.yy}.) The +declaration and implementation of the C++ parser class. The basename +and extension of these two files follow the same rules as with regular C +parsers (@pxref{Invocation}). + +The header is @emph{mandatory}; you must either pass +@option{-d}/@option{--defines} to @command{bison}, or use the +@samp{%defines} directive. +@end table + +All these files are documented using Doxygen; run @command{doxygen} +for a complete and accurate documentation. + +@node C++ Semantic Values +@subsection C++ Semantic Values +@c - No objects in unions +@c - YYSTYPE +@c - Printer and destructor + +The @code{%union} directive works as for C, see @ref{Union Decl, ,The +Collection of Value Types}. In particular it produces a genuine +@code{union}@footnote{In the future techniques to allow complex types +within pseudo-unions (similar to Boost variants) might be implemented to +alleviate these issues.}, which have a few specific features in C++. +@itemize @minus +@item +The type @code{YYSTYPE} is defined but its use is discouraged: rather +you should refer to the parser's encapsulated type +@code{yy::parser::semantic_type}. +@item +Non POD (Plain Old Data) types cannot be used. C++ forbids any +instance of classes with constructors in unions: only @emph{pointers} +to such objects are allowed. +@end itemize + +Because objects have to be stored via pointers, memory is not +reclaimed automatically: using the @code{%destructor} directive is the +only means to avoid leaks. @xref{Destructor Decl, , Freeing Discarded +Symbols}. + + +@node C++ Location Values +@subsection C++ Location Values +@c - %locations +@c - class Position +@c - class Location +@c - %define filename_type "const symbol::Symbol" + +When the directive @code{%locations} is used, the C++ parser supports +location tracking, see @ref{Locations, , Locations Overview}. Two +auxiliary classes define a @code{position}, a single point in a file, +and a @code{location}, a range composed of a pair of +@code{position}s (possibly spanning several files). + +@deftypemethod {position} {std::string*} file +The name of the file. It will always be handled as a pointer, the +parser will never duplicate nor deallocate it. As an experimental +feature you may change it to @samp{@var{type}*} using @samp{%define +filename_type "@var{type}"}. +@end deftypemethod + +@deftypemethod {position} {unsigned int} line +The line, starting at 1. +@end deftypemethod + +@deftypemethod {position} {unsigned int} lines (int @var{height} = 1) +Advance by @var{height} lines, resetting the column number. +@end deftypemethod + +@deftypemethod {position} {unsigned int} column +The column, starting at 0. +@end deftypemethod + +@deftypemethod {position} {unsigned int} columns (int @var{width} = 1) +Advance by @var{width} columns, without changing the line number. +@end deftypemethod + +@deftypemethod {position} {position&} operator+= (position& @var{pos}, int @var{width}) +@deftypemethodx {position} {position} operator+ (const position& @var{pos}, int @var{width}) +@deftypemethodx {position} {position&} operator-= (const position& @var{pos}, int @var{width}) +@deftypemethodx {position} {position} operator- (position& @var{pos}, int @var{width}) +Various forms of syntactic sugar for @code{columns}. +@end deftypemethod + +@deftypemethod {position} {position} operator<< (std::ostream @var{o}, const position& @var{p}) +Report @var{p} on @var{o} like this: +@samp{@var{file}:@var{line}.@var{column}}, or +@samp{@var{line}.@var{column}} if @var{file} is null. +@end deftypemethod + +@deftypemethod {location} {position} begin +@deftypemethodx {location} {position} end +The first, inclusive, position of the range, and the first beyond. +@end deftypemethod + +@deftypemethod {location} {unsigned int} columns (int @var{width} = 1) +@deftypemethodx {location} {unsigned int} lines (int @var{height} = 1) +Advance the @code{end} position. +@end deftypemethod + +@deftypemethod {location} {location} operator+ (const location& @var{begin}, const location& @var{end}) +@deftypemethodx {location} {location} operator+ (const location& @var{begin}, int @var{width}) +@deftypemethodx {location} {location} operator+= (const location& @var{loc}, int @var{width}) +Various forms of syntactic sugar. +@end deftypemethod + +@deftypemethod {location} {void} step () +Move @code{begin} onto @code{end}. +@end deftypemethod + + +@node C++ Parser Interface +@subsection C++ Parser Interface +@c - define parser_class_name +@c - Ctor +@c - parse, error, set_debug_level, debug_level, set_debug_stream, +@c debug_stream. +@c - Reporting errors + +The output files @file{@var{output}.hh} and @file{@var{output}.cc} +declare and define the parser class in the namespace @code{yy}. The +class name defaults to @code{parser}, but may be changed using +@samp{%define parser_class_name "@var{name}"}. The interface of +this class is detailed below. It can be extended using the +@code{%parse-param} feature: its semantics is slightly changed since +it describes an additional member of the parser class, and an +additional argument for its constructor. + +@defcv {Type} {parser} {semantic_value_type} +@defcvx {Type} {parser} {location_value_type} +The types for semantics value and locations. +@end defcv + +@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...) +Build a new parser object. There are no arguments by default, unless +@samp{%parse-param @{@var{type1} @var{arg1}@}} was used. +@end deftypemethod + +@deftypemethod {parser} {int} parse () +Run the syntactic analysis, and return 0 on success, 1 otherwise. +@end deftypemethod + +@deftypemethod {parser} {std::ostream&} debug_stream () +@deftypemethodx {parser} {void} set_debug_stream (std::ostream& @var{o}) +Get or set the stream used for tracing the parsing. It defaults to +@code{std::cerr}. +@end deftypemethod + +@deftypemethod {parser} {debug_level_type} debug_level () +@deftypemethodx {parser} {void} set_debug_level (debug_level @var{l}) +Get or set the tracing level. Currently its value is either 0, no trace, +or nonzero, full tracing. +@end deftypemethod + +@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m}) +The definition for this member function must be supplied by the user: +the parser uses it to report a parser error occurring at @var{l}, +described by @var{m}. +@end deftypemethod + + +@node C++ Scanner Interface +@subsection C++ Scanner Interface +@c - prefix for yylex. +@c - Pure interface to yylex +@c - %lex-param + +The parser invokes the scanner by calling @code{yylex}. Contrary to C +parsers, C++ parsers are always pure: there is no point in using the +@code{%define api.pure} directive. Therefore the interface is as follows. + +@deftypemethod {parser} {int} yylex (semantic_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...) +Return the next token. Its type is the return value, its semantic +value and location being @var{yylval} and @var{yylloc}. Invocations of +@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments. +@end deftypemethod + + +@node A Complete C++ Example +@subsection A Complete C++ Example + +This section demonstrates the use of a C++ parser with a simple but +complete example. This example should be available on your system, +ready to compile, in the directory @dfn{../bison/examples/calc++}. It +focuses on the use of Bison, therefore the design of the various C++ +classes is very naive: no accessors, no encapsulation of members etc. +We will use a Lex scanner, and more precisely, a Flex scanner, to +demonstrate the various interaction. A hand written scanner is +actually easier to interface with. + +@menu +* Calc++ --- C++ Calculator:: The specifications +* Calc++ Parsing Driver:: An active parsing context +* Calc++ Parser:: A parser class +* Calc++ Scanner:: A pure C++ Flex scanner +* Calc++ Top Level:: Conducting the band +@end menu + +@node Calc++ --- C++ Calculator +@subsubsection Calc++ --- C++ Calculator + +Of course the grammar is dedicated to arithmetics, a single +expression, possibly preceded by variable assignments. An +environment containing possibly predefined variables such as +@code{one} and @code{two}, is exchanged with the parser. An example +of valid input follows. + +@example +three := 3 +seven := one + two * three +seven * seven +@end example + +@node Calc++ Parsing Driver +@subsubsection Calc++ Parsing Driver +@c - An env +@c - A place to store error messages +@c - A place for the result + +To support a pure interface with the parser (and the scanner) the +technique of the ``parsing context'' is convenient: a structure +containing all the data to exchange. Since, in addition to simply +launch the parsing, there are several auxiliary tasks to execute (open +the file for parsing, instantiate the parser etc.), we recommend +transforming the simple parsing context structure into a fully blown +@dfn{parsing driver} class. + +The declaration of this driver class, @file{calc++-driver.hh}, is as +follows. The first part includes the CPP guard and imports the +required standard library components, and the declaration of the parser +class. + +@comment file: calc++-driver.hh +@example +#ifndef CALCXX_DRIVER_HH +# define CALCXX_DRIVER_HH +# include +# include +# include "calc++-parser.hh" +@end example + + +@noindent +Then comes the declaration of the scanning function. Flex expects +the signature of @code{yylex} to be defined in the macro +@code{YY_DECL}, and the C++ parser expects it to be declared. We can +factor both as follows. + +@comment file: calc++-driver.hh +@example +// Tell Flex the lexer's prototype ... +# define YY_DECL \ + yy::calcxx_parser::token_type \ + yylex (yy::calcxx_parser::semantic_type* yylval, \ + yy::calcxx_parser::location_type* yylloc, \ + calcxx_driver& driver) +// ... and declare it for the parser's sake. +YY_DECL; +@end example + +@noindent +The @code{calcxx_driver} class is then declared with its most obvious +members. + +@comment file: calc++-driver.hh +@example +// Conducting the whole scanning and parsing of Calc++. +class calcxx_driver +@{ +public: + calcxx_driver (); + virtual ~calcxx_driver (); + + std::map variables; + + int result; +@end example + +@noindent +To encapsulate the coordination with the Flex scanner, it is useful to +have two members function to open and close the scanning phase. + +@comment file: calc++-driver.hh +@example + // Handling the scanner. + void scan_begin (); + void scan_end (); + bool trace_scanning; +@end example + +@noindent +Similarly for the parser itself. + +@comment file: calc++-driver.hh +@example + // Run the parser. Return 0 on success. + int parse (const std::string& f); + std::string file; + bool trace_parsing; +@end example + +@noindent +To demonstrate pure handling of parse errors, instead of simply +dumping them on the standard error output, we will pass them to the +compiler driver using the following two member functions. Finally, we +close the class declaration and CPP guard. + +@comment file: calc++-driver.hh +@example + // Error handling. + void error (const yy::location& l, const std::string& m); + void error (const std::string& m); +@}; +#endif // ! CALCXX_DRIVER_HH +@end example + +The implementation of the driver is straightforward. The @code{parse} +member function deserves some attention. The @code{error} functions +are simple stubs, they should actually register the located error +messages and set error state. + +@comment file: calc++-driver.cc +@example +#include "calc++-driver.hh" +#include "calc++-parser.hh" + +calcxx_driver::calcxx_driver () + : trace_scanning (false), trace_parsing (false) +@{ + variables["one"] = 1; + variables["two"] = 2; +@} + +calcxx_driver::~calcxx_driver () +@{ +@} + +int +calcxx_driver::parse (const std::string &f) +@{ + file = f; + scan_begin (); + yy::calcxx_parser parser (*this); + parser.set_debug_level (trace_parsing); + int res = parser.parse (); + scan_end (); + return res; +@} + +void +calcxx_driver::error (const yy::location& l, const std::string& m) +@{ + std::cerr << l << ": " << m << std::endl; +@} + +void +calcxx_driver::error (const std::string& m) +@{ + std::cerr << m << std::endl; +@} +@end example + +@node Calc++ Parser +@subsubsection Calc++ Parser + +The parser definition file @file{calc++-parser.yy} starts by asking for +the C++ LALR(1) skeleton, the creation of the parser header file, and +specifies the name of the parser class. Because the C++ skeleton +changed several times, it is safer to require the version you designed +the grammar for. + +@comment file: calc++-parser.yy +@example +%skeleton "lalr1.cc" /* -*- C++ -*- */ +%require "@value{VERSION}" +%defines +%define parser_class_name "calcxx_parser" +@end example + +@noindent +@findex %code requires +Then come the declarations/inclusions needed to define the +@code{%union}. Because the parser uses the parsing driver and +reciprocally, both cannot include the header of the other. Because the +driver's header needs detailed knowledge about the parser class (in +particular its inner types), it is the parser's header which will simply +use a forward declaration of the driver. +@xref{Decl Summary, ,%code}. + +@comment file: calc++-parser.yy +@example +%code requires @{ +# include +class calcxx_driver; +@} +@end example + +@noindent +The driver is passed by reference to the parser and to the scanner. +This provides a simple but effective pure interface, not relying on +global variables. + +@comment file: calc++-parser.yy +@example +// The parsing context. +%parse-param @{ calcxx_driver& driver @} +%lex-param @{ calcxx_driver& driver @} +@end example + +@noindent +Then we request the location tracking feature, and initialize the +first location's file name. Afterwards new locations are computed +relatively to the previous locations: the file name will be +automatically propagated. + +@comment file: calc++-parser.yy +@example +%locations +%initial-action +@{ + // Initialize the initial location. + @@$.begin.filename = @@$.end.filename = &driver.file; +@}; +@end example + +@noindent +Use the two following directives to enable parser tracing and verbose +error messages. + +@comment file: calc++-parser.yy +@example +%debug +%error-verbose +@end example + +@noindent +Semantic values cannot use ``real'' objects, but only pointers to +them. + +@comment file: calc++-parser.yy +@example +// Symbols. +%union +@{ + int ival; + std::string *sval; +@}; +@end example + +@noindent +@findex %code +The code between @samp{%code @{} and @samp{@}} is output in the +@file{*.cc} file; it needs detailed knowledge about the driver. + +@comment file: calc++-parser.yy +@example +%code @{ +# include "calc++-driver.hh" +@} +@end example + + +@noindent +The token numbered as 0 corresponds to end of file; the following line +allows for nicer error messages referring to ``end of file'' instead +of ``$end''. Similarly user friendly named are provided for each +symbol. Note that the tokens names are prefixed by @code{TOKEN_} to +avoid name clashes. + +@comment file: calc++-parser.yy +@example +%token END 0 "end of file" +%token ASSIGN ":=" +%token IDENTIFIER "identifier" +%token NUMBER "number" +%type exp +@end example + +@noindent +To enable memory deallocation during error recovery, use +@code{%destructor}. + +@c FIXME: Document %printer, and mention that it takes a braced-code operand. +@comment file: calc++-parser.yy +@example +%printer @{ debug_stream () << *$$; @} "identifier" +%destructor @{ delete $$; @} "identifier" + +%printer @{ debug_stream () << $$; @} +@end example + +@noindent +The grammar itself is straightforward. + +@comment file: calc++-parser.yy +@example +%% +%start unit; +unit: assignments exp @{ driver.result = $2; @}; + +assignments: assignments assignment @{@} + | /* Nothing. */ @{@}; + +assignment: + "identifier" ":=" exp + @{ driver.variables[*$1] = $3; delete $1; @}; + +%left '+' '-'; +%left '*' '/'; +exp: exp '+' exp @{ $$ = $1 + $3; @} + | exp '-' exp @{ $$ = $1 - $3; @} + | exp '*' exp @{ $$ = $1 * $3; @} + | exp '/' exp @{ $$ = $1 / $3; @} + | '(' exp ')' @{ $$ = $2; @} + | "identifier" @{ $$ = driver.variables[*$1]; delete $1; @} + | "number" @{ $$ = $1; @}; +%% +@end example + +@noindent +Finally the @code{error} member function registers the errors to the +driver. + +@comment file: calc++-parser.yy +@example +void +yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l, + const std::string& m) +@{ + driver.error (l, m); +@} +@end example + +@node Calc++ Scanner +@subsubsection Calc++ Scanner + +The Flex scanner first includes the driver declaration, then the +parser's to get the set of defined tokens. + +@comment file: calc++-scanner.ll +@example +%@{ /* -*- C++ -*- */ +# include +# include +# include +# include +# include "calc++-driver.hh" +# include "calc++-parser.hh" + +/* Work around an incompatibility in flex (at least versions + 2.5.31 through 2.5.33): it generates code that does + not conform to C89. See Debian bug 333231 + . */ +# undef yywrap +# define yywrap() 1 + +/* By default yylex returns int, we use token_type. + Unfortunately yyterminate by default returns 0, which is + not of token_type. */ +#define yyterminate() return token::END +%@} +@end example + +@noindent +Because there is no @code{#include}-like feature we don't need +@code{yywrap}, we don't need @code{unput} either, and we parse an +actual file, this is not an interactive session with the user. +Finally we enable the scanner tracing features. + +@comment file: calc++-scanner.ll +@example +%option noyywrap nounput batch debug +@end example + +@noindent +Abbreviations allow for more readable rules. + +@comment file: calc++-scanner.ll +@example +id [a-zA-Z][a-zA-Z_0-9]* +int [0-9]+ +blank [ \t] +@end example + +@noindent +The following paragraph suffices to track locations accurately. Each +time @code{yylex} is invoked, the begin position is moved onto the end +position. Then when a pattern is matched, the end position is +advanced of its width. In case it matched ends of lines, the end +cursor is adjusted, and each time blanks are matched, the begin cursor +is moved onto the end cursor to effectively ignore the blanks +preceding tokens. Comments would be treated equally. + +@comment file: calc++-scanner.ll +@example +%@{ +# define YY_USER_ACTION yylloc->columns (yyleng); +%@} +%% +%@{ + yylloc->step (); +%@} +@{blank@}+ yylloc->step (); +[\n]+ yylloc->lines (yyleng); yylloc->step (); +@end example + +@noindent +The rules are simple, just note the use of the driver to report errors. +It is convenient to use a typedef to shorten +@code{yy::calcxx_parser::token::identifier} into +@code{token::identifier} for instance. + +@comment file: calc++-scanner.ll +@example +%@{ + typedef yy::calcxx_parser::token token; +%@} + /* Convert ints to the actual type of tokens. */ +[-+*/()] return yy::calcxx_parser::token_type (yytext[0]); +":=" return token::ASSIGN; +@{int@} @{ + errno = 0; + long n = strtol (yytext, NULL, 10); + if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE)) + driver.error (*yylloc, "integer is out of range"); + yylval->ival = n; + return token::NUMBER; +@} +@{id@} yylval->sval = new std::string (yytext); return token::IDENTIFIER; +. driver.error (*yylloc, "invalid character"); +%% +@end example + +@noindent +Finally, because the scanner related driver's member function depend +on the scanner's data, it is simpler to implement them in this file. + +@comment file: calc++-scanner.ll +@example +void +calcxx_driver::scan_begin () +@{ + yy_flex_debug = trace_scanning; + if (file == "-") + yyin = stdin; + else if (!(yyin = fopen (file.c_str (), "r"))) + @{ + error (std::string ("cannot open ") + file); + exit (1); + @} +@} + +void +calcxx_driver::scan_end () +@{ + fclose (yyin); +@} +@end example + +@node Calc++ Top Level +@subsubsection Calc++ Top Level + +The top level file, @file{calc++.cc}, poses no problem. + +@comment file: calc++.cc +@example +#include +#include "calc++-driver.hh" + +int +main (int argc, char *argv[]) +@{ + int res = 0; + calcxx_driver driver; + for (++argv; argv[0]; ++argv) + if (*argv == std::string ("-p")) + driver.trace_parsing = true; + else if (*argv == std::string ("-s")) + driver.trace_scanning = true; + else if (!driver.parse (*argv)) + std::cout << driver.result << std::endl; + else + res = 1; + return res; +@} +@end example + +@node Java Parsers +@section Java Parsers + +@menu +* Java Bison Interface:: Asking for Java parser generation +* Java Semantic Values:: %type and %token vs. Java +* Java Location Values:: The position and location classes +* Java Parser Interface:: Instantiating and running the parser +* Java Scanner Interface:: Specifying the scanner for the parser +* Java Action Features:: Special features for use in actions +* Java Differences:: Differences between C/C++ and Java Grammars +* Java Declarations Summary:: List of Bison declarations used with Java +@end menu + +@node Java Bison Interface +@subsection Java Bison Interface +@c - %language "Java" + +(The current Java interface is experimental and may evolve. +More user feedback will help to stabilize it.) + +The Java parser skeletons are selected using the @code{%language "Java"} +directive or the @option{-L java}/@option{--language=java} option. + +@c FIXME: Documented bug. +When generating a Java parser, @code{bison @var{basename}.y} will create +a single Java source file named @file{@var{basename}.java}. Using an +input file without a @file{.y} suffix is currently broken. The basename +of the output file can be changed by the @code{%file-prefix} directive +or the @option{-p}/@option{--name-prefix} option. The entire output file +name can be changed by the @code{%output} directive or the +@option{-o}/@option{--output} option. The output file contains a single +class for the parser. + +You can create documentation for generated parsers using Javadoc. + +Contrary to C parsers, Java parsers do not use global variables; the +state of the parser is always local to an instance of the parser class. +Therefore, all Java parsers are ``pure'', and the @code{%pure-parser} +and @code{%define api.pure} directives does not do anything when used in +Java. + +Push parsers are currently unsupported in Java and @code{%define +api.push_pull} have no effect. + +@acronym{GLR} parsers are currently unsupported in Java. Do not use the +@code{glr-parser} directive. + +No header file can be generated for Java parsers. Do not use the +@code{%defines} directive or the @option{-d}/@option{--defines} options. + +@c FIXME: Possible code change. +Currently, support for debugging is always compiled +in. Thus the @code{%debug} and @code{%token-table} directives and the +@option{-t}/@option{--debug} and @option{-k}/@option{--token-table} +options have no effect. This may change in the future to eliminate +unused code in the generated parser, so use @code{%debug} explicitly +if needed. Also, in the future the +@code{%token-table} directive might enable a public interface to +access the token names and codes. + +Getting a ``code too large'' error from the Java compiler means the code +hit the 64KB bytecode per method limination of the Java class file. +Try reducing the amount of code in actions and static initializers; +otherwise, report a bug so that the parser skeleton will be improved. + + +@node Java Semantic Values +@subsection Java Semantic Values +@c - No %union, specify type in %type/%token. +@c - YYSTYPE +@c - Printer and destructor + +There is no @code{%union} directive in Java parsers. Instead, the +semantic values' types (class names) should be specified in the +@code{%type} or @code{%token} directive: + +@example +%type expr assignment_expr term factor +%type number +@end example -@item YYBACKUP -Macro to discard a value from the parser stack and fake a look-ahead -token. @xref{Action Features, ,Special Features for Use in Actions}. +By default, the semantic stack is declared to have @code{Object} members, +which means that the class types you specify can be of any class. +To improve the type safety of the parser, you can declare the common +superclass of all the semantic values using the @code{%define stype} +directive. For example, after the following declaration: -@item YYERROR -Macro to pretend that a syntax error has just been detected: call -@code{yyerror} and then perform normal error recovery if possible -(@pxref{Error Recovery}), or (if recovery is impossible) make -@code{yyparse} return 1. @xref{Error Recovery}. +@example +%define stype "ASTNode" +@end example -@item YYERROR_VERBOSE -Macro that you define with @code{#define} in the Bison declarations -section to request verbose, specific error message strings when -@code{yyerror} is called. +@noindent +any @code{%type} or @code{%token} specifying a semantic type which +is not a subclass of ASTNode, will cause a compile-time error. + +@c FIXME: Documented bug. +Types used in the directives may be qualified with a package name. +Primitive data types are accepted for Java version 1.5 or later. Note +that in this case the autoboxing feature of Java 1.5 will be used. +Generic types may not be used; this is due to a limitation in the +implementation of Bison, and may change in future releases. + +Java parsers do not support @code{%destructor}, since the language +adopts garbage collection. The parser will try to hold references +to semantic values for as little time as needed. + +Java parsers do not support @code{%printer}, as @code{toString()} +can be used to print the semantic values. This however may change +(in a backwards-compatible way) in future versions of Bison. + + +@node Java Location Values +@subsection Java Location Values +@c - %locations +@c - class Position +@c - class Location + +When the directive @code{%locations} is used, the Java parser +supports location tracking, see @ref{Locations, , Locations Overview}. +An auxiliary user-defined class defines a @dfn{position}, a single point +in a file; Bison itself defines a class representing a @dfn{location}, +a range composed of a pair of positions (possibly spanning several +files). The location class is an inner class of the parser; the name +is @code{Location} by default, and may also be renamed using +@code{%define location_type "@var{class-name}}. + +The location class treats the position as a completely opaque value. +By default, the class name is @code{Position}, but this can be changed +with @code{%define position_type "@var{class-name}"}. This class must +be supplied by the user. + + +@deftypeivar {Location} {Position} begin +@deftypeivarx {Location} {Position} end +The first, inclusive, position of the range, and the first beyond. +@end deftypeivar + +@deftypeop {Constructor} {Location} {} Location (Position @var{loc}) +Create a @code{Location} denoting an empty range located at a given point. +@end deftypeop + +@deftypeop {Constructor} {Location} {} Location (Position @var{begin}, Position @var{end}) +Create a @code{Location} from the endpoints of the range. +@end deftypeop + +@deftypemethod {Location} {String} toString () +Prints the range represented by the location. For this to work +properly, the position class should override the @code{equals} and +@code{toString} methods appropriately. +@end deftypemethod + + +@node Java Parser Interface +@subsection Java Parser Interface +@c - define parser_class_name +@c - Ctor +@c - parse, error, set_debug_level, debug_level, set_debug_stream, +@c debug_stream. +@c - Reporting errors + +The name of the generated parser class defaults to @code{YYParser}. The +@code{YY} prefix may be changed using the @code{%name-prefix} directive +or the @option{-p}/@option{--name-prefix} option. Alternatively, use +@code{%define parser_class_name "@var{name}"} to give a custom name to +the class. The interface of this class is detailed below. + +By default, the parser class has package visibility. A declaration +@code{%define public} will change to public visibility. Remember that, +according to the Java language specification, the name of the @file{.java} +file should match the name of the class in this case. Similarly, you can +use @code{abstract}, @code{final} and @code{strictfp} with the +@code{%define} declaration to add other modifiers to the parser class. +A single @code{%define annotations "@var{annotations}"} directive can +be used to add any number of annotations to the parser class. + +The Java package name of the parser class can be specified using the +@code{%define package} directive. The superclass and the implemented +interfaces of the parser class can be specified with the @code{%define +extends} and @code{%define implements} directives. + +The parser class defines an inner class, @code{Location}, that is used +for location tracking (see @ref{Java Location Values}), and a inner +interface, @code{Lexer} (see @ref{Java Scanner Interface}). Other than +these inner class/interface, and the members described in the interface +below, all the other members and fields are preceded with a @code{yy} or +@code{YY} prefix to avoid clashes with user code. + +The parser class can be extended using the @code{%parse-param} +directive. Each occurrence of the directive will add a @code{protected +final} field to the parser class, and an argument to its constructor, +which initialize them automatically. + +@deftypeop {Constructor} {YYParser} {} YYParser (@var{lex_param}, @dots{}, @var{parse_param}, @dots{}) +Build a new parser object with embedded @code{%code lexer}. There are +no parameters, unless @code{%parse-param}s and/or @code{%lex-param}s are +used. + +Use @code{%code init} for code added to the start of the constructor +body. This is especially useful to initialize superclasses. Use +@code{%define init_throws} to specify any uncatch exceptions. +@end deftypeop + +@deftypeop {Constructor} {YYParser} {} YYParser (Lexer @var{lexer}, @var{parse_param}, @dots{}) +Build a new parser object using the specified scanner. There are no +additional parameters unless @code{%parse-param}s are used. + +If the scanner is defined by @code{%code lexer}, this constructor is +declared @code{protected} and is called automatically with a scanner +created with the correct @code{%lex-param}s. + +Use @code{%code init} for code added to the start of the constructor +body. This is especially useful to initialize superclasses. Use +@code{%define init_throws} to specify any uncatch exceptions. +@end deftypeop + +@deftypemethod {YYParser} {boolean} parse () +Run the syntactic analysis, and return @code{true} on success, +@code{false} otherwise. +@end deftypemethod + +@deftypemethod {YYParser} {boolean} getErrorVerbose () +@deftypemethodx {YYParser} {void} setErrorVerbose (boolean @var{verbose}) +Get or set the option to produce verbose error messages. These are only +available with the @code{%error-verbose} directive, which also turn on +verbose error messages. +@end deftypemethod + +@deftypemethod {YYParser} {void} yyerror (String @var{msg}) +@deftypemethodx {YYParser} {void} yyerror (Position @var{pos}, String @var{msg}) +@deftypemethodx {YYParser} {void} yyerror (Location @var{loc}, String @var{msg}) +Print an error message using the @code{yyerror} method of the scanner +instance in use. The @code{Location} and @code{Position} parameters are +available only if location tracking is active. +@end deftypemethod + +@deftypemethod {YYParser} {boolean} recovering () +During the syntactic analysis, return @code{true} if recovering +from a syntax error. +@xref{Error Recovery}. +@end deftypemethod + +@deftypemethod {YYParser} {java.io.PrintStream} getDebugStream () +@deftypemethodx {YYParser} {void} setDebugStream (java.io.printStream @var{o}) +Get or set the stream used for tracing the parsing. It defaults to +@code{System.err}. +@end deftypemethod + +@deftypemethod {YYParser} {int} getDebugLevel () +@deftypemethodx {YYParser} {void} setDebugLevel (int @var{l}) +Get or set the tracing level. Currently its value is either 0, no trace, +or nonzero, full tracing. +@end deftypemethod + +@deftypecv {Constant} {YYParser} {String} {bisonVersion} +@deftypecvx {Constant} {YYParser} {String} {bisonSkeleton} +Identify the Bison version and skeleton used to generate this parser. +@end deftypecv + + +@node Java Scanner Interface +@subsection Java Scanner Interface +@c - %code lexer +@c - %lex-param +@c - Lexer interface + +There are two possible ways to interface a Bison-generated Java parser +with a scanner: the scanner may be defined by @code{%code lexer}, or +defined elsewhere. In either case, the scanner has to implement the +@code{Lexer} inner interface of the parser class. This interface also +contain constants for all user-defined token names and the predefined +@code{EOF} token. + +In the first case, the body of the scanner class is placed in +@code{%code lexer} blocks. If you want to pass parameters from the +parser constructor to the scanner constructor, specify them with +@code{%lex-param}; they are passed before @code{%parse-param}s to the +constructor. + +In the second case, the scanner has to implement the @code{Lexer} interface, +which is defined within the parser class (e.g., @code{YYParser.Lexer}). +The constructor of the parser object will then accept an object +implementing the interface; @code{%lex-param} is not used in this +case. + +In both cases, the scanner has to implement the following methods. + +@deftypemethod {Lexer} {void} yyerror (Location @var{loc}, String @var{msg}) +This method is defined by the user to emit an error message. The first +parameter is omitted if location tracking is not active. Its type can be +changed using @code{%define location_type "@var{class-name}".} +@end deftypemethod + +@deftypemethod {Lexer} {int} yylex () +Return the next token. Its type is the return value, its semantic +value and location are saved and returned by the ther methods in the +interface. + +Use @code{%define lex_throws} to specify any uncaught exceptions. +Default is @code{java.io.IOException}. +@end deftypemethod + +@deftypemethod {Lexer} {Position} getStartPos () +@deftypemethodx {Lexer} {Position} getEndPos () +Return respectively the first position of the last token that +@code{yylex} returned, and the first position beyond it. These +methods are not needed unless location tracking is active. + +The return type can be changed using @code{%define position_type +"@var{class-name}".} +@end deftypemethod + +@deftypemethod {Lexer} {Object} getLVal () +Return the semantical value of the last token that yylex returned. + +The return type can be changed using @code{%define stype +"@var{class-name}".} +@end deftypemethod + + +@node Java Action Features +@subsection Special Features for Use in Java Actions + +The following special constructs can be uses in Java actions. +Other analogous C action features are currently unavailable for Java. + +Use @code{%define throws} to specify any uncaught exceptions from parser +actions, and initial actions specified by @code{%initial-action}. + +@defvar $@var{n} +The semantic value for the @var{n}th component of the current rule. +This may not be assigned to. +@xref{Java Semantic Values}. +@end defvar + +@defvar $<@var{typealt}>@var{n} +Like @code{$@var{n}} but specifies a alternative type @var{typealt}. +@xref{Java Semantic Values}. +@end defvar + +@defvar $$ +The semantic value for the grouping made by the current rule. As a +value, this is in the base type (@code{Object} or as specified by +@code{%define stype}) as in not cast to the declared subtype because +casts are not allowed on the left-hand side of Java assignments. +Use an explicit Java cast if the correct subtype is needed. +@xref{Java Semantic Values}. +@end defvar + +@defvar $<@var{typealt}>$ +Same as @code{$$} since Java always allow assigning to the base type. +Perhaps we should use this and @code{$<>$} for the value and @code{$$} +for setting the value but there is currently no easy way to distinguish +these constructs. +@xref{Java Semantic Values}. +@end defvar + +@defvar @@@var{n} +The location information of the @var{n}th component of the current rule. +This may not be assigned to. +@xref{Java Location Values}. +@end defvar + +@defvar @@$ +The location information of the grouping made by the current rule. +@xref{Java Location Values}. +@end defvar + +@deffn {Statement} {return YYABORT;} +Return immediately from the parser, indicating failure. +@xref{Java Parser Interface}. +@end deffn + +@deffn {Statement} {return YYACCEPT;} +Return immediately from the parser, indicating success. +@xref{Java Parser Interface}. +@end deffn + +@deffn {Statement} {return YYERROR;} +Start error recovery without printing an error message. +@xref{Error Recovery}. +@end deffn -@item YYINITDEPTH -Macro for specifying the initial size of the parser stack. -@xref{Stack Overflow}. +@deffn {Statement} {return YYFAIL;} +Print an error message and start error recovery. +@xref{Error Recovery}. +@end deffn -@item YYLTYPE -Macro for the data type of @code{yylloc}; a structure with four -members. @xref{Token Positions, ,Textual Positions of Tokens}. +@deftypefn {Function} {boolean} recovering () +Return whether error recovery is being done. In this state, the parser +reads token until it reaches a known state, and then restarts normal +operation. +@xref{Error Recovery}. +@end deftypefn -@item YYMAXDEPTH -Macro for specifying the maximum size of the parser stack. -@xref{Stack Overflow}. +@deftypefn {Function} {void} yyerror (String @var{msg}) +@deftypefnx {Function} {void} yyerror (Position @var{loc}, String @var{msg}) +@deftypefnx {Function} {void} yyerror (Location @var{loc}, String @var{msg}) +Print an error message using the @code{yyerror} method of the scanner +instance in use. The @code{Location} and @code{Position} parameters are +available only if location tracking is active. +@end deftypefn -@item YYRECOVERING -Macro whose value indicates whether the parser is recovering from a -syntax error. @xref{Action Features, ,Special Features for Use in Actions}. -@item YYSTYPE -Macro for the data type of semantic values; @code{int} by default. -@xref{Value Type, ,Data Types of Semantic Values}. +@node Java Differences +@subsection Differences between C/C++ and Java Grammars -@item yychar -External integer variable that contains the integer value of the -current look-ahead token. (In a pure parser, it is a local variable -within @code{yyparse}.) Error-recovery rule actions may examine this -variable. @xref{Action Features, ,Special Features for Use in Actions}. +The different structure of the Java language forces several differences +between C/C++ grammars, and grammars designed for Java parsers. This +section summarizes these differences. -@item yyclearin -Macro used in error-recovery rule actions. It clears the previous -look-ahead token. @xref{Error Recovery}. +@itemize +@item +Java lacks a preprocessor, so the @code{YYERROR}, @code{YYACCEPT}, +@code{YYABORT} symbols (@pxref{Table of Symbols}) cannot obviously be +macros. Instead, they should be preceded by @code{return} when they +appear in an action. The actual definition of these symbols is +opaque to the Bison grammar, and it might change in the future. The +only meaningful operation that you can do, is to return them. +See @pxref{Java Action Features}. + +Note that of these three symbols, only @code{YYACCEPT} and +@code{YYABORT} will cause a return from the @code{yyparse} +method@footnote{Java parsers include the actions in a separate +method than @code{yyparse} in order to have an intuitive syntax that +corresponds to these C macros.}. -@item yydebug -External integer variable set to zero by default. If @code{yydebug} -is given a nonzero value, the parser will output information on input -symbols and parser action. @xref{Debugging, ,Debugging Your Parser}. +@item +Java lacks unions, so @code{%union} has no effect. Instead, semantic +values have a common base type: @code{Object} or as specified by +@code{%define stype}. Angle backets on @code{%token}, @code{type}, +@code{$@var{n}} and @code{$$} specify subtypes rather than fields of +an union. The type of @code{$$}, even with angle brackets, is the base +type since Java casts are not allow on the left-hand side of assignments. +Also, @code{$@var{n}} and @code{@@@var{n}} are not allowed on the +left-hand side of assignments. See @pxref{Java Semantic Values} and +@pxref{Java Action Features}. -@item yyerrok -Macro to cause parser to recover immediately to its normal mode -after a parse error. @xref{Error Recovery}. +@item +The prolog declarations have a different meaning than in C/C++ code. +@table @asis +@item @code{%code imports} +blocks are placed at the beginning of the Java source code. They may +include copyright notices. For a @code{package} declarations, it is +suggested to use @code{%define package} instead. + +@item unqualified @code{%code} +blocks are placed inside the parser class. + +@item @code{%code lexer} +blocks, if specified, should include the implementation of the +scanner. If there is no such block, the scanner can be any class +that implements the appropriate interface (see @pxref{Java Scanner +Interface}). +@end table -@item yyerror -User-supplied function to be called by @code{yyparse} on error. The -function receives one argument, a pointer to a character string -containing an error message. @xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +Other @code{%code} blocks are not supported in Java parsers. +In particular, @code{%@{ @dots{} %@}} blocks should not be used +and may give an error in future versions of Bison. -@item yylex -User-supplied lexical analyzer function, called with no arguments -to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}. +The epilogue has the same meaning as in C/C++ code and it can +be used to define other classes used by the parser @emph{outside} +the parser class. +@end itemize -@item yylval -External variable in which @code{yylex} should place the semantic -value associated with a token. (In a pure parser, it is a local -variable within @code{yyparse}, and its address is passed to -@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}. -@item yylloc -External variable in which @code{yylex} should place the line and -column numbers associated with a token. (In a pure parser, it is a -local variable within @code{yyparse}, and its address is passed to -@code{yylex}.) You can ignore this variable if you don't use the -@samp{@@} feature in the grammar actions. @xref{Token Positions, ,Textual Positions of Tokens}. +@node Java Declarations Summary +@subsection Java Declarations Summary + +This summary only include declarations specific to Java or have special +meaning when used in a Java parser. + +@deffn {Directive} {%language "Java"} +Generate a Java class for the parser. +@end deffn + +@deffn {Directive} %lex-param @{@var{type} @var{name}@} +A parameter for the lexer class defined by @code{%code lexer} +@emph{only}, added as parameters to the lexer constructor and the parser +constructor that @emph{creates} a lexer. Default is none. +@xref{Java Scanner Interface}. +@end deffn + +@deffn {Directive} %name-prefix "@var{prefix}" +The prefix of the parser class name @code{@var{prefix}Parser} if +@code{%define parser_class_name} is not used. Default is @code{YY}. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} %parse-param @{@var{type} @var{name}@} +A parameter for the parser class added as parameters to constructor(s) +and as fields initialized by the constructor(s). Default is none. +@xref{Java Parser Interface}. +@end deffn + +@deffn {Directive} %token <@var{type}> @var{token} @dots{} +Declare tokens. Note that the angle brackets enclose a Java @emph{type}. +@xref{Java Semantic Values}. +@end deffn + +@deffn {Directive} %type <@var{type}> @var{nonterminal} @dots{} +Declare the type of nonterminals. Note that the angle brackets enclose +a Java @emph{type}. +@xref{Java Semantic Values}. +@end deffn + +@deffn {Directive} %code @{ @var{code} @dots{} @} +Code appended to the inside of the parser class. +@xref{Java Differences}. +@end deffn + +@deffn {Directive} {%code imports} @{ @var{code} @dots{} @} +Code inserted just after the @code{package} declaration. +@xref{Java Differences}. +@end deffn + +@deffn {Directive} {%code init} @{ @var{code} @dots{} @} +Code inserted at the beginning of the parser constructor body. +@xref{Java Parser Interface}. +@end deffn + +@deffn {Directive} {%code lexer} @{ @var{code} @dots{} @} +Code added to the body of a inner lexer class within the parser class. +@xref{Java Scanner Interface}. +@end deffn + +@deffn {Directive} %% @var{code} @dots{} +Code (after the second @code{%%}) appended to the end of the file, +@emph{outside} the parser class. +@xref{Java Differences}. +@end deffn + +@deffn {Directive} %@{ @var{code} @dots{} %@} +Not supported. Use @code{%code imports} instead. +@xref{Java Differences}. +@end deffn + +@deffn {Directive} {%define abstract} +Whether the parser class is declared @code{abstract}. Default is false. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define annotations} "@var{annotations}" +The Java annotations for the parser class. Default is none. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define extends} "@var{superclass}" +The superclass of the parser class. Default is none. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define final} +Whether the parser class is declared @code{final}. Default is false. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define implements} "@var{interfaces}" +The implemented interfaces of the parser class, a comma-separated list. +Default is none. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define init_throws} "@var{exceptions}" +The exceptions thrown by @code{%code init} from the parser class +constructor. Default is none. +@xref{Java Parser Interface}. +@end deffn + +@deffn {Directive} {%define lex_throws} "@var{exceptions}" +The exceptions thrown by the @code{yylex} method of the lexer, a +comma-separated list. Default is @code{java.io.IOException}. +@xref{Java Scanner Interface}. +@end deffn + +@deffn {Directive} {%define location_type} "@var{class}" +The name of the class used for locations (a range between two +positions). This class is generated as an inner class of the parser +class by @command{bison}. Default is @code{Location}. +@xref{Java Location Values}. +@end deffn + +@deffn {Directive} {%define package} "@var{package}" +The package to put the parser class in. Default is none. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define parser_class_name} "@var{name}" +The name of the parser class. Default is @code{YYParser} or +@code{@var{name-prefix}Parser}. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define position_type} "@var{class}" +The name of the class used for positions. This class must be supplied by +the user. Default is @code{Position}. +@xref{Java Location Values}. +@end deffn + +@deffn {Directive} {%define public} +Whether the parser class is declared @code{public}. Default is false. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define stype} "@var{class}" +The base type of semantic values. Default is @code{Object}. +@xref{Java Semantic Values}. +@end deffn + +@deffn {Directive} {%define strictfp} +Whether the parser class is declared @code{strictfp}. Default is false. +@xref{Java Bison Interface}. +@end deffn + +@deffn {Directive} {%define throws} "@var{exceptions}" +The exceptions thrown by user-supplied parser actions and +@code{%initial-action}, a comma-separated list. Default is none. +@xref{Java Parser Interface}. +@end deffn + + +@c ================================================= FAQ + +@node FAQ +@chapter Frequently Asked Questions +@cindex frequently asked questions +@cindex questions + +Several questions about Bison come up occasionally. Here some of them +are addressed. -@item yynerrs -Global variable which Bison increments each time there is a parse -error. (In a pure parser, it is a local variable within -@code{yyparse}.) @xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@menu +* Memory Exhausted:: Breaking the Stack Limits +* How Can I Reset the Parser:: @code{yyparse} Keeps some State +* Strings are Destroyed:: @code{yylval} Loses Track of Strings +* Implementing Gotos/Loops:: Control Flow in the Calculator +* Multiple start-symbols:: Factoring closely related grammars +* Secure? Conform?:: Is Bison @acronym{POSIX} safe? +* I can't build Bison:: Troubleshooting +* Where can I find help?:: Troubleshouting +* Bug Reports:: Troublereporting +* More Languages:: Parsers in C++, Java, and so on +* Beta Testing:: Experimenting development versions +* Mailing Lists:: Meeting other Bison users +@end menu -@item yyparse -The parser function produced by Bison; call this function to start -parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@node Memory Exhausted +@section Memory Exhausted + +@display +My parser returns with error with a @samp{memory exhausted} +message. What can I do? +@end display + +This question is already addressed elsewhere, @xref{Recursion, +,Recursive Rules}. + +@node How Can I Reset the Parser +@section How Can I Reset the Parser + +The following phenomenon has several symptoms, resulting in the +following typical questions: + +@display +I invoke @code{yyparse} several times, and on correct input it works +properly; but when a parse error is found, all the other calls fail +too. How can I reset the error flag of @code{yyparse}? +@end display + +@noindent +or + +@display +My parser includes support for an @samp{#include}-like feature, in +which case I run @code{yyparse} from @code{yyparse}. This fails +although I did specify @code{%define api.pure}. +@end display + +These problems typically come not from Bison itself, but from +Lex-generated scanners. Because these scanners use large buffers for +speed, they might not notice a change of input file. As a +demonstration, consider the following source file, +@file{first-line.l}: + +@verbatim +%{ +#include +#include +%} +%% +.*\n ECHO; return 1; +%% +int +yyparse (char const *file) +{ + yyin = fopen (file, "r"); + if (!yyin) + exit (2); + /* One token only. */ + yylex (); + if (fclose (yyin) != 0) + exit (3); + return 0; +} + +int +main (void) +{ + yyparse ("input"); + yyparse ("input"); + return 0; +} +@end verbatim + +@noindent +If the file @file{input} contains + +@verbatim +input:1: Hello, +input:2: World! +@end verbatim + +@noindent +then instead of getting the first line twice, you get: + +@example +$ @kbd{flex -ofirst-line.c first-line.l} +$ @kbd{gcc -ofirst-line first-line.c -ll} +$ @kbd{./first-line} +input:1: Hello, +input:2: World! +@end example + +Therefore, whenever you change @code{yyin}, you must tell the +Lex-generated scanner to discard its current buffer and switch to the +new one. This depends upon your implementation of Lex; see its +documentation for more. For Flex, it suffices to call +@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}. If your +Flex-generated scanner needs to read from several input streams to +handle features like include files, you might consider using Flex +functions like @samp{yy_switch_to_buffer} that manipulate multiple +input buffers. + +If your Flex-generated scanner uses start conditions (@pxref{Start +conditions, , Start conditions, flex, The Flex Manual}), you might +also want to reset the scanner's state, i.e., go back to the initial +start condition, through a call to @samp{BEGIN (0)}. + +@node Strings are Destroyed +@section Strings are Destroyed + +@display +My parser seems to destroy old strings, or maybe it loses track of +them. Instead of reporting @samp{"foo", "bar"}, it reports +@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}. +@end display + +This error is probably the single most frequent ``bug report'' sent to +Bison lists, but is only concerned with a misunderstanding of the role +of the scanner. Consider the following Lex code: + +@verbatim +%{ +#include +char *yylval = NULL; +%} +%% +.* yylval = yytext; return 1; +\n /* IGNORE */ +%% +int +main () +{ + /* Similar to using $1, $2 in a Bison action. */ + char *fst = (yylex (), yylval); + char *snd = (yylex (), yylval); + printf ("\"%s\", \"%s\"\n", fst, snd); + return 0; +} +@end verbatim + +If you compile and run this code, you get: + +@example +$ @kbd{flex -osplit-lines.c split-lines.l} +$ @kbd{gcc -osplit-lines split-lines.c -ll} +$ @kbd{printf 'one\ntwo\n' | ./split-lines} +"one +two", "two" +@end example + +@noindent +this is because @code{yytext} is a buffer provided for @emph{reading} +in the action, but if you want to keep it, you have to duplicate it +(e.g., using @code{strdup}). Note that the output may depend on how +your implementation of Lex handles @code{yytext}. For instance, when +given the Lex compatibility option @option{-l} (which triggers the +option @samp{%array}) Flex generates a different behavior: + +@example +$ @kbd{flex -l -osplit-lines.c split-lines.l} +$ @kbd{gcc -osplit-lines split-lines.c -ll} +$ @kbd{printf 'one\ntwo\n' | ./split-lines} +"two", "two" +@end example + + +@node Implementing Gotos/Loops +@section Implementing Gotos/Loops + +@display +My simple calculator supports variables, assignments, and functions, +but how can I implement gotos, or loops? +@end display + +Although very pedagogical, the examples included in the document blur +the distinction to make between the parser---whose job is to recover +the structure of a text and to transmit it to subsequent modules of +the program---and the processing (such as the execution) of this +structure. This works well with so called straight line programs, +i.e., precisely those that have a straightforward execution model: +execute simple instructions one after the others. + +@cindex abstract syntax tree +@cindex @acronym{AST} +If you want a richer model, you will probably need to use the parser +to construct a tree that does represent the structure it has +recovered; this tree is usually called the @dfn{abstract syntax tree}, +or @dfn{@acronym{AST}} for short. Then, walking through this tree, +traversing it in various ways, will enable treatments such as its +execution or its translation, which will result in an interpreter or a +compiler. + +This topic is way beyond the scope of this manual, and the reader is +invited to consult the dedicated literature. + + +@node Multiple start-symbols +@section Multiple start-symbols + +@display +I have several closely related grammars, and I would like to share their +implementations. In fact, I could use a single grammar but with +multiple entry points. +@end display + +Bison does not support multiple start-symbols, but there is a very +simple means to simulate them. If @code{foo} and @code{bar} are the two +pseudo start-symbols, then introduce two new tokens, say +@code{START_FOO} and @code{START_BAR}, and use them as switches from the +real start-symbol: + +@example +%token START_FOO START_BAR; +%start start; +start: START_FOO foo + | START_BAR bar; +@end example + +These tokens prevents the introduction of new conflicts. As far as the +parser goes, that is all that is needed. + +Now the difficult part is ensuring that the scanner will send these +tokens first. If your scanner is hand-written, that should be +straightforward. If your scanner is generated by Lex, them there is +simple means to do it: recall that anything between @samp{%@{ ... %@}} +after the first @code{%%} is copied verbatim in the top of the generated +@code{yylex} function. Make sure a variable @code{start_token} is +available in the scanner (e.g., a global variable or using +@code{%lex-param} etc.), and use the following: + +@example + /* @r{Prologue.} */ +%% +%@{ + if (start_token) + @{ + int t = start_token; + start_token = 0; + return t; + @} +%@} + /* @r{The rules.} */ +@end example + + +@node Secure? Conform? +@section Secure? Conform? + +@display +Is Bison secure? Does it conform to POSIX? +@end display + +If you're looking for a guarantee or certification, we don't provide it. +However, Bison is intended to be a reliable program that conforms to the +@acronym{POSIX} specification for Yacc. If you run into problems, +please send us a bug report. + +@node I can't build Bison +@section I can't build Bison + +@display +I can't build Bison because @command{make} complains that +@code{msgfmt} is not found. +What should I do? +@end display + +Like most GNU packages with internationalization support, that feature +is turned on by default. If you have problems building in the @file{po} +subdirectory, it indicates that your system's internationalization +support is lacking. You can re-configure Bison with +@option{--disable-nls} to turn off this support, or you can install GNU +gettext from @url{ftp://ftp.gnu.org/gnu/gettext/} and re-configure +Bison. See the file @file{ABOUT-NLS} for more information. + + +@node Where can I find help? +@section Where can I find help? -@item %left -Bison declaration to assign left associativity to token(s). +@display +I'm having trouble using Bison. Where can I find help? +@end display + +First, read this fine manual. Beyond that, you can send mail to +@email{help-bison@@gnu.org}. This mailing list is intended to be +populated with people who are willing to answer questions about using +and installing Bison. Please keep in mind that (most of) the people on +the list have aspects of their lives which are not related to Bison (!), +so you may not receive an answer to your question right away. This can +be frustrating, but please try not to honk them off; remember that any +help they provide is purely voluntary and out of the kindness of their +hearts. + +@node Bug Reports +@section Bug Reports + +@display +I found a bug. What should I include in the bug report? +@end display + +Before you send a bug report, make sure you are using the latest +version. Check @url{ftp://ftp.gnu.org/pub/gnu/bison/} or one of its +mirrors. Be sure to include the version number in your bug report. If +the bug is present in the latest version but not in a previous version, +try to determine the most recent version which did not contain the bug. + +If the bug is parser-related, you should include the smallest grammar +you can which demonstrates the bug. The grammar file should also be +complete (i.e., I should be able to run it through Bison without having +to edit or add anything). The smaller and simpler the grammar, the +easier it will be to fix the bug. + +Include information about your compilation environment, including your +operating system's name and version and your compiler's name and +version. If you have trouble compiling, you should also include a +transcript of the build session, starting with the invocation of +`configure'. Depending on the nature of the bug, you may be asked to +send additional files as well (such as `config.h' or `config.cache'). + +Patches are most welcome, but not required. That is, do not hesitate to +send a bug report just because you can not provide a fix. + +Send bug reports to @email{bug-bison@@gnu.org}. + +@node More Languages +@section More Languages + +@display +Will Bison ever have C++ and Java support? How about @var{insert your +favorite language here}? +@end display + +C++ and Java support is there now, and is documented. We'd love to add other +languages; contributions are welcome. + +@node Beta Testing +@section Beta Testing + +@display +What is involved in being a beta tester? +@end display + +It's not terribly involved. Basically, you would download a test +release, compile it, and use it to build and run a parser or two. After +that, you would submit either a bug report or a message saying that +everything is okay. It is important to report successes as well as +failures because test releases eventually become mainstream releases, +but only if they are adequately tested. If no one tests, development is +essentially halted. + +Beta testers are particularly needed for operating systems to which the +developers do not have easy access. They currently have easy access to +recent GNU/Linux and Solaris versions. Reports about other operating +systems are especially welcome. + +@node Mailing Lists +@section Mailing Lists + +@display +How do I join the help-bison and bug-bison mailing lists? +@end display + +See @url{http://lists.gnu.org/}. + +@c ================================================= Table of Symbols + +@node Table of Symbols +@appendix Bison Symbols +@cindex Bison symbols, table of +@cindex symbols in Bison, table of + +@deffn {Variable} @@$ +In an action, the location of the left-hand side of the rule. +@xref{Locations, , Locations Overview}. +@end deffn + +@deffn {Variable} @@@var{n} +In an action, the location of the @var{n}-th symbol of the right-hand +side of the rule. @xref{Locations, , Locations Overview}. +@end deffn + +@deffn {Variable} $$ +In an action, the semantic value of the left-hand side of the rule. +@xref{Actions}. +@end deffn + +@deffn {Variable} $@var{n} +In an action, the semantic value of the @var{n}-th symbol of the +right-hand side of the rule. @xref{Actions}. +@end deffn + +@deffn {Delimiter} %% +Delimiter used to separate the grammar rule section from the +Bison declarations section or the epilogue. +@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. +@end deffn + +@c Don't insert spaces, or check the DVI output. +@deffn {Delimiter} %@{@var{code}%@} +All code listed between @samp{%@{} and @samp{%@}} is copied directly to +the output file uninterpreted. Such code forms the prologue of the input +file. @xref{Grammar Outline, ,Outline of a Bison +Grammar}. +@end deffn + +@deffn {Construct} /*@dots{}*/ +Comment delimiters, as in C. +@end deffn + +@deffn {Delimiter} : +Separates a rule's result from its components. @xref{Rules, ,Syntax of +Grammar Rules}. +@end deffn + +@deffn {Delimiter} ; +Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. +@end deffn + +@deffn {Delimiter} | +Separates alternate rules for the same result nonterminal. +@xref{Rules, ,Syntax of Grammar Rules}. +@end deffn + +@deffn {Directive} <*> +Used to define a default tagged @code{%destructor} or default tagged +@code{%printer}. + +This feature is experimental. +More user feedback will help to determine whether it should become a permanent +feature. + +@xref{Destructor Decl, , Freeing Discarded Symbols}. +@end deffn + +@deffn {Directive} <> +Used to define a default tagless @code{%destructor} or default tagless +@code{%printer}. + +This feature is experimental. +More user feedback will help to determine whether it should become a permanent +feature. + +@xref{Destructor Decl, , Freeing Discarded Symbols}. +@end deffn + +@deffn {Symbol} $accept +The predefined nonterminal whose only rule is @samp{$accept: @var{start} +$end}, where @var{start} is the start symbol. @xref{Start Decl, , The +Start-Symbol}. It cannot be used in the grammar. +@end deffn + +@deffn {Directive} %code @{@var{code}@} +@deffnx {Directive} %code @var{qualifier} @{@var{code}@} +Insert @var{code} verbatim into output parser source. +@xref{Decl Summary,,%code}. +@end deffn + +@deffn {Directive} %debug +Equip the parser for debugging. @xref{Decl Summary}. +@end deffn + +@deffn {Directive} %debug +Equip the parser for debugging. @xref{Decl Summary}. +@end deffn + +@ifset defaultprec +@deffn {Directive} %default-prec +Assign a precedence to rules that lack an explicit @samp{%prec} +modifier. @xref{Contextual Precedence, ,Context-Dependent +Precedence}. +@end deffn +@end ifset + +@deffn {Directive} %define @var{define-variable} +@deffnx {Directive} %define @var{define-variable} @var{value} +Define a variable to adjust Bison's behavior. +@xref{Decl Summary,,%define}. +@end deffn + +@deffn {Directive} %defines +Bison declaration to create a header file meant for the scanner. +@xref{Decl Summary}. +@end deffn + +@deffn {Directive} %defines @var{defines-file} +Same as above, but save in the file @var{defines-file}. +@xref{Decl Summary}. +@end deffn + +@deffn {Directive} %destructor +Specify how the parser should reclaim the memory associated to +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +@end deffn + +@deffn {Directive} %dprec +Bison declaration to assign a precedence to a rule that is used at parse +time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing +@acronym{GLR} Parsers}. +@end deffn + +@deffn {Symbol} $end +The predefined token marking the end of the token stream. It cannot be +used in the grammar. +@end deffn + +@deffn {Symbol} error +A token name reserved for error recovery. This token may be used in +grammar rules so as to allow the Bison parser to recognize an error in +the grammar without halting the process. In effect, a sentence +containing an error may be recognized as valid. On a syntax error, the +token @code{error} becomes the current lookahead token. Actions +corresponding to @code{error} are then executed, and the lookahead +token is reset to the token that originally caused the violation. +@xref{Error Recovery}. +@end deffn + +@deffn {Directive} %error-verbose +Bison declaration to request verbose, specific error message strings +when @code{yyerror} is called. +@end deffn + +@deffn {Directive} %file-prefix "@var{prefix}" +Bison declaration to set the prefix of the output files. @xref{Decl +Summary}. +@end deffn + +@deffn {Directive} %glr-parser +Bison declaration to produce a @acronym{GLR} parser. @xref{GLR +Parsers, ,Writing @acronym{GLR} Parsers}. +@end deffn + +@deffn {Directive} %initial-action +Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}. +@end deffn + +@deffn {Directive} %language +Specify the programming language for the generated parser. +@xref{Decl Summary}. +@end deffn + +@deffn {Directive} %left +Bison declaration to assign precedence and left associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. +@end deffn + +@deffn {Directive} %lex-param @{@var{argument-declaration}@} +Bison declaration to specifying an additional parameter that +@code{yylex} should accept. @xref{Pure Calling,, Calling Conventions +for Pure Parsers}. +@end deffn + +@deffn {Directive} %merge +Bison declaration to assign a merging function to a rule. If there is a +reduce/reduce conflict with a rule having the same merging function, the +function is applied to the two semantic values to get a single result. +@xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}. +@end deffn + +@deffn {Directive} %name-prefix "@var{prefix}" +Bison declaration to rename the external symbols. @xref{Decl Summary}. +@end deffn + +@ifset defaultprec +@deffn {Directive} %no-default-prec +Do not assign a precedence to rules that lack an explicit @samp{%prec} +modifier. @xref{Contextual Precedence, ,Context-Dependent +Precedence}. +@end deffn +@end ifset -@item %nonassoc -Bison declaration to assign nonassociativity to token(s). +@deffn {Directive} %no-lines +Bison declaration to avoid generating @code{#line} directives in the +parser file. @xref{Decl Summary}. +@end deffn + +@deffn {Directive} %nonassoc +Bison declaration to assign precedence and nonassociativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. +@end deffn + +@deffn {Directive} %output "@var{file}" +Bison declaration to set the name of the parser file. @xref{Decl +Summary}. +@end deffn -@item %prec +@deffn {Directive} %parse-param @{@var{argument-declaration}@} +Bison declaration to specifying an additional parameter that +@code{yyparse} should accept. @xref{Parser Function,, The Parser +Function @code{yyparse}}. +@end deffn + +@deffn {Directive} %prec Bison declaration to assign a precedence to a specific rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}. +@end deffn -@item %pure_parser -Bison declaration to request a pure (reentrant) parser. -@xref{Pure Decl, ,A Pure (Reentrant) Parser}. +@deffn {Directive} %precedence +Bison declaration to assign precedence to token(s), but no associativity +@xref{Precedence Decl, ,Operator Precedence}. +@end deffn + +@deffn {Directive} %pure-parser +Deprecated version of @code{%define api.pure} (@pxref{Decl Summary, ,%define}), +for which Bison is more careful to warn about unreasonable usage. +@end deffn -@item %right -Bison declaration to assign right associativity to token(s). +@deffn {Directive} %require "@var{version}" +Require version @var{version} or higher of Bison. @xref{Require Decl, , +Require a Version of Bison}. +@end deffn + +@deffn {Directive} %right +Bison declaration to assign precedence and right associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. +@end deffn + +@deffn {Directive} %skeleton +Specify the skeleton to use; usually for development. +@xref{Decl Summary}. +@end deffn -@item %start -Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}. +@deffn {Directive} %start +Bison declaration to specify the start symbol. @xref{Start Decl, ,The +Start-Symbol}. +@end deffn -@item %token +@deffn {Directive} %token Bison declaration to declare token(s) without specifying precedence. @xref{Token Decl, ,Token Type Names}. +@end deffn + +@deffn {Directive} %token-table +Bison declaration to include a token name table in the parser file. +@xref{Decl Summary}. +@end deffn + +@deffn {Directive} %type +Bison declaration to declare nonterminals. @xref{Type Decl, +,Nonterminal Symbols}. +@end deffn -@item %type -Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}. +@deffn {Symbol} $undefined +The predefined token onto which all undefined values returned by +@code{yylex} are mapped. It cannot be used in the grammar, rather, use +@code{error}. +@end deffn -@item %union +@deffn {Directive} %union Bison declaration to specify several possible data types for semantic values. @xref{Union Decl, ,The Collection of Value Types}. -@end table +@end deffn -These are the punctuation and delimiters used in Bison input: +@deffn {Macro} YYABORT +Macro to pretend that an unrecoverable syntax error has occurred, by +making @code{yyparse} return 1 immediately. The error reporting +function @code{yyerror} is not called. @xref{Parser Function, ,The +Parser Function @code{yyparse}}. -@table @samp -@item %% -Delimiter used to separate the grammar rule section from the -Bison declarations section or the additional C code section. -@xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. +For Java parsers, this functionality is invoked using @code{return YYABORT;} +instead. +@end deffn -@item %@{ %@} -All code listed between @samp{%@{} and @samp{%@}} is copied directly -to the output file uninterpreted. Such code forms the ``C -declarations'' section of the input file. @xref{Grammar Outline, ,Outline of a Bison Grammar}. +@deffn {Macro} YYACCEPT +Macro to pretend that a complete utterance of the language has been +read, by making @code{yyparse} return 0 immediately. +@xref{Parser Function, ,The Parser Function @code{yyparse}}. -@item /*@dots{}*/ -Comment delimiters, as in C. +For Java parsers, this functionality is invoked using @code{return YYACCEPT;} +instead. +@end deffn -@item : -Separates a rule's result from its components. @xref{Rules, ,Syntax of Grammar Rules}. +@deffn {Macro} YYBACKUP +Macro to discard a value from the parser stack and fake a lookahead +token. @xref{Action Features, ,Special Features for Use in Actions}. +@end deffn -@item ; -Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. +@deffn {Variable} yychar +External integer variable that contains the integer value of the +lookahead token. (In a pure parser, it is a local variable within +@code{yyparse}.) Error-recovery rule actions may examine this variable. +@xref{Action Features, ,Special Features for Use in Actions}. +@end deffn -@item | -Separates alternate rules for the same result nonterminal. -@xref{Rules, ,Syntax of Grammar Rules}. -@end table +@deffn {Variable} yyclearin +Macro used in error-recovery rule actions. It clears the previous +lookahead token. @xref{Error Recovery}. +@end deffn + +@deffn {Macro} YYDEBUG +Macro to define to equip the parser with tracing code. @xref{Tracing, +,Tracing Your Parser}. +@end deffn + +@deffn {Variable} yydebug +External integer variable set to zero by default. If @code{yydebug} +is given a nonzero value, the parser will output information on input +symbols and parser action. @xref{Tracing, ,Tracing Your Parser}. +@end deffn + +@deffn {Macro} yyerrok +Macro to cause parser to recover immediately to its normal mode +after a syntax error. @xref{Error Recovery}. +@end deffn + +@deffn {Macro} YYERROR +Macro to pretend that a syntax error has just been detected: call +@code{yyerror} and then perform normal error recovery if possible +(@pxref{Error Recovery}), or (if recovery is impossible) make +@code{yyparse} return 1. @xref{Error Recovery}. + +For Java parsers, this functionality is invoked using @code{return YYERROR;} +instead. +@end deffn + +@deffn {Function} yyerror +User-supplied function to be called by @code{yyparse} on error. +@xref{Error Reporting, ,The Error +Reporting Function @code{yyerror}}. +@end deffn + +@deffn {Macro} YYERROR_VERBOSE +An obsolete macro that you define with @code{#define} in the prologue +to request verbose, specific error message strings +when @code{yyerror} is called. It doesn't matter what definition you +use for @code{YYERROR_VERBOSE}, just whether you define it. Using +@code{%error-verbose} is preferred. +@end deffn + +@deffn {Macro} YYINITDEPTH +Macro for specifying the initial size of the parser stack. +@xref{Memory Management}. +@end deffn + +@deffn {Function} yylex +User-supplied lexical analyzer function, called with no arguments to get +the next token. @xref{Lexical, ,The Lexical Analyzer Function +@code{yylex}}. +@end deffn + +@deffn {Macro} YYLEX_PARAM +An obsolete macro for specifying an extra argument (or list of extra +arguments) for @code{yyparse} to pass to @code{yylex}. The use of this +macro is deprecated, and is supported only for Yacc like parsers. +@xref{Pure Calling,, Calling Conventions for Pure Parsers}. +@end deffn + +@deffn {Variable} yylloc +External variable in which @code{yylex} should place the line and column +numbers associated with a token. (In a pure parser, it is a local +variable within @code{yyparse}, and its address is passed to +@code{yylex}.) +You can ignore this variable if you don't use the @samp{@@} feature in the +grammar actions. +@xref{Token Locations, ,Textual Locations of Tokens}. +In semantic actions, it stores the location of the lookahead token. +@xref{Actions and Locations, ,Actions and Locations}. +@end deffn + +@deffn {Type} YYLTYPE +Data type of @code{yylloc}; by default, a structure with four +members. @xref{Location Type, , Data Types of Locations}. +@end deffn + +@deffn {Variable} yylval +External variable in which @code{yylex} should place the semantic +value associated with a token. (In a pure parser, it is a local +variable within @code{yyparse}, and its address is passed to +@code{yylex}.) +@xref{Token Values, ,Semantic Values of Tokens}. +In semantic actions, it stores the semantic value of the lookahead token. +@xref{Actions, ,Actions}. +@end deffn + +@deffn {Macro} YYMAXDEPTH +Macro for specifying the maximum size of the parser stack. @xref{Memory +Management}. +@end deffn + +@deffn {Variable} yynerrs +Global variable which Bison increments each time it reports a syntax error. +(In a pure parser, it is a local variable within @code{yyparse}. In a +pure push parser, it is a member of yypstate.) +@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@end deffn + +@deffn {Function} yyparse +The parser function produced by Bison; call this function to start +parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn + +@deffn {Function} yypstate_delete +The function to delete a parser instance, produced by Bison in push mode; +call this function to delete the memory associated with a parser. +@xref{Parser Delete Function, ,The Parser Delete Function +@code{yypstate_delete}}. +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) +@end deffn + +@deffn {Function} yypstate_new +The function to create a parser instance, produced by Bison in push mode; +call this function to create a new parser. +@xref{Parser Create Function, ,The Parser Create Function +@code{yypstate_new}}. +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) +@end deffn + +@deffn {Function} yypull_parse +The parser function produced by Bison in push mode; call this function to +parse the rest of the input stream. +@xref{Pull Parser Function, ,The Pull Parser Function +@code{yypull_parse}}. +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) +@end deffn + +@deffn {Function} yypush_parse +The parser function produced by Bison in push mode; call this function to +parse a single token. @xref{Push Parser Function, ,The Push Parser Function +@code{yypush_parse}}. +(The current push parsing interface is experimental and may evolve. +More user feedback will help to stabilize it.) +@end deffn + +@deffn {Macro} YYPARSE_PARAM +An obsolete macro for specifying the name of a parameter that +@code{yyparse} should accept. The use of this macro is deprecated, and +is supported only for Yacc like parsers. @xref{Pure Calling,, Calling +Conventions for Pure Parsers}. +@end deffn + +@deffn {Macro} YYRECOVERING +The expression @code{YYRECOVERING ()} yields 1 when the parser +is recovering from a syntax error, and 0 otherwise. +@xref{Action Features, ,Special Features for Use in Actions}. +@end deffn + +@deffn {Macro} YYSTACK_USE_ALLOCA +Macro used to control the use of @code{alloca} when the C +@acronym{LALR}(1) parser needs to extend its stacks. If defined to 0, +the parser will use @code{malloc} to extend its stacks. If defined to +1, the parser will use @code{alloca}. Values other than 0 and 1 are +reserved for future Bison extensions. If not defined, +@code{YYSTACK_USE_ALLOCA} defaults to 0. + +In the all-too-common case where your code may run on a host with a +limited stack and with unreliable stack-overflow checking, you should +set @code{YYMAXDEPTH} to a value that cannot possibly result in +unchecked stack overflow on any of your target hosts when +@code{alloca} is called. You can inspect the code that Bison +generates in order to determine the proper numeric values. This will +require some expertise in low-level implementation details. +@end deffn + +@deffn {Type} YYSTYPE +Data type of semantic values; @code{int} by default. +@xref{Value Type, ,Data Types of Semantic Values}. +@end deffn -@node Glossary, Index, Table of Symbols, Top +@node Glossary @appendix Glossary @cindex glossary @table @asis -@item Backus-Naur Form (BNF) -Formal method of specifying context-free grammars. BNF was first used -in the @cite{ALGOL-60} report, 1963. @xref{Language and Grammar, ,Languages and Context-Free Grammars}. +@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'') +Formal method of specifying context-free grammars originally proposed +by John Backus, and slightly improved by Peter Naur in his 1960-01-02 +committee document contributing to what became the Algol 60 report. +@xref{Language and Grammar, ,Languages and Context-Free Grammars}. @item Context-free grammars Grammars specified as rules that can be applied regardless of context. Thus, if there is a rule which says that an integer can be used as an expression, integers are allowed @emph{anywhere} an expression is -permitted. @xref{Language and Grammar, ,Languages and Context-Free Grammars}. +permitted. @xref{Language and Grammar, ,Languages and Context-Free +Grammars}. @item Dynamic allocation Allocation of memory that occurs during execution, rather than at @@ -4997,11 +10327,20 @@ each instant in time. As input to the machine is processed, the machine moves from state to state as specified by the logic of the machine. In the case of the parser, the input is the language being parsed, and the states correspond to various stages in the grammar -rules. @xref{Algorithm, ,The Bison Parser Algorithm }. +rules. @xref{Algorithm, ,The Bison Parser Algorithm}. + +@item Generalized @acronym{LR} (@acronym{GLR}) +A parsing algorithm that can handle all context-free grammars, including those +that are not @acronym{LALR}(1). It resolves situations that Bison's +usual @acronym{LALR}(1) +algorithm cannot by effectively splitting off multiple parsers, trying all +possible parsers, and discarding those that fail in the light of additional +right context. @xref{Generalized LR Parsing, ,Generalized +@acronym{LR} Parsing}. @item Grouping A language construct that is (in general) grammatically divisible; -for example, `expression' or `declaration' in C. +for example, `expression' or `declaration' in C@. @xref{Language and Grammar, ,Languages and Context-Free Grammars}. @item Infix operator @@ -5022,12 +10361,13 @@ Operators having left associativity are analyzed from left to right: @samp{c}. @xref{Precedence, ,Operator Precedence}. @item Left recursion -A rule whose result symbol is also its first component symbol; -for example, @samp{expseq1 : expseq1 ',' exp;}. @xref{Recursion, ,Recursive Rules}. +A rule whose result symbol is also its first component symbol; for +example, @samp{expseq1 : expseq1 ',' exp;}. @xref{Recursion, ,Recursive +Rules}. @item Left-to-right parsing Parsing a sentence of a language by analyzing it token by token from -left to right. @xref{Algorithm, ,The Bison Parser Algorithm }. +left to right. @xref{Algorithm, ,The Bison Parser Algorithm}. @item Lexical analyzer (scanner) A function that reads an input stream and returns tokens one by one. @@ -5037,27 +10377,27 @@ A function that reads an input stream and returns tokens one by one. A flag, set by actions in the grammar rules, which alters the way tokens are parsed. @xref{Lexical Tie-ins}. -@item Look-ahead token -A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead Tokens}. +@item Literal string token +A token which consists of two or more fixed characters. @xref{Symbols}. + +@item Lookahead token +A token already read but not yet shifted. @xref{Lookahead, ,Lookahead +Tokens}. -@item LALR(1) +@item @acronym{LALR}(1) The class of context-free grammars that Bison (like most other parser -generators) can handle; a subset of LR(1). @xref{Mystery Conflicts, , -Mysterious Reduce/Reduce Conflicts}. +generators) can handle; a subset of @acronym{LR}(1). @xref{Mystery +Conflicts, ,Mysterious Reduce/Reduce Conflicts}. -@item LR(1) +@item @acronym{LR}(1) The class of context-free grammars in which at most one token of -look-ahead is needed to disambiguate the parsing of any piece of input. +lookahead is needed to disambiguate the parsing of any piece of input. @item Nonterminal symbol A grammar symbol standing for a grammatical construct that can be expressed through rules in terms of smaller constructs; in other words, a construct that is not a token. @xref{Symbols}. -@item Parse error -An error encountered during parsing of an input stream due to invalid -syntax. @xref{Error Recovery}. - @item Parser A function that recognizes valid sentences of a language by analyzing the syntax structure of a set of tokens passed to it from a lexical @@ -5069,7 +10409,8 @@ performs some operation. @item Reduction Replacing a string of nonterminals and/or terminals with a single -nonterminal, according to a grammar rule. @xref{Algorithm, ,The Bison Parser Algorithm }. +nonterminal, according to a grammar rule. @xref{Algorithm, ,The Bison +Parser Algorithm}. @item Reentrant A reentrant subprogram is a subprogram which can be in invoked any @@ -5080,8 +10421,9 @@ invocations. @xref{Pure Decl, ,A Pure (Reentrant) Parser}. A language in which all operators are postfix operators. @item Right recursion -A rule whose result symbol is also its last component symbol; -for example, @samp{expseq1: exp ',' expseq1;}. @xref{Recursion, ,Recursive Rules}. +A rule whose result symbol is also its last component symbol; for +example, @samp{expseq1: exp ',' expseq1;}. @xref{Recursion, ,Recursive +Rules}. @item Semantics In computer languages, the semantics are specified by the actions @@ -5091,7 +10433,7 @@ each statement. @xref{Semantics, ,Defining Language Semantics}. @item Shift A parser is said to shift when it makes the choice of analyzing further input from the stream rather than reducing immediately some -already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm }. +already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm}. @item Single-character literal A single character that is recognized and interpreted as is. @@ -5100,7 +10442,7 @@ A single character that is recognized and interpreted as is. @item Start symbol The nonterminal symbol that stands for a complete valid utterance in the language being parsed. The start symbol is usually listed as the -first nonterminal symbol in a language specification. +first nonterminal symbol in a language specification. @xref{Start Decl, ,The Start-Symbol}. @item Symbol table @@ -5108,6 +10450,10 @@ A data structure where symbol names and associated data are stored during parsing to allow for recognition and use of existing information in repeated uses of a symbol. @xref{Multi-function Calc}. +@item Syntax error +An error encountered during parsing of an input stream due to invalid +syntax. @xref{Error Recovery}. + @item Token A basic, grammatically indivisible unit of a language. The symbol that describes a token in the grammar is a terminal symbol. @@ -5115,44 +10461,48 @@ The input of the Bison parser is a stream of tokens which comes from the lexical analyzer. @xref{Symbols}. @item Terminal symbol -A grammar symbol that has no rules in the grammar and therefore -is grammatically indivisible. The piece of text it represents -is a token. @xref{Language and Grammar, ,Languages and Context-Free Grammars}. +A grammar symbol that has no rules in the grammar and therefore is +grammatically indivisible. The piece of text it represents is a token. +@xref{Language and Grammar, ,Languages and Context-Free Grammars}. @end table -@node Index, , Glossary, Top +@node Copying This Manual +@appendix Copying This Manual +@include fdl.texi + +@node Index @unnumbered Index @printindex cp -@contents - @bye - - - -@c old menu - -* Introduction:: -* Conditions:: -* Copying:: The GNU General Public License says - how you can copy and share Bison - -Tutorial sections: -* Concepts:: Basic concepts for understanding Bison. -* Examples:: Three simple explained examples of using Bison. - -Reference sections: -* Grammar File:: Writing Bison declarations and rules. -* Interface:: C-language interface to the parser function @code{yyparse}. -* Algorithm:: How the Bison parser works at run-time. -* Error Recovery:: Writing rules for error recovery. -* Context Dependency::What to do if your language syntax is too - messy for Bison to handle straightforwardly. -* Debugging:: Debugging Bison parsers that parse wrong. -* Invocation:: How to run Bison (to produce the parser source file). -* Table of Symbols:: All the keywords of the Bison language are explained. -* Glossary:: Basic concepts are explained. -* Index:: Cross-references to the text. - +@c LocalWords: texinfo setfilename settitle setchapternewpage finalout +@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex +@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry +@c LocalWords: ifset vskip pt filll insertcopying sp ISBN Etienne Suvasa +@c LocalWords: ifnottex yyparse detailmenu GLR RPN Calc var Decls Rpcalc +@c LocalWords: rpcalc Lexer Expr ltcalc mfcalc yylex +@c LocalWords: yyerror pxref LR yylval cindex dfn LALR samp gpl BNF xref +@c LocalWords: const int paren ifnotinfo AC noindent emph expr stmt findex +@c LocalWords: glr YYSTYPE TYPENAME prog dprec printf decl init stmtMerge +@c LocalWords: pre STDC GNUC endif yy YY alloca lf stddef stdlib YYDEBUG +@c LocalWords: NUM exp subsubsection kbd Ctrl ctype EOF getchar isdigit +@c LocalWords: ungetc stdin scanf sc calc ulator ls lm cc NEG prec yyerrok +@c LocalWords: longjmp fprintf stderr yylloc YYLTYPE cos ln +@c LocalWords: smallexample symrec val tptr FNCT fnctptr func struct sym +@c LocalWords: fnct putsym getsym fname arith fncts atan ptr malloc sizeof +@c LocalWords: strlen strcpy fctn strcmp isalpha symbuf realloc isalnum +@c LocalWords: ptypes itype YYPRINT trigraphs yytname expseq vindex dtype +@c LocalWords: Rhs YYRHSLOC LE nonassoc op deffn typeless yynerrs +@c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES +@c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param +@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP +@c LocalWords: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword +@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH +@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm notype +@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args +@c LocalWords: infile ypp yxx outfile itemx tex leaderfill +@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll +@c LocalWords: nbar yytext fst snd osplit ntwo strdup AST +@c LocalWords: YYSTACK DVI fdl printindex