X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/2a2e87dbe0a067663bab544f0fb85fa71f7c438f..10e5b8bd0a5e704006b371ce7fb9e4bea6c79a06:/doc/bison.texinfo?ds=sidebyside diff --git a/doc/bison.texinfo b/doc/bison.texinfo index 0fccc16e..be4d69e2 100644 --- a/doc/bison.texinfo +++ b/doc/bison.texinfo @@ -1,20 +1,17 @@ \input texinfo @c -*-texinfo-*- @comment %**start of header @setfilename bison.info -@settitle Bison 1.20 +@include version.texi +@settitle Bison @value{VERSION} @setchapternewpage odd -@iftex @finalout -@end iftex -@c SMALL BOOK version +@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 @@ -24,6 +21,7 @@ @c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo @c titlepage; should NOT be changed in the GPL. --mew +@c FIXME: I don't understand this `iftex'. Obsolete? --akim. @iftex @syncodeindex fn cp @syncodeindex vr cp @@ -36,10 +34,20 @@ @end ifinfo @comment %**end of header +@ifinfo +@format +START-INFO-DIR-ENTRY +* bison: (bison). GNU Project parser generator (yacc replacement). +END-INFO-DIR-ENTRY +@end format +@end ifinfo + @ifinfo This file documents the Bison parser generator. -Copyright (C) 1988, 1989, 1990, 1991, 1992 Free Software Foundation, Inc. +Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999, +2000, 2001, 2002 +Free Software Foundation, Inc. Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice @@ -73,21 +81,22 @@ instead of in the original English. @titlepage @title Bison @subtitle The YACC-compatible Parser Generator -@subtitle December 1992, Bison Version 1.20 +@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 +Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, +1999, 2000, 2001, 2002 +Free Software Foundation, Inc. @sp 2 Published by the Free Software Foundation @* -675 Massachusetts Avenue @* -Cambridge, MA 02139 USA @* -Printed copies are available for $15 each.@* -ISBN-1-882114-30-2 +59 Temple Place, Suite 330 @* +Boston, MA 02111-1307 USA @* +Printed copies are available from the Free Software Foundation.@* +ISBN 1-882114-44-2 Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice @@ -116,17 +125,20 @@ instead of in the original English. @sp 2 Cover art by Etienne Suvasa. @end titlepage -@page -@node Top, Introduction, (dir), (dir) +@contents + +@ifnottex +@node Top +@top Bison -@ifinfo -This manual documents version 1.20 of Bison. -@end ifinfo +This manual documents version @value{VERSION} of Bison, updated +@value{UPDATED}. +@end ifnottex @menu -* Introduction:: -* Conditions:: +* Introduction:: +* Conditions:: * Copying:: The GNU General Public License says how you can copy and share Bison @@ -141,13 +153,14 @@ Reference sections: * Error Recovery:: Writing rules for error recovery. * Context Dependency:: What to do if your language syntax is too messy for Bison to handle straightforwardly. -* Debugging:: Debugging Bison parsers that parse wrong. +* Debugging:: Understanding or debugging Bison parsers. * Invocation:: How to run Bison (to produce the parser source file). * Table of Symbols:: All the keywords of the Bison language are explained. * Glossary:: Basic concepts are explained. +* Copying This Manual:: License for copying this manual. * Index:: Cross-references to the text. - --- The Detailed Node Listing --- +@detailmenu --- The Detailed Node Listing --- The Concepts of Bison @@ -170,13 +183,14 @@ Examples * Infix Calc:: Infix (algebraic) notation calculator. Operator precedence is introduced. * Simple Error Recovery:: Continuing after syntax errors. +* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$. * 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. Reverse Polish Notation Calculator -* Decls: Rpcalc Decls. Bison and C declarations for rpcalc. +* Decls: Rpcalc Decls. Prologue (declarations) for rpcalc. * Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation. * Lexer: Rpcalc Lexer. The lexical analyzer. * Main: Rpcalc Main. The controlling function. @@ -186,9 +200,15 @@ Reverse Polish Notation Calculator Grammar Rules for @code{rpcalc} -* Rpcalc Input:: -* Rpcalc Line:: -* Rpcalc Expr:: +* Rpcalc Input:: +* Rpcalc Line:: +* Rpcalc Expr:: + +Location Tracking Calculator: @code{ltcalc} + +* Decls: Ltcalc Decls. Bison and C declarations for ltcalc. +* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations. +* Lexer: Ltcalc Lexer. The lexical analyzer. Multi-Function Calculator: @code{mfcalc} @@ -208,10 +228,10 @@ Bison Grammar Files Outline of a Bison Grammar -* C Declarations:: Syntax and usage of the C declarations section. +* Prologue:: Syntax and usage of the prologue (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. +* Epilogue:: Syntax and usage of the epilogue (additional code section). Defining Language Semantics @@ -237,7 +257,7 @@ 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} +* 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. @@ -253,7 +273,7 @@ The Lexical Analyzer Function @code{yylex} * 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. * Shift/Reduce:: Conflicts: when either shifting or reduction is valid. @@ -278,15 +298,26 @@ Handling Context Dependencies * Tie-in Recovery:: Lexical tie-ins have implications for how error recovery rules must be written. +Understanding or Debugging Your Parser + +* Understanding:: Understanding the structure of your parser. +* Tracing:: Tracing the execution of your parser. + Invoking Bison -* Bison Options:: All the options described in detail, +* 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. + +Copying This Manual + +* GNU Free Documentation License:: License for copying this manual. + +@end detailmenu @end menu -@node Introduction, Conditions, Top, Top +@node Introduction @unnumbered Introduction @cindex introduction @@ -306,440 +337,43 @@ 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. 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SHOULD THE -PROGRAM PROVE DEFECTIVE, YOU ASSUME THE COST OF ALL NECESSARY SERVICING, -REPAIR OR CORRECTION. - -@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 - -@iftex -@heading END OF TERMS AND CONDITIONS -@end iftex -@ifinfo -@center END OF TERMS AND CONDITIONS -@end ifinfo - -@page -@unnumberedsec How to Apply These Terms to Your New Programs - - 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. - - 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. - -@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 - -Also add information on how to contact you by electronic and paper mail. - -If the program is interactive, make it output a short notice like this -when it starts in an interactive mode: - -@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 - -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. - -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. - -@var{signature of Ty Coon}, 1 April 1989 -Ty Coon, President of Vice -@end smallexample +This edition corresponds to version @value{VERSION} of 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. +@node Conditions +@unnumbered Conditions for Using Bison -@node Concepts, Examples, Copying, Top +As of Bison version 1.24, we have changed the distribution terms for +@code{yyparse} to permit using Bison's output in nonfree programs. +Formerly, Bison parsers could be used only in programs that were free +software. + +The other GNU programming tools, such as the 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 +@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.) When we applied the GPL terms to the code for @code{yyparse}, +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 GNU tools. + +@include gpl.texi + +@node Concepts @chapter The Concepts of Bison This chapter introduces many of the basic concepts without which the @@ -754,17 +388,16 @@ use Bison or Yacc, we suggest you start by reading this chapter carefully. 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. +* 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 @@ -804,7 +437,7 @@ 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 @@ -818,16 +451,26 @@ 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, identifier,} + @r{identifier, close-paren} */ +@{ /* @r{open-brace} */ + return x * x; /* @r{keyword `return', identifier, asterisk, + 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, identifier, 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 @@ -871,7 +514,7 @@ 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 @@ -892,13 +535,16 @@ 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 @@ -913,7 +559,7 @@ 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 @@ -923,7 +569,7 @@ 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 @@ -935,7 +581,7 @@ 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 @@ -955,7 +601,7 @@ 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 @@ -965,7 +611,7 @@ 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 @@ -986,7 +632,36 @@ 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 +@node Locations Overview +@section Locations +@cindex location +@cindex textual position +@cindex position, 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 position}, 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 @@ -1005,12 +680,13 @@ 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 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 @@ -1022,7 +698,7 @@ 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 variable and function names used in the Bison parser file +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. @@ -1031,7 +707,15 @@ 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. -@node Stages, Grammar Layout, Bison Parser, Concepts +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-@sc{gnu} hosts, @code{}, +@code{}, and @code{} are included as needed to +declare memory allocators and related types. 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 @@ -1042,15 +726,16 @@ 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. +(@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. +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. @@ -1073,7 +758,7 @@ Compile the code output by Bison, as well as any other source files. Link the object files to produce the finished product. @end enumerate -@node Grammar Layout, , Stages, Concepts +@node Grammar Layout @section The Overall Layout of a Bison Grammar @cindex grammar file @cindex file format @@ -1085,7 +770,7 @@ general form of a Bison grammar file is as follows: @example %@{ -@var{C declarations} +@var{Prologue} %@} @var{Bison declarations} @@ -1093,15 +778,15 @@ general form of a Bison grammar file is as follows: %% @var{Grammar rules} %% -@var{Additional C code} +@var{Epilogue} @end example @noindent The @samp{%%}, @samp{%@{} and @samp{%@}} are punctuation that appears in every Bison grammar file to separate the sections. -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 +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. The Bison declarations declare the names of the terminal and nonterminal @@ -1111,12 +796,12 @@ semantic values of various symbols. The grammar rules define how to construct each nonterminal symbol from its parts. -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 epilogue can contain any 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. -@node Examples, Grammar File, Concepts, Top +@node Examples @chapter Examples @cindex simple examples @cindex examples, simple @@ -1140,12 +825,13 @@ to try them. * 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. @end menu -@node RPN Calc, Infix Calc, , Examples +@node RPN Calc @section Reverse Polish Notation Calculator @cindex reverse polish notation @cindex polish notation calculator @@ -1161,7 +847,7 @@ 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. +* Decls: Rpcalc Decls. Prologue (declarations) for rpcalc. * Rules: Rpcalc Rules. Grammar Rules for rpcalc, with explanation. * Lexer: Rpcalc Lexer. The lexical analyzer. * Main: Rpcalc Main. The controlling function. @@ -1170,7 +856,7 @@ The source code for this calculator is named @file{rpcalc.y}. The * Comp: Rpcalc Compile. Run the C compiler on the output code. @end menu -@node Rpcalc Decls, Rpcalc Rules, , RPN Calc +@node Rpcalc Decls @subsection Declarations for @code{rpcalc} Here are the C and Bison declarations for the reverse polish notation @@ -1189,28 +875,30 @@ calculator. As in C, comments are placed between @samp{/*@dots{}*/}. %% /* Grammar rules and actions follow */ @end example -The C declarations section (@pxref{C Declarations, ,The C Declarations Section}) contains two +The declarations section (@pxref{Prologue, , The prologue}) contains two preprocessor directives. 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 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. @@ -1256,12 +944,12 @@ 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}: @@ -1295,7 +983,7 @@ 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. -@node Rpcalc Line, Rpcalc Expr, Rpcalc Input, Rpcalc Rules +@node Rpcalc Line @subsubsection Explanation of @code{line} Now consider the definition of @code{line}: @@ -1320,7 +1008,7 @@ 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. @@ -1379,14 +1067,15 @@ exp: NUM @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 lexical analyzer skips blanks and tabs, then reads in numbers as @@ -1398,16 +1087,17 @@ 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 Decls, +,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 @@ -1417,26 +1107,27 @@ 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 +/* 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. Skips all blanks and tabs, returns 0 for EOF. */ #include @end group @group -yylex () +int +yylex (void) @{ int c; /* skip white space */ - while ((c = getchar ()) == ' ' || c == '\t') + while ((c = getchar ()) == ' ' || c == '\t') ; @end group @group /* process numbers */ - if (c == '.' || isdigit (c)) + if (c == '.' || isdigit (c)) @{ ungetc (c, stdin); scanf ("%lf", &yylval); @@ -1445,15 +1136,15 @@ yylex () @end group @group /* return end-of-file */ - if (c == EOF) + if (c == EOF) return 0; /* return single chars */ - return c; + 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 @@ -1464,28 +1155,30 @@ 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{"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: @example @group #include -yyerror (s) /* Called by yyparse on error */ - char *s; +void +yyerror (const char *s) /* Called by yyparse on error */ @{ printf ("%s\n", s); @} @@ -1497,17 +1190,18 @@ 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 Gen @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. @@ -1527,7 +1221,7 @@ 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 @@ -1536,19 +1230,19 @@ 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 rpcalc.tab.c -lm -o rpcalc} @end group @group # @r{List files again.} -% ls +$ @kbd{ls} rpcalc rpcalc.tab.c rpcalc.y @end group @end example @@ -1557,22 +1251,22 @@ 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} @@ -1621,8 +1315,8 @@ exp: NUM @{ $$ = $1; @} @end example @noindent -The functions @code{yylex}, @code{yyerror} and @code{main} can be the same -as before. +The functions @code{yylex}, @code{yyerror} and @code{main} can be the +same as before. There are two important new features shown in this code. @@ -1638,36 +1332,38 @@ Operator precedence is determined by the line ordering of the declarations; the higher the line number of the declaration (lower on the page or screen), the higher the precedence. Hence, exponentiation has the highest precedence, unary minus (@code{NEG}) is next, followed -by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator Precedence}. +by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator +Precedence}. -The other important new feature is the @code{%prec} in the grammar section -for the unary minus operator. The @code{%prec} simply instructs Bison that -the rule @samp{| '-' exp} has the same precedence as @code{NEG}---in this -case the next-to-highest. @xref{Contextual Precedence, ,Context-Dependent Precedence}. +The other important new feature is the @code{%prec} in the grammar +section for the unary minus operator. The @code{%prec} simply instructs +Bison that the rule @samp{| '-' exp} has the same precedence as +@code{NEG}---in this case the next-to-highest. @xref{Contextual +Precedence, ,Context-Dependent Precedence}. Here is a sample run of @file{calc.y}: @need 500 @example -% calc -4 + 4.5 - (34/(8*3+-3)) +$ @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 @@ -1682,14 +1378,15 @@ 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 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. This form of error recovery deals with syntax errors. There are other kinds of errors; for example, division by zero, which raises an exception @@ -1699,7 +1396,206 @@ 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 +@node Location Tracking Calc +@section Location Tracking Calculator: @code{ltcalc} +@cindex location tracking calculator +@cindex @code{ltcalc} +@cindex calculator, location tracking + +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 +analyser. + +@menu +* Decls: Ltcalc Decls. Bison and C declarations for ltcalc. +* Rules: Ltcalc Rules. Grammar rules for ltcalc, with explanations. +* Lexer: Ltcalc Lexer. The lexical analyzer. +@end menu + +@node Ltcalc Decls +@subsection Declarations for @code{ltcalc} + +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. */ + +%@{ +#define YYSTYPE int +#include +%@} + +/* Bison declarations. */ +%token NUM + +%left '-' '+' +%left '*' '/' +%left NEG +%right '^' + +%% /* Grammar follows */ +@end example + +@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}. + +@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. + +@example +@group +input : /* empty */ + | input line +; +@end group + +@group +line : '\n' + | exp '\n' @{ printf ("%d\n", $1); @} +; +@end group + +@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 %preg NEG @{ $$ = -$2; @} + | exp '^' exp @{ $$ = pow ($1, $3); @} + | '(' exp ')' @{ $$ = $2; @} +@end group +@end example + +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. + +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 analyser, 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; + + /* skip white space */ + while ((c = getchar ()) == ' ' || c == '\t') + ++yylloc.last_column; + + /* 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-file */ + if (c == EOF) + return 0; + + /* return single chars 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} @@ -1713,7 +1609,7 @@ 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 +back all nonnumber 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: @@ -1727,20 +1623,20 @@ to create named variables, store values in them, and use them later. Here is a sample session with the multi-function calculator: @example -% mfcalc -pi = 3.141592653589 +$ @kbd{mfcalc} +@kbd{pi = 3.141592653589} 3.1415926536 -sin(pi) +@kbd{sin(pi)} 0.0000000000 -alpha = beta1 = 2.3 +@kbd{alpha = beta1 = 2.3} 2.3000000000 -alpha +@kbd{alpha} 2.3000000000 -ln(alpha) +@kbd{ln(alpha)} 0.8329091229 -exp(ln(beta1)) +@kbd{exp(ln(beta1))} 2.3000000000 -% +$ @end example Note that multiple assignment and nested function calls are permitted. @@ -1751,7 +1647,7 @@ Note that multiple assignment and nested function calls are permitted. * Symtab: Mfcalc Symtab. Symbol table management subroutines. @end menu -@node Mfcalc Decl, Mfcalc Rules, , Multi-function Calc +@node Mfcalc Decl @subsection Declarations for @code{mfcalc} Here are the C and Bison declarations for the multi-function calculator. @@ -1796,13 +1692,14 @@ 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}. +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, Mfcalc Symtab, Mfcalc Decl, Multi-function Calc +@node Mfcalc Rules @subsection Grammar Rules for @code{mfcalc} Here are the grammar rules for the multi-function calculator. @@ -1836,7 +1733,7 @@ exp: NUM @{ $$ = $1; @} %% @end smallexample -@node Mfcalc Symtab, , Mfcalc Rules, Multi-function Calc +@node Mfcalc Symtab @subsection The @code{mfcalc} Symbol Table @cindex symbol table example @@ -1851,14 +1748,18 @@ provides for either functions or variables to be placed in the table. @smallexample @group +/* Fonctions type. */ +typedef double (*func_t) (double); + /* Data type for links in the chain of symbols. */ struct symrec @{ 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 */ @}; @@ -1870,8 +1771,8 @@ typedef struct symrec symrec; /* The symbol table: a chain of `struct symrec'. */ extern symrec *sym_table; -symrec *putsym (); -symrec *getsym (); +symrec *putsym (const char *, func_t); +symrec *getsym (const char *); @end group @end smallexample @@ -1883,16 +1784,17 @@ function that initializes the symbol table. Here it is, and @group #include -main () +int +main (void) @{ init_table (); - yyparse (); + return yyparse (); @} @end group @group -yyerror (s) /* Called by yyparse on error */ - char *s; +void +yyerror (const char *s) /* Called by yyparse on error */ @{ printf ("%s\n", s); @} @@ -1900,28 +1802,30 @@ yyerror (s) /* Called by yyparse on error */ struct init @{ char *fname; - double (*fnct)(); + 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 arith_fncts[] = +@{ + "sin", sin, + "cos", cos, + "atan", atan, + "ln", log, + "exp", exp, + "sqrt", sqrt, + 0, 0 +@}; /* The symbol table: a chain of `struct symrec'. */ -symrec *sym_table = (symrec *)0; +symrec *sym_table = (symrec *) 0; @end group @group -init_table () /* puts arithmetic functions in table. */ +/* Put arithmetic functions in table. */ +void +init_table (void) @{ int i; symrec *ptr; @@ -1946,9 +1850,7 @@ 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 *sym_name, int sym_type) @{ symrec *ptr; ptr = (symrec *) malloc (sizeof (symrec)); @@ -1962,8 +1864,7 @@ putsym (sym_name,sym_type) @} symrec * -getsym (sym_name) - char *sym_name; +getsym (const char *sym_name) @{ symrec *ptr; for (ptr = sym_table; ptr != (symrec *) 0; @@ -1976,7 +1877,7 @@ 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 non-digit 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 @@ -1984,7 +1885,7 @@ 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}. @@ -1992,7 +1893,9 @@ operators in @code{yylex}. @smallexample @group #include -yylex () + +int +yylex (void) @{ int c; @@ -2068,10 +1971,10 @@ yylex () @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. +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 @@ -2089,13 +1992,14 @@ 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. @@ -2103,11 +2007,12 @@ 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 @@ -2115,7 +2020,7 @@ appropriate delimiters: @example %@{ -@var{C declarations} +@var{Prologue} %@} @var{Bison declarations} @@ -2124,24 +2029,25 @@ 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. @menu -* C Declarations:: Syntax and usage of the C declarations section. +* Prologue:: Syntax and usage of the prologue. * 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. +* 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, Bison Declarations, , Grammar Outline +@subsection The prologue +@cindex declarations section +@cindex Prologue +@cindex declarations -The @var{C declarations} section contains macro definitions and +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 @@ -2149,7 +2055,7 @@ that they precede the definition of @code{yyparse}. You can use need any C declarations, you may omit the @samp{%@{} and @samp{%@}} delimiters that bracket this section. -@node Bison Declarations, Grammar Rules, C Declarations, Grammar Outline +@node Bison Declarations @subsection The Bison Declarations Section @cindex Bison declarations (introduction) @cindex declarations, Bison (introduction) @@ -2159,7 +2065,7 @@ 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 @@ -2171,17 +2077,18 @@ 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, , Grammar Rules, Grammar Outline +@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. +@xref{Interface, ,Parser C-Language Interface}. If the last section is empty, you may omit the @samp{%%} that separates it from the grammar rules. @@ -2189,9 +2096,9 @@ 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. +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 @@ -2216,7 +2123,7 @@ 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 @@ -2229,12 +2136,13 @@ 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 @@ -2244,8 +2152,38 @@ 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, is the code @code{yylex} +returns for end-of-input (@pxref{Calling Convention, ,Calling Convention +for @code{yylex}}). + +@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. 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 @@ -2255,11 +2193,11 @@ 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 numeric code for a character token type is simply the numeric code of 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 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 @@ -2268,11 +2206,27 @@ 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}. +The @code{yylex} function must use the same character set and encoding +that was used by Bison. For example, if you run Bison in an +@sc{ascii} environment, but then compile and run the resulting program +in an environment that uses an incompatible character set like +@sc{ebcdic}, the resulting program will probably not work because the +tables generated by Bison will assume @sc{ascii} numeric values for +character tokens. Portable grammars should avoid non-@sc{ascii} +character tokens, as implementations in practice often use different +and incompatible extensions in this area. However, it is standard +practice for software distributions to contain C source files that +were generated by Bison in an @sc{ascii} environment, so installers on +platforms that are incompatible with @sc{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 @@ -2281,14 +2235,16 @@ 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, @@ -2365,15 +2321,15 @@ 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: +of a particular thing. Consider this recursive definition of a +comma-separated sequence of one or more expressions: @example @group @@ -2399,20 +2355,20 @@ 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: @@ -2434,10 +2390,10 @@ 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 @@ -2458,7 +2414,7 @@ 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 @@ -2467,7 +2423,8 @@ 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}). +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 specify some other type, define @code{YYSTYPE} as a macro, like this: @@ -2477,10 +2434,10 @@ 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}). +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 @@ -2494,16 +2451,18 @@ 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}). +@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of +Value Types}). @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 $$ @@ -2515,10 +2474,11 @@ is to compute a semantic value for the grouping built by the rule from the semantic values associated with tokens or smaller groupings. An action consists of C statements surrounded by braces, much like a -compound statement in C. It can be placed at any position in the rule; it -is executed at that position. Most rules have just one action at the end -of the rule, following all the components. Actions in the middle of a rule -are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}). +compound statement in C. It can be placed at any position in the rule; +it is executed at that position. Most rules have just one action at the +end of the rule, following all the components. Actions in the middle of +a rule are tricky and used only for special purposes (@pxref{Mid-Rule +Actions, ,Actions in Mid-Rule}). The C code in an action can refer to the semantic values of the components matched by the rule with the construct @code{$@var{n}}, which stands for @@ -2545,7 +2505,19 @@ which are the first and third symbols on the right hand side of the rule. The sum is stored into @code{$$} 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}. + +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: + +@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: @@ -2579,7 +2551,7 @@ 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 +@node Action Types @subsection Data Types of Values in Actions @cindex action data types @cindex data types in actions @@ -2591,7 +2563,7 @@ 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{$$} or @code{$@var{n}}, its data type is determined by which symbol it refers to -in the rule. In this example,@refill +in the rule. In this example, @example @group @@ -2605,7 +2577,7 @@ exp: @dots{} @code{$1} and @code{$3} refer to instances of @code{exp}, so they all have the data type declared for the nonterminal symbol @code{exp}. If @code{$2} were used, it would have the data type declared for the -terminal symbol @code{'+'}, whatever that might be.@refill +terminal symbol @code{'+'}, whatever that might be. Alternatively, you can specify the data type when you refer to the value, by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the @@ -2624,7 +2596,7 @@ reference. For example, if you have defined types as shown here: then you can write @code{$1} to refer to the first subunit of the rule as an integer, or @code{$1} to refer to it as a double. -@node Mid-Rule Actions, , Action Types, Semantics +@node Mid-Rule Actions @subsection Actions in Mid-Rule @cindex actions in mid-rule @cindex mid-rule actions @@ -2647,8 +2619,8 @@ The mid-rule action can also have a semantic value. The action can set its value with an assignment to @code{$$}, 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 @@ -2786,70 +2758,226 @@ 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. -@node Declarations, Multiple Parsers, Semantics, Grammar File -@section Bison Declarations -@cindex declarations, Bison -@cindex Bison declarations +@node Locations +@section Tracking Locations +@cindex location +@cindex textual position +@cindex position, textual -The @dfn{Bison declarations} section of a Bison grammar defines the symbols -used in formulating the grammar and the data types of semantic values. -@xref{Symbols}. +Though grammar rules and semantic actions are enough to write a fully +functional parser, it can be useful to process some additionnal informations, +especially symbol locations. -All token type names (but not single-character literal tokens such as -@code{'+'} and @code{'*'}) must be declared. Nonterminal symbols must be -declared if you need to specify which data type to use for the semantic -value (@pxref{Multiple Types, ,More Than One Value Type}). +@c (terminal or not) ? -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}). +The way locations are handled is defined by providing a data type, and +actions to take when rules are matched. @menu -* 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. -* Start Decl:: Specifying the start symbol. -* Pure Decl:: Requesting a reentrant parser. -* Decl Summary:: Table of all Bison declarations. +* 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 Token Decl, Precedence Decl, , Declarations -@subsection Token Type Names -@cindex declaring token type names -@cindex token type names, declaring -@findex %token +@node Location Type +@subsection Data Type of Locations +@cindex data type of locations +@cindex default location type -The basic way to declare a token type name (terminal symbol) is as follows: +Defining a data type for locations is much simpler than for semantic values, +since all tokens and groupings always use the same type. + +The type of locations is specified by defining a macro called @code{YYLTYPE}. +When @code{YYLTYPE} is not defined, Bison uses a default structure type with +four members: @example -%token @var{name} +struct +@{ + int first_line; + int first_column; + int last_line; + int last_column; +@} @end example -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. +@node Actions and Locations +@subsection Actions and Locations +@cindex location actions +@cindex actions, location +@vindex @@$ +@vindex @@@var{n} -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}. +Actions are not only useful for defining language semantics, but also for +describing the behavior of the output parser with locations. -You can explicitly specify the numeric code for a token type by appending -an integer value in the field immediately following the token name: +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{@@$}. -@example -%token NUM 300 -@end example +Here is a basic example using the default data type for locations: -@noindent +@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 + @{ + $$ = 1; + printf("Division by zero, l%d,c%d-l%d,c%d", + @@3.first_line, @@3.first_column, + @@3.last_line, @@3.last_column); + @} + @} +@end group +@end example + +As for semantic values, there is a default action for locations that is +run each time a rule is matched. It sets the beginning of @code{@@$} to the +beginning of the first symbol, and the end of @code{@@$} to the end of the +last symbol. + +With this default action, the location tracking can be fully automatic. The +example above simply rewrites this way: + +@example +@group +exp: @dots{} + | exp '/' exp + @{ + if ($3) + $$ = $1 / $3; + else + @{ + $$ = 1; + printf("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 + +@node Location Default Action +@subsection Default Action for Locations +@vindex 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. + +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). The second one +is an array holding locations of all right hand side elements of the rule +being matched. The last one is the size of the right hand side rule. + +By default, it is defined this way: + +@example +@group +#define YYLLOC_DEFAULT(Current, Rhs, N) \ + Current.first_line = Rhs[1].first_line; \ + Current.first_column = Rhs[1].first_column; \ + Current.last_line = Rhs[N].last_line; \ + Current.last_column = Rhs[N].last_column; +@end group +@end example + +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 for the location +array range from 1 to @var{n}. +@end itemize + +@node Declarations +@section Bison Declarations +@cindex declarations, Bison +@cindex Bison declarations + +The @dfn{Bison declarations} section of a Bison grammar defines the symbols +used in formulating the grammar and the data types of semantic values. +@xref{Symbols}. + +All token type names (but not single-character literal tokens such as +@code{'+'} and @code{'*'}) must be declared. Nonterminal symbols must be +declared if you need to specify which data type to use for the semantic +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}). + +@menu +* 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. +* Start Decl:: Specifying the start symbol. +* Pure Decl:: Requesting a reentrant parser. +* Decl Summary:: Table of all Bison declarations. +@end menu + +@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: + +@example +%token @var{name} +@end example + +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 +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: + +@example +%token NUM 300 +@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: @@ -2863,7 +2991,31 @@ 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}). + +@node Precedence Decl @subsection Operator Precedence @cindex precedence declarations @cindex declaring operator precedence @@ -2872,7 +3024,8 @@ For example: Use the @code{%left}, @code{%right} or @code{%nonassoc} 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 @code{%token}: either @@ -2912,7 +3065,7 @@ 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 +@node Union Decl @subsection The Collection of Value Types @cindex declaring value types @cindex value types, declaring @@ -2921,7 +3074,7 @@ the one declared later has the higher precedence and is grouped first. 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. +C. For example: @@ -2943,7 +3096,7 @@ 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 a semicolon after the closing brace. -@node Type Decl, Expect Decl, Union Decl, Declarations +@node Type Decl @subsection Nonterminal Symbols @cindex declaring value types, nonterminals @cindex value types, nonterminals, declaring @@ -2959,13 +3112,19 @@ 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. - -@node Expect Decl, Start Decl, Type Decl, Declarations +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 Expect Decl @subsection Suppressing Conflict Warnings @cindex suppressing conflict warnings @cindex preventing warnings about conflicts @@ -2974,11 +3133,11 @@ spaces to separate the symbol names. @findex %expect 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: @@ -2986,10 +3145,11 @@ 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 +no warning if there are @var{n} shift/reduce conflicts and no +reduce/reduce conflicts. An error, instead of the usual warning, is +given if there are either more or fewer conflicts, or if there are any +reduce/reduce conflicts. In general, using @code{%expect} involves these steps: @@ -3013,7 +3173,7 @@ 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. -@node Start Decl, Pure Decl, Expect Decl, Declarations +@node Start Decl @subsection The Start-Symbol @cindex declaring the start symbol @cindex start symbol, declaring @@ -3028,44 +3188,53 @@ 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 %pure-parser 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) code. Reentrancy is important whenever asynchronous execution is possible; -for example, a nonreentrant program may not be safe to call from a signal -handler. In systems with multiple threads of control, a nonreentrant +for example, a non-reentrant program may not be safe to call from a signal +handler. In systems with multiple threads of control, a non-reentrant 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{%pure-parser} says that you want the parser to be +reentrant. It looks like this: @example -%pure_parser +%pure-parser @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} also +becomes local in @code{yyparse} (@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 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 @@ -3094,17 +3263,147 @@ Declare the type of semantic values for a nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}). @item %start -Specify the grammar's start symbol (@pxref{Start Decl, ,The Start-Symbol}). +Specify the grammar's start symbol (@pxref{Start Decl, ,The +Start-Symbol}). @item %expect Declare the expected number of shift-reduce conflicts (@pxref{Expect Decl, ,Suppressing Conflict Warnings}). +@end table + +@sp 1 +@noindent +In order to change the behavior of @command{bison}, use the following +directives: + +@table @code +@item %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 %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}. -@item %pure_parser -Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). +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}. + +@item %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}. + +@c @item %header-extension +@c Specify the extension of the parser header file generated when +@c @code{%define} or @samp{-d} are used. +@c +@c For example, a grammar file named @file{foo.ypp} and containing a +@c @code{%header-extension .hh} directive will produce a header file +@c named @file{foo.tab.hh} + +@item %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 parse error messages. + +@item %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{yynerrs}, +@code{yylval}, @code{yychar}, @code{yydebug}, and possible +@code{yylloc}. For example, if you use @samp{%name-prefix="c_"}, the +names become @code{c_parse}, @code{c_lex}, and so on. @xref{Multiple +Parsers, ,Multiple Parsers in the Same Program}. + +@item %no-parser +Do not include any C code in the parser file; generate tables only. The +parser file contains just @code{#define} directives and static variable +declarations. + +This option also tells Bison to write the C code for the grammar actions +into a file named @file{@var{filename}.act}, in the form of a +brace-surrounded body fit for a @code{switch} statement. + +@item %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. + +@item %output="@var{filename}" +Specify the @var{filename} for the parser file. + +@item %pure-parser +Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure +(Reentrant) Parser}). + +@c @item %source-extension +@c Specify the extension of the parser output file. +@c +@c For example, a grammar file named @file{foo.yy} and containing a +@c @code{%source-extension .cpp} directive will produce a parser file +@c named @file{foo.tab.cpp} + +@item %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} are always @code{"$"}, @code{"error"}, and +@code{"$illegal"}; after these come the symbols defined in the grammar +file. + +For single-character literal tokens and literal string tokens, the name +in the table includes the single-quote or double-quote characters: for +example, @code{"'+'"} is a single-character literal and @code{"\"<=\""} +is a literal string token. All the characters of the literal string +token appear verbatim in the string found in the table; even +double-quote characters are not escaped. For example, if the token +consists of three characters @samp{*"*}, its string in @code{yytname} +contains @samp{"*"*"}. (In C, that would be written 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 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 -@node Multiple Parsers, , Declarations, Grammar File +@item %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. @xref{Understanding, , Understanding Your Parser}, for more +information. + + + +@item %yacc +Pretend the option @option{--yacc} was given, i.e., imitate Yacc, +including its naming conventions. @xref{Bison Options}, for more. +@end table + + + + +@node Multiple Parsers @section Multiple Parsers in the Same Program Most programs that use Bison parse only one language and therefore contain @@ -3113,15 +3412,15 @@ 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. +(@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{yylval}, @code{yychar} and @code{yydebug}. For -example, if you use @samp{-p c}, the names become @code{cparse}, -@code{clex}, and so on. +@code{yyerror}, @code{yynerrs}, @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. @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 @@ -3134,7 +3433,7 @@ 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, Algorithm, Grammar File, Top +@node Interface @chapter Parser C-Language Interface @cindex C-language interface @cindex interface @@ -3145,26 +3444,26 @@ 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 additional -C code in the grammar file, you are likely to run into trouble. +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. -* Lexical:: You must supply a function @code{yylex} +* 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 -@node Parser Function, Lexical, , Interface +@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. +write an action which directs @code{yyparse} to return immediately +without reading further. The value returned by @code{yyparse} is 0 if parsing was successful (return is due to end-of-input). @@ -3184,7 +3483,7 @@ Return immediately with value 0 (to report success). Return immediately with value 1 (to report failure). @end table -@node Lexical, Error Reporting, Parser Function, Interface +@node Lexical @section The Lexical Analyzer Function @code{yylex} @findex yylex @cindex lexical analyzer @@ -3200,7 +3499,7 @@ 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}. @@ -3213,7 +3512,7 @@ that need it. @xref{Invocation, ,Invoking Bison}.@refill 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 @@ -3233,7 +3532,8 @@ signifies end-of-input. Here is an example showing these things: @example -yylex () +int +yylex (void) @{ @dots{} if (c == EOF) /* Detect end of file. */ @@ -3251,11 +3551,49 @@ 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 not escaped in any way; they appear verbatim in +the contents of the string in the table. + +Here's code for looking up a token in @code{yytname}, assuming that the +characters of the token are stored in @code{token_buffer}. + +@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 -In an ordinary (nonreentrant) parser, the semantic value of the token must +In an ordinary (non-reentrant) parser, the semantic value of the token must be stored into the global variable @code{yylval}. When you are using just one data type for semantic values, @code{yylval} has that type. Thus, if the type is @code{int} (the default), you might write this in @@ -3271,9 +3609,10 @@ Thus, if the type is @code{int} (the default), you might write this in @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 @@ -3297,38 +3636,41 @@ then the code in @code{yylex} might look like this: @end group @end example -@node Token Positions, Pure Calling, Token Values, Lexical +@node Token Positions @subsection Textual Positions 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{%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. @example -yylex (lvalp, llocp) - YYSTYPE *lvalp; - YYLTYPE *llocp; +int +yylex (YYSTYPE *lvalp, YYLTYPE *llocp) @{ @dots{} *lvalp = value; /* Put value onto Bison stack. */ @@ -3342,7 +3684,87 @@ textual positions, 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 +@vindex YYPARSE_PARAM +If you use a reentrant parser, you can optionally pass additional +parameter information to it in a reentrant way. To do so, define the +macro @code{YYPARSE_PARAM} as a variable name. This modifies the +@code{yyparse} function to accept one argument, of type @code{void *}, +with that name. + +When you call @code{yyparse}, pass the address of an object, casting the +address to @code{void *}. The grammar actions can refer to the contents +of the object by casting the pointer value back to its proper type and +then dereferencing it. Here's an example. Write this in the parser: + +@example +%@{ +struct parser_control +@{ + int nastiness; + int randomness; +@}; + +#define YYPARSE_PARAM parm +%@} +@end example + +@noindent +Then call the parser like this: + +@example +struct parser_control +@{ + int nastiness; + int randomness; +@}; + +@dots{} + +@{ + struct parser_control foo; + @dots{} /* @r{Store proper data in @code{foo}.} */ + value = yyparse ((void *) &foo); + @dots{} +@} +@end example + +@noindent +In the grammar actions, use expressions like this to refer to the data: + +@example +((struct parser_control *) parm)->randomness +@end example + +@vindex YYLEX_PARAM +If you wish to pass the additional parameter data to @code{yylex}, +define the macro @code{YYLEX_PARAM} just like @code{YYPARSE_PARAM}, as +shown here: + +@example +%@{ +struct parser_control +@{ + int nastiness; + int randomness; +@}; + +#define YYPARSE_PARAM parm +#define YYLEX_PARAM parm +%@} +@end example + +You should then define @code{yylex} to accept one additional +argument---the value of @code{parm}. (This makes either two or three +arguments in total, depending on whether an argument of type +@code{YYLTYPE} is passed.) You can declare the argument as a pointer to +the proper object type, or you can declare it as @code{void *} and +access the contents as shown above. + +You can use @samp{%pure-parser} to request a reentrant parser without +also using @code{YYPARSE_PARAM}. Then you should call @code{yyparse} +with no arguments, as usual. + +@node Error Reporting @section The Error Reporting Function @code{yyerror} @cindex error reporting function @findex yyerror @@ -3350,9 +3772,10 @@ only one argument. @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 +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 @@ -3362,10 +3785,11 @@ receives one argument. For a parse error, the string is normally @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. +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 @@ -3379,8 +3803,8 @@ The following definition suffices in simple programs: @example @group -yyerror (s) - char *s; +void +yyerror (char *s) @{ @end group @group @@ -3397,10 +3821,10 @@ immediately return 1. @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. +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 @@ -3419,12 +3843,13 @@ Acts like a variable that contains the semantic value for the @item $<@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}. @item $<@var{typealt}>@var{n} Like @code{$@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}. @item YYABORT; Return immediately from @code{yyparse}, indicating failure. @@ -3478,36 +3903,45 @@ error rules. @xref{Error Recovery}. @item 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} -@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: +@item @@$ +@findex @@$ +Acts like a structure variable containing information on the textual position +of the grouping made by the current rule. @xref{Locations, , +Tracking Locations}. -@example -struct @{ - int first_line, last_line; - int first_column, last_column; -@}; -@end example +@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 -Thus, to get the starting line number of the third component, use -@samp{@@3.first_line}. +@c Thus, to get the starting line number of the third component, you would +@c use @samp{@@3.first_line}. -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. +@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. + +@item @@@var{n} +@findex @@@var{n} +Acts like a structure variable containing information on the textual position +of the @var{n}th component of the current rule. @xref{Locations, , +Tracking Locations}. -The use of this feature makes the parser noticeably slower. @end table -@node Algorithm, Error Recovery, Interface, Top -@chapter The Bison Parser Algorithm -@cindex Bison parser algorithm +@node Algorithm +@chapter The Bison Parser Algorithm +@cindex Bison parser algorithm @cindex algorithm of parser @cindex shifting @cindex reduction @@ -3572,7 +4006,7 @@ This kind of parser is known in the literature as a bottom-up parser. * Stack Overflow:: What happens when stack gets full. How to avoid it. @end menu -@node Look-Ahead, Shift/Reduce, , Algorithm +@node Look-Ahead @section Look-Ahead Tokens @cindex look-ahead token @@ -3627,7 +4061,7 @@ doing so would produce on the stack the sequence of symbols @code{expr The current look-ahead token is stored in the variable @code{yychar}. @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 @@ -3723,7 +4157,7 @@ expr: variable ; @end example -@node Precedence, Contextual Precedence, Shift/Reduce, Algorithm +@node Precedence @section Operator Precedence @cindex operator precedence @cindex precedence of operators @@ -3740,7 +4174,7 @@ shift and when to reduce. * 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 @@ -3759,35 +4193,35 @@ 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 look-ahead token is @samp{-}: shifting +makes right-associativity. + +@node Using Precedence @subsection Specifying Operator Precedence @findex %left @findex %right @@ -3808,7 +4242,7 @@ order in which they are declared. The first @code{%left} or 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 +@node Precedence Examples @subsection Precedence Examples In our example, we would want the following declarations: @@ -3834,28 +4268,29 @@ 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 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. 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. -@node Contextual Precedence, Parser States, Precedence, Algorithm +@node Contextual Precedence @section Context-Dependent Precedence @cindex context-dependent precedence @cindex unary operator precedence @@ -3872,7 +4307,7 @@ The Bison precedence declarations, @code{%left}, @code{%right} and @code{%nonassoc}, 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. @@ -3913,7 +4348,7 @@ exp: @dots{} @end group @end example -@node Parser States, Reduce/Reduce, Contextual Precedence, Algorithm +@node Parser States @section Parser States @cindex finite-state machine @cindex parser state @@ -3939,7 +4374,7 @@ There is one other alternative: the table can say that the look-ahead 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 @@ -4054,7 +4489,7 @@ 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. @@ -4093,7 +4528,7 @@ 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 look-ahead: 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). @@ -4162,7 +4597,7 @@ return_spec: ; @end example -@node Stack Overflow, , Mystery Conflicts, Algorithm +@node Stack Overflow @section Stack Overflow, and How to Avoid It @cindex stack overflow @cindex parser stack overflow @@ -4196,7 +4631,7 @@ 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. -@node Error Recovery, Context Dependency, Algorithm, Top +@node Error Recovery @chapter Error Recovery @cindex error recovery @cindex recovery from errors @@ -4220,7 +4655,7 @@ 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: @@ -4314,7 +4749,7 @@ 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. -@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 @@ -4333,7 +4768,7 @@ 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 @@ -4376,11 +4811,11 @@ static int foo (lose); /* @r{redeclare @code{foo} as function} */ 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: @@ -4408,7 +4843,7 @@ 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 @@ -4457,12 +4892,11 @@ 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. @@ -4517,9 +4951,414 @@ 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 @sc{vcg} +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 that @samp{calc.y contains 1 useless nonterminal +and 1 useless rule} and that @samp{calc.y contains 7 shift/reduce +conflicts}. 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 contains 1 shift/reduce conflict. +State 9 contains 1 shift/reduce conflict. +State 10 contains 1 shift/reduce conflict. +State 11 contains 4 shift/reduce conflicts. +@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 ``not used'' +below): + +@example +Useless nonterminals: + useless + +Terminals which are not used: + STR + +Useless rules: +#6 useless: STR; +@end example + +@noindent +The next section reproduces the exact grammar that Bison used: + +@example +Grammar + + Number, Line, Rule + 0 5 $axiom -> exp $ + 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 + +$ (0) 0 +'*' (42) 3 +'+' (43) 1 +'-' (45) 2 +'/' (47) 4 +error (256) +NUM (258) 5 + +Nonterminals, with rules where they appear + +$axiom (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 + + $axiom -> . 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 parse 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 symbol 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 + + $axiom -> . 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) + + $default reduce using rule 5 (exp) +@end example + +@noindent +the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead +(@samp{$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 + + $axiom -> 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 parse error. + +The state 3 is named the @dfn{final state}, or the @dfn{accepting +state}: + +@example +state 3 + + $axiom -> 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 contains 1 +shift/reduce conflict}: + +@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)] + $default reduce using rule 1 (exp) +@end example + +Indeed, there are two actions associated to the lookahead @samp{/}: +either shifting (and going to state 7), or reducing rule 1. The +conflict means that either the grammar is ambiguous, or the parser lacks +information to make the right decision. Indeed the grammar is +ambiguous, as, since we did not specify the precedence of @samp{/}, the +sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM / +NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) / +NUM}, which corresponds to reducing rule 1. + +Because in LALR(1) parsing a single decision can be made, Bison +arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, , +Shift/Reduce Conflicts}. Discarded actions are reported in between +square brackets. + +Note that all the previous states had a single possible action: either +shifting the next token and going to the corresponding state, or +reducing a single rule. In the other cases, i.e., when shifting +@emph{and} reducing is possible or when @emph{several} reductions are +possible, the lookahead is required to select the action. State 8 is +one such state: if the lookahead is @samp{*} or @samp{/} then the action +is shifting, otherwise the action is reducing rule 1. In other words, +the first two items, corresponding to rule 1, are not eligible when the +lookahead is @samp{*}, since we specified that @samp{*} has higher +precedence that @samp{+}. More generally, some items are eligible only +with some set of possible lookaheads. When run with +@option{--report=lookahead}, Bison specifies these lookaheads: + +@example +state 8 + + exp -> exp . '+' exp [$, '+', '-', '/'] (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 due to the lack of precedence +of @samp{/} wrt @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 @@ -4527,16 +5366,39 @@ 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 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 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. Useless POSIX and Yacc portability matter to you, this is +the preferred solution. +@end table -The trace facility uses @code{stderr}, so you must add @w{@code{#include -}} to the C declarations section unless it is already there. +We suggest that you always enable the debug option so that debugging is +always possible. + +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 +arguments. If you define @code{YYDEBUG} to a nonzero value but do not +define @code{YYFPRINTF}, @code{} is automatically included +and @code{YYPRINTF} 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}. @@ -4561,13 +5423,14 @@ 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 @@ -4590,10 +5453,7 @@ calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}): #define YYPRINT(file, type, value) yyprint (file, type, value) static void -yyprint (file, type, value) - FILE *file; - int type; - YYSTYPE value; +yyprint (FILE *file, int type, YYSTYPE value) @{ if (type == VAR) fprintf (file, " %s", value.tptr->name); @@ -4602,7 +5462,9 @@ yyprint (file, type, value) @} @end smallexample -@node Invocation, Table of Symbols, Debugging, Top +@c ================================================= Invoking Bison + +@node Invocation @chapter Invoking Bison @cindex invoking Bison @cindex Bison invocation @@ -4618,16 +5480,37 @@ 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 +@file{hack/foo.tab.c}. It's is 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 extention like +the given one as input (repectively @file{foo.tab.cpp} and @file{foo.tab.c++}). +This feature takes effect with all options that manipulate filenames 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 @var{infile.y} -o @var{output.c++} +@end example +@noindent +will produce @file{output.c++} and @file{outfile.h++}. + @menu -* Bison Options:: All the options described in detail, +* Bison Options:: All the options described in detail, in alphabetical order by short options. +* Environment Variables:: Variables which affect Bison execution. * Option Cross Key:: Alphabetical list of long options. * VMS Invocation:: Bison command syntax on VMS. @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 @@ -4641,25 +5524,54 @@ 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}. +@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 -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. +@item -V +@itemx --version +Print the version number of Bison and exit. -If the parser output file is named @file{@var{name}.c} then this file -is named @file{@var{name}.h}.@refill +@need 1750 +@item -y +@itemx --yacc +Equivalent to @samp{-o y.tab.c}; 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 option is to imitate Yacc's output +file name conventions. Thus, the following shell script can substitute +for Yacc: -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 +@example +bison -y $* +@end example +@end table + +@noindent +Tuning the parser: + +@table @option +@item -S @var{file} +@itemx --skeleton=@var{file} +Specify the skeleton to use. You probably don't need this option unless +you are developing Bison. + +@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 --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 @@ -4667,72 +5579,99 @@ 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. +parser file, treating it as 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. +@item -n +@itemx --no-parser +Pretend that @code{%no-parser} was specified. @xref{Decl Summary}. -The other output files' names are constructed from @var{outfile} -as described under the @samp{-v} and @samp{-d} switches. +@item -k +@itemx --token-table +Pretend that @code{%token-table} was specified. @xref{Decl Summary}. +@end table -@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}. +@noindent +Adjust the output: -For example, if you use @samp{-p c}, the names become @code{cparse}, -@code{clex}, and so on. +@table @option +@item -d +@itemx --defines +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 and the semantic value type @code{YYSTYPE}, as well as a few +@code{extern} variable declarations. @xref{Decl Summary}. -@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}. +@item --defines=@var{defines-file} +Same as above, but save in the file @var{defines-file}. -@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 -b @var{file-prefix} +@itemx --file-prefix=@var{prefix} +Pretend that @code{%verbose} was specified, i.e, specify prefix to use +for all Bison output file names. @xref{Decl Summary}. -@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. +@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: -This file also describes all the conflicts, both those resolved by -operator precedence and the unresolved ones. +@table @code +@item state +Description of the grammar, conflicts (resolved and unresolved), and +LALR automaton. -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 +@item lookahead +Implies @code{state} and augments the description of the automaton with +each rule's lookahead set. -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 +@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 -V -@itemx --version -Print the version number of Bison and exit. +For instance, on the following grammar -@item -h -@itemx --help -Print a summary of the command-line options to Bison and exit. +@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{filename} +@itemx --output=@var{filename} +Specify the @var{filename} for the parser file. + +The other output files' names are constructed from @var{filename} as +described under the @samp{-v} and @samp{-d} options. + +@item -g +Output a VCG definition of the LALR(1) grammar automaton computed by +Bison. If the grammar file is @file{foo.y}, the VCG output file will +be @file{foo.vcg}. + +@item --graph=@var{graph-file} +The behaviour of @var{--graph} is the same than @samp{-g}. The only +difference is that it has an optionnal argument which is the name of +the output graph filename. +@end table -@need 1750 -@item -y -@itemx --yacc -@itemx --fixed-output-files -Equivalent to @samp{-o y.tab.c}; 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 +@node Environment Variables +@section Environment Variables +@cindex environment variables +@cindex BISON_SIMPLE -@example -bison -y $* -@end example +Here is a list of environment variables which affect the way Bison +runs. + +@table @samp +@item BISON_SIMPLE +Much of the parser generated by Bison is copied verbatim from a file +called @file{bison.simple}. If Bison cannot find that file, or if you +would like to direct Bison to use a different copy, setting the +environment variable @code{BISON_SIMPLE} to the path of the file will +cause Bison to use that copy instead. @end table -@node Option Cross Key, VMS Invocation, Bison Options, Invocation +@node Option Cross Key @section Option Cross Key Here is a list of options, alphabetized by long option, to help you find @@ -4745,11 +5684,13 @@ the corresponding short option. \line{ --debug \leaderfill -t} \line{ --defines \leaderfill -d} \line{ --file-prefix \leaderfill -b} -\line{ --fixed-output-files \leaderfill -y} +\line{ --graph \leaderfill -g} \line{ --help \leaderfill -h} \line{ --name-prefix \leaderfill -p} \line{ --no-lines \leaderfill -l} -\line{ --output-file \leaderfill -o} +\line{ --no-parser \leaderfill -n} +\line{ --output \leaderfill -o} +\line{ --token-table \leaderfill -k} \line{ --verbose \leaderfill -v} \line{ --version \leaderfill -V} \line{ --yacc \leaderfill -y} @@ -4759,19 +5700,22 @@ the corresponding short option. @ifinfo @example --debug -t ---defines -d +--defines=@var{defines-file} -d --file-prefix=@var{prefix} -b @var{file-prefix} ---fixed-output-files --yacc -y +--graph=@var{graph-file} -d --help -h ---name-prefix -p +--name-prefix=@var{prefix} -p @var{name-prefix} --no-lines -l ---output-file=@var{outfile} -o @var{outfile} +--no-parser -n +--output=@var{outfile} -o @var{outfile} +--token-table -k --verbose -v --version -V +--yacc -y @end example @end ifinfo -@node VMS Invocation, , Option Cross Key, Invocation +@node VMS Invocation @section Invoking Bison under VMS @cindex invoking Bison under VMS @cindex VMS @@ -4799,12 +5743,28 @@ 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 +@node Table of Symbols @appendix Bison Symbols @cindex Bison symbols, table of @cindex symbols in Bison, table of @table @code +@item @@$ +In an action, the location of the left-hand side of the rule. + @xref{Locations, , Locations Overview}. + +@item @@@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}. + +@item $$ +In an action, the semantic value of the left-hand side of the rule. +@xref{Actions}. + +@item $@var{n} +In an action, the semantic value of the @var{n}-th symbol of the +right-hand side of the rule. @xref{Actions}. + @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 @@ -4818,17 +5778,22 @@ token is reset to the token that originally caused the violation. @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}}. +function @code{yyerror} is not called. @xref{Parser Function, ,The +Parser Function @code{yyparse}}. @item YYACCEPT Macro to pretend that a complete utterance of the language has been -read, by making @code{yyparse} return 0 immediately. +read, by making @code{yyparse} return 0 immediately. @xref{Parser Function, ,The Parser Function @code{yyparse}}. @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}. +@item YYDEBUG +Macro to define to equip the parser with tracing code. @xref{Tracing, +,Tracing Your Parser}. + @item YYERROR Macro to pretend that a syntax error has just been detected: call @code{yyerror} and then perform normal error recovery if possible @@ -4844,27 +5809,45 @@ section to request verbose, specific error message strings when Macro for specifying the initial size of the parser stack. @xref{Stack Overflow}. +@item YYLEX_PARAM +Macro for specifying an extra argument (or list of extra arguments) for +@code{yyparse} to pass to @code{yylex}. @xref{Pure Calling,, Calling +Conventions for Pure Parsers}. + @item YYLTYPE Macro for the data type of @code{yylloc}; a structure with four -members. @xref{Token Positions, ,Textual Positions of Tokens}. +members. @xref{Location Type, , Data Types of Locations}. + +@item yyltype +Default value for YYLTYPE. @item YYMAXDEPTH Macro for specifying the maximum size of the parser stack. @xref{Stack Overflow}. +@item YYPARSE_PARAM +Macro for specifying the name of a parameter that @code{yyparse} should +accept. @xref{Pure Calling,, Calling Conventions for Pure Parsers}. + @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 YYSTACK_USE_ALLOCA +Macro used to control the use of @code{alloca}. If defined to @samp{0}, +the parser will not use @code{alloca} but @code{malloc} when trying to +grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA} +to anything else. + @item YYSTYPE Macro for the data type of semantic values; @code{int} by default. @xref{Value Type, ,Data Types of Semantic Values}. @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}. +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}. @item yyclearin Macro used in error-recovery rule actions. It clears the previous @@ -4873,7 +5856,7 @@ look-ahead token. @xref{Error Recovery}. @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}. +symbols and parser action. @xref{Tracing, ,Tracing Your Parser}. @item yyerrok Macro to cause parser to recover immediately to its normal mode @@ -4882,11 +5865,13 @@ after a parse error. @xref{Error Recovery}. @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}}. +containing an error message. @xref{Error Reporting, ,The Error +Reporting Function @code{yyerror}}. @item yylex -User-supplied lexical analyzer function, called with no arguments -to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}. +User-supplied lexical analyzer function, called with no arguments to get +the next token. @xref{Lexical, ,The Lexical Analyzer Function +@code{yylex}}. @item yylval External variable in which @code{yylex} should place the semantic @@ -4895,34 +5880,65 @@ 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 +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}. +@samp{@@} feature in the grammar actions. @xref{Token Positions, +,Textual Positions of Tokens}. @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}}. +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}}. @item yyparse The parser function produced by Bison; call this function to start parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@item %debug +Equip the parser for debugging. @xref{Decl Summary}. + +@item %defines +Bison declaration to create a header file meant for the scanner. +@xref{Decl Summary}. + +@item %file-prefix="@var{prefix}" +Bison declaration to set tge prefix of the output files. @xref{Decl +Summary}. + +@c @item %source-extension +@c Bison declaration to specify the generated parser output file extension. +@c @xref{Decl Summary}. +@c +@c @item %header-extension +@c Bison declaration to specify the generated parser header file extension +@c if required. @xref{Decl Summary}. + @item %left Bison declaration to assign left associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. +@item %name-prefix="@var{prefix}" +Bison declaration to rename the external symbols. @xref{Decl Summary}. + +@item %no-lines +Bison declaration to avoid generating @code{#line} directives in the +parser file. @xref{Decl Summary}. + @item %nonassoc -Bison declaration to assign nonassociativity to token(s). +Bison declaration to assign non-associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. +@item %output="@var{filename}" +Bison declaration to set the name of the parser file. @xref{Decl +Summary}. + @item %prec Bison declaration to assign a precedence to a specific rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}. -@item %pure_parser +@item %pure-parser Bison declaration to request a pure (reentrant) parser. @xref{Pure Decl, ,A Pure (Reentrant) Parser}. @@ -4931,38 +5947,48 @@ Bison declaration to assign right associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. @item %start -Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}. +Bison declaration to specify the start symbol. @xref{Start Decl, ,The +Start-Symbol}. @item %token Bison declaration to declare token(s) without specifying precedence. @xref{Token Decl, ,Token Type Names}. +@item %token-table +Bison declaration to include a token name table in the parser file. +@xref{Decl Summary}. + @item %type -Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}. +Bison declaration to declare nonterminals. @xref{Type Decl, +,Nonterminal Symbols}. @item %union Bison declaration to specify several possible data types for semantic values. @xref{Union Decl, ,The Collection of Value Types}. @end table +@sp 1 + These are the punctuation and delimiters used in Bison input: @table @samp @item %% Delimiter used to separate the grammar rule section from the -Bison declarations section or the additional C code section. +Bison declarations section or the epilogue. @xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. @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}. +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}. @item /*@dots{}*/ Comment delimiters, as in C. @item : -Separates a rule's result from its components. @xref{Rules, ,Syntax of Grammar Rules}. +Separates a rule's result from its components. @xref{Rules, ,Syntax of +Grammar Rules}. @item ; Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. @@ -4972,20 +5998,22 @@ Separates alternate rules for the same result nonterminal. @xref{Rules, ,Syntax of Grammar Rules}. @end table -@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}. +in the @cite{ALGOL-60} report, 1963. @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 @@ -5005,7 +6033,7 @@ rules. @xref{Algorithm, ,The Bison Parser Algorithm }. @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 @@ -5026,8 +6054,9 @@ 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 @@ -5041,8 +6070,12 @@ 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 Literal string token +A token which consists of two or more fixed characters. @xref{Symbols}. + @item Look-ahead token -A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead Tokens}. +A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead +Tokens}. @item LALR(1) The class of context-free grammars that Bison (like most other parser @@ -5073,7 +6106,8 @@ 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 @@ -5084,8 +6118,9 @@ 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 @@ -5104,7 +6139,7 @@ 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 @@ -5119,44 +6154,23 @@ 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 -@unnumbered Index +@node Copying This Manual +@appendix Copying This Manual -@printindex cp - -@contents - -@bye - - - - -@c old menu +@menu +* GNU Free Documentation License:: License for copying this manual. +@end menu -* Introduction:: -* Conditions:: -* Copying:: The GNU General Public License says - how you can copy and share Bison +@include fdl.texi -Tutorial sections: -* Concepts:: Basic concepts for understanding Bison. -* Examples:: Three simple explained examples of using Bison. +@node Index +@unnumbered Index -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. +@printindex cp +@bye