X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/fdc6758b794cc59f7352aa14b7ea5f169b6ee530..fae437e835febad97544793497ebd1a645a4e78b:/doc/bison.texinfo diff --git a/doc/bison.texinfo b/doc/bison.texinfo index ed683241..03d3cc90 100644 --- a/doc/bison.texinfo +++ b/doc/bison.texinfo @@ -5,9 +5,7 @@ @settitle Bison @value{VERSION} @setchapternewpage odd -@iftex @finalout -@end iftex @c SMALL BOOK version @c This edition has been formatted so that you can format and print it in @@ -23,6 +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 @@ -35,45 +34,33 @@ @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 +@copying -@ifinfo -This file documents the Bison parser generator. - -Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999, 2000 -Free Software Foundation, Inc. - -Permission is granted to make and distribute verbatim copies of -this manual provided the copyright notice and this permission notice -are preserved on all copies. - -@ignore -Permission is granted to process this file through Tex and print the -results, provided the printed document carries copying permission -notice identical to this one except for the removal of this paragraph -(this paragraph not being relevant to the printed manual). - -@end ignore -Permission is granted to copy and distribute modified versions of this -manual under the conditions for verbatim copying, provided also that the -sections entitled ``GNU General Public License'' and ``Conditions for -Using Bison'' are included exactly as in the original, and provided that -the entire resulting derived work is distributed under the terms of a -permission notice identical to this one. - -Permission is granted to copy and distribute translations of this manual -into another language, under the above conditions for modified versions, -except that the sections entitled ``GNU General Public License'', -``Conditions for Using Bison'' and this permission notice may be -included in translations approved by the Free Software Foundation -instead of in the original English. -@end ifinfo +This manual is for GNU Bison (version @value{VERSION}, @value{UPDATED}), +the GNU parser generator. + +Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, +1999, 2000, 2001, 2002 Free Software Foundation, Inc. + +@quotation +Permission is granted to copy, distribute and/or modify this document +under the terms of the GNU Free Documentation License, Version 1.1 or +any later version published by the Free Software Foundation; with no +Invariant Sections, with the Front-Cover texts being ``A GNU Manual,'' +and with the Back-Cover Texts as in (a) below. A copy of the +license is included in the section entitled ``GNU Free Documentation +License.'' + +(a) The FSF's Back-Cover Text is: ``You have freedom to copy and modify +this GNU Manual, like GNU software. Copies published by the Free +Software Foundation raise funds for GNU development.'' +@end quotation +@end copying + +@dircategory GNU programming tools +@direntry +* bison: (bison). GNU parser generator (yacc replacement). +@end direntry @ifset shorttitlepage-enabled @shorttitlepage Bison @@ -87,52 +74,24 @@ instead of in the original English. @page @vskip 0pt plus 1filll -Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, -1999, 2000 -Free Software Foundation, Inc. - +@insertcopying @sp 2 Published by the Free Software Foundation @* 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 -are preserved on all copies. - -@ignore -Permission is granted to process this file through TeX and print the -results, provided the printed document carries copying permission -notice identical to this one except for the removal of this paragraph -(this paragraph not being relevant to the printed manual). - -@end ignore -Permission is granted to copy and distribute modified versions of this -manual under the conditions for verbatim copying, provided also that the -sections entitled ``GNU General Public License'' and ``Conditions for -Using Bison'' are included exactly as in the original, and provided that -the entire resulting derived work is distributed under the terms of a -permission notice identical to this one. - -Permission is granted to copy and distribute translations of this manual -into another language, under the above conditions for modified versions, -except that the sections entitled ``GNU General Public License'', -``Conditions for Using Bison'' and this permission notice may be -included in translations approved by the Free Software Foundation -instead of in the original English. @sp 2 Cover art by Etienne Suvasa. @end titlepage @contents -@node Top, Introduction, (dir), (dir) - -@ifinfo -This manual documents version @value{VERSION} of Bison. -@end ifinfo +@ifnottex +@node Top +@top Bison +@insertcopying +@end ifnottex @menu * Introduction:: @@ -151,13 +110,14 @@ 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 @@ -180,13 +140,14 @@ 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. @@ -200,6 +161,12 @@ Grammar Rules for @code{rpcalc} * 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} * Decl: Mfcalc Decl. Bison declarations for multi-function calculator. @@ -218,10 +185,10 @@ 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 @@ -272,6 +239,7 @@ The Bison Parser Algorithm * Parser States:: The parser is a finite-state-machine with stack. * Reduce/Reduce:: When two rules are applicable in the same situation. * Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. +* Generalized LR Parsing:: Parsing arbitrary context-free grammars. * Stack Overflow:: What happens when stack gets full. How to avoid it. Operator Precedence @@ -288,15 +256,26 @@ 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, 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 @@ -322,7 +301,7 @@ multi-character string literals and other features. This edition corresponds to version @value{VERSION} of Bison. -@node Conditions, Copying, Introduction, Top +@node Conditions @unnumbered Conditions for Using Bison As of Bison version 1.24, we have changed the distribution terms for @@ -350,400 +329,9 @@ 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. -@node Copying, Concepts, Conditions, Top -@unnumbered GNU GENERAL PUBLIC LICENSE -@center Version 2, June 1991 - -@display -Copyright @copyright{} 1989, 1991 Free Software Foundation, Inc. -59 Temple Place - Suite 330, Boston, MA 02111-1307, 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. 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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., 59 Temple Place - Suite 330, -Boston, MA 02111-1307, 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: - -@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 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. +@include gpl.texi -@node Concepts, Examples, Copying, Top +@node Concepts @chapter The Concepts of Bison This chapter introduces many of the basic concepts without which the @@ -758,6 +346,7 @@ 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. +* GLR Parsers:: Writing parsers for general context-free languages * Locations Overview:: Tracking Locations. * Bison Parser:: What are Bison's input and output, how is the output used? @@ -765,7 +354,7 @@ use Bison or Yacc, we suggest you start by reading this chapter carefully. * 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 @cindex context-free grammar @@ -788,8 +377,12 @@ specify the language Algol 60. Any grammar expressed in BNF is a context-free grammar. The input to Bison is essentially machine-readable BNF. -Not all context-free languages can be handled by Bison, only those -that are LALR(1). In brief, this means that it must be possible to +@cindex LALR(1) grammars +@cindex LR(1) grammars +There are various important subclasses of context-free grammar. Although it +can handle almost all context-free grammars, Bison is optimized for what +are called LALR(1) grammars. +In brief, in these grammars, it must be possible to tell how to parse any portion of an input string with just a single token of look-ahead. Strictly speaking, that is a description of an LR(1) grammar, and LALR(1) involves additional restrictions that are @@ -797,6 +390,24 @@ hard to explain simply; but it is rare in actual practice to find an LR(1) grammar that fails to be LALR(1). @xref{Mystery Conflicts, , Mysterious Reduce/Reduce Conflicts}, for more information on this. +@cindex GLR parsing +@cindex generalized LR (GLR) parsing +@cindex ambiguous grammars +@cindex non-deterministic parsing +Parsers for LALR(1) grammars are @dfn{deterministic}, meaning roughly that +the next grammar rule to apply at any point in the input is uniquely +determined by the preceding input and a fixed, finite portion (called +a @dfn{look-ahead}) of the remaining input. +A context-free grammar can be @dfn{ambiguous}, meaning that +there are multiple ways to apply the grammar rules to get the some inputs. +Even unambiguous grammars can be @dfn{non-deterministic}, meaning that no +fixed look-ahead always suffices to determine the next grammar rule to apply. +With the proper declarations, Bison is also able to parse these more general +context-free grammars, using a technique known as GLR parsing (for +Generalized LR). Bison's GLR parsers are able to handle any context-free +grammar for which the number of possible parses of any given string +is finite. + @cindex symbols (abstract) @cindex token @cindex syntactic grouping @@ -807,7 +418,7 @@ 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 @@ -821,16 +432,26 @@ lexicography, not grammar.) Here is a simple C function subdivided into tokens: +@ifinfo @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,} + @r{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 ifinfo +@ifnotinfo +@example +int /* @r{keyword `int'} */ +square (int x) /* @r{identifier, open-paren, identifier, identifier, close-paren} */ +@{ /* @r{open-brace} */ + 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 @@ -874,7 +495,7 @@ 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 @@ -919,7 +540,7 @@ 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 @@ -929,7 +550,7 @@ 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 @@ -941,7 +562,7 @@ 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 @@ -961,7 +582,7 @@ 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, Locations Overview, Semantic Values, Concepts +@node Semantic Actions @section Semantic Actions @cindex semantic actions @cindex actions, semantic @@ -992,7 +613,181 @@ 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 Locations Overview, Bison Parser, Semantic Actions, Concepts +@node GLR Parsers +@section Writing GLR Parsers +@cindex GLR parsing +@cindex generalized LR (GLR) parsing +@findex %glr-parser +@cindex conflicts +@cindex shift/reduce conflicts + +In some grammars, there will be cases where Bison's standard LALR(1) +parsing algorithm cannot decide whether to apply a certain grammar rule +at a given point. That is, it may not be able to decide (on the basis +of the input read so far) which of two possible reductions (applications +of a grammar rule) applies, or whether to apply a reduction or read more +of the input and apply a reduction later in the input. These are known +respectively as @dfn{reduce/reduce} conflicts (@pxref{Reduce/Reduce}), +and @dfn{shift/reduce} conflicts (@pxref{Shift/Reduce}). + +To use a grammar that is not easily modified to be LALR(1), a more +general parsing algorithm is sometimes necessary. If you include +@code{%glr-parser} among the Bison declarations in your file +(@pxref{Grammar Outline}), the result will be a Generalized LR (GLR) +parser. These parsers handle Bison grammars that contain no unresolved +conflicts (i.e., after applying precedence declarations) identically to +LALR(1) parsers. However, when faced with unresolved shift/reduce and +reduce/reduce conflicts, GLR parsers use the simple expedient of doing +both, effectively cloning the parser to follow both possibilities. Each +of the resulting parsers can again split, so that at any given time, +there can be any number of possible parses being explored. The parsers +proceed in lockstep; that is, all of them consume (shift) a given input +symbol before any of them proceed to the next. Each of the cloned +parsers eventually meets one of two possible fates: either it runs into +a parsing error, in which case it simply vanishes, or it merges with +another parser, because the two of them have reduced the input to an +identical set of symbols. + +During the time that there are multiple parsers, semantic actions are +recorded, but not performed. When a parser disappears, its recorded +semantic actions disappear as well, and are never performed. When a +reduction makes two parsers identical, causing them to merge, Bison +records both sets of semantic actions. Whenever the last two parsers +merge, reverting to the single-parser case, Bison resolves all the +outstanding actions either by precedences given to the grammar rules +involved, or by performing both actions, and then calling a designated +user-defined function on the resulting values to produce an arbitrary +merged result. + +Let's consider an example, vastly simplified from C++. + +@example +%@{ + #define YYSTYPE const char* +%@} + +%token TYPENAME ID + +%right '=' +%left '+' + +%glr-parser + +%% + +prog : + | prog stmt @{ printf ("\n"); @} + ; + +stmt : expr ';' %dprec 1 + | decl %dprec 2 + ; + +expr : ID @{ printf ("%s ", $$); @} + | TYPENAME '(' expr ')' + @{ printf ("%s ", $1); @} + | expr '+' expr @{ printf ("+ "); @} + | expr '=' expr @{ printf ("= "); @} + ; + +decl : TYPENAME declarator ';' + @{ printf ("%s ", $1); @} + | TYPENAME declarator '=' expr ';' + @{ printf ("%s ", $1); @} + ; + +declarator : ID @{ printf ("\"%s\" ", $1); @} + | '(' declarator ')' + ; +@end example + +@noindent +This models a problematic part of the C++ grammar---the ambiguity between +certain declarations and statements. For example, + +@example +T (x) = y+z; +@end example + +@noindent +parses as either an @code{expr} or a @code{stmt} +(assuming that @samp{T} is recognized as a TYPENAME and @samp{x} as an ID). +Bison detects this as a reduce/reduce conflict between the rules +@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the +time it encounters @code{x} in the example above. The two @code{%dprec} +declarations, however, give precedence to interpreting the example as a +@code{decl}, which implies that @code{x} is a declarator. +The parser therefore prints + +@example +"x" y z + T +@end example + +Consider a different input string for this parser: + +@example +T (x) + y; +@end example + +@noindent +Here, there is no ambiguity (this cannot be parsed as a declaration). +However, at the time the Bison parser encounters @code{x}, it does not +have enough information to resolve the reduce/reduce conflict (again, +between @code{x} as an @code{expr} or a @code{declarator}). In this +case, no precedence declaration is used. Instead, the parser splits +into two, one assuming that @code{x} is an @code{expr}, and the other +assuming @code{x} is a @code{declarator}. The second of these parsers +then vanishes when it sees @code{+}, and the parser prints + +@example +x T y + +@end example + +Suppose that instead of resolving the ambiguity, you wanted to see all +the possibilities. For this purpose, we must @dfn{merge} the semantic +actions of the two possible parsers, rather than choosing one over the +other. To do so, you could change the declaration of @code{stmt} as +follows: + +@example +stmt : expr ';' %merge + | decl %merge + ; +@end example + +@noindent + +and define the @code{stmtMerge} function as: + +@example +static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1) +@{ + printf (" "); + return ""; +@} +@end example + +@noindent +with an accompanying forward declaration +in the C declarations at the beginning of the file: + +@example +%@{ + #define YYSTYPE const char* + static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1); +%@} +@end example + +@noindent +With these declarations, the resulting parser will parse the first example +as both an @code{expr} and a @code{decl}, and print + +@example +"x" y z + T x T y z + = +@end example + + +@node Locations Overview @section Locations @cindex location @cindex textual position @@ -1021,7 +816,7 @@ 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, Stages, Locations Overview, Concepts +@node Bison Parser @section Bison Output: the Parser File @cindex Bison parser @cindex Bison utility @@ -1040,12 +835,13 @@ 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 @@ -1057,7 +853,7 @@ 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. @@ -1066,7 +862,15 @@ 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 @@ -1077,15 +881,16 @@ 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. @@ -1108,7 +913,7 @@ 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 @@ -1120,7 +925,7 @@ general form of a Bison grammar file is as follows: @example %@{ -@var{C declarations} +@var{Prologue} %@} @var{Bison declarations} @@ -1128,15 +933,15 @@ 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 @@ -1146,12 +951,12 @@ 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 @@ -1175,12 +980,13 @@ 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 @@ -1196,7 +1002,7 @@ 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. @@ -1205,7 +1011,7 @@ 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 @@ -1224,28 +1030,30 @@ 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. @@ -1296,7 +1104,7 @@ rule are referred to as @code{$1}, @code{$2}, and so on. * 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}: @@ -1330,7 +1138,7 @@ 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}: @@ -1355,7 +1163,7 @@ 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. @@ -1414,14 +1222,15 @@ 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 @@ -1433,16 +1242,17 @@ 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 @@ -1453,8 +1263,8 @@ 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 + 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 @@ -1489,7 +1299,7 @@ yylex (void) @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 @@ -1508,7 +1318,7 @@ main (void) @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 @@ -1537,7 +1347,7 @@ 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 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) @@ -1545,8 +1355,8 @@ 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}). +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. @@ -1566,7 +1376,7 @@ 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 @@ -1575,19 +1385,19 @@ 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 @@ -1596,22 +1406,22 @@ 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} @@ -1677,27 +1487,29 @@ 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 @@ -1729,7 +1541,7 @@ 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 +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 @@ -1739,7 +1551,206 @@ 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} @@ -1767,20 +1778,20 @@ 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. @@ -1791,7 +1802,7 @@ 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. @@ -1836,13 +1847,14 @@ 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. @@ -1876,7 +1888,7 @@ 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 @@ -2028,7 +2040,7 @@ 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}. @@ -2114,10 +2126,10 @@ yylex (void) @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 @@ -2135,7 +2147,7 @@ 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 @@ -2155,7 +2167,7 @@ The Bison grammar input file conventionally has a name ending in @samp{.y}. * 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 @@ -2163,7 +2175,7 @@ appropriate delimiters: @example %@{ -@var{C declarations} +@var{Prologue} %@} @var{Bison declarations} @@ -2172,24 +2184,25 @@ 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 @@ -2197,7 +2210,34 @@ 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 +You may have more than one @var{Prologue} section, intermixed with the +@var{Bison declarations}. This allows you to have C and Bison +declarations that refer to each other. For example, the @code{%union} +declaration may use types defined in a header file, and you may wish to +prototype functions that take arguments of type @code{YYSTYPE}. This +can be done with two @var{Prologue} blocks, one before and one after the +@code{%union} declaration. + +@smallexample +%@{ +#include +#include "ptypes.h" +%@} + +%union @{ + long n; + tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */ +@} + +%@{ +static void yyprint(FILE *, int, YYSTYPE); +#define YYPRINT(F, N, L) yyprint(F, N, L) +%@} + +@dots{} +@end smallexample + +@node Bison Declarations @subsection The Bison Declarations Section @cindex Bison declarations (introduction) @cindex declarations, Bison (introduction) @@ -2207,7 +2247,7 @@ 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 @@ -2219,18 +2259,18 @@ 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. @@ -2238,9 +2278,9 @@ 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 @@ -2294,7 +2334,7 @@ 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} +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}}). @@ -2335,7 +2375,7 @@ 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. @@ -2348,11 +2388,27 @@ 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 @@ -2447,7 +2503,7 @@ 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 @@ -2481,14 +2537,14 @@ 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 @@ -2516,7 +2572,7 @@ primary: constant defines two mutually-recursive nonterminals, since each refers to the other. -@node Semantics, Locations, Recursion, Grammar File +@node Semantics @section Defining Language Semantics @cindex defining language semantics @cindex language semantics, defining @@ -2540,7 +2596,7 @@ 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 @@ -2549,7 +2605,8 @@ 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: @@ -2559,10 +2616,10 @@ 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 @@ -2576,7 +2633,8 @@ 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 @@ -2586,7 +2644,7 @@ 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 $$ @@ -2598,10 +2656,11 @@ is to compute a semantic value for the grouping built by the rule from the semantic values associated with tokens or smaller groupings. An action consists of C statements surrounded by braces, much like a -compound statement in C. It can be placed at any position in the rule; it -is executed at that position. Most rules have just one action at the end -of the rule, following all the components. Actions in the middle of a rule -are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}). +compound statement in C. It can be placed at any position in the rule; +it is executed at that position. Most rules have just one action at the +end of the rule, following all the components. Actions in the middle of +a rule are tricky and used only for special purposes (@pxref{Mid-Rule +Actions, ,Actions in Mid-Rule}). The C code in an action can refer to the semantic values of the components matched by the rule with the construct @code{$@var{n}}, which stands for @@ -2628,7 +2687,19 @@ which are the first and third symbols on the right hand side of the rule. The sum is stored into @code{$$} 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: @@ -2662,7 +2733,7 @@ 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 @@ -2674,7 +2745,7 @@ 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 @@ -2688,7 +2759,7 @@ 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 @@ -2707,7 +2778,7 @@ 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 @@ -2869,7 +2940,7 @@ 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 Locations, Declarations, Semantics, Grammar File +@node Locations @section Tracking Locations @cindex location @cindex textual position @@ -2877,10 +2948,12 @@ actually does to implement mid-rule actions. Though grammar rules and semantic actions are enough to write a fully functional parser, it can be useful to process some additionnal informations, -especially locations of tokens and groupings. +especially symbol locations. + +@c (terminal or not) ? -The way locations are handled is defined by providing a data type, and actions -to take when rules are matched. +The way locations are handled is defined by providing a data type, and +actions to take when rules are matched. @menu * Location Type:: Specifying a data type for locations. @@ -2888,7 +2961,7 @@ to take when rules are matched. * Location Default Action:: Defining a general way to compute locations. @end menu -@node Location Type, Actions and Locations, , Locations +@node Location Type @subsection Data Type of Locations @cindex data type of locations @cindex default location type @@ -2910,7 +2983,7 @@ struct @} @end example -@node Actions and Locations, Location Default Action, Location Type, Locations +@node Actions and Locations @subsection Actions and Locations @cindex location actions @cindex actions, location @@ -2927,61 +3000,112 @@ The location of the @var{n}th component of the right hand side is @code{@@@var{n}}, while the location of the left hand side grouping is @code{@@$}. -Here is a simple example using the default data type for locations: +Here is a basic example using the default data type for locations: @example @group exp: @dots{} - | exp '+' exp + | exp '/' exp @{ + @@$.first_column = @@1.first_column; + @@$.first_line = @@1.first_line; @@$.last_column = @@3.last_column; @@$.last_line = @@3.last_line; - $$ = $1 + $3; + 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 -@noindent -In the example above, there is no need to set the beginning of @code{@@$}. The -output parser always sets @code{@@$} to @code{@@1} before executing the C -code of a given action, whether you provide a processing for locations or not. +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, , Actions and Locations, Locations +@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 define a default action to take for each rule. The @code{YYLLOC_DEFAULT} -macro is called each time a rule is matched, before the associated action is -run. +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. -@c Documentation for the old (?) YYLLOC_DEFAULT +By default, it is defined this way for simple LALR(1) parsers: -This macro takes two parameters, the first one being the location of the -grouping (the result of the computation), and the second one being the -location of the last element matched. Of course, before @code{YYLLOC_DEFAULT} -is run, the result is set to the location of the first component matched. +@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 -By default, this macro computes a location that ranges from the beginning of -the first element to the end of the last element. It is defined this way: +@noindent +and like this for GLR parsers: @example @group -#define YYLLOC_DEFAULT(Current, Last) \ - Current.last_line = Last.last_line; \ - Current.last_column = Last.last_column; +#define YYLLOC_DEFAULT(Current, Rhs, N) \ + Current.first_line = YYRHSLOC(Rhs,1).first_line; \ + Current.first_column = YYRHSLOC(Rhs,1).first_column; \ + Current.last_line = YYRHSLOC(Rhs,N).last_line; \ + Current.last_column = YYRHSLOC(Rhs,N).last_column; @end group @end example -@c not Documentation for the old (?) YYLLOC_DEFAULT +When defining @code{YYLLOC_DEFAULT}, you should consider that: -@noindent +@itemize @bullet +@item +All arguments are free of side-effects. However, only the first one (the +result) should be modified by @code{YYLLOC_DEFAULT}. -Most of the time, the default action for locations is general enough to -suppress location dedicated code from most actions. +@item +For consistency with semantic actions, valid indexes for the location +array range from 1 to @var{n}. +@end itemize -@node Declarations, Multiple Parsers, Locations, Grammar File +@node Declarations @section Bison Declarations @cindex declarations, Bison @cindex Bison declarations @@ -2997,7 +3121,8 @@ value (@pxref{Multiple Types, ,More Than One Value Type}). The first rule in the file also specifies the start symbol, by default. If you want some other symbol to be the start symbol, you must declare -it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars}). +it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free +Grammars}). @menu * Token Decl:: Declaring terminal symbols. @@ -3010,7 +3135,7 @@ it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars * Decl Summary:: Table of all Bison declarations. @end menu -@node Token Decl, Precedence Decl, , Declarations +@node Token Decl @subsection Token Type Names @cindex declaring token type names @cindex token type names, declaring @@ -3042,11 +3167,12 @@ an integer value in the field immediately following the token name: @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: @@ -3084,7 +3210,7 @@ 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, Union Decl, Token Decl, Declarations +@node Precedence Decl @subsection Operator Precedence @cindex precedence declarations @cindex declaring operator precedence @@ -3093,7 +3219,8 @@ obtain the token type code number (@pxref{Calling Convention}). 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 @@ -3133,7 +3260,7 @@ 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 @@ -3164,7 +3291,7 @@ 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 @@ -3180,18 +3307,19 @@ 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. +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, Start Decl, Type Decl, Declarations +@node Expect Decl @subsection Suppressing Conflict Warnings @cindex suppressing conflict warnings @cindex preventing warnings about conflicts @@ -3200,11 +3328,11 @@ terminal symbol. All kinds of token declarations allow @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: @@ -3212,10 +3340,11 @@ 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: @@ -3239,7 +3368,7 @@ 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 @@ -3254,11 +3383,11 @@ may override this restriction with the @code{%start} declaration as follows: %start @var{symbol} @end example -@node Pure Decl, Decl Summary, Start Decl, Declarations +@node Pure Decl @subsection A Pure (Reentrant) Parser @cindex reentrant parser @cindex pure parser -@findex %pure_parser +@findex %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) @@ -3274,11 +3403,11 @@ statically allocated variables for communication with @code{yylex}, including @code{yylval} and @code{yylloc}.) Alternatively, you can generate a pure, reentrant parser. The Bison -declaration @code{%pure_parser} says that you want the parser to be +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 result is that the communication variables @code{yylval} and @@ -3294,13 +3423,13 @@ 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, , Pure Decl, Declarations +@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 @@ -3335,11 +3464,43 @@ Start-Symbol}). @item %expect Declare the expected number of shift-reduce conflicts (@pxref{Expect Decl, ,Suppressing Conflict Warnings}). +@end table -@item %yacc -@itemx %fixed_output_files -Pretend the option @option{--yacc} was given, i.e., imitate Yacc, -including its naming conventions. @xref{Bison Options}, for more. +@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}. + +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, @@ -3348,11 +3509,16 @@ 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 %pure_parser -Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure -(Reentrant) Parser}). +@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 +@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. @@ -3361,7 +3527,7 @@ 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 +@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 @@ -3369,40 +3535,21 @@ 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 %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 %defines -Write an extra output file containing macro definitions for the token -type names defined in the grammar and the semantic value type -@code{YYSTYPE}, as well as a few @code{extern} variable declarations. - -If the parser output file is named @file{@var{name}.c} then this file -is named @file{@var{name}.h}.@refill - -This output file is essential if you wish to put the definition of -@code{yylex} in a separate source file, because @code{yylex} needs to -be able to refer to token type codes and the variable -@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill - -@item %verbose -Write an extra output file containing verbose descriptions of the -parser states and what is done for each type of look-ahead token in -that state. - -This file also describes all the conflicts, both those resolved by -operator precedence and the unresolved ones. +@item %output="@var{filename}" +Specify the @var{filename} for the parser file. -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 %pure-parser +Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure +(Reentrant) Parser}). -Therefore, if the input file is @file{foo.y}, then the parser file is -called @file{foo.tab.c} by default. As a consequence, the verbose -output file is called @file{foo.output}.@refill +@c @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 +@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 @@ -3420,7 +3567,7 @@ 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 +When you specify @code{%token-table}, Bison also generates macro definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and @code{YYNRULES}, and @code{YYNSTATES}: @@ -3434,9 +3581,24 @@ The number of grammar rules, @item YYNSTATES The number of parser states (@pxref{Parser States}). @end table + +@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,, Declarations, Grammar File + + + +@node Multiple Parsers @section Multiple Parsers in the Same Program Most programs that use Bison parse only one language and therefore contain @@ -3445,10 +3607,10 @@ 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{yynerrs}, @code{yylval}, @code{yychar} and @@ -3466,7 +3628,7 @@ 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 @@ -3477,8 +3639,8 @@ 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. @@ -3488,7 +3650,7 @@ C code in the grammar file, you are likely to run into trouble. * 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 @@ -3516,7 +3678,7 @@ 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 @@ -3532,7 +3694,7 @@ 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}. @@ -3545,7 +3707,7 @@ 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 @@ -3619,10 +3781,10 @@ for (i = 0; i < YYNTOKENS; i++) @end smallexample The @code{yytname} table is generated only if you use the -@code{%token_table} declaration. @xref{Decl Summary}. +@code{%token-table} declaration. @xref{Decl Summary}. @end itemize -@node Token Values, Token Positions, Calling Convention, Lexical +@node Token Values @subsection Semantic Values of Tokens @vindex yylval @@ -3642,9 +3804,10 @@ 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 @@ -3668,7 +3831,7 @@ 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 @@ -3689,10 +3852,10 @@ 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 +@node Pure Calling @subsection Calling Conventions for Pure Parsers -When you use the Bison declaration @code{%pure_parser} to request a +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 @@ -3792,11 +3955,11 @@ arguments in total, depending on whether an argument of type 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 +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, Action Features, Lexical, Interface +@node Error Reporting @section The Error Reporting Function @code{yyerror} @cindex error reporting function @findex yyerror @@ -3853,10 +4016,10 @@ 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 @@ -3875,12 +4038,13 @@ 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 +@xref{Action Types, ,Data Types of Values in Actions}. @item YYABORT; Return immediately from @code{yyparse}, indicating failure. @@ -3894,6 +4058,7 @@ Return immediately from @code{yyparse}, indicating success. @findex YYBACKUP Unshift a token. This macro is allowed only for rules that reduce a single value, and only when there is no look-ahead token. +It is also disallowed in GLR parsers. It installs a look-ahead token with token type @var{token} and semantic value @var{value}; then it discards the value that was going to be reduced by this rule. @@ -3970,7 +4135,7 @@ Tracking Locations}. @end table -@node Algorithm, Error Recovery, Interface, Top +@node Algorithm @chapter The Bison Parser Algorithm @cindex Bison parser algorithm @cindex algorithm of parser @@ -4034,10 +4199,11 @@ This kind of parser is known in the literature as a bottom-up parser. * Parser States:: The parser is a finite-state-machine with stack. * Reduce/Reduce:: When two rules are applicable in the same situation. * Mystery Conflicts:: Reduce/reduce conflicts that look unjustified. +* Generalized LR Parsing:: Parsing arbitrary context-free grammars. * 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 @@ -4092,7 +4258,7 @@ 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 @@ -4188,7 +4354,7 @@ expr: variable ; @end example -@node Precedence, Contextual Precedence, Shift/Reduce, Algorithm +@node Precedence @section Operator Precedence @cindex operator precedence @cindex precedence of operators @@ -4205,7 +4371,7 @@ 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 @@ -4252,7 +4418,7 @@ 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 +@node Using Precedence @subsection Specifying Operator Precedence @findex %left @findex %right @@ -4273,7 +4439,7 @@ 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: @@ -4299,28 +4465,29 @@ 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 @@ -4337,7 +4504,7 @@ 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. @@ -4378,7 +4545,7 @@ 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 @@ -4404,7 +4571,7 @@ 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 @@ -4519,7 +4686,7 @@ 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. @@ -4627,7 +4794,83 @@ return_spec: ; @end example -@node Stack Overflow, , Mystery Conflicts, Algorithm +@node Generalized LR Parsing +@section Generalized LR (GLR) Parsing +@cindex GLR parsing +@cindex generalized LR (GLR) parsing +@cindex ambiguous grammars +@cindex non-deterministic parsing + +Bison produces @emph{deterministic} parsers that choose uniquely +when to reduce and which reduction to apply +based on a summary of the preceding input and on one extra token of lookahead. +As a result, normal Bison handles a proper subset of the family of +context-free languages. +Ambiguous grammars, since they have strings with more than one possible +sequence of reductions cannot have deterministic parsers in this sense. +The same is true of languages that require more than one symbol of +lookahead, since the parser lacks the information necessary to make a +decision at the point it must be made in a shift-reduce parser. +Finally, as previously mentioned (@pxref{Mystery Conflicts}), +there are languages where Bison's particular choice of how to +summarize the input seen so far loses necessary information. + +When you use the @samp{%glr-parser} declaration in your grammar file, +Bison generates a parser that uses a different algorithm, called +Generalized LR (or GLR). A Bison GLR parser uses the same basic +algorithm for parsing as an ordinary Bison parser, but behaves +differently in cases where there is a shift-reduce conflict that has not +been resolved by precedence rules (@pxref{Precedence}) or a +reduce-reduce conflict. When a GLR parser encounters such a situation, it +effectively @emph{splits} into a several parsers, one for each possible +shift or reduction. These parsers then proceed as usual, consuming +tokens in lock-step. Some of the stacks may encounter other conflicts +and split further, with the result that instead of a sequence of states, +a Bison GLR parsing stack is what is in effect a tree of states. + +In effect, each stack represents a guess as to what the proper parse +is. Additional input may indicate that a guess was wrong, in which case +the appropriate stack silently disappears. Otherwise, the semantics +actions generated in each stack are saved, rather than being executed +immediately. When a stack disappears, its saved semantic actions never +get executed. When a reduction causes two stacks to become equivalent, +their sets of semantic actions are both saved with the state that +results from the reduction. We say that two stacks are equivalent +when they both represent the same sequence of states, +and each pair of corresponding states represents a +grammar symbol that produces the same segment of the input token +stream. + +Whenever the parser makes a transition from having multiple +states to having one, it reverts to the normal LALR(1) parsing +algorithm, after resolving and executing the saved-up actions. +At this transition, some of the states on the stack will have semantic +values that are sets (actually multisets) of possible actions. The +parser tries to pick one of the actions by first finding one whose rule +has the highest dynamic precedence, as set by the @samp{%dprec} +declaration. Otherwise, if the alternative actions are not ordered by +precedence, but there the same merging function is declared for both +rules by the @samp{%merge} declaration, +Bison resolves and evaluates both and then calls the merge function on +the result. Otherwise, it reports an ambiguity. + +It is possible to use a data structure for the GLR parsing tree that +permits the processing of any LALR(1) grammar in linear time (in the +size of the input), any unambiguous (not necessarily LALR(1)) grammar in +quadratic worst-case time, and any general (possibly ambiguous) +context-free grammar in cubic worst-case time. However, Bison currently +uses a simpler data structure that requires time proportional to the +length of the input times the maximum number of stacks required for any +prefix of the input. Thus, really ambiguous or non-deterministic +grammars can require exponential time and space to process. Such badly +behaving examples, however, are not generally of practical interest. +Usually, non-determinism in a grammar is local---the parser is ``in +doubt'' only for a few tokens at a time. Therefore, the current data +structure should generally be adequate. On LALR(1) portions of a +grammar, in particular, it is only slightly slower than with the default +Bison parser. + +@node Stack Overflow @section Stack Overflow, and How to Avoid It @cindex stack overflow @cindex parser stack overflow @@ -4661,7 +4904,7 @@ 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 @@ -4779,7 +5022,7 @@ 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 @@ -4798,7 +5041,7 @@ 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 @@ -4873,7 +5116,7 @@ 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 @@ -4922,12 +5165,11 @@ 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. @@ -4982,9 +5224,414 @@ 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 @@ -4992,16 +5639,39 @@ 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}. @@ -5026,13 +5696,14 @@ 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 @@ -5064,7 +5735,9 @@ yyprint (FILE *file, int type, YYSTYPE value) @} @end smallexample -@node Invocation, Table of Symbols, Debugging, Top +@c ================================================= Invoking Bison + +@node Invocation @chapter Invoking Bison @cindex invoking Bison @cindex Bison invocation @@ -5080,8 +5753,8 @@ 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}. It's is also possible, in case you are writting -C++ code instead of C in your grammar file, to name it @file{foo.ypp} +@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 @@ -5105,12 +5778,11 @@ will produce @file{output.c++} and @file{outfile.h++}. @menu * 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, Environment Variables, , Invocation +@node Bison Options @section Bison Options Bison supports both traditional single-letter options and mnemonic long @@ -5139,12 +5811,11 @@ Print the version number of Bison and exit. @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 option is to imitate Yacc's output file name conventions. Thus, the following shell script can substitute -for Yacc:@refill +for Yacc: @example bison -y $* @@ -5162,24 +5833,17 @@ you are developing Bison. @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}. +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{%locactions} was specified. @xref{Decl Summary}. +Pretend that @code{%locations} was specified. @xref{Decl Summary}. @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{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. - -@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}. +Pretend that @code{%name-prefix="@var{prefix}"} was specified. +@xref{Decl Summary}. @item -l @itemx --no-lines @@ -5191,11 +5855,11 @@ parser file, treating it as an independent source file in its own right. @item -n @itemx --no-parser -Pretend that @code{%no_parser} was specified. @xref{Decl Summary}. +Pretend that @code{%no-parser} was specified. @xref{Decl Summary}. @item -k @itemx --token-table -Pretend that @code{%token_table} was specified. @xref{Decl Summary}. +Pretend that @code{%token-table} was specified. @xref{Decl Summary}. @end table @noindent @@ -5204,56 +5868,65 @@ Adjust the output: @table @option @item -d @itemx --defines -Pretend that @code{%verbose} was specified, i.e., write an extra output +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}. +@item --defines=@var{defines-file} +Same as above, but save in the file @var{defines-file}. + @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}. +Pretend that @code{%verbose} was specified, i.e, specify prefix to use +for all Bison output file names. @xref{Decl Summary}. + +@item -r @var{things} +@itemx --report=@var{things} +Write an extra output file containing verbose description of the comma +separated list of @var{things} among: + +@table @code +@item state +Description of the grammar, conflicts (resolved and unresolved), and +LALR automaton. + +@item lookahead +Implies @code{state} and augments the description of the automaton with +each rule's lookahead set. + +@item itemset +Implies @code{state} and augments the description of the automaton with +the full set of items for each state, instead of its core only. +@end table + +For instance, on the following grammar @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}, for more. - -@item -o @var{outfile} -@itemx --output-file=@var{outfile} -Specify the name @var{outfile} for the parser file. +parser. @xref{Decl Summary}. -The other output files' names are constructed from @var{outfile} -as described under the @samp{-v} and @samp{-d} options. -@end table - -@node Environment Variables, Option Cross Key, Bison Options, Invocation -@section Environment Variables -@cindex environment variables -@cindex BISON_HAIRY -@cindex BISON_SIMPLE +@item -o @var{filename} +@itemx --output=@var{filename} +Specify the @var{filename} for the parser file. -Here is a list of environment variables which affect the way Bison -runs. +The other output files' names are constructed from @var{filename} as +described under the @samp{-v} and @samp{-d} options. -@table @samp -@item BISON_SIMPLE -@itemx BISON_HAIRY -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. - -When the @samp{%semantic_parser} declaration is used, Bison copies from -a file called @file{bison.hairy} instead. The location of this file can -also be specified or overridden in a similar fashion, with the -@code{BISON_HAIRY} environment variable. +@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 -@node Option Cross Key, VMS Invocation, Environment Variables, Invocation +@node Option Cross Key @section Option Cross Key Here is a list of options, alphabetized by long option, to help you find @@ -5266,12 +5939,12 @@ 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{ --no-parser \leaderfill -n} -\line{ --output-file \leaderfill -o} +\line{ --output \leaderfill -o} \line{ --token-table \leaderfill -k} \line{ --verbose \leaderfill -v} \line{ --version \leaderfill -V} @@ -5282,21 +5955,22 @@ 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=@var{prefix} -p @var{name-prefix} --no-lines -l --no-parser -n ---output-file=@var{outfile} -o @var{outfile} +--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 @@ -5324,12 +5998,28 @@ 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 @@ -5355,6 +6045,10 @@ read, by making @code{yyparse} return 0 immediately. 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 @@ -5394,6 +6088,12 @@ accept. @xref{Pure Calling,, Calling Conventions for Pure Parsers}. 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}. @@ -5411,7 +6111,7 @@ 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 @@ -5424,8 +6124,9 @@ 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 @@ -5457,11 +6158,39 @@ Equip the parser for debugging. @xref{Decl Summary}. Bison declaration to create a header file meant for the scanner. @xref{Decl Summary}. +@item %dprec +Bison declaration to assign a precedence to a rule that is used at parse +time to resolve reduce/reduce conflicts. @xref{GLR Parsers}. + +@item %file-prefix="@var{prefix}" +Bison declaration to set the prefix of the output files. @xref{Decl +Summary}. + +@item %glr-parser +Bison declaration to produce a GLR parser. @xref{GLR Parsers}. + +@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 %no_lines +@item %merge +Bison declaration to assign a merging function to a rule. If there is a +reduce/reduce conflict with a rule having the same merging function, the +function is applied to the two semantic values to get a single result. +@xref{GLR Parsers}. + +@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}. @@ -5469,11 +6198,15 @@ parser file. @xref{Decl Summary}. 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}. @@ -5482,36 +6215,40 @@ 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 +@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 +the output file uninterpreted. Such code forms the prologue of the input +file. @xref{Grammar Outline, ,Outline of a Bison Grammar}. @item /*@dots{}*/ @@ -5529,7 +6266,7 @@ 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 @@ -5562,6 +6299,13 @@ machine. In the case of the parser, the input is the language being parsed, and the states correspond to various stages in the grammar rules. @xref{Algorithm, ,The Bison Parser Algorithm }. +@item Generalized LR (GLR) +A parsing algorithm that can handle all context-free grammars, including those +that are not LALR(1). It resolves situations that Bison's usual LALR(1) +algorithm cannot by effectively splitting off multiple parsers, trying all +possible parsers, and discarding those that fail in the light of additional +right context. @xref{Generalized LR Parsing, ,Generalized LR Parsing}. + @item Grouping A language construct that is (in general) grammatically divisible; for example, `expression' or `declaration' in C. @@ -5690,7 +6434,16 @@ grammatically indivisible. The piece of text it represents is a token. @xref{Language and Grammar, ,Languages and Context-Free Grammars}. @end table -@node Index, , Glossary, Top +@node Copying This Manual +@appendix Copying This Manual + +@menu +* GNU Free Documentation License:: License for copying this manual. +@end menu + +@include fdl.texi + +@node Index @unnumbered Index @printindex cp