X-Git-Url: https://git.saurik.com/bison.git/blobdiff_plain/3ded9a63e320a0f6142c1112b248a7926569d791..231897ad21e478b0b0821636aa49fe49ca0f3301:/doc/bison.texinfo?ds=inline diff --git a/doc/bison.texinfo b/doc/bison.texinfo index 58186349..3ed6b7f5 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 @@ -35,94 +34,54 @@ @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, 2001, 2002 -Free Software Foundation, Inc. - -Permission is granted to make and distribute verbatim copies of -this manual provided the copyright notice and this permission notice -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 @acronym{GNU} Bison (version @value{VERSION}, +@value{UPDATED}), the @acronym{GNU} parser generator. + +Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 2003, +1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc. + +@quotation +Permission is granted to copy, distribute and/or modify this document +under the terms of the @acronym{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 @acronym{GNU} Manual,'' and with the Back-Cover Texts as in +(a) below. A copy of the license is included in the section entitled +``@acronym{GNU} Free Documentation License.'' + +(a) The @acronym{FSF}'s Back-Cover Text is: ``You have freedom to copy +and modify this @acronym{GNU} Manual, like @acronym{GNU} software. +Copies published by the Free Software Foundation raise funds for +@acronym{GNU} development.'' +@end quotation +@end copying + +@dircategory GNU programming tools +@direntry +* bison: (bison). @acronym{GNU} parser generator (Yacc replacement). +@end direntry @ifset shorttitlepage-enabled @shorttitlepage Bison @end ifset @titlepage @title Bison -@subtitle The YACC-compatible Parser Generator +@subtitle The Yacc-compatible Parser Generator @subtitle @value{UPDATED}, Bison Version @value{VERSION} @author by Charles Donnelly and Richard Stallman @page @vskip 0pt plus 1filll -Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, -1999, 2000, 2001, 2002 -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. +@acronym{ISBN} 1-882114-44-2 @sp 2 Cover art by Etienne Suvasa. @end titlepage @@ -132,15 +91,13 @@ Cover art by Etienne Suvasa. @ifnottex @node Top @top Bison - -This manual documents version @value{VERSION} of Bison, updated -@value{UPDATED}. +@insertcopying @end ifnottex @menu * Introduction:: * Conditions:: -* Copying:: The GNU General Public License says +* Copying:: The @acronym{GNU} General Public License says how you can copy and share Bison Tutorial sections: @@ -154,14 +111,16 @@ 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. +* FAQ:: Frequently Asked Questions * Copying This Manual:: License for copying this manual. * Index:: Cross-references to the text. -@detailmenu --- The Detailed Node Listing --- +@detailmenu + --- The Detailed Node Listing --- The Concepts of Bison @@ -172,6 +131,8 @@ The Concepts of Bison a semantic value (the value of an integer, the name of an identifier, etc.). * Semantic Actions:: Each rule can have an action containing C code. +* GLR Parsers:: Writing parsers for general context-free languages +* Locations Overview:: Tracking Locations. * Bison Parser:: What are Bison's input and output, how is the output used? * Stages:: Stages in writing and running Bison grammars. @@ -185,8 +146,8 @@ Examples 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. +* 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 @@ -224,15 +185,16 @@ Bison Grammar Files * Rules:: How to write grammar rules. * Recursion:: Writing recursive rules. * Semantics:: Semantic values and actions. +* Locations:: Locations and actions. * Declarations:: All kinds of Bison declarations are described here. * Multiple Parsers:: Putting more than one Bison parser in one program. Outline of a Bison Grammar -* Prologue:: Syntax and usage of the prologue (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. -* Epilogue:: Syntax and usage of the epilogue (additional code section). +* Epilogue:: Syntax and usage of the epilogue. Defining Language Semantics @@ -244,12 +206,19 @@ Defining Language Semantics This says when, why and how to use the exceptional action in the middle of a rule. +Tracking Locations + +* Location Type:: Specifying a data type for locations. +* Actions and Locations:: Using locations in actions. +* Location Default Action:: Defining a general way to compute locations. + Bison Declarations * Token Decl:: Declaring terminal symbols. * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. +* Destructor Decl:: Declaring how symbols are freed. * Expect Decl:: Suppressing warnings about shift/reduce conflicts. * Start Decl:: Specifying the start symbol. * Pure Decl:: Requesting a reentrant parser. @@ -268,9 +237,9 @@ The Lexical Analyzer Function @code{yylex} * Calling Convention:: How @code{yyparse} calls @code{yylex}. * Token Values:: How @code{yylex} must return the semantic value of the token it has read. -* Token Positions:: How @code{yylex} must return the text position +* Token Locations:: How @code{yylex} must return the text location (line number, etc.) of the token, if the - actions want that. + actions want that. * Pure Calling:: How the calling convention differs in a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). @@ -283,6 +252,7 @@ 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 @@ -299,12 +269,23 @@ Handling Context Dependencies * Tie-in Recovery:: Lexical tie-ins have implications for how error recovery rules must be written. +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. + in alphabetical order by short options. * Option Cross Key:: Alphabetical list of long options. -* VMS Invocation:: Bison command syntax on VMS. +* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}. + +Frequently Asked Questions + +* Parser Stack Overflow:: Breaking the Stack Limits +* How Can I Reset @code{yyparse}:: @code{yyparse} Keeps some State +* Strings are Destroyed:: @code{yylval} Loses Track of Strings Copying This Manual @@ -318,7 +299,7 @@ Copying This Manual @cindex introduction @dfn{Bison} is a general-purpose parser generator that converts a -grammar description for an LALR(1) context-free grammar into a C +grammar description for an @acronym{LALR}(1) context-free grammar into a C program to parse that grammar. Once you are proficient with Bison, you may use it to develop a wide range of language parsers, from those used in simple desk calculators to complex programming languages. @@ -343,11 +324,12 @@ This edition corresponds to version @value{VERSION} of Bison. @unnumbered Conditions for Using Bison As of Bison version 1.24, we have changed the distribution terms for -@code{yyparse} to permit using Bison's output in nonfree programs. -Formerly, Bison parsers could be used only in programs that were free -software. +@code{yyparse} to permit using Bison's output in nonfree programs when +Bison is generating C code for @acronym{LALR}(1) parsers. Formerly, these +parsers could be used only in programs that were free software. -The other GNU programming tools, such as the GNU C compiler, have never +The other @acronym{GNU} programming tools, such as the @acronym{GNU} C +compiler, have never had such a requirement. They could always be used for nonfree software. The reason Bison was different was not due to a special policy decision; it resulted from applying the usual General Public @@ -357,7 +339,8 @@ The output of the Bison utility---the Bison parser file---contains a verbatim copy of a sizable piece of Bison, which is the code for the @code{yyparse} function. (The actions from your grammar are inserted into this function at one point, but the rest of the function is not -changed.) When we applied the GPL terms to the code for @code{yyparse}, +changed.) When we applied the @acronym{GPL} terms to the code for +@code{yyparse}, the effect was to restrict the use of Bison output to free software. We didn't change the terms because of sympathy for people who want to @@ -365,7 +348,15 @@ make software proprietary. @strong{Software should be free.} But we concluded that limiting Bison's use to free software was doing little to encourage people to make other software free. So we decided to make the practical conditions for using Bison match the practical conditions for -using the other GNU tools. +using the other @acronym{GNU} tools. + +This exception applies only when Bison is generating C code for a +@acronym{LALR}(1) parser; otherwise, the @acronym{GPL} terms operate +as usual. You can +tell whether the exception applies to your @samp{.c} output file by +inspecting it to see whether it says ``As a special exception, when +this file is copied by Bison into a Bison output file, you may use +that output file without restriction.'' @include gpl.texi @@ -384,6 +375,7 @@ 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? @@ -406,43 +398,68 @@ can be made of a minus sign and another expression''. Another would be, recursive, but there must be at least one rule which leads out of the recursion. -@cindex BNF +@cindex @acronym{BNF} @cindex Backus-Naur form The most common formal system for presenting such rules for humans to read -is @dfn{Backus-Naur Form} or ``BNF'', which was developed in order to -specify the language Algol 60. Any grammar expressed in BNF is a -context-free grammar. The input to Bison is essentially machine-readable -BNF. - -Not all context-free languages can be handled by Bison, only those -that are LALR(1). In brief, this means that it must be possible to +is @dfn{Backus-Naur Form} or ``@acronym{BNF}'', which was developed in +order to specify the language Algol 60. Any grammar expressed in +@acronym{BNF} is a context-free grammar. The input to Bison is +essentially machine-readable @acronym{BNF}. + +@cindex @acronym{LALR}(1) grammars +@cindex @acronym{LR}(1) grammars +There are various important subclasses of context-free grammar. Although it +can handle almost all context-free grammars, Bison is optimized for what +are called @acronym{LALR}(1) grammars. +In brief, in these grammars, it must be possible to tell how to parse any portion of an input string with just a single token of look-ahead. Strictly speaking, that is a description of an -LR(1) grammar, and LALR(1) involves additional restrictions that are +@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional +restrictions that are hard to explain simply; but it is rare in actual practice to find an -LR(1) grammar that fails to be LALR(1). @xref{Mystery Conflicts, , -Mysterious Reduce/Reduce Conflicts}, for more information on this. +@acronym{LR}(1) grammar that fails to be @acronym{LALR}(1). +@xref{Mystery Conflicts, ,Mysterious Reduce/Reduce Conflicts}, for +more information on this. + +@cindex @acronym{GLR} parsing +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing +@cindex ambiguous grammars +@cindex non-deterministic parsing + +Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning +roughly that the next grammar rule to apply at any point in the input is +uniquely determined by the preceding input and a fixed, finite portion +(called a @dfn{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 @acronym{GLR} +parsing (for Generalized @acronym{LR}). Bison's @acronym{GLR} parsers +are able to handle any context-free grammar for which the number of +possible parses of any given string is finite. @cindex symbols (abstract) @cindex token @cindex syntactic grouping @cindex grouping, syntactic -In the formal grammatical rules for a language, each kind of syntactic unit -or grouping is named by a @dfn{symbol}. Those which are built by grouping -smaller constructs according to grammatical rules are called +In the formal grammatical rules for a language, each kind of syntactic +unit or grouping is named by a @dfn{symbol}. Those which are built by +grouping smaller constructs according to grammatical rules are called @dfn{nonterminal symbols}; those which can't be subdivided are called @dfn{terminal symbols} or @dfn{token types}. We call a piece of input corresponding to a single terminal symbol a @dfn{token}, and a piece corresponding to a single nonterminal symbol a @dfn{grouping}. We can use the C language as an example of what symbols, terminal and -nonterminal, mean. The tokens of C are identifiers, constants (numeric and -string), and the various keywords, arithmetic operators and punctuation -marks. So the terminal symbols of a grammar for C include `identifier', -`number', `string', plus one symbol for each keyword, operator or -punctuation mark: `if', `return', `const', `static', `int', `char', -`plus-sign', `open-brace', `close-brace', `comma' and many more. (These -tokens can be subdivided into characters, but that is a matter of +nonterminal, mean. The tokens of C are identifiers, constants (numeric +and string), and the various keywords, arithmetic operators and +punctuation marks. So the terminal symbols of a grammar for C include +`identifier', `number', `string', plus one symbol for each keyword, +operator or punctuation mark: `if', `return', `const', `static', `int', +`char', `plus-sign', `open-brace', `close-brace', `comma' and many more. +(These tokens can be subdivided into characters, but that is a matter of lexicography, not grammar.) Here is a simple C function subdivided into tokens: @@ -521,7 +538,7 @@ for Bison, you must write a file expressing the grammar in Bison syntax: a @dfn{Bison grammar} file. @xref{Grammar File, ,Bison Grammar Files}. A nonterminal symbol in the formal grammar is represented in Bison input -as an identifier, like an identifier in C. By convention, it should be +as an identifier, like an identifier in C@. By convention, it should be in lower case, such as @code{expr}, @code{stmt} or @code{declaration}. The Bison representation for a terminal symbol is also called a @dfn{token @@ -570,7 +587,8 @@ grammatical. But the precise value is very important for what the input means once it is parsed. A compiler is useless if it fails to distinguish between 4, 1 and 3989 as constants in the program! Therefore, each token in a Bison grammar -has both a token type and a @dfn{semantic value}. @xref{Semantics, ,Defining Language Semantics}, +has both a token type and a @dfn{semantic value}. @xref{Semantics, +,Defining Language Semantics}, for details. The token type is a terminal symbol defined in the grammar, such as @@ -628,32 +646,240 @@ 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 GLR Parsers +@section Writing @acronym{GLR} Parsers +@cindex @acronym{GLR} parsing +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing +@findex %glr-parser +@cindex conflicts +@cindex shift/reduce conflicts + +In some grammars, there will be cases where Bison's standard +@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a +certain grammar rule at a given point. That is, it may not be able to +decide (on the basis of the input read so far) which of two possible +reductions (applications of a grammar rule) applies, or whether to apply +a reduction or read more of the input and apply a reduction later in the +input. These are known respectively as @dfn{reduce/reduce} conflicts +(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts +(@pxref{Shift/Reduce}). + +To use a grammar that is not easily modified to be @acronym{LALR}(1), a +more general parsing algorithm is sometimes necessary. If you include +@code{%glr-parser} among the Bison declarations in your file +(@pxref{Grammar Outline}), the result will be a Generalized @acronym{LR} +(@acronym{GLR}) parser. These parsers handle Bison grammars that +contain no unresolved conflicts (i.e., after applying precedence +declarations) identically to @acronym{LALR}(1) parsers. However, when +faced with unresolved shift/reduce and reduce/reduce conflicts, +@acronym{GLR} parsers use the simple expedient of doing both, +effectively cloning the parser to follow both possibilities. Each of +the resulting parsers can again split, so that at any given time, there +can be any number of possible parses being explored. The parsers +proceed in lockstep; that is, all of them consume (shift) a given input +symbol before any of them proceed to the next. Each of the cloned +parsers eventually meets one of two possible fates: either it runs into +a parsing error, in which case it simply vanishes, or it merges with +another parser, because the two of them have reduced the input to an +identical set of symbols. + +During the time that there are multiple parsers, semantic actions are +recorded, but not performed. When a parser disappears, its recorded +semantic actions disappear as well, and are never performed. When a +reduction makes two parsers identical, causing them to merge, Bison +records both sets of semantic actions. Whenever the last two parsers +merge, reverting to the single-parser case, Bison resolves all the +outstanding actions either by precedences given to the grammar rules +involved, or by performing both actions, and then calling a designated +user-defined function on the resulting values to produce an arbitrary +merged result. + +Let's consider an example, vastly simplified from a C++ grammar. + +@example +%@{ + #include + #define YYSTYPE char const * + int yylex (void); + void yyerror (char const *); +%@} + +%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 @code{TYPENAME} and +@samp{x} as an @code{ID}). +Bison detects this as a reduce/reduce conflict between the rules +@code{expr : ID} and @code{declarator : ID}, which it cannot resolve at the +time it encounters @code{x} in the example above. 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 char const * + 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 + +@sp 1 + +@cindex @code{incline} +@cindex @acronym{GLR} parsers and @code{inline} +The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or +later. In addition, they use the @code{inline} keyword, which is not +C89, but is C99 and is a common extension in pre-C99 compilers. It is +up to the user of these parsers to handle +portability issues. For instance, if using Autoconf and the Autoconf +macro @code{AC_C_INLINE}, a mere + +@example +%@{ + #include +%@} +@end example + +@noindent +will suffice. Otherwise, we suggest + +@example +%@{ + #if __STDC_VERSION__ < 199901 && ! defined __GNUC__ && ! defined inline + #define inline + #endif +%@} +@end example + @node Locations Overview @section Locations @cindex location -@cindex textual position -@cindex position, textual +@cindex textual location +@cindex location, textual Many applications, like interpreters or compilers, have to produce verbose -and useful error messages. To achieve this, one must be able to keep track of -the @dfn{textual position}, or @dfn{location}, of each syntactic construct. +and useful error messages. To achieve this, one must be able to keep track of +the @dfn{textual location}, or @dfn{location}, of each syntactic construct. Bison provides a mechanism for handling these locations. -Each token has a semantic value. In a similar fashion, each token has an +Each token has a semantic value. In a similar fashion, each token has an associated location, but the type of locations is the same for all tokens and -groupings. Moreover, the output parser is equipped with a default data +groupings. Moreover, the output parser is equipped with a default data structure for storing locations (@pxref{Locations}, for more details). Like semantic values, locations can be reached in actions using a dedicated -set of constructs. In the example above, the location of the whole grouping +set of constructs. In the example above, the location of the whole grouping is @code{@@$}, while the locations of the subexpressions are @code{@@1} and @code{@@3}. When a rule is matched, a default action is used to compute the semantic value -of its left hand side (@pxref{Actions}). In the same way, another default -action is used for locations. However, the action for locations is general +of its left hand side (@pxref{Actions}). In the same way, another default +action is used for locations. However, the action for locations is general enough for most cases, meaning there is usually no need to describe for each -rule how @code{@@$} should be formed. When building a new location for a given +rule how @code{@@$} 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. @@ -705,11 +931,11 @@ this manual. 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{}, +headers. On some non-@acronym{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{Debugging, ,Debugging Your Parser}). +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 @@ -766,7 +992,7 @@ general form of a Bison grammar file is as follows: @example %@{ -@var{Prologue (declarations)} +@var{Prologue} %@} @var{Bison declarations} @@ -774,16 +1000,19 @@ general form of a Bison grammar file is as follows: %% @var{Grammar rules} %% -@var{Epilogue (additional 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 prologue may define types and variables used in the actions. You can +The prologue may define types and variables used in the actions. You can also use preprocessor commands to define macros used there, and use @code{#include} to include header files that do any of these things. +You need to declare the lexical analyzer @code{yylex} and the error +printer @code{yyerror} here, along with any other global identifiers +used by the actions in the grammar rules. The Bison declarations declare the names of the terminal and nonterminal symbols, and may also describe operator precedence and the data types of @@ -792,10 +1021,9 @@ semantic values of various symbols. The grammar rules define how to construct each nonterminal symbol from its parts. -The epilogue can contain any code you want to use. Often the 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 +definitions of functions declared in the prologue go here. In a +simple program, all the rest of the program can go here. @node Examples @chapter Examples @@ -859,20 +1087,22 @@ Here are the C and Bison declarations for the reverse polish notation calculator. As in C, comments are placed between @samp{/*@dots{}*/}. @example -/* Reverse polish notation calculator. */ +/* Reverse polish notation calculator. */ %@{ -#define YYSTYPE double -#include + #define YYSTYPE double + #include + int yylex (void); + void yyerror (char const *); %@} %token NUM -%% /* Grammar rules and actions follow */ +%% /* Grammar rules and actions follow. */ @end example The declarations section (@pxref{Prologue, , The prologue}) contains two -preprocessor directives. +preprocessor directives and two forward declarations. The @code{#define} directive defines the macro @code{YYSTYPE}, thus specifying the C data type for semantic values of both tokens and @@ -885,6 +1115,12 @@ which is a floating point number. The @code{#include} directive is used to declare the exponentiation function @code{pow}. +The forward declarations for @code{yylex} and @code{yyerror} are +needed because the C language requires that functions be declared +before they are used. These functions will be defined in the +epilogue, but the parser calls them so they must be declared in the +prologue. + The second section, Bison declarations, provides information to Bison about the token types (@pxref{Bison Declarations, ,The Bison Declarations Section}). Each terminal symbol that is not a @@ -905,18 +1141,18 @@ input: /* empty */ ; line: '\n' - | exp '\n' @{ printf ("\t%.10g\n", $1); @} + | exp '\n' @{ printf ("\t%.10g\n", $1); @} ; -exp: NUM @{ $$ = $1; @} - | exp exp '+' @{ $$ = $1 + $2; @} - | exp exp '-' @{ $$ = $1 - $2; @} - | exp exp '*' @{ $$ = $1 * $2; @} - | exp exp '/' @{ $$ = $1 / $2; @} - /* Exponentiation */ - | exp exp '^' @{ $$ = pow ($1, $2); @} - /* Unary minus */ - | exp 'n' @{ $$ = -$1; @} +exp: NUM @{ $$ = $1; @} + | exp exp '+' @{ $$ = $1 + $2; @} + | exp exp '-' @{ $$ = $1 - $2; @} + | exp exp '*' @{ $$ = $1 * $2; @} + | exp exp '/' @{ $$ = $1 / $2; @} + /* Exponentiation */ + | exp exp '^' @{ $$ = pow ($1, $2); @} + /* Unary minus */ + | exp 'n' @{ $$ = -$1; @} ; %% @end example @@ -977,7 +1213,7 @@ more times. The parser function @code{yyparse} continues to process input until a grammatical error is seen or the lexical analyzer says there are no more -input tokens; we will arrange for the latter to happen at end of file. +input tokens; we will arrange for the latter to happen at end-of-input. @node Rpcalc Line @subsubsection Explanation of @code{line} @@ -1044,7 +1280,7 @@ action, Bison by default copies the value of @code{$1} into @code{$$}. This is what happens in the first rule (the one that uses @code{NUM}). The formatting shown here is the recommended convention, but Bison does -not require it. You can add or change whitespace as much as you wish. +not require it. You can add or change white space as much as you wish. For example, this: @example @@ -1073,7 +1309,8 @@ or sequences of characters into tokens. The Bison parser gets its tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}. -Only a simple lexical analyzer is needed for the RPN calculator. This +Only a simple lexical analyzer is needed for the @acronym{RPN} +calculator. This lexical analyzer skips blanks and tabs, then reads in numbers as @code{double} and returns them as @code{NUM} tokens. Any other character that isn't part of a number is a separate token. Note that the token-code @@ -1095,18 +1332,17 @@ 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 -input.) +A token type code of zero is returned if the end-of-input is encountered. +(Bison recognizes any nonpositive value as indicating end-of-input.) Here is the code for the lexical analyzer: @example @group -/* Lexical analyzer returns a double floating point +/* The lexical analyzer returns a double floating point number on the stack and the token NUM, or the numeric code - of the character read if not a number. Skips all blanks - and tabs, returns 0 for EOF. */ + of the character read if not a number. It skips all blanks + and tabs, and returns 0 for end-of-input. */ #include @end group @@ -1117,12 +1353,12 @@ yylex (void) @{ int c; - /* skip white space */ + /* Skip white space. */ while ((c = getchar ()) == ' ' || c == '\t') ; @end group @group - /* process numbers */ + /* Process numbers. */ if (c == '.' || isdigit (c)) @{ ungetc (c, stdin); @@ -1131,10 +1367,10 @@ yylex (void) @} @end group @group - /* return end-of-file */ + /* Return end-of-input. */ if (c == EOF) return 0; - /* return single chars */ + /* Return a single char. */ return c; @} @end group @@ -1165,7 +1401,7 @@ main (void) When @code{yyparse} detects a syntax error, it calls the error reporting function @code{yyerror} to print an error message (usually but not -always @code{"parse error"}). It is up to the programmer to supply +always @code{"syntax error"}). It is up to the programmer to supply @code{yyerror} (@pxref{Interface, ,Parser C-Language Interface}), so here is the definition we will use: @@ -1173,8 +1409,9 @@ here is the definition we will use: @group #include +/* Called by yyparse on error. */ void -yyerror (const char *s) /* Called by yyparse on error */ +yyerror (char const *s) @{ printf ("%s\n", s); @} @@ -1211,8 +1448,8 @@ bison @var{file_name}.y @noindent In this example the file was called @file{rpcalc.y} (for ``Reverse Polish -CALCulator''). Bison produces a file named @file{@var{file_name}.tab.c}, -removing the @samp{.y} from the original file name. The file output by +@sc{calc}ulator''). Bison produces a file named @file{@var{file_name}.tab.c}, +removing the @samp{.y} from the original file name. The file output by Bison contains the source code for @code{yyparse}. The additional functions in the input file (@code{yylex}, @code{yyerror} and @code{main}) are copied verbatim to the output. @@ -1233,7 +1470,7 @@ rpcalc.tab.c rpcalc.y @group # @r{Compile the Bison parser.} # @r{@samp{-lm} tells compiler to search math library for @code{pow}.} -$ @kbd{cc rpcalc.tab.c -lm -o rpcalc} +$ @kbd{cc -lm -o rpcalc rpcalc.tab.c} @end group @group @@ -1274,23 +1511,25 @@ parentheses nested to arbitrary depth. Here is the Bison code for @file{calc.y}, an infix desk-top calculator. @example -/* Infix notation calculator--calc */ +/* Infix notation calculator. */ %@{ -#define YYSTYPE double -#include + #define YYSTYPE double + #include + #include + int yylex (void); + void yyerror (char const *); %@} -/* BISON Declarations */ +/* Bison declarations. */ %token NUM %left '-' '+' %left '*' '/' %left NEG /* negation--unary minus */ -%right '^' /* exponentiation */ +%right '^' /* exponentiation */ -/* Grammar follows */ -%% -input: /* empty string */ +%% /* The grammar follows. */ +input: /* empty */ | input line ; @@ -1375,7 +1614,7 @@ line: '\n' @end example This addition to the grammar allows for simple error recovery in the -event of a parse error. If an expression that cannot be evaluated is +event of a syntax error. If an expression that cannot be evaluated is read, the error will be recognized by the third rule for @code{line}, and parsing will continue. (The @code{yyerror} function is still called upon to print its message as well.) The action executes the statement @@ -1402,7 +1641,7 @@ 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. +analyzer. @menu * Decls: Ltcalc Decls. Bison and C declarations for ltcalc. @@ -1420,8 +1659,10 @@ the same as the declarations for the infix notation calculator. /* Location tracking calculator. */ %@{ -#define YYSTYPE int -#include + #define YYSTYPE int + #include + int yylex (void); + void yyerror (char const *); %@} /* Bison declarations. */ @@ -1432,7 +1673,7 @@ the same as the declarations for the infix notation calculator. %left NEG %right '^' -%% /* Grammar follows */ +%% /* The grammar follows. */ @end example @noindent @@ -1510,7 +1751,7 @@ hand. @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 +tracking. The next step is to rewrite the lexical analyzer, and make it able to feed the parser with the token locations, as it already does for semantic values. @@ -1523,18 +1764,22 @@ int yylex (void) @{ int c; +@end group - /* skip white space */ +@group + /* Skip white space. */ while ((c = getchar ()) == ' ' || c == '\t') ++yylloc.last_column; +@end group - /* step */ +@group + /* Step. */ yylloc.first_line = yylloc.last_line; yylloc.first_column = yylloc.last_column; @end group @group - /* process numbers */ + /* Process numbers. */ if (isdigit (c)) @{ yylval = c - '0'; @@ -1549,11 +1794,11 @@ yylex (void) @} @end group - /* return end-of-file */ + /* Return end-of-input. */ if (c == EOF) return 0; - /* return single chars and update location */ + /* Return a single char, and update location. */ if (c == '\n') @{ ++yylloc.last_line; @@ -1571,7 +1816,7 @@ 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 +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: @@ -1649,28 +1894,32 @@ Note that multiple assignment and nested function calls are permitted. Here are the C and Bison declarations for the multi-function calculator. @smallexample +@group %@{ -#include /* For math functions, cos(), sin(), etc. */ -#include "calc.h" /* Contains definition of `symrec' */ + #include /* For math functions, cos(), sin(), etc. */ + #include "calc.h" /* Contains definition of `symrec'. */ + int yylex (void); + void yyerror (char const *); %@} +@end group +@group %union @{ -double val; /* For returning numbers. */ -symrec *tptr; /* For returning symbol-table pointers */ + double val; /* For returning numbers. */ + symrec *tptr; /* For returning symbol-table pointers. */ @} - -%token NUM /* Simple double precision number */ -%token VAR FNCT /* Variable and Function */ +@end group +%token NUM /* Simple double precision number. */ +%token VAR FNCT /* Variable and Function. */ %type exp +@group %right '=' %left '-' '+' %left '*' '/' -%left NEG /* Negation--unary minus */ -%right '^' /* Exponentiation */ - -/* Grammar follows */ - -%% +%left NEG /* negation--unary minus */ +%right '^' /* exponentiation */ +@end group +%% /* The grammar follows. */ @end smallexample The above grammar introduces only two new features of the Bison language. @@ -1703,16 +1952,21 @@ Most of them are copied directly from @code{calc}; three rules, those which mention @code{VAR} or @code{FNCT}, are new. @smallexample +@group input: /* empty */ | input line ; +@end group +@group line: '\n' | exp '\n' @{ printf ("\t%.10g\n", $1); @} | error '\n' @{ yyerrok; @} ; +@end group +@group exp: NUM @{ $$ = $1; @} | VAR @{ $$ = $1->value.var; @} | VAR '=' exp @{ $$ = $3; $1->value.var = $3; @} @@ -1725,7 +1979,8 @@ exp: NUM @{ $$ = $1; @} | exp '^' exp @{ $$ = pow ($1, $3); @} | '(' exp ')' @{ $$ = $2; @} ; -/* End of grammar */ +@end group +/* End of grammar. */ %% @end smallexample @@ -1744,31 +1999,33 @@ provides for either functions or variables to be placed in the table. @smallexample @group -/* Fonctions type. */ +/* Function type. */ typedef double (*func_t) (double); +@end group -/* Data type for links in the chain of symbols. */ +@group +/* Data type for links in the chain of symbols. */ struct symrec @{ - char *name; /* name of symbol */ + char *name; /* name of symbol */ int type; /* type of symbol: either VAR or FNCT */ union @{ - double var; /* value of a VAR */ - func_t fnctptr; /* value of a FNCT */ + double var; /* value of a VAR */ + func_t fnctptr; /* value of a FNCT */ @} value; - struct symrec *next; /* link field */ + struct symrec *next; /* link field */ @}; @end group @group typedef struct symrec symrec; -/* The symbol table: a chain of `struct symrec'. */ +/* The symbol table: a chain of `struct symrec'. */ extern symrec *sym_table; -symrec *putsym (const char *, func_t); -symrec *getsym (const char *); +symrec *putsym (char const *, func_t); +symrec *getsym (char const *); @end group @end smallexample @@ -1777,33 +2034,27 @@ function that initializes the symbol table. Here it is, and @code{init_table} as well: @smallexample -@group #include -int -main (void) -@{ - init_table (); - return yyparse (); -@} -@end group - @group +/* Called by yyparse on error. */ void -yyerror (const char *s) /* Called by yyparse on error */ +yyerror (char const *s) @{ printf ("%s\n", s); @} +@end group +@group struct init @{ - char *fname; - double (*fnct)(double); + char const *fname; + double (*fnct) (double); @}; @end group @group -struct init arith_fncts[] = +struct init const arith_fncts[] = @{ "sin", sin, "cos", cos, @@ -1813,13 +2064,15 @@ struct init arith_fncts[] = "sqrt", sqrt, 0, 0 @}; +@end group +@group /* The symbol table: a chain of `struct symrec'. */ -symrec *sym_table = (symrec *) 0; +symrec *sym_table; @end group @group -/* Put arithmetic functions in table. */ +/* Put arithmetic functions in table. */ void init_table (void) @{ @@ -1832,6 +2085,15 @@ init_table (void) @} @} @end group + +@group +int +main (void) +@{ + init_table (); + return yyparse (); +@} +@end group @end smallexample By simply editing the initialization list and adding the necessary include @@ -1846,21 +2108,21 @@ found, a pointer to that symbol is returned; otherwise zero is returned. @smallexample symrec * -putsym (char *sym_name, int sym_type) +putsym (char const *sym_name, int sym_type) @{ symrec *ptr; ptr = (symrec *) malloc (sizeof (symrec)); ptr->name = (char *) malloc (strlen (sym_name) + 1); strcpy (ptr->name,sym_name); ptr->type = sym_type; - ptr->value.var = 0; /* set value to 0 even if fctn. */ + ptr->value.var = 0; /* Set value to 0 even if fctn. */ ptr->next = (struct symrec *)sym_table; sym_table = ptr; return ptr; @} symrec * -getsym (const char *sym_name) +getsym (char const *sym_name) @{ symrec *ptr; for (ptr = sym_table; ptr != (symrec *) 0; @@ -1889,13 +2151,15 @@ operators in @code{yylex}. @smallexample @group #include +@end group +@group int yylex (void) @{ int c; - /* Ignore whitespace, get first nonwhite character. */ + /* Ignore white space, get first nonwhite character. */ while ((c = getchar ()) == ' ' || c == '\t'); if (c == EOF) @@ -1937,7 +2201,7 @@ yylex (void) if (i == length) @{ length *= 2; - symbuf = (char *)realloc (symbuf, length + 1); + symbuf = (char *) realloc (symbuf, length + 1); @} /* Add this character to the buffer. */ symbuf[i++] = c; @@ -1946,7 +2210,7 @@ yylex (void) @} @end group @group - while (c != EOF && isalnum (c)); + while (isalnum (c)); ungetc (c, stdin); symbuf[i] = '\0'; @@ -1966,7 +2230,7 @@ yylex (void) @end group @end smallexample -This program is both powerful and flexible. You may easily add new +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. @@ -2016,7 +2280,7 @@ appropriate delimiters: @example %@{ -@var{Prologue} + @var{Prologue} %@} @var{Bison declarations} @@ -2029,6 +2293,8 @@ appropriate delimiters: @end example Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections. +As a @acronym{GNU} extension, @samp{//} introduces a comment that +continues until end of line. @menu * Prologue:: Syntax and usage of the prologue. @@ -2037,13 +2303,13 @@ Comments enclosed in @samp{/* @dots{} */} may appear in any of the sections. * Epilogue:: Syntax and usage of the epilogue. @end menu -@node Prologue, Bison Declarations, , Grammar Outline +@node Prologue @subsection The prologue @cindex declarations section @cindex Prologue @cindex declarations -The @var{prologue} 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 @@ -2051,6 +2317,33 @@ 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. +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 print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(F, N, L) print_token_value (F, N, L) +%@} + +@dots{} +@end smallexample + @node Bison Declarations @subsection The Bison Declarations Section @cindex Bison declarations (introduction) @@ -2073,24 +2366,27 @@ 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 Epilogue, , Grammar Rules, Grammar Outline +@node Epilogue @subsection The epilogue @cindex additional C code section @cindex epilogue @cindex C code, section for additional -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 +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. +definitions of @code{yylex} and @code{yyerror} often go here. Because +C requires functions to be declared before being used, you often need +to declare functions like @code{yylex} and @code{yyerror} in the Prologue, +even if you define them int he Epilogue. @xref{Interface, ,Parser C-Language Interface}. If the last section is empty, you may omit the @samp{%%} that separates it from the grammar rules. -The Bison parser itself contains many static variables whose names start -with @samp{yy} and many macros whose names start with @samp{YY}. It is a +The Bison parser itself contains many macros and identifiers whose +names start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using any such names (except those documented in this manual) in the epilogue of the grammar file. @@ -2124,7 +2420,7 @@ There are three ways of writing terminal symbols in the grammar: @itemize @bullet @item A @dfn{named token type} is written with an identifier, like an -identifier in C. By convention, it should be all upper case. Each +identifier in C@. By convention, it should be all upper case. Each such name must be defined with a Bison declaration such as @code{%token}. @xref{Token Decl, ,Token Type Names}. @@ -2148,9 +2444,11 @@ 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 numeric code, zero, is the code @code{yylex} -returns for end-of-input (@pxref{Calling Convention, ,Calling Convention -for @code{yylex}}). +character literal because its numeric code, zero, signifies +end-of-input (@pxref{Calling Convention, ,Calling Convention +for @code{yylex}}). Also, unlike standard C, trigraphs have no +special meaning in Bison character literals, nor is backslash-newline +allowed. @item @cindex string token @@ -2168,7 +2466,7 @@ Declarations}). If you don't do that, the lexical analyzer has to retrieve the token number for the literal string token from the @code{yytname} table (@pxref{Calling Convention}). -@strong{WARNING}: literal string tokens do not work in Yacc. +@strong{Warning}: literal string tokens do not work in Yacc. By convention, a literal string token is used only to represent a token that consists of that particular string. Thus, you should use the token @@ -2177,21 +2475,25 @@ does not enforce this convention, but if you depart from it, people who read your program will be confused. All the escape sequences used in string literals in C can be used in -Bison as well. A literal string token must contain two or more -characters; for a token containing just one character, use a character -token (see above). +Bison as well. However, unlike Standard C, trigraphs have no special +meaning in Bison string literals, nor is backslash-newline allowed. A +literal string token must contain two or more characters; for a token +containing just one character, use a character token (see above). @end itemize How you choose to write a terminal symbol has no effect on its grammatical meaning. That depends only on where it appears in rules and on when the parser function returns that symbol. -The value returned by @code{yylex} is always one of the terminal symbols -(or 0 for end-of-input). Whichever way you write the token type in the -grammar rules, you write it the same way in the definition of @code{yylex}. -The numeric code for a character token type is simply the 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 value returned by @code{yylex} is always one of the terminal +symbols, except that a zero or negative value signifies end-of-input. +Whichever way you write the token type in the grammar rules, you write +it the same way in the definition of @code{yylex}. The numeric code +for a character token type is simply the positive numeric code of the +character, so @code{yylex} can use the identical value to generate the +requisite code, though you may need to convert it to @code{unsigned +char} to avoid sign-extension on hosts where @code{char} is signed. +Each named token type becomes a C macro in the parser file, so @code{yylex} can use the name to stand for the code. (This is why periods don't make sense in terminal symbols.) @xref{Calling Convention, ,Calling Convention for @code{yylex}}. @@ -2202,27 +2504,33 @@ 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 +If you want to write a grammar that is portable to any Standard C +host, you must use only non-null character tokens taken from the basic +execution character set of Standard C@. This set consists of the ten +digits, the 52 lower- and upper-case English letters, and the +characters in the following C-language string: + +@example +"\a\b\t\n\v\f\r !\"#%&'()*+,-./:;<=>?[\\]^_@{|@}~" +@end example + +The @code{yylex} function and Bison must use a consistent character +set and encoding for character tokens. For example, if you run Bison in an +@acronym{ASCII} environment, but then compile and run the resulting program in an environment that uses an incompatible character set like -@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 +@acronym{EBCDIC}, the resulting program may not work because the +tables generated by Bison will assume @acronym{ASCII} numeric values for +character tokens. It is standard practice for software distributions to contain C source files that -were generated by Bison in an @sc{ascii} environment, so installers on -platforms that are incompatible with @sc{ascii} must rebuild those +were generated by Bison in an @acronym{ASCII} environment, so installers on +platforms that are incompatible with @acronym{ASCII} must rebuild those files before compiling them. The symbol @code{error} is a terminal symbol reserved for error recovery (@pxref{Error Recovery}); you shouldn't use it for any other purpose. -In particular, @code{yylex} should never return this value. -The default value of the error token is 256, so in the -unlikely event that you need to use a character token with numeric -value 256 you must reassign the error token's value with a -@code{%token} declaration. +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 @section Syntax of Grammar Rules @@ -2257,8 +2565,8 @@ exp: exp '+' exp says that two groupings of type @code{exp}, with a @samp{+} token in between, can be combined into a larger grouping of type @code{exp}. -Whitespace in rules is significant only to separate symbols. You can add -extra whitespace as you wish. +White space in rules is significant only to separate symbols. You can add +extra white space as you wish. Scattered among the components can be @var{actions} that determine the semantics of the rule. An action looks like this: @@ -2353,14 +2661,14 @@ 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 @@ -2421,7 +2729,7 @@ 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 +@acronym{RPN} and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish Notation Calculator}). Bison's default is to use type @code{int} for all semantic values. To @@ -2472,7 +2780,13 @@ 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; +compound statement in C@. An action can contain any sequence of C +statements. Bison does not look for trigraphs, though, so if your C +code uses trigraphs you should ensure that they do not affect the +nesting of braces or the boundaries of comments, strings, or character +literals. + +An action 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 @@ -2519,11 +2833,11 @@ a-or-b: 'a'|'b' @{ a_or_b_found = 1; @}; @cindex default action If you don't specify an action for a rule, Bison supplies a default: -@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule becomes -the value of the whole rule. Of course, the default rule is valid only -if the two data types match. There is no meaningful default action for -an empty rule; every empty rule must have an explicit action unless the -rule's value does not matter. +@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule +becomes the value of the whole rule. Of course, the default action is +valid only if the two data types match. There is no meaningful default +action for an empty rule; every empty rule must have an explicit action +unless the rule's value does not matter. @code{$@var{n}} with @var{n} zero or negative is allowed for reference to tokens and groupings on the stack @emph{before} those that match the @@ -2759,11 +3073,11 @@ actually does to implement mid-rule actions. @node Locations @section Tracking Locations @cindex location -@cindex textual position -@cindex position, textual +@cindex textual location +@cindex location, textual Though grammar rules and semantic actions are enough to write a fully -functional parser, it can be useful to process some additionnal informations, +functional parser, it can be useful to process some additional information, especially symbol locations. @c (terminal or not) ? @@ -2790,13 +3104,13 @@ When @code{YYLTYPE} is not defined, Bison uses a default structure type with four members: @example -struct +typedef struct YYLTYPE @{ int first_line; int first_column; int last_line; int last_column; -@} +@} YYLTYPE; @end example @node Actions and Locations @@ -2810,7 +3124,7 @@ Actions are not only useful for defining language semantics, but also for describing the behavior of the output parser with locations. The most obvious way for building locations of syntactic groupings is very -similar to the way semantic values are computed. In a given rule, several +similar to the way semantic values are computed. In a given rule, several constructs can be used to access the locations of the elements being matched. The location of the @var{n}th component of the right hand side is @code{@@@var{n}}, while the location of the left hand side grouping is @@ -2841,11 +3155,11 @@ exp: @dots{} @end example As for semantic values, there is a default action for locations that is -run each time a rule is matched. It sets the beginning of @code{@@$} to the +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 +With this default action, the location tracking can be fully automatic. The example above simply rewrites this way: @example @@ -2870,27 +3184,48 @@ exp: @dots{} @subsection Default Action for Locations @vindex YYLLOC_DEFAULT -Actually, actions are not the best place to compute locations. Since +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. +rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is +matched, before the associated action is run. It is also invoked +while processing a syntax error, to compute the error's location. 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. +The @code{YYLLOC_DEFAULT} macro takes three parameters. The first one is +the location of the grouping (the result of the computation). When a +rule is matched, the second parameter is an array holding locations of +all right hand side elements of the rule being matched, and the third +parameter is the size of the rule's right hand side. When processing +a syntax error, the second parameter is an array holding locations of +the symbols that were discarded during error processing, and the third +parameter is the number of discarded symbols. + +By default, @code{YYLLOC_DEFAULT} is defined this way for simple +@acronym{LALR}(1) parsers: + +@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, it is defined this way: +@noindent +and like this for @acronym{GLR} parsers: @example @group -#define YYLLOC_DEFAULT(Current, Rhs, N) \ - Current.last_line = Rhs[N].last_line; \ - Current.last_column = Rhs[N].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 @@ -2898,16 +3233,12 @@ When defining @code{YYLLOC_DEFAULT}, you should consider that: @itemize @bullet @item -All arguments are free of side-effects. However, only the first one (the +All arguments are free of side-effects. However, only the first one (the result) should be modified by @code{YYLLOC_DEFAULT}. @item -Before @code{YYLLOC_DEFAULT} is executed, the output parser sets @code{@@$} -to @code{@@1}. - -@item -For consistency with semantic actions, valid indexes for the location array -range from 1 to @var{n}. +For consistency with semantic actions, valid indexes for the location +array range from 1 to @var{n}. @end itemize @node Declarations @@ -2934,6 +3265,7 @@ Grammars}). * Precedence Decl:: Declaring terminals with precedence and associativity. * Union Decl:: Declaring the set of all semantic value types. * Type Decl:: Declaring the choice of type for a nonterminal symbol. +* Destructor Decl:: Declaring how symbols are freed. * Expect Decl:: Suppressing warnings about shift/reduce conflicts. * Start Decl:: Specifying the start symbol. * Pure Decl:: Requesting a reentrant parser. @@ -3093,7 +3425,23 @@ This says that the two alternative types are @code{double} and @code{symrec in the @code{%token} and @code{%type} declarations to pick one of the types for a terminal or nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}). -Note that, unlike making a @code{union} declaration in C, you do not write +As an extension to @acronym{POSIX}, a tag is allowed after the +@code{union}. For example: + +@example +@group +%union value @{ + double val; + symrec *tptr; +@} +@end group +@end example + +specifies the union tag @code{value}, so the corresponding C type is +@code{union value}. If you do not specify a tag, it defaults to +@code{YYSTYPE}. + +Note that, unlike making a @code{union} declaration in C, you need not write a semicolon after the closing brace. @node Type Decl @@ -3124,6 +3472,71 @@ use the same @code{<@var{type}>} construction in a declaration for the terminal symbol. All kinds of token declarations allow @code{<@var{type}>}. +@node Destructor Decl +@subsection Freeing Discarded Symbols +@cindex freeing discarded symbols +@findex %destructor + +Some symbols can be discarded by the parser, typically during error +recovery (@pxref{Error Recovery}). Basically, during error recovery, +embarrassing symbols already pushed on the stack, and embarrassing +tokens coming from the rest of the file are thrown away until the parser +falls on its feet. If these symbols convey heap based information, this +memory is lost. While this behavior is tolerable for batch parsers, +such as in compilers, it is unacceptable for parsers that can +possibility ``never end'' such as shells, or implementations of +communication protocols. + +The @code{%destructor} directive allows for the definition of code that +is called when a symbol is thrown away. + +@deffn {Directive} %destructor @{ @var{code} @} @var{symbols} +@findex %destructor +Declare that the @var{code} must be invoked for each of the +@var{symbols} that will be discarded by the parser. The @var{code} +should use @code{$$} to designate the semantic value associated to the +@var{symbols}. The additional parser parameters are also avaible +(@pxref{Parser Function, , The Parser Function @code{yyparse}}). + +@strong{Warning:} as of Bison 1.875, this feature is still considered as +experimental, as there was not enough user feedback. In particular, +the syntax might still change. +@end deffn + +For instance: + +@smallexample +%union +@{ + char *string; +@} +%token STRING +%type string +%destructor @{ free ($$); @} STRING string +@end smallexample + +@noindent +guarantees that when a @code{STRING} or a @code{string} will be discarded, +its associated memory will be freed. + +Note that in the future, Bison might also consider that right hand side +members that are not mentioned in the action can be destroyed. For +instance, in: + +@smallexample +comment: "/*" STRING "*/"; +@end smallexample + +@noindent +the parser is entitled to destroy the semantic value of the +@code{string}. Of course, this will not apply to the default action; +compare: + +@smallexample +typeless: string; // $$ = $1 does not apply; $1 is destroyed. +typefull: string; // $$ = $1 applies, $1 is not destroyed. +@end smallexample + @node Expect Decl @subsection Suppressing Conflict Warnings @cindex suppressing conflict warnings @@ -3147,7 +3560,7 @@ The declaration looks like this: 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 +reduce/reduce conflicts. The usual warning is given if there are either more or fewer conflicts, or if there are any reduce/reduce conflicts. @@ -3169,9 +3582,9 @@ Add an @code{%expect} declaration, copying the number @var{n} from the number which Bison printed. @end itemize -Now Bison will stop annoying you about the conflicts you have checked, but -it will warn you again if changes in the grammar result in additional -conflicts. +Now Bison will stop annoying you if you do not change the number of +conflicts, but it will warn you again if changes in the grammar result +in more or fewer conflicts. @node Start Decl @subsection The Start-Symbol @@ -3202,8 +3615,8 @@ handler. In systems with multiple threads of control, a non-reentrant program must be called only within interlocks. Normally, Bison generates a parser which is not reentrant. This is -suitable for most uses, and it permits compatibility with YACC. (The -standard YACC interfaces are inherently nonreentrant, because they use +suitable for most uses, and it permits compatibility with Yacc. (The +standard Yacc interfaces are inherently nonreentrant, because they use statically allocated variables for communication with @code{yylex}, including @code{yylval} and @code{yylloc}.) @@ -3236,53 +3649,60 @@ valid grammar. Here is a summary of the declarations used to define a grammar: -@table @code -@item %union +@deffn {Directive} %union Declare the collection of data types that semantic values may have (@pxref{Union Decl, ,The Collection of Value Types}). +@end deffn -@item %token +@deffn {Directive} %token Declare a terminal symbol (token type name) with no precedence or associativity specified (@pxref{Token Decl, ,Token Type Names}). +@end deffn -@item %right +@deffn {Directive} %right Declare a terminal symbol (token type name) that is right-associative (@pxref{Precedence Decl, ,Operator Precedence}). +@end deffn -@item %left +@deffn {Directive} %left Declare a terminal symbol (token type name) that is left-associative (@pxref{Precedence Decl, ,Operator Precedence}). +@end deffn -@item %nonassoc +@deffn {Directive} %nonassoc Declare a terminal symbol (token type name) that is nonassociative (using it in a way that would be associative is a syntax error) +@end deffn (@pxref{Precedence Decl, ,Operator Precedence}). -@item %type +@deffn {Directive} %type Declare the type of semantic values for a nonterminal symbol (@pxref{Type Decl, ,Nonterminal Symbols}). +@end deffn -@item %start +@deffn {Directive} %start Specify the grammar's start symbol (@pxref{Start Decl, ,The Start-Symbol}). +@end deffn -@item %expect +@deffn {Directive} %expect Declare the expected number of shift-reduce conflicts (@pxref{Expect Decl, ,Suppressing Conflict Warnings}). -@end table +@end deffn + @sp 1 @noindent In order to change the behavior of @command{bison}, use the following directives: -@table @code -@item %debug +@deffn {Directive} %debug In the parser file, define the macro @code{YYDEBUG} to 1 if it is not already defined, so that the debugging facilities are compiled. -@xref{Debugging, ,Debugging Your Parser}. +@end deffn +@xref{Tracing, ,Tracing Your Parser}. -@item %defines +@deffn {Directive} %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. @@ -3294,36 +3714,38 @@ 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}. +@end deffn -@item %file-prefix="@var{prefix}" +@deffn {Directive} %destructor +Specifying how the parser should reclaim the memory associated to +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +@end deffn + +@deffn {Directive} %file-prefix="@var{prefix}" Specify a prefix to use for all Bison output file names. The names are chosen as if the input file were named @file{@var{prefix}.y}. +@end deffn -@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 +@deffn {Directive} %locations Generate the code processing the locations (@pxref{Action Features, ,Special Features for Use in Actions}). This mode is enabled as soon as the grammar uses the special @samp{@@@var{n}} tokens, but if your grammar does not use it, using @samp{%locations} allows for more -accurate parse error messages. +accurate syntax error messages. +@end deffn -@item %name-prefix="@var{prefix}" +@deffn {Directive} %name-prefix="@var{prefix}" Rename the external symbols used in the parser so that they start with @var{prefix} instead of @samp{yy}. The precise list of symbols renamed 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 +@code{yylval}, @code{yylloc}, @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}. +@end deffn + +@deffn {Directive} %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. @@ -3331,36 +3753,34 @@ declarations. This option also tells Bison to write the C code for the grammar actions into a file named @file{@var{filename}.act}, in the form of a brace-surrounded body fit for a @code{switch} statement. +@end deffn -@item %no-lines +@deffn {Directive} %no-lines Don't generate any @code{#line} preprocessor commands in the parser file. Ordinarily Bison writes these commands in the parser file so that the C compiler and debuggers will associate errors and object code with your source file (the grammar file). This directive causes them to associate errors with the parser file, treating it an independent source file in its own right. +@end deffn -@item %output="@var{filename}" +@deffn {Directive} %output="@var{filename}" Specify the @var{filename} for the parser file. +@end deffn -@item %pure-parser +@deffn {Directive} %pure-parser Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}). +@end deffn -@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 +@deffn {Directive} %token-table Generate an array of token names in the parser file. The name of the array is @code{yytname}; @code{yytname[@var{i}]} is the name of the -token whose internal Bison token code number is @var{i}. The first three -elements of @code{yytname} are always @code{"$"}, @code{"error"}, and -@code{"$illegal"}; after these come the symbols defined in the grammar -file. +token whose internal Bison token code number is @var{i}. The first +three elements of @code{yytname} correspond to the predefined tokens +@code{"$end"}, +@code{"error"}, and @code{"$undefined"}; after these come the symbols +defined in the grammar file. For single-character literal tokens and literal string tokens, the name in the table includes the single-quote or double-quote characters: for @@ -3386,28 +3806,19 @@ The number of grammar rules, @item YYNSTATES The number of parser states (@pxref{Parser States}). @end table +@end deffn -@item %verbose +@deffn {Directive} %verbose Write an extra output file containing verbose descriptions of the parser states and what is done for each type of look-ahead token in -that state. - -This file also describes all the conflicts, both those resolved by -operator precedence and the unresolved ones. - -The file's name is made by removing @samp{.tab.c} or @samp{.c} from -the parser output file name, and adding @samp{.output} instead. - -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}. +that state. @xref{Understanding, , Understanding Your Parser}, for more +information. +@end deffn -@item %yacc +@deffn {Directive} %yacc Pretend the option @option{--yacc} was given, i.e., imitate Yacc, including its naming conventions. @xref{Bison Options}, for more. -@end table - - +@end deffn @node Multiple Parsers @@ -3425,9 +3836,9 @@ 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 -@code{yydebug}. For example, if you use @samp{-p c}, the names become -@code{cparse}, @code{clex}, and so on. +@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yylloc}, +@code{yychar} and @code{yydebug}. For example, if you use @samp{-p c}, +the names become @code{cparse}, @code{clex}, and so on. @strong{All the other variables and macros associated with Bison are not renamed.} These others are not global; there is no conflict if the same @@ -3472,38 +3883,81 @@ encounters end-of-input or an unrecoverable syntax error. You can also write an action which directs @code{yyparse} to return immediately without reading further. + +@deftypefun int yyparse (void) The value returned by @code{yyparse} is 0 if parsing was successful (return is due to end-of-input). The value is 1 if parsing failed (return is due to a syntax error). +@end deftypefun In an action, you can cause immediate return from @code{yyparse} by using these macros: -@table @code -@item YYACCEPT +@defmac YYACCEPT @findex YYACCEPT Return immediately with value 0 (to report success). +@end defmac -@item YYABORT +@defmac YYABORT @findex YYABORT Return immediately with value 1 (to report failure). -@end table +@end defmac -@node Lexical -@section The Lexical Analyzer Function @code{yylex} -@findex yylex -@cindex lexical analyzer +If you use a reentrant parser, you can optionally pass additional +parameter information to it in a reentrant way. To do so, use the +declaration @code{%parse-param}: -The @dfn{lexical analyzer} function, @code{yylex}, recognizes tokens from -the input stream and returns them to the parser. Bison does not create -this function automatically; you must write it so that @code{yyparse} can -call it. The function is sometimes referred to as a lexical scanner. +@deffn {Directive} %parse-param @{@var{argument-declaration}@} +@findex %parse-param +Declare that an argument declared by @code{argument-declaration} is an +additional @code{yyparse} argument. +The @var{argument-declaration} is used when declaring +functions or prototypes. The last identifier in +@var{argument-declaration} must be the argument name. +@end deffn -In simple programs, @code{yylex} is often defined at the end of the Bison -grammar file. If @code{yylex} is defined in a separate source file, you -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 +Here's an example. Write this in the parser: + +@example +%parse-param @{int *nastiness@} +%parse-param @{int *randomness@} +@end example + +@noindent +Then call the parser like this: + +@example +@{ + int nastiness, randomness; + @dots{} /* @r{Store proper data in @code{nastiness} and @code{randomness}.} */ + value = yyparse (&nastiness, &randomness); + @dots{} +@} +@end example + +@noindent +In the grammar actions, use expressions like this to refer to the data: + +@example +exp: @dots{} @{ @dots{}; *randomness += 1; @dots{} @} +@end example + + +@node Lexical +@section The Lexical Analyzer Function @code{yylex} +@findex yylex +@cindex lexical analyzer + +The @dfn{lexical analyzer} function, @code{yylex}, recognizes tokens from +the input stream and returns them to the parser. Bison does not create +this function automatically; you must write it so that @code{yyparse} can +call it. The function is sometimes referred to as a lexical scanner. + +In simple programs, @code{yylex} is often defined at the end of the Bison +grammar file. If @code{yylex} is defined in a separate source file, you +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}. @@ -3512,7 +3966,7 @@ that need it. @xref{Invocation, ,Invoking Bison}. * Calling Convention:: How @code{yyparse} calls @code{yylex}. * Token Values:: How @code{yylex} must return the semantic value of the token it has read. -* Token Positions:: How @code{yylex} must return the text position +* Token Locations:: How @code{yylex} must return the text location (line number, etc.) of the token, if the actions want that. * Pure Calling:: How the calling convention differs @@ -3522,8 +3976,9 @@ that need it. @xref{Invocation, ,Invoking Bison}. @node Calling Convention @subsection Calling Convention for @code{yylex} -The value that @code{yylex} returns must be the numeric code for the type -of token it has just found, or 0 for end-of-input. +The value that @code{yylex} returns must be the positive numeric code +for the type of token it has just found; a zero or negative value +signifies end-of-input. When a token is referred to in the grammar rules by a name, that name in the parser file becomes a C macro whose definition is the proper @@ -3532,8 +3987,9 @@ to indicate that type. @xref{Symbols}. When a token is referred to in the grammar rules by a character literal, the numeric code for that character is also the code for the token type. -So @code{yylex} can simply return that character code. The null character -must not be used this way, because its code is zero and that is what +So @code{yylex} can simply return that character code, possibly converted +to @code{unsigned char} to avoid sign-extension. The null character +must not be used this way, because its code is zero and that signifies end-of-input. Here is an example showing these things: @@ -3543,13 +3999,13 @@ int yylex (void) @{ @dots{} - if (c == EOF) /* Detect end of file. */ + if (c == EOF) /* Detect end-of-input. */ return 0; @dots{} if (c == '+' || c == '-') - return c; /* Assume token type for `+' is '+'. */ + return c; /* Assume token type for `+' is '+'. */ @dots{} - return INT; /* Return the type of the token. */ + return INT; /* Return the type of the token. */ @dots{} @} @end example @@ -3584,8 +4040,8 @@ for (i = 0; i < YYNTOKENS; i++) @{ if (yytname[i] != 0 && yytname[i][0] == '"' - && strncmp (yytname[i] + 1, token_buffer, - strlen (token_buffer)) + && ! strncmp (yytname[i] + 1, token_buffer, + strlen (token_buffer)) && yytname[i][strlen (token_buffer) + 1] == '"' && yytname[i][strlen (token_buffer) + 2] == 0) break; @@ -3609,8 +4065,8 @@ Thus, if the type is @code{int} (the default), you might write this in @example @group @dots{} - yylval = value; /* Put value onto Bison stack. */ - return INT; /* Return the type of the token. */ + yylval = value; /* Put value onto Bison stack. */ + return INT; /* Return the type of the token. */ @dots{} @end group @end example @@ -3637,14 +4093,14 @@ then the code in @code{yylex} might look like this: @example @group @dots{} - yylval.intval = value; /* Put value onto Bison stack. */ - return INT; /* Return the type of the token. */ + yylval.intval = value; /* Put value onto Bison stack. */ + return INT; /* Return the type of the token. */ @dots{} @end group @end example -@node Token Positions -@subsection Textual Positions of Tokens +@node Token Locations +@subsection Textual Locations of Tokens @vindex yylloc If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, , @@ -3687,89 +4143,51 @@ yylex (YYSTYPE *lvalp, YYLTYPE *llocp) @end example If the grammar file does not use the @samp{@@} constructs to refer to -textual positions, then the type @code{YYLTYPE} will not be defined. In +textual locations, then the type @code{YYLTYPE} will not be defined. In this case, omit the second argument; @code{yylex} will be called with only one argument. -@vindex YYPARSE_PARAM -If you use a reentrant parser, you can optionally pass additional -parameter information to it in a reentrant way. To do so, define the -macro @code{YYPARSE_PARAM} as a variable name. This modifies the -@code{yyparse} function to accept one argument, of type @code{void *}, -with that name. -When you call @code{yyparse}, pass the address of an object, casting the -address to @code{void *}. The grammar actions can refer to the contents -of the object by casting the pointer value back to its proper type and -then dereferencing it. Here's an example. Write this in the parser: - -@example -%@{ -struct parser_control -@{ - int nastiness; - int randomness; -@}; +If you wish to pass the additional parameter data to @code{yylex}, use +@code{%lex-param} just like @code{%parse-param} (@pxref{Parser +Function}). -#define YYPARSE_PARAM parm -%@} -@end example +@deffn {Directive} lex-param @{@var{argument-declaration}@} +@findex %lex-param +Declare that @code{argument-declaration} is an additional @code{yylex} +argument declaration. +@end deffn -@noindent -Then call the parser like this: +For instance: @example -struct parser_control -@{ - int nastiness; - int randomness; -@}; - -@dots{} - -@{ - struct parser_control foo; - @dots{} /* @r{Store proper data in @code{foo}.} */ - value = yyparse ((void *) &foo); - @dots{} -@} +%parse-param @{int *nastiness@} +%lex-param @{int *nastiness@} +%parse-param @{int *randomness@} @end example @noindent -In the grammar actions, use expressions like this to refer to the data: +results in the following signature: @example -((struct parser_control *) parm)->randomness +int yylex (int *nastiness); +int yyparse (int *nastiness, int *randomness); @end example -@vindex YYLEX_PARAM -If you wish to pass the additional parameter data to @code{yylex}, -define the macro @code{YYLEX_PARAM} just like @code{YYPARSE_PARAM}, as -shown here: +If @code{%pure-parser} is added: @example -%@{ -struct parser_control -@{ - int nastiness; - int randomness; -@}; - -#define YYPARSE_PARAM parm -#define YYLEX_PARAM parm -%@} +int yylex (YYSTYPE *lvalp, int *nastiness); +int yyparse (int *nastiness, int *randomness); @end example -You should then define @code{yylex} to accept one additional -argument---the value of @code{parm}. (This makes either two or three -arguments in total, depending on whether an argument of type -@code{YYLTYPE} is passed.) You can declare the argument as a pointer to -the proper object type, or you can declare it as @code{void *} and -access the contents as shown above. +@noindent +and finally, if both @code{%pure-parser} and @code{%locations} are used: -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. +@example +int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness); +int yyparse (int *nastiness, int *randomness); +@end example @node Error Reporting @section The Error Reporting Function @code{yyerror} @@ -3778,7 +4196,7 @@ with no arguments, as usual. @cindex parse error @cindex syntax error -The Bison parser detects a @dfn{parse error} or @dfn{syntax error} +The Bison parser detects a @dfn{syntax error} or @dfn{parse error} whenever it reads a token which cannot satisfy any syntax rule. An action in the grammar can also explicitly proclaim an error, using the macro @code{YYERROR} (@pxref{Action Features, ,Special Features for Use @@ -3787,16 +4205,14 @@ in Actions}). The Bison parser expects to report the error by calling an error reporting function named @code{yyerror}, which you must supply. It is called by @code{yyparse} whenever a syntax error is found, and it -receives one argument. For a parse error, the string is normally -@w{@code{"parse error"}}. +receives one argument. For a syntax error, the string is normally +@w{@code{"syntax error"}}. -@findex YYERROR_VERBOSE -If you define the macro @code{YYERROR_VERBOSE} in the Bison declarations -section (@pxref{Bison Declarations, ,The Bison Declarations Section}), -then Bison provides a more verbose and specific error message string -instead of just plain @w{@code{"parse error"}}. It doesn't matter what -definition you use for @code{YYERROR_VERBOSE}, just whether you define -it. +@findex %error-verbose +If you invoke the directive @code{%error-verbose} in the Bison +declarations section (@pxref{Bison Declarations, ,The Bison Declarations +Section}), then Bison provides a more verbose and specific error message +string instead of just plain @w{@code{"syntax error"}}. The parser can detect one other kind of error: stack overflow. This happens when the input contains constructions that are very deeply @@ -3811,7 +4227,7 @@ The following definition suffices in simple programs: @example @group void -yyerror (char *s) +yyerror (char const *s) @{ @end group @group @@ -3825,6 +4241,63 @@ error recovery if you have written suitable error recovery grammar rules (@pxref{Error Recovery}). If recovery is impossible, @code{yyparse} will immediately return 1. +Obviously, in location tracking pure parsers, @code{yyerror} should have +an access to the current location. This is indeed the case for the GLR +parsers, but not for the Yacc parser, for historical reasons. I.e., if +@samp{%locations %pure-parser} is passed then the prototypes for +@code{yyerror} are: + +@example +void yyerror (char const *msg); /* Yacc parsers. */ +void yyerror (YYLTYPE *locp, char const *msg); /* GLR parsers. */ +@end example + +If @samp{%parse-param @{int *nastiness@}} is used, then: + +@example +void yyerror (int *nastiness, char const *msg); /* Yacc parsers. */ +void yyerror (int *nastiness, char const *msg); /* GLR parsers. */ +@end example + +Finally, GLR and Yacc parsers share the same @code{yyerror} calling +convention for absolutely pure parsers, i.e., when the calling +convention of @code{yylex} @emph{and} the calling convention of +@code{%pure-parser} are pure. I.e.: + +@example +/* Location tracking. */ +%locations +/* Pure yylex. */ +%pure-parser +%lex-param @{int *nastiness@} +/* Pure yyparse. */ +%parse-param @{int *nastiness@} +%parse-param @{int *randomness@} +@end example + +@noindent +results in the following signatures for all the parser kinds: + +@example +int yylex (YYSTYPE *lvalp, YYLTYPE *llocp, int *nastiness); +int yyparse (int *nastiness, int *randomness); +void yyerror (YYLTYPE *locp, + int *nastiness, int *randomness, + char const *msg); +@end example + +@noindent +The prototypes are only indications of how the code produced by Bison +uses @code{yyerror}. Bison-generated code always ignores the returned +value, so @code{yyerror} can return any type, including @code{void}. +Also, @code{yyerror} can be a variadic function; that is why the +message is always passed last. + +Traditionally @code{yyerror} returns an @code{int} that is always +ignored, but this is purely for historical reasons, and @code{void} is +preferable since it more accurately describes the return type for +@code{yyerror}. + @vindex yynerrs The variable @code{yynerrs} contains the number of syntax errors encountered so far. Normally this variable is global; but if you @@ -3839,37 +4312,43 @@ then it is a local variable which only the actions can access. Here is a table of Bison constructs, variables and macros that are useful in actions. -@table @samp -@item $$ +@deffn {Variable} $$ Acts like a variable that contains the semantic value for the grouping made by the current rule. @xref{Actions}. +@end deffn -@item $@var{n} +@deffn {Variable} $@var{n} Acts like a variable that contains the semantic value for the @var{n}th component of the current rule. @xref{Actions}. +@end deffn -@item $<@var{typealt}>$ +@deffn {Variable} $<@var{typealt}>$ Like @code{$$} but specifies alternative @var{typealt} in the union specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}. +@end deffn -@item $<@var{typealt}>@var{n} +@deffn {Variable} $<@var{typealt}>@var{n} Like @code{$@var{n}} but specifies alternative @var{typealt} in the union specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}. +@end deffn -@item YYABORT; +@deffn {Macro} YYABORT; Return immediately from @code{yyparse}, indicating failure. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@item YYACCEPT; +@deffn {Macro} YYACCEPT; Return immediately from @code{yyparse}, indicating success. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@item YYBACKUP (@var{token}, @var{value}); +@deffn {Macro} YYBACKUP (@var{token}, @var{value}); @findex YYBACKUP Unshift a token. This macro is allowed only for rules that reduce a single value, and only when there is no look-ahead token. +It is also disallowed in @acronym{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. @@ -3880,42 +4359,49 @@ a message @samp{cannot back up} and performs ordinary error recovery. In either case, the rest of the action is not executed. +@end deffn -@item YYEMPTY +@deffn {Macro} YYEMPTY @vindex YYEMPTY Value stored in @code{yychar} when there is no look-ahead token. +@end deffn -@item YYERROR; +@deffn {Macro} YYERROR; @findex YYERROR Cause an immediate syntax error. This statement initiates error recovery just as if the parser itself had detected an error; however, it does not call @code{yyerror}, and does not print any message. If you want to print an error message, call @code{yyerror} explicitly before the @samp{YYERROR;} statement. @xref{Error Recovery}. +@end deffn -@item YYRECOVERING +@deffn {Macro} YYRECOVERING This macro stands for an expression that has the value 1 when the parser is recovering from a syntax error, and 0 the rest of the time. @xref{Error Recovery}. +@end deffn -@item yychar +@deffn {Variable} yychar Variable containing the current look-ahead token. (In a pure parser, this is actually a local variable within @code{yyparse}.) When there is no look-ahead token, the value @code{YYEMPTY} is stored in the variable. @xref{Look-Ahead, ,Look-Ahead Tokens}. +@end deffn -@item yyclearin; +@deffn {Macro} yyclearin; Discard the current look-ahead token. This is useful primarily in error rules. @xref{Error Recovery}. +@end deffn -@item yyerrok; +@deffn {Macro} yyerrok; Resume generating error messages immediately for subsequent syntax errors. This is useful primarily in error rules. @xref{Error Recovery}. +@end deffn -@item @@$ +@deffn {Value} @@$ @findex @@$ -Acts like a structure variable containing information on the textual position +Acts like a structure variable containing information on the textual location of the grouping made by the current rule. @xref{Locations, , Tracking Locations}. @@ -3937,14 +4423,15 @@ Tracking Locations}. @c those members. @c The use of this feature makes the parser noticeably slower. +@end deffn -@item @@@var{n} +@deffn {Value} @@@var{n} @findex @@@var{n} -Acts like a structure variable containing information on the textual position +Acts like a structure variable containing information on the textual location of the @var{n}th component of the current rule. @xref{Locations, , Tracking Locations}. +@end deffn -@end table @node Algorithm @chapter The Bison Parser Algorithm @@ -4010,6 +4497,7 @@ 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 @@ -4537,12 +5025,13 @@ name_list: It would seem that this grammar can be parsed with only a single token of look-ahead: when a @code{param_spec} is being read, an @code{ID} is a @code{name} if a comma or colon follows, or a @code{type} if another -@code{ID} follows. In other words, this grammar is LR(1). +@code{ID} follows. In other words, this grammar is @acronym{LR}(1). -@cindex LR(1) -@cindex LALR(1) +@cindex @acronym{LR}(1) +@cindex @acronym{LALR}(1) However, Bison, like most parser generators, cannot actually handle all -LR(1) grammars. In this grammar, two contexts, that after an @code{ID} +@acronym{LR}(1) grammars. In this grammar, two contexts, that after +an @code{ID} at the beginning of a @code{param_spec} and likewise at the beginning of a @code{return_spec}, are similar enough that Bison assumes they are the same. They appear similar because the same set of rules would be @@ -4551,11 +5040,12 @@ a @code{type}. Bison is unable to determine at that stage of processing that the rules would require different look-ahead tokens in the two contexts, so it makes a single parser state for them both. Combining the two contexts causes a conflict later. In parser terminology, this -occurrence means that the grammar is not LALR(1). +occurrence means that the grammar is not @acronym{LALR}(1). In general, it is better to fix deficiencies than to document them. But this particular deficiency is intrinsically hard to fix; parser -generators that can handle LR(1) grammars are hard to write and tend to +generators that can handle @acronym{LR}(1) grammars are hard to write +and tend to produce parsers that are very large. In practice, Bison is more useful as it is now. @@ -4604,6 +5094,92 @@ return_spec: ; @end example +@node Generalized LR Parsing +@section Generalized @acronym{LR} (@acronym{GLR}) Parsing +@cindex @acronym{GLR} parsing +@cindex generalized @acronym{LR} (@acronym{GLR}) parsing +@cindex ambiguous grammars +@cindex 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 @acronym{LR} (or @acronym{GLR}). A Bison @acronym{GLR} +parser uses the same basic +algorithm for parsing as an ordinary Bison parser, but behaves +differently in cases where there is a shift-reduce conflict that has not +been resolved by precedence rules (@pxref{Precedence}) or a +reduce-reduce conflict. When a @acronym{GLR} parser encounters such a +situation, it +effectively @emph{splits} into a several parsers, one for each possible +shift or reduction. These parsers then proceed as usual, consuming +tokens in lock-step. Some of the stacks may encounter other conflicts +and split further, with the result that instead of a sequence of states, +a Bison @acronym{GLR} parsing stack is what is in effect a tree of states. + +In effect, each stack represents a guess as to what the proper parse +is. Additional input may indicate that a guess was wrong, in which case +the appropriate stack silently disappears. Otherwise, the semantics +actions generated in each stack are saved, rather than being executed +immediately. When a stack disappears, its saved semantic actions never +get executed. When a reduction causes two stacks to become equivalent, +their sets of semantic actions are both saved with the state that +results from the reduction. We say that two stacks are equivalent +when they both represent the same sequence of states, +and each pair of corresponding states represents a +grammar symbol that produces the same segment of the input token +stream. + +Whenever the parser makes a transition from having multiple +states to having one, it reverts to the normal @acronym{LALR}(1) parsing +algorithm, after resolving and executing the saved-up actions. +At this transition, some of the states on the stack will have semantic +values that are sets (actually multisets) of possible actions. The +parser tries to pick one of the actions by first finding one whose rule +has the highest dynamic precedence, as set by the @samp{%dprec} +declaration. Otherwise, if the alternative actions are not ordered by +precedence, but there the same merging function is declared for both +rules by the @samp{%merge} declaration, +Bison resolves and evaluates both and then calls the merge function on +the result. Otherwise, it reports an ambiguity. + +It is possible to use a data structure for the @acronym{GLR} parsing tree that +permits the processing of any @acronym{LALR}(1) grammar in linear time (in the +size of the input), any unambiguous (not necessarily +@acronym{LALR}(1)) grammar in +quadratic worst-case time, and any general (possibly ambiguous) +context-free grammar in cubic worst-case time. However, Bison currently +uses a simpler data structure that requires time proportional to the +length of the input times the maximum number of stacks required for any +prefix of the input. Thus, really ambiguous or 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 @acronym{LALR}(1) portions of a +grammar, in particular, it is only slightly slower than with the default +Bison parser. + +For a more detailed exposition of GLR parsers, please see: Elizabeth +Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style +Generalised @acronym{LR} Parsers, Royal Holloway, University of +London, Department of Computer Science, TR-00-12, +@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps}, +(2000-12-24). + @node Stack Overflow @section Stack Overflow, and How to Avoid It @cindex stack overflow @@ -4615,6 +5191,10 @@ not reduced. When this happens, the parser function @code{yyparse} returns a nonzero value, pausing only to call @code{yyerror} to report the overflow. +Because Bison parsers have growing stacks, hitting the upper limit +usually results from using a right recursion instead of a left +recursion, @xref{Recursion, ,Recursive Rules}. + @vindex YYMAXDEPTH By defining the macro @code{YYMAXDEPTH}, you can control how deep the parser stack can become before a stack overflow occurs. Define the @@ -4638,12 +5218,20 @@ 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. +@c FIXME: C++ output. +Because of semantical differences between C and C++, the +@acronym{LALR}(1) parsers +in C produced by Bison by compiled as C++ cannot grow. In this precise +case (compiling a C parser as C++) you are suggested to grow +@code{YYINITDEPTH}. In the near future, a C++ output output will be +provided which addresses this issue. + @node Error Recovery @chapter Error Recovery @cindex error recovery @cindex recovery from errors -It is not usually acceptable to have a program terminate on a parse +It is not usually acceptable to have a program terminate on a syntax error. For example, a compiler should recover sufficiently to parse the rest of the input file and check it for errors; a calculator should accept another expression. @@ -4685,22 +5273,24 @@ will be tokens to read before the next newline. So the rule is not applicable in the ordinary way. But Bison can force the situation to fit the rule, by discarding part of -the semantic context and part of the input. First it discards states and -objects from the stack until it gets back to a state in which the +the semantic context and part of the input. First it discards states +and objects from the stack until it gets back to a state in which the @code{error} token is acceptable. (This means that the subexpressions -already parsed are discarded, back to the last complete @code{stmnts}.) At -this point the @code{error} token can be shifted. Then, if the old +already parsed are discarded, back to the last complete @code{stmnts}.) +At this point the @code{error} token can be shifted. Then, if the old look-ahead token is not acceptable to be shifted next, the parser reads tokens and discards them until it finds a token which is acceptable. In -this example, Bison reads and discards input until the next newline -so that the fourth rule can apply. +this example, Bison reads and discards input until the next newline so +that the fourth rule can apply. Note that discarded symbols are +possible sources of memory leaks, see @ref{Destructor Decl, , Freeing +Discarded Symbols}, for a means to reclaim this memory. The choice of error rules in the grammar is a choice of strategies for error recovery. A simple and useful strategy is simply to skip the rest of the current input line or current statement if an error is detected: @example -stmnt: error ';' /* on error, skip until ';' is read */ +stmnt: error ';' /* On error, skip until ';' is read. */ @end example It is also useful to recover to the matching close-delimiter of an @@ -4744,7 +5334,7 @@ this is unacceptable, then the macro @code{yyclearin} may be used to clear this token. Write the statement @samp{yyclearin;} in the error rule's action. -For example, suppose that on a parse error, an error handling routine is +For example, suppose that on a syntax error, an error handling routine is called that advances the input stream to some point where parsing should once again commence. The next symbol returned by the lexical scanner is probably correct. The previous look-ahead token ought to be discarded @@ -4789,7 +5379,7 @@ This looks like a function call statement, but if @code{foo} is a typedef name, then this is actually a declaration of @code{x}. How can a Bison parser for C decide how to parse this input? -The method used in GNU C is to have two different token types, +The method used in @acronym{GNU} C is to have two different token types, @code{IDENTIFIER} and @code{TYPENAME}. When @code{yylex} finds an identifier, it looks up the current declaration of the identifier in order to decide which token type to return: @code{TYPENAME} if the identifier is @@ -4867,7 +5457,9 @@ as an identifier if it appears in that context. Here is how you can do it: @example @group %@{ -int hexflag; + int hexflag; + int yylex (void); + void yyerror (char const *); %@} %% @dots{} @@ -4958,8 +5550,421 @@ make sure your error recovery rules are not of this kind. Each rule must be such that you can be sure that it always will, or always won't, have to clear the flag. +@c ================================================== Debugging Your Parser + @node Debugging @chapter Debugging Your Parser + +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 @acronym{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: + +@example +calc.y: warning: 1 useless nonterminal and 1 useless rule +calc.y:11.1-7: warning: useless nonterminal: useless +calc.y:11.10-12: warning: useless rule: useless: STR +calc.y: conflicts: 7 shift/reduce +@end example + +When given @option{--report=state}, in addition to @file{calc.tab.c}, it +creates a file @file{calc.output} with contents detailed below. The +order of the output and the exact presentation might vary, but the +interpretation is the same. + +The first section includes details on conflicts that were solved thanks +to precedence and/or associativity: + +@example +Conflict in state 8 between rule 2 and token '+' resolved as reduce. +Conflict in state 8 between rule 2 and token '-' resolved as reduce. +Conflict in state 8 between rule 2 and token '*' resolved as shift. +@exdent @dots{} +@end example + +@noindent +The next section lists states that still have conflicts. + +@example +State 8 conflicts: 1 shift/reduce +State 9 conflicts: 1 shift/reduce +State 10 conflicts: 1 shift/reduce +State 11 conflicts: 4 shift/reduce +@end example + +@noindent +@cindex token, useless +@cindex useless token +@cindex nonterminal, useless +@cindex useless nonterminal +@cindex rule, useless +@cindex useless rule +The next section reports useless tokens, nonterminal and rules. Useless +nonterminals and rules are removed in order to produce a smaller parser, +but useless tokens are preserved, since they might be used by the +scanner (note the difference between ``useless'' and ``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 $accept -> exp $end + 1 5 exp -> exp '+' exp + 2 6 exp -> exp '-' exp + 3 7 exp -> exp '*' exp + 4 8 exp -> exp '/' exp + 5 9 exp -> NUM +@end example + +@noindent +and reports the uses of the symbols: + +@example +Terminals, with rules where they appear + +$end (0) 0 +'*' (42) 3 +'+' (43) 1 +'-' (45) 2 +'/' (47) 4 +error (256) +NUM (258) 5 + +Nonterminals, with rules where they appear + +$accept (8) + on left: 0 +exp (9) + on left: 1 2 3 4 5, on right: 0 1 2 3 4 +@end example + +@noindent +@cindex item +@cindex pointed rule +@cindex rule, pointed +Bison then proceeds onto the automaton itself, describing each state +with it set of @dfn{items}, also known as @dfn{pointed rules}. Each +item is a production rule together with a point (marked by @samp{.}) +that the input cursor. + +@example +state 0 + + $accept -> . exp $ (rule 0) + + NUM shift, and go to state 1 + + exp go to state 2 +@end example + +This reads as follows: ``state 0 corresponds to being at the very +beginning of the parsing, in the initial rule, right before the start +symbol (here, @code{exp}). When the parser returns to this state right +after having reduced a rule that produced an @code{exp}, the control +flow jumps to state 2. If there is no such transition on a nonterminal +symbol, and the lookahead is a @code{NUM}, then this token is shifted on +the parse stack, and the control flow jumps to state 1. Any other +lookahead triggers a syntax error.'' + +@cindex core, item set +@cindex item set core +@cindex kernel, item set +@cindex item set core +Even though the only active rule in state 0 seems to be rule 0, the +report lists @code{NUM} as a lookahead 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 + + $accept -> . exp $ (rule 0) + exp -> . exp '+' exp (rule 1) + exp -> . exp '-' exp (rule 2) + exp -> . exp '*' exp (rule 3) + exp -> . exp '/' exp (rule 4) + exp -> . NUM (rule 5) + + NUM shift, and go to state 1 + + exp go to state 2 +@end example + +@noindent +In the state 1... + +@example +state 1 + + exp -> NUM . (rule 5) + + $default reduce using rule 5 (exp) +@end example + +@noindent +the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead +(@samp{$default}), the parser will reduce it. If it was coming from +state 0, then, after this reduction it will return to state 0, and will +jump to state 2 (@samp{exp: go to state 2}). + +@example +state 2 + + $accept -> exp . $ (rule 0) + exp -> exp . '+' exp (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp . '/' exp (rule 4) + + $ shift, and go to state 3 + '+' shift, and go to state 4 + '-' shift, and go to state 5 + '*' shift, and go to state 6 + '/' shift, and go to state 7 +@end example + +@noindent +In state 2, the automaton can only shift a symbol. For instance, +because of the item @samp{exp -> exp . '+' exp}, if the lookahead if +@samp{+}, it will be shifted on the parse stack, and the automaton +control will jump to state 4, corresponding to the item @samp{exp -> exp +'+' . exp}. Since there is no default action, any other token than +those listed above will trigger a syntax error. + +The state 3 is named the @dfn{final state}, or the @dfn{accepting +state}: + +@example +state 3 + + $accept -> exp $ . (rule 0) + + $default accept +@end example + +@noindent +the initial rule is completed (the start symbol and the end +of input were read), the parsing exits successfully. + +The interpretation of states 4 to 7 is straightforward, and is left to +the reader. + +@example +state 4 + + exp -> exp '+' . exp (rule 1) + + NUM shift, and go to state 1 + + exp go to state 8 + +state 5 + + exp -> exp '-' . exp (rule 2) + + NUM shift, and go to state 1 + + exp go to state 9 + +state 6 + + exp -> exp '*' . exp (rule 3) + + NUM shift, and go to state 1 + + exp go to state 10 + +state 7 + + exp -> exp '/' . exp (rule 4) + + NUM shift, and go to state 1 + + exp go to state 11 +@end example + +As was announced in beginning of the report, @samp{State 8 conflicts: +1 shift/reduce}: + +@example +state 8 + + exp -> exp . '+' exp (rule 1) + exp -> exp '+' exp . (rule 1) + exp -> exp . '-' exp (rule 2) + exp -> exp . '*' exp (rule 3) + exp -> exp . '/' exp (rule 4) + + '*' shift, and go to state 6 + '/' shift, and go to state 7 + + '/' [reduce using rule 1 (exp)] + $default reduce using rule 1 (exp) +@end example + +Indeed, there are two actions associated to the lookahead @samp{/}: +either shifting (and going to state 7), or reducing rule 1. The +conflict means that either the grammar is ambiguous, or the parser lacks +information to make the right decision. Indeed the grammar is +ambiguous, as, since we did not specify the precedence of @samp{/}, the +sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM / +NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) / +NUM}, which corresponds to reducing rule 1. + +Because in @acronym{LALR}(1) parsing a single decision can be made, Bison +arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, , +Shift/Reduce Conflicts}. Discarded actions are reported in between +square brackets. + +Note that all the previous states had a single possible action: either +shifting the next token and going to the corresponding state, or +reducing a single rule. In the other cases, i.e., when shifting +@emph{and} reducing is possible or when @emph{several} reductions are +possible, the lookahead is required to select the action. State 8 is +one such state: if the lookahead is @samp{*} or @samp{/} then the action +is shifting, otherwise the action is reducing rule 1. In other words, +the first two items, corresponding to rule 1, are not eligible when the +lookahead is @samp{*}, since we specified that @samp{*} has higher +precedence that @samp{+}. More generally, some items are eligible only +with some set of possible lookaheads. When run with +@option{--report=lookahead}, Bison specifies these lookaheads: + +@example +state 8 + + exp -> exp . '+' exp [$, '+', '-', '/'] (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 @@ -4973,21 +5978,22 @@ There are several means to enable compilation of trace facilities: @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 +parser. This is compliant with @acronym{POSIX} Yacc. You could use @samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define YYDEBUG 1} in the 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. +,Invoking Bison}). This is @acronym{POSIX} compliant too. @item the directive @samp{%debug} @findex %debug Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison Declaration Summary}). This is a Bison extension, which will prove useful when Bison will output parsers for languages that don't use a -preprocessor. Useless POSIX and Yacc portability matter to you, this is +preprocessor. Unless @acronym{POSIX} and Yacc portability matter to +you, this is the preferred solution. @end table @@ -5051,18 +6057,25 @@ Here is an example of @code{YYPRINT} suitable for the multi-function calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}): @smallexample -#define YYPRINT(file, type, value) yyprint (file, type, value) +%@{ + static void print_token_value (FILE *, int, YYSTYPE); + #define YYPRINT(file, type, value) print_token_value (file, type, value) +%@} + +@dots{} %% @dots{} %% @dots{} static void -yyprint (FILE *file, int type, YYSTYPE value) +print_token_value (FILE *file, int type, YYSTYPE value) @{ if (type == VAR) - fprintf (file, " %s", value.tptr->name); + fprintf (file, "%s", value.tptr->name); else if (type == NUM) - fprintf (file, " %d", value.val); + fprintf (file, "%d", value.val); @} @end smallexample +@c ================================================= Invoking Bison + @node Invocation @chapter Invoking Bison @cindex invoking Bison @@ -5079,10 +6092,11 @@ 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 writing +@file{hack/foo.tab.c}. It's also possible, in case you are writing C++ code instead of C in your grammar file, to name it @file{foo.ypp} -or @file{foo.y++}. Then, the output files will take an extention like -the given one as input (repectively @file{foo.tab.cpp} and @file{foo.tab.c++}). +or @file{foo.y++}. Then, the output files will take an extension like +the given one as input (respectively @file{foo.tab.cpp} and +@file{foo.tab.c++}). This feature takes effect with all options that manipulate filenames like @samp{-o} or @samp{-d}. @@ -5092,21 +6106,23 @@ For example : bison -d @var{infile.yxx} @end example @noindent -will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}. and +will produce @file{infile.tab.cxx} and @file{infile.tab.hxx}, and @example -bison -d @var{infile.y} -o @var{output.c++} +bison -d -o @var{output.c++} @var{infile.y} @end example @noindent will produce @file{output.c++} and @file{outfile.h++}. +For compatibility with @acronym{POSIX}, the standard Bison +distribution also contains a shell script called @command{yacc} that +invokes Bison with the @option{-y} option. @menu * Bison Options:: All the options described in detail, - in alphabetical order by short options. -* Environment Variables:: Variables which affect Bison execution. + in alphabetical order by short options. * Option Cross Key:: Alphabetical list of long options. -* VMS Invocation:: Bison command syntax on VMS. +* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}. @end menu @node Bison Options @@ -5142,10 +6158,12 @@ 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: +for Yacc, and the Bison distribution contains such a script for +compatibility with @acronym{POSIX}: @example -bison -y $* +#! /bin/sh +bison -y "$@" @end example @end table @@ -5162,7 +6180,7 @@ you are developing Bison. @itemx --debug In the parser file, define the macro @code{YYDEBUG} to 1 if it is not already defined, so that the debugging facilities are compiled. -@xref{Debugging, ,Debugging Your Parser}. +@xref{Tracing, ,Tracing Your Parser}. @item --locations Pretend that @code{%locations} was specified. @xref{Decl Summary}. @@ -5206,13 +6224,34 @@ Same as above, but save in the file @var{defines-file}. @item -b @var{file-prefix} @itemx --file-prefix=@var{prefix} Pretend that @code{%verbose} was specified, i.e, specify prefix to use -for all Bison output file names. @xref{Decl Summary}. +for all Bison output file names. @xref{Decl Summary}. + +@item -r @var{things} +@itemx --report=@var{things} +Write an extra output file containing verbose description of the comma +separated list of @var{things} among: + +@table @code +@item state +Description of the grammar, conflicts (resolved and unresolved), and +@acronym{LALR} automaton. + +@item lookahead +Implies @code{state} and augments the description of the automaton with +each rule's lookahead set. + +@item itemset +Implies @code{state} and augments the description of the automaton with +the full set of items for each state, instead of its core only. +@end table + +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}. +parser. @xref{Decl Summary}. @item -o @var{filename} @itemx --output=@var{filename} @@ -5222,41 +6261,17 @@ The other output files' names are constructed from @var{filename} as described under the @samp{-v} and @samp{-d} options. @item -g -Output a VCG definition of the LALR(1) grammar automaton computed by -Bison. If the grammar file is @file{foo.y}, the VCG output file will +Output a @acronym{VCG} definition of the @acronym{LALR}(1) grammar +automaton computed by Bison. If the grammar file is @file{foo.y}, the +@acronym{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 behavior of @var{--graph} is the same than @samp{-g}. The only +difference is that it has an optional argument which is the name of the output graph filename. @end table -@node Environment Variables -@section Environment Variables -@cindex environment variables -@cindex BISON_HAIRY -@cindex BISON_SIMPLE - -Here is a list of environment variables which affect the way Bison -runs. - -@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. - -@end table - @node Option Cross Key @section Option Cross Key @@ -5301,298 +6316,566 @@ the corresponding short option. @end example @end ifinfo -@node VMS Invocation -@section Invoking Bison under VMS -@cindex invoking Bison under VMS -@cindex VMS +@node Yacc Library +@section Yacc Library + +The Yacc library contains default implementations of the +@code{yyerror} and @code{main} functions. These default +implementations are normally not useful, but @acronym{POSIX} requires +them. To use the Yacc library, link your program with the +@option{-ly} option. Note that Bison's implementation of the Yacc +library is distributed under the terms of the @acronym{GNU} General +Public License (@pxref{Copying}). + +If you use the Yacc library's @code{yyerror} function, you should +declare @code{yyerror} as follows: -The command line syntax for Bison on VMS is a variant of the usual -Bison command syntax---adapted to fit VMS conventions. +@example +int yyerror (char const *); +@end example -To find the VMS equivalent for any Bison option, start with the long -option, and substitute a @samp{/} for the leading @samp{--}, and -substitute a @samp{_} for each @samp{-} in the name of the long option. -For example, the following invocation under VMS: +Bison ignores the @code{int} value returned by this @code{yyerror}. +If you use the Yacc library's @code{main} function, your +@code{yyparse} function should have the following type signature: @example -bison /debug/name_prefix=bar foo.y +int yyparse (void); @end example +@c ================================================= Invoking Bison + +@node FAQ +@chapter Frequently Asked Questions +@cindex frequently asked questions +@cindex questions + +Several questions about Bison come up occasionally. Here some of them +are addressed. + +@menu +* Parser Stack Overflow:: Breaking the Stack Limits +* How Can I Reset @code{yyparse}:: @code{yyparse} Keeps some State +* Strings are Destroyed:: @code{yylval} Loses Track of Strings +@end menu + +@node Parser Stack Overflow +@section Parser Stack Overflow + +@display +My parser returns with error with a @samp{parser stack overflow} +message. What can I do? +@end display + +This question is already addressed elsewhere, @xref{Recursion, +,Recursive Rules}. + +@node How Can I Reset @code{yyparse} +@section How Can I Reset @code{yyparse} + +The following phenomenon gives raise to several incarnations, +resulting in the following typical questions: + +@display +I invoke @code{yyparse} several times, and on correct input it works +properly; but when a parse error is found, all the other calls fail +too. How can I reset @code{yyparse}'s error flag? +@end display + +@noindent +or + +@display +My parser includes support for a @samp{#include} like feature, in +which case I run @code{yyparse} from @code{yyparse}. This fails +although I did specify I needed a @code{%pure-parser}. +@end display + +These problems are not related to Bison itself, but with the Lex +generated scanners. Because these scanners use large buffers for +speed, they might not notice a change of input file. As a +demonstration, consider the following source file, +@file{first-line.l}: + +@verbatim +%{ +#include +#include +%} +%% +.*\n ECHO; return 1; +%% +int +yyparse (const char *file) +{ + yyin = fopen (file, "r"); + if (!yyin) + exit (2); + /* One token only. */ + yylex (); + if (!fclose (yyin)) + exit (3); + return 0; +} + +int +main () +{ + yyparse ("input"); + yyparse ("input"); + return 0; +} +@end verbatim + @noindent -is equivalent to the following command under POSIX. +If the file @file{input} contains + +@verbatim +input:1: Hello, +input:2: World! +@end verbatim + +@noindent +then instead of getting twice the first line, you get: @example -bison --debug --name-prefix=bar foo.y +$ @kbd{flex -ofirst-line.c first-line.l} +$ @kbd{gcc -ofirst-line first-line.c -ll} +$ @kbd{./first-line} +input:1: Hello, +input:2: World! @end example -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}. +Therefore, whenever you change @code{yyin}, you must tell the Lex +generated scanner to discard its current buffer, and to switch to the +new one. This depends upon your implementation of Lex, see its +documentation for more. For instance, in the case of Flex, a simple +call @samp{yyrestart (yyin)} suffices after each change to +@code{yyin}. + +@node Strings are Destroyed +@section Strings are Destroyed + +@display +My parser seems to destroy old strings, or maybe it losses track of +them. Instead of reporting @samp{"foo", "bar"}, it reports +@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}. +@end display + +This error is probably the single most frequent ``bug report'' sent to +Bison lists, but is only concerned with a misunderstanding of the role +of scanner. Consider the following Lex code: + +@verbatim +%{ +#include +char *yylval = NULL; +%} +%% +.* yylval = yytext; return 1; +\n /* IGNORE */ +%% +int +main () +{ + /* Similar to using $1, $2 in a Bison action. */ + char *fst = (yylex (), yylval); + char *snd = (yylex (), yylval); + printf ("\"%s\", \"%s\"\n", fst, snd); + return 0; +} +@end verbatim + +If you compile and run this code, you get: + +@example +$ @kbd{flex -osplit-lines.c split-lines.l} +$ @kbd{gcc -osplit-lines split-lines.c -ll} +$ @kbd{printf 'one\ntwo\n' | ./split-lines} +"one +two", "two" +@end example + +@noindent +this is because @code{yytext} is a buffer provided for @emph{reading} +in the action, but if you want to keep it, you have to duplicate it +(e.g., using @code{strdup}). Note that the output may depend on how +your implementation of Lex handles @code{yytext}. For instance, when +given the Lex compatibility option @option{-l} (which triggers the +option @samp{%array}) Flex generates a different behavior: + +@example +$ @kbd{flex -l -osplit-lines.c split-lines.l} +$ @kbd{gcc -osplit-lines split-lines.c -ll} +$ @kbd{printf 'one\ntwo\n' | ./split-lines} +"two", "two" +@end example + + +@c ================================================= Table of Symbols @node Table of Symbols @appendix Bison Symbols @cindex Bison symbols, table of @cindex symbols in Bison, table of -@table @code -@item @@$ +@deffn {Variable} @@$ In an action, the location of the left-hand side of the rule. - @xref{Locations, , Locations Overview}. +@xref{Locations, , Locations Overview}. +@end deffn -@item @@@var{n} +@deffn {Variable} @@@var{n} In an action, the location of the @var{n}-th symbol of the right-hand side of the rule. @xref{Locations, , Locations Overview}. +@end deffn -@item $$ +@deffn {Variable} $$ In an action, the semantic value of the left-hand side of the rule. @xref{Actions}. +@end deffn -@item $@var{n} +@deffn {Variable} $@var{n} In an action, the semantic value of the @var{n}-th symbol of the right-hand side of the rule. @xref{Actions}. - -@item error +@end deffn + +@deffn {Symbol} $accept +The predefined nonterminal whose only rule is @samp{$accept: @var{start} +$end}, where @var{start} is the start symbol. @xref{Start Decl, , The +Start-Symbol}. It cannot be used in the grammar. +@end deffn + +@deffn {Symbol} $end +The predefined token marking the end of the token stream. It cannot be +used in the grammar. +@end deffn + +@deffn {Symbol} $undefined +The predefined token onto which all undefined values returned by +@code{yylex} are mapped. It cannot be used in the grammar, rather, use +@code{error}. +@end deffn + +@deffn {Symbol} error A token name reserved for error recovery. This token may be used in grammar rules so as to allow the Bison parser to recognize an error in the grammar without halting the process. In effect, a sentence -containing an error may be recognized as valid. On a parse error, the +containing an error may be recognized as valid. On a syntax error, the token @code{error} becomes the current look-ahead token. Actions corresponding to @code{error} are then executed, and the look-ahead token is reset to the token that originally caused the violation. @xref{Error Recovery}. +@end deffn -@item YYABORT +@deffn {Macro} YYABORT Macro to pretend that an unrecoverable syntax error has occurred, by making @code{yyparse} return 1 immediately. The error reporting function @code{yyerror} is not called. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@item YYACCEPT +@deffn {Macro} YYACCEPT Macro to pretend that a complete utterance of the language has been read, by making @code{yyparse} return 0 immediately. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@item YYBACKUP +@deffn {Macro} YYBACKUP Macro to discard a value from the parser stack and fake a look-ahead token. @xref{Action Features, ,Special Features for Use in Actions}. +@end deffn -@item YYDEBUG -Macro to define to equip the parser with tracing code. @xref{Debugging, -,Debugging Your Parser}. +@deffn {Macro} YYDEBUG +Macro to define to equip the parser with tracing code. @xref{Tracing, +,Tracing Your Parser}. +@end deffn -@item YYERROR +@deffn {Macro} YYERROR Macro to pretend that a syntax error has just been detected: call @code{yyerror} and then perform normal error recovery if possible (@pxref{Error Recovery}), or (if recovery is impossible) make @code{yyparse} return 1. @xref{Error Recovery}. +@end deffn -@item YYERROR_VERBOSE -Macro that you define with @code{#define} in the Bison declarations -section to request verbose, specific error message strings when -@code{yyerror} is called. +@deffn {Macro} YYERROR_VERBOSE +An obsolete macro that you define with @code{#define} in the prologue +to request verbose, specific error message strings +when @code{yyerror} is called. It doesn't matter what definition you +use for @code{YYERROR_VERBOSE}, just whether you define it. Using +@code{%error-verbose} is preferred. +@end deffn -@item YYINITDEPTH +@deffn {Macro} YYINITDEPTH Macro for specifying the initial size of the parser stack. @xref{Stack Overflow}. +@end deffn -@item YYLEX_PARAM -Macro for specifying an extra argument (or list of extra arguments) for -@code{yyparse} to pass to @code{yylex}. @xref{Pure Calling,, Calling -Conventions for Pure Parsers}. +@deffn {Macro} YYLEX_PARAM +An obsolete macro for specifying an extra argument (or list of extra +arguments) for @code{yyparse} to pass to @code{yylex}. he use of this +macro is deprecated, and is supported only for Yacc like parsers. +@xref{Pure Calling,, Calling Conventions for Pure Parsers}. +@end deffn -@item YYLTYPE -Macro for the data type of @code{yylloc}; a structure with four +@deffn {Type} YYLTYPE +Data type of @code{yylloc}; by default, a structure with four members. @xref{Location Type, , Data Types of Locations}. +@end deffn -@item yyltype -Default value for YYLTYPE. +@deffn {Macro} YYMAXDEPTH +Macro for specifying the maximum size of the parser stack. @xref{Stack +Overflow}. +@end deffn -@item YYMAXDEPTH -Macro for specifying the maximum size of the parser stack. -@xref{Stack Overflow}. - -@item YYPARSE_PARAM -Macro for specifying the name of a parameter that @code{yyparse} should -accept. @xref{Pure Calling,, Calling Conventions for Pure Parsers}. +@deffn {Macro} YYPARSE_PARAM +An obsolete macro for specifying the name of a parameter that +@code{yyparse} should accept. The use of this macro is deprecated, and +is supported only for Yacc like parsers. @xref{Pure Calling,, Calling +Conventions for Pure Parsers}. +@end deffn -@item YYRECOVERING +@deffn {Macro} YYRECOVERING Macro whose value indicates whether the parser is recovering from a syntax error. @xref{Action Features, ,Special Features for Use in Actions}. +@end deffn -@item YYSTACK_USE_ALLOCA -Macro used to control the use of @code{alloca}. If defined to @samp{0}, +@deffn {Macro} 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} +grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA} to anything else. +@end deffn -@item YYSTYPE -Macro for the data type of semantic values; @code{int} by default. +@deffn {Type} YYSTYPE +Data type of semantic values; @code{int} by default. @xref{Value Type, ,Data Types of Semantic Values}. +@end deffn -@item yychar +@deffn {Variable} yychar External integer variable that contains the integer value of the current look-ahead token. (In a pure parser, it is a local variable within @code{yyparse}.) Error-recovery rule actions may examine this variable. @xref{Action Features, ,Special Features for Use in Actions}. +@end deffn -@item yyclearin +@deffn {Variable} yyclearin Macro used in error-recovery rule actions. It clears the previous look-ahead token. @xref{Error Recovery}. +@end deffn -@item yydebug +@deffn {Variable} yydebug External integer variable set to zero by default. If @code{yydebug} is given a nonzero value, the parser will output information on input -symbols and parser action. @xref{Debugging, ,Debugging Your Parser}. +symbols and parser action. @xref{Tracing, ,Tracing Your Parser}. +@end deffn -@item yyerrok +@deffn {Macro} yyerrok Macro to cause parser to recover immediately to its normal mode -after a parse error. @xref{Error Recovery}. +after a syntax error. @xref{Error Recovery}. +@end deffn -@item yyerror -User-supplied function to be called by @code{yyparse} on error. The -function receives one argument, a pointer to a character string -containing an error message. @xref{Error Reporting, ,The Error +@deffn {Function} yyerror +User-supplied function to be called by @code{yyparse} on error. +@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@end deffn -@item yylex +@deffn {Function} yylex User-supplied lexical analyzer function, called with no arguments to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}. +@end deffn -@item yylval +@deffn {Variable} yylval External variable in which @code{yylex} should place the semantic value associated with a token. (In a pure parser, it is a local variable within @code{yyparse}, and its address is passed to @code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}. +@end deffn -@item yylloc +@deffn {Variable} yylloc External variable in which @code{yylex} should place the line and column numbers associated with a token. (In a pure parser, it is a local variable within @code{yyparse}, and its address is passed to @code{yylex}.) You can ignore this variable if you don't use the -@samp{@@} feature in the grammar actions. @xref{Token Positions, -,Textual Positions of Tokens}. +@samp{@@} feature in the grammar actions. @xref{Token Locations, +,Textual Locations of Tokens}. +@end deffn -@item yynerrs -Global variable which Bison increments each time there is a parse error. +@deffn {Variable} yynerrs +Global variable which Bison increments each time there is a syntax error. (In a pure parser, it is a local variable within @code{yyparse}.) @xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}. +@end deffn -@item yyparse +@deffn {Function} yyparse The parser function produced by Bison; call this function to start parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}. +@end deffn -@item %debug +@deffn {Directive} %debug Equip the parser for debugging. @xref{Decl Summary}. +@end deffn -@item %defines +@deffn {Directive} %defines Bison declaration to create a header file meant for the scanner. @xref{Decl Summary}. - -@item %file-prefix="@var{prefix}" -Bison declaration to set tge prefix of the output files. @xref{Decl +@end deffn + +@deffn {Directive} %destructor +Specifying how the parser should reclaim the memory associated to +discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}. +@end deffn + +@deffn {Directive} %dprec +Bison declaration to assign a precedence to a rule that is used at parse +time to resolve reduce/reduce conflicts. @xref{GLR Parsers, ,Writing +@acronym{GLR} Parsers}. +@end deffn + +@deffn {Directive} %error-verbose +Bison declaration to request verbose, specific error message strings +when @code{yyerror} is called. +@end deffn + +@deffn {Directive} %file-prefix="@var{prefix}" +Bison declaration to set the prefix of the output files. @xref{Decl Summary}. +@end deffn -@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}. +@deffn {Directive} %glr-parser +Bison declaration to produce a @acronym{GLR} parser. @xref{GLR +Parsers, ,Writing @acronym{GLR} Parsers}. +@end deffn -@item %left +@deffn {Directive} %left Bison declaration to assign left associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. - -@item %name-prefix="@var{prefix}" -Bison declaration to rename the external symbols. @xref{Decl Summary}. - -@item %no-lines +@end deffn + +@deffn {Directive} %lex-param @{@var{argument-declaration}@} +Bison declaration to specifying an additional parameter that +@code{yylex} should accept. @xref{Pure Calling,, Calling Conventions +for Pure Parsers}. +@end deffn + +@deffn {Directive} %merge +Bison declaration to assign a merging function to a rule. If there is a +reduce/reduce conflict with a rule having the same merging function, the +function is applied to the two semantic values to get a single result. +@xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}. +@end deffn + +@deffn {Directive} %name-prefix="@var{prefix}" +Bison declaration to rename the external symbols. @xref{Decl Summary}. +@end deffn + +@deffn {Directive} %no-lines Bison declaration to avoid generating @code{#line} directives in the parser file. @xref{Decl Summary}. +@end deffn -@item %nonassoc +@deffn {Directive} %nonassoc Bison declaration to assign non-associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. +@end deffn -@item %output="@var{filename}" -Bison declaration to set the name of the parser file. @xref{Decl +@deffn {Directive} %output="@var{filename}" +Bison declaration to set the name of the parser file. @xref{Decl Summary}. +@end deffn -@item %prec +@deffn {Directive} %parse-param @{@var{argument-declaration}@} +Bison declaration to specifying an additional parameter that +@code{yyparse} should accept. @xref{Parser Function,, The Parser +Function @code{yyparse}}. +@end deffn + +@deffn {Directive} %prec Bison declaration to assign a precedence to a specific rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}. +@end deffn -@item %pure-parser +@deffn {Directive} %pure-parser Bison declaration to request a pure (reentrant) parser. @xref{Pure Decl, ,A Pure (Reentrant) Parser}. +@end deffn -@item %right +@deffn {Directive} %right Bison declaration to assign right associativity to token(s). @xref{Precedence Decl, ,Operator Precedence}. +@end deffn -@item %start +@deffn {Directive} %start Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}. +@end deffn -@item %token +@deffn {Directive} %token Bison declaration to declare token(s) without specifying precedence. @xref{Token Decl, ,Token Type Names}. +@end deffn -@item %token-table +@deffn {Directive} %token-table Bison declaration to include a token name table in the parser file. @xref{Decl Summary}. +@end deffn -@item %type +@deffn {Directive} %type Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}. +@end deffn -@item %union +@deffn {Directive} %union Bison declaration to specify several possible data types for semantic values. @xref{Union Decl, ,The Collection of Value Types}. -@end table +@end deffn @sp 1 These are the punctuation and delimiters used in Bison input: -@table @samp -@item %% +@deffn {Delimiter} %% Delimiter used to separate the grammar rule section from the Bison declarations section or the epilogue. @xref{Grammar Layout, ,The Overall Layout of a Bison Grammar}. +@end deffn -@item %@{ %@} +@c Don't insert spaces, or check the DVI output. +@deffn {Delimiter} %@{@var{code}%@} All code listed between @samp{%@{} and @samp{%@}} is copied directly to the output file uninterpreted. Such code forms the prologue of the input file. @xref{Grammar Outline, ,Outline of a Bison Grammar}. +@end deffn -@item /*@dots{}*/ +@deffn {Construct} /*@dots{}*/ Comment delimiters, as in C. +@end deffn -@item : +@deffn {Delimiter} : Separates a rule's result from its components. @xref{Rules, ,Syntax of Grammar Rules}. +@end deffn -@item ; +@deffn {Delimiter} ; Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}. +@end deffn -@item | +@deffn {Delimiter} | Separates alternate rules for the same result nonterminal. @xref{Rules, ,Syntax of Grammar Rules}. -@end table +@end deffn @node Glossary @appendix Glossary @cindex glossary @table @asis -@item Backus-Naur Form (BNF) -Formal method of specifying context-free grammars. BNF was first used -in the @cite{ALGOL-60} report, 1963. @xref{Language and Grammar, -,Languages and Context-Free Grammars}. +@item Backus-Naur Form (@acronym{BNF}; also called ``Backus Normal Form'') +Formal method of specifying context-free grammars originally proposed +by John Backus, and slightly improved by Peter Naur in his 1960-01-02 +committee document contributing to what became the Algol 60 report. +@xref{Language and Grammar, ,Languages and Context-Free Grammars}. @item Context-free grammars Grammars specified as rules that can be applied regardless of context. @@ -5615,11 +6898,20 @@ each instant in time. As input to the machine is processed, the machine moves from state to state as specified by the logic of the machine. In the case of the parser, the input is the language being parsed, and the states correspond to various stages in the grammar -rules. @xref{Algorithm, ,The Bison Parser Algorithm }. +rules. @xref{Algorithm, ,The Bison Parser Algorithm}. + +@item Generalized @acronym{LR} (@acronym{GLR}) +A parsing algorithm that can handle all context-free grammars, including those +that are not @acronym{LALR}(1). It resolves situations that Bison's +usual @acronym{LALR}(1) +algorithm cannot by effectively splitting off multiple parsers, trying all +possible parsers, and discarding those that fail in the light of additional +right context. @xref{Generalized LR Parsing, ,Generalized +@acronym{LR} Parsing}. @item Grouping A language construct that is (in general) grammatically divisible; -for example, `expression' or `declaration' in C. +for example, `expression' or `declaration' in C@. @xref{Language and Grammar, ,Languages and Context-Free Grammars}. @item Infix operator @@ -5646,7 +6938,7 @@ Rules}. @item Left-to-right parsing Parsing a sentence of a language by analyzing it token by token from -left to right. @xref{Algorithm, ,The Bison Parser Algorithm }. +left to right. @xref{Algorithm, ,The Bison Parser Algorithm}. @item Lexical analyzer (scanner) A function that reads an input stream and returns tokens one by one. @@ -5663,12 +6955,12 @@ A token which consists of two or more fixed characters. @xref{Symbols}. A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead Tokens}. -@item LALR(1) +@item @acronym{LALR}(1) The class of context-free grammars that Bison (like most other parser -generators) can handle; a subset of LR(1). @xref{Mystery Conflicts, , -Mysterious Reduce/Reduce Conflicts}. +generators) can handle; a subset of @acronym{LR}(1). @xref{Mystery +Conflicts, ,Mysterious Reduce/Reduce Conflicts}. -@item LR(1) +@item @acronym{LR}(1) The class of context-free grammars in which at most one token of look-ahead is needed to disambiguate the parsing of any piece of input. @@ -5677,10 +6969,6 @@ A grammar symbol standing for a grammatical construct that can be expressed through rules in terms of smaller constructs; in other words, a construct that is not a token. @xref{Symbols}. -@item Parse error -An error encountered during parsing of an input stream due to invalid -syntax. @xref{Error Recovery}. - @item Parser A function that recognizes valid sentences of a language by analyzing the syntax structure of a set of tokens passed to it from a lexical @@ -5693,7 +6981,7 @@ performs some operation. @item Reduction Replacing a string of nonterminals and/or terminals with a single nonterminal, according to a grammar rule. @xref{Algorithm, ,The Bison -Parser Algorithm }. +Parser Algorithm}. @item Reentrant A reentrant subprogram is a subprogram which can be in invoked any @@ -5716,7 +7004,7 @@ each statement. @xref{Semantics, ,Defining Language Semantics}. @item Shift A parser is said to shift when it makes the choice of analyzing further input from the stream rather than reducing immediately some -already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm }. +already-recognized rule. @xref{Algorithm, ,The Bison Parser Algorithm}. @item Single-character literal A single character that is recognized and interpreted as is. @@ -5733,6 +7021,10 @@ A data structure where symbol names and associated data are stored during parsing to allow for recognition and use of existing information in repeated uses of a symbol. @xref{Multi-function Calc}. +@item Syntax error +An error encountered during parsing of an input stream due to invalid +syntax. @xref{Error Recovery}. + @item Token A basic, grammatically indivisible unit of a language. The symbol that describes a token in the grammar is a terminal symbol.