@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
@c @clear shorttitlepage-enabled
@c @set shorttitlepage-enabled
+@c Set following if you want to document %default-prec and %no-default-prec.
+@c This feature is experimental and may change in future Bison versions.
+@c @set defaultprec
+
@c ISPELL CHECK: done, 14 Jan 1993 --bob
@c Check COPYRIGHT dates. should be updated in the titlepage, ifinfo
@c titlepage; should NOT be changed in the GPL. --mew
+@c FIXME: I don't understand this `iftex'. Obsolete? --akim.
@iftex
@syncodeindex fn cp
@syncodeindex vr cp
@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
-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,
+1999, 2000, 2001, 2002, 2003, 2004 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
-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
@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:
* 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
a semantic value (the value of an integer,
the name of an identifier, etc.).
* Semantic Actions:: Each rule can have an action containing C code.
+* GLR Parsers:: Writing parsers for general context-free languages.
+* Locations Overview:: Tracking Locations.
* Bison Parser:: What are Bison's input and output,
how is the output used?
* Stages:: Stages in writing and running Bison grammars.
* Grammar Layout:: Overall structure of a Bison grammar file.
+Writing @acronym{GLR} Parsers
+
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
+
Examples
* RPN Calc:: Reverse polish notation calculator;
Operator precedence is introduced.
* Simple Error Recovery:: Continuing after syntax errors.
* Location Tracking Calc:: Demonstrating the use of @@@var{n} and @@$.
-* Multi-function Calc:: Calculator with memory and trig functions.
- It uses multiple data-types for semantic values.
+* 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
* 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
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.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Initial Action Decl:: Code run before parsing starts.
+* Destructor Decl:: Declaring how symbols are freed.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
* 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}).
* 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
* 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 the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* C++ Parsers:: Compiling Parsers with C++ Compilers
+* Implementing Gotos/Loops:: Control Flow in the Calculator
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.
@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
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
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 an
+@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
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?
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}.@refill
+corresponding to a single nonterminal symbol a @dfn{grouping}.
We can use the C language as an example of what symbols, terminal and
-nonterminal, mean. The tokens of C are identifiers, constants (numeric and
-string), and the various keywords, arithmetic operators and punctuation
-marks. So the terminal symbols of a grammar for C include `identifier',
-`number', `string', plus one symbol for each keyword, operator or
-punctuation mark: `if', `return', `const', `static', `int', `char',
-`plus-sign', `open-brace', `close-brace', `comma' and many more. (These
-tokens can be subdivided into characters, but that is a matter of
+nonterminal, mean. The tokens of C are identifiers, constants (numeric
+and string), and the various keywords, arithmetic operators and
+punctuation marks. So the terminal symbols of a grammar for C include
+`identifier', `number', `string', plus one symbol for each keyword,
+operator or punctuation mark: `if', `return', `const', `static', `int',
+`char', `plus-sign', `open-brace', `close-brace', `comma' and many more.
+(These tokens can be subdivided into characters, but that is a matter of
lexicography, not grammar.)
Here is a simple C function subdivided into tokens:
@ifinfo
@example
int /* @r{keyword `int'} */
-square (int x) /* @r{identifier, open-paren, identifier,}
+square (int x) /* @r{identifier, open-paren, keyword `int',}
@r{identifier, close-paren} */
@{ /* @r{open-brace} */
return x * x; /* @r{keyword `return', identifier, asterisk,
@ifnotinfo
@example
int /* @r{keyword `int'} */
-square (int x) /* @r{identifier, open-paren, identifier, identifier, close-paren} */
+square (int x) /* @r{identifier, open-paren, keyword `int', identifier, close-paren} */
@{ /* @r{open-brace} */
return x * x; /* @r{keyword `return', identifier, asterisk, identifier, semicolon} */
@} /* @r{close-brace} */
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
@emph{any} integer constant is grammatically valid in that position. The
precise value of the constant is irrelevant to how to parse the input: if
@samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally
-grammatical.@refill
+grammatical.
But the precise value is very important for what the input means once it is
parsed. A compiler is useless if it fails to distinguish between 4, 1 and
3989 as constants in the program! Therefore, each token in a Bison grammar
-has both a token type and a @dfn{semantic value}. @xref{Semantics, ,Defining Language Semantics},
+has both a token type and a @dfn{semantic value}. @xref{Semantics,
+,Defining Language Semantics},
for details.
The token type is a terminal symbol defined in the grammar, such as
@code{INTEGER}, @code{IDENTIFIER} or @code{','}. It tells everything
you need to know to decide where the token may validly appear and how to
group it with other tokens. The grammar rules know nothing about tokens
-except their types.@refill
+except their types.
The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
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
+@cindex reduce/reduce conflicts
+
+In some grammars, Bison's standard
+@acronym{LALR}(1) parsing algorithm cannot decide whether to apply a
+certain grammar rule at a given point. That is, it may not be able to
+decide (on the basis of the input read so far) which of two possible
+reductions (applications of a grammar rule) applies, or whether to apply
+a reduction or read more of the input and apply a reduction later in the
+input. These are known respectively as @dfn{reduce/reduce} conflicts
+(@pxref{Reduce/Reduce}), and @dfn{shift/reduce} conflicts
+(@pxref{Shift/Reduce}).
+
+To use a grammar that is not easily modified to be @acronym{LALR}(1), a
+more general parsing algorithm is sometimes necessary. If you include
+@code{%glr-parser} among the Bison declarations in your file
+(@pxref{Grammar Outline}), the result is a Generalized @acronym{LR}
+(@acronym{GLR}) parser. These parsers handle Bison grammars that
+contain no unresolved conflicts (i.e., after applying precedence
+declarations) identically to @acronym{LALR}(1) parsers. However, when
+faced with unresolved shift/reduce and reduce/reduce conflicts,
+@acronym{GLR} parsers use the simple expedient of doing both,
+effectively cloning the parser to follow both possibilities. Each of
+the resulting parsers can again split, so that at any given time, there
+can be any number of possible parses being explored. The parsers
+proceed in lockstep; that is, all of them consume (shift) a given input
+symbol before any of them proceed to the next. Each of the cloned
+parsers eventually meets one of two possible fates: either it runs into
+a parsing error, in which case it simply vanishes, or it merges with
+another parser, because the two of them have reduced the input to an
+identical set of symbols.
+
+During the time that there are multiple parsers, semantic actions are
+recorded, but not performed. When a parser disappears, its recorded
+semantic actions disappear as well, and are never performed. When a
+reduction makes two parsers identical, causing them to merge, Bison
+records both sets of semantic actions. Whenever the last two parsers
+merge, reverting to the single-parser case, Bison resolves all the
+outstanding actions either by precedences given to the grammar rules
+involved, or by performing both actions, and then calling a designated
+user-defined function on the resulting values to produce an arbitrary
+merged result.
+
+@menu
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler
+@end menu
+
+@node Simple GLR Parsers
+@subsection Using @acronym{GLR} on Unambiguous Grammars
+@cindex @acronym{GLR} parsing, unambiguous grammars
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, unambiguous grammars
+@findex %glr-parser
+@findex %expect-rr
+@cindex conflicts
+@cindex reduce/reduce conflicts
+@cindex shift/reduce conflicts
+
+In the simplest cases, you can use the @acronym{GLR} algorithm
+to parse grammars that are unambiguous, but fail to be @acronym{LALR}(1).
+Such grammars typically require more than one symbol of look-ahead,
+or (in rare cases) fall into the category of grammars in which the
+@acronym{LALR}(1) algorithm throws away too much information (they are in
+@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).
+
+Consider a problem that
+arises in the declaration of enumerated and subrange types in the
+programming language Pascal. Here are some examples:
+
+@example
+type subrange = lo .. hi;
+type enum = (a, b, c);
+@end example
+
+@noindent
+The original language standard allows only numeric
+literals and constant identifiers for the subrange bounds (@samp{lo}
+and @samp{hi}), but Extended Pascal (@acronym{ISO}/@acronym{IEC}
+10206) and many other
+Pascal implementations allow arbitrary expressions there. This gives
+rise to the following situation, containing a superfluous pair of
+parentheses:
+
+@example
+type subrange = (a) .. b;
+@end example
+
+@noindent
+Compare this to the following declaration of an enumerated
+type with only one value:
+
+@example
+type enum = (a);
+@end example
+
+@noindent
+(These declarations are contrived, but they are syntactically
+valid, and more-complicated cases can come up in practical programs.)
+
+These two declarations look identical until the @samp{..} token.
+With normal @acronym{LALR}(1) one-token look-ahead it is not
+possible to decide between the two forms when the identifier
+@samp{a} is parsed. It is, however, desirable
+for a parser to decide this, since in the latter case
+@samp{a} must become a new identifier to represent the enumeration
+value, while in the former case @samp{a} must be evaluated with its
+current meaning, which may be a constant or even a function call.
+
+You could parse @samp{(a)} as an ``unspecified identifier in parentheses'',
+to be resolved later, but this typically requires substantial
+contortions in both semantic actions and large parts of the
+grammar, where the parentheses are nested in the recursive rules for
+expressions.
+
+You might think of using the lexer to distinguish between the two
+forms by returning different tokens for currently defined and
+undefined identifiers. But if these declarations occur in a local
+scope, and @samp{a} is defined in an outer scope, then both forms
+are possible---either locally redefining @samp{a}, or using the
+value of @samp{a} from the outer scope. So this approach cannot
+work.
+
+A simple solution to this problem is to declare the parser to
+use the @acronym{GLR} algorithm.
+When the @acronym{GLR} parser reaches the critical state, it
+merely splits into two branches and pursues both syntax rules
+simultaneously. Sooner or later, one of them runs into a parsing
+error. If there is a @samp{..} token before the next
+@samp{;}, the rule for enumerated types fails since it cannot
+accept @samp{..} anywhere; otherwise, the subrange type rule
+fails since it requires a @samp{..} token. So one of the branches
+fails silently, and the other one continues normally, performing
+all the intermediate actions that were postponed during the split.
+
+If the input is syntactically incorrect, both branches fail and the parser
+reports a syntax error as usual.
+
+The effect of all this is that the parser seems to ``guess'' the
+correct branch to take, or in other words, it seems to use more
+look-ahead than the underlying @acronym{LALR}(1) algorithm actually allows
+for. In this example, @acronym{LALR}(2) would suffice, but also some cases
+that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
+
+In general, a @acronym{GLR} parser can take quadratic or cubic worst-case time,
+and the current Bison parser even takes exponential time and space
+for some grammars. In practice, this rarely happens, and for many
+grammars it is possible to prove that it cannot happen.
+The present example contains only one conflict between two
+rules, and the type-declaration context containing the conflict
+cannot be nested. So the number of
+branches that can exist at any time is limited by the constant 2,
+and the parsing time is still linear.
+
+Here is a Bison grammar corresponding to the example above. It
+parses a vastly simplified form of Pascal type declarations.
+
+@example
+%token TYPE DOTDOT ID
+
+@group
+%left '+' '-'
+%left '*' '/'
+@end group
+
+%%
+
+@group
+type_decl : TYPE ID '=' type ';'
+ ;
+@end group
+
+@group
+type : '(' id_list ')'
+ | expr DOTDOT expr
+ ;
+@end group
+
+@group
+id_list : ID
+ | id_list ',' ID
+ ;
+@end group
+
+@group
+expr : '(' expr ')'
+ | expr '+' expr
+ | expr '-' expr
+ | expr '*' expr
+ | expr '/' expr
+ | ID
+ ;
+@end group
+@end example
+
+When used as a normal @acronym{LALR}(1) grammar, Bison correctly complains
+about one reduce/reduce conflict. In the conflicting situation the
+parser chooses one of the alternatives, arbitrarily the one
+declared first. Therefore the following correct input is not
+recognized:
+
+@example
+type t = (a) .. b;
+@end example
+
+The parser can be turned into a @acronym{GLR} parser, while also telling Bison
+to be silent about the one known reduce/reduce conflict, by
+adding these two declarations to the Bison input file (before the first
+@samp{%%}):
+
+@example
+%glr-parser
+%expect-rr 1
+@end example
+
+@noindent
+No change in the grammar itself is required. Now the
+parser recognizes all valid declarations, according to the
+limited syntax above, transparently. In fact, the user does not even
+notice when the parser splits.
+
+So here we have a case where we can use the benefits of @acronym{GLR}, almost
+without disadvantages. Even in simple cases like this, however, there
+are at least two potential problems to beware.
+First, always analyze the conflicts reported by
+Bison to make sure that @acronym{GLR} splitting is only done where it is
+intended. A @acronym{GLR} parser splitting inadvertently may cause
+problems less obvious than an @acronym{LALR} parser statically choosing the
+wrong alternative in a conflict.
+Second, consider interactions with the lexer (@pxref{Semantic Tokens})
+with great care. Since a split parser consumes tokens
+without performing any actions during the split, the lexer cannot
+obtain information via parser actions. Some cases of
+lexer interactions can be eliminated by using @acronym{GLR} to
+shift the complications from the lexer to the parser. You must check
+the remaining cases for correctness.
+
+In our example, it would be safe for the lexer to return tokens
+based on their current meanings in some symbol table, because no new
+symbols are defined in the middle of a type declaration. Though it
+is possible for a parser to define the enumeration
+constants as they are parsed, before the type declaration is
+completed, it actually makes no difference since they cannot be used
+within the same enumerated type declaration.
+
+@node Merging GLR Parses
+@subsection Using @acronym{GLR} to Resolve Ambiguities
+@cindex @acronym{GLR} parsing, ambiguous grammars
+@cindex generalized @acronym{LR} (@acronym{GLR}) parsing, ambiguous grammars
+@findex %dprec
+@findex %merge
+@cindex conflicts
+@cindex reduce/reduce conflicts
+
+Let's consider an example, vastly simplified from a C++ grammar.
+
+@example
+%@{
+ #include <stdio.h>
+ #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 <cast> ", $1); @}
+ | expr '+' expr @{ printf ("+ "); @}
+ | expr '=' expr @{ printf ("= "); @}
+ ;
+
+decl : TYPENAME declarator ';'
+ @{ printf ("%s <declare> ", $1); @}
+ | TYPENAME declarator '=' expr ';'
+ @{ printf ("%s <init-declare> ", $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. Since this is a
+@acronym{GLR} parser, it therefore splits the problem into two parses, one for
+each choice of resolving the reduce/reduce conflict.
+Unlike the example from the previous section (@pxref{Simple GLR Parsers}),
+however, neither of these parses ``dies,'' because the grammar as it stands is
+ambiguous. One of the parsers eventually reduces @code{stmt : expr ';'} and
+the other reduces @code{stmt : decl}, after which both parsers are in an
+identical state: they've seen @samp{prog stmt} and have the same unprocessed
+input remaining. We say that these parses have @dfn{merged.}
+
+At this point, the @acronym{GLR} parser requires a specification in the
+grammar of how to choose between the competing parses.
+In the example above, the two @code{%dprec}
+declarations specify that Bison is to give precedence
+to the parse that interprets the example as a
+@code{decl}, which implies that @code{x} is a declarator.
+The parser therefore prints
+
+@example
+"x" y z + T <init-declare>
+@end example
+
+The @code{%dprec} declarations only come into play when more than one
+parse survives. Consider a different input string for this parser:
+
+@example
+T (x) + y;
+@end example
+
+@noindent
+This is another example of using @acronym{GLR} to parse an unambiguous
+construct, as shown in the previous section (@pxref{Simple GLR Parsers}).
+Here, there is no ambiguity (this cannot be parsed as a declaration).
+However, at the time the Bison parser encounters @code{x}, it does not
+have enough information to resolve the reduce/reduce conflict (again,
+between @code{x} as an @code{expr} or a @code{declarator}). In this
+case, no precedence declaration is used. Again, the parser splits
+into two, one assuming that @code{x} is an @code{expr}, and the other
+assuming @code{x} is a @code{declarator}. The second of these parsers
+then vanishes when it sees @code{+}, and the parser prints
+
+@example
+x T <cast> y +
+@end example
+
+Suppose that instead of resolving the ambiguity, you wanted to see all
+the possibilities. For this purpose, you must merge the semantic
+actions of the two possible parsers, rather than choosing one over the
+other. To do so, you could change the declaration of @code{stmt} as
+follows:
+
+@example
+stmt : expr ';' %merge <stmtMerge>
+ | decl %merge <stmtMerge>
+ ;
+@end example
+
+@noindent
+and define the @code{stmtMerge} function as:
+
+@example
+static YYSTYPE
+stmtMerge (YYSTYPE x0, YYSTYPE x1)
+@{
+ printf ("<OR> ");
+ return "";
+@}
+@end example
+
+@noindent
+with an accompanying forward declaration
+in the C declarations at the beginning of the file:
+
+@example
+%@{
+ #define YYSTYPE char const *
+ static YYSTYPE stmtMerge (YYSTYPE x0, YYSTYPE x1);
+%@}
+@end example
+
+@noindent
+With these declarations, the resulting parser parses the first example
+as both an @code{expr} and a @code{decl}, and prints
+
+@example
+"x" y z + T <init-declare> x T <cast> y z + = <OR>
+@end example
+
+Bison requires that all of the
+productions that participate in any particular merge have identical
+@samp{%merge} clauses. Otherwise, the ambiguity would be unresolvable,
+and the parser will report an error during any parse that results in
+the offending merge.
+
+@node Compiler Requirements
+@subsection Considerations when Compiling @acronym{GLR} Parsers
+@cindex @code{inline}
+@cindex @acronym{GLR} parsers and @code{inline}
+
+The @acronym{GLR} parsers require a compiler for @acronym{ISO} C89 or
+later. In addition, they use the @code{inline} keyword, which is not
+C89, but is C99 and is a common extension in pre-C99 compilers. It is
+up to the user of these parsers to handle
+portability issues. For instance, if using Autoconf and the Autoconf
+macro @code{AC_C_INLINE}, a mere
+
+@example
+%@{
+ #include <config.h>
+%@}
+@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.
expressions. As it does this, it runs the actions for the grammar rules it
uses.
-The tokens come from a function called the @dfn{lexical analyzer} that you
-must supply in some fashion (such as by writing it in C). The Bison parser
-calls the lexical analyzer each time it wants a new token. It doesn't know
-what is ``inside'' the tokens (though their semantic values may reflect
-this). Typically the lexical analyzer makes the tokens by parsing
-characters of text, but Bison does not depend on this. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The tokens come from a function called the @dfn{lexical analyzer} that
+you must supply in some fashion (such as by writing it in C). The Bison
+parser calls the lexical analyzer each time it wants a new token. It
+doesn't know what is ``inside'' the tokens (though their semantic values
+may reflect this). Typically the lexical analyzer makes the tokens by
+parsing characters of text, but Bison does not depend on this.
+@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
The Bison parser file is C code which defines a function named
@code{yyparse} which implements that grammar. This function does not make
@xref{Interface, ,Parser C-Language Interface}.
Aside from the token type names and the symbols in the actions you
-write, all variable and function names used in the Bison parser file
+write, all symbols defined in the Bison parser file itself
begin with @samp{yy} or @samp{YY}. This includes interface functions
such as the lexical analyzer function @code{yylex}, the error reporting
function @code{yyerror} and the parser function @code{yyparse} itself.
or @samp{YY} in the Bison grammar file except for the ones defined in
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-@acronym{GNU} hosts, @code{<alloca.h>},
+@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
+declare memory allocators and related types. Other system headers may
+be included if you define @code{YYDEBUG} to a nonzero value
+(@pxref{Tracing, ,Tracing Your Parser}).
+
@node Stages
@section Stages in Using Bison
@cindex stages in using Bison
@enumerate
@item
Formally specify the grammar in a form recognized by Bison
-(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule in the language,
-describe the action that is to be taken when an instance of that rule
-is recognized. The action is described by a sequence of C statements.
+(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule
+in the language, describe the action that is to be taken when an
+instance of that rule is recognized. The action is described by a
+sequence of C statements.
@item
-Write a lexical analyzer to process input and pass tokens to the
-parser. The lexical analyzer may be written by hand in C
-(@pxref{Lexical, ,The Lexical Analyzer Function @code{yylex}}). It could also be produced using Lex, but the use
-of Lex is not discussed in this manual.
+Write a lexical analyzer to process input and pass tokens to the parser.
+The lexical analyzer may be written by hand in C (@pxref{Lexical, ,The
+Lexical Analyzer Function @code{yylex}}). It could also be produced
+using Lex, but the use of Lex is not discussed in this manual.
@item
Write a controlling function that calls the Bison-produced parser.
@example
%@{
-@var{Prologue (declarations)}
+@var{Prologue}
%@}
@var{Bison declarations}
%%
@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
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
calculator. As in C, comments are placed between @samp{/*@dots{}*/}.
@example
-/* Reverse polish notation calculator. */
+/* Reverse polish notation calculator. */
%@{
-#define YYSTYPE double
-#include <math.h>
+ #define YYSTYPE double
+ #include <math.h>
+ 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
The @code{#include} directive is used to declare the exponentiation
function @code{pow}.
-The second section, Bison declarations, provides information to Bison about
-the token types (@pxref{Bison Declarations, ,The Bison Declarations Section}). Each terminal symbol that is
-not a single-character literal must be declared here. (Single-character
+The forward declarations for @code{yylex} and @code{yyerror} are
+needed because the C language requires that functions be declared
+before they are used. These functions will be defined in the
+epilogue, but the parser calls them so they must be declared in the
+prologue.
+
+The second section, Bison declarations, provides information to Bison
+about the token types (@pxref{Bison Declarations, ,The Bison
+Declarations Section}). Each terminal symbol that is not a
+single-character literal must be declared here. (Single-character
literals normally don't need to be declared.) In this example, all the
arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is @code{NUM}, the token
;
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' @{ $$ = -$1; @}
;
%%
@end example
The parser function @code{yyparse} continues to process input until a
grammatical error is seen or the lexical analyzer says there are no more
-input tokens; we will arrange for the latter to happen at end of file.
+input tokens; we will arrange for the latter to happen at end-of-input.
@node Rpcalc Line
@subsubsection Explanation of @code{line}
This is what happens in the first rule (the one that uses @code{NUM}).
The formatting shown here is the recommended convention, but Bison does
-not require it. You can add or change whitespace as much as you wish.
+not require it. You can add or change white space as much as you wish.
For example, this:
@example
-exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{}
+exp : NUM | exp exp '+' @{$$ = $1 + $2; @} | @dots{} ;
@end example
@noindent
exp: NUM
| exp exp '+' @{ $$ = $1 + $2; @}
| @dots{}
+;
@end example
@noindent
@cindex writing a lexical analyzer
@cindex lexical analyzer, writing
-The lexical analyzer's job is low-level parsing: converting characters or
-sequences of characters into tokens. The Bison parser gets its tokens by
-calling the lexical analyzer. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The lexical analyzer's job is low-level parsing: converting characters
+or sequences of characters into tokens. The Bison parser gets its
+tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical
+Analyzer Function @code{yylex}}.
-Only a simple lexical analyzer is needed for the RPN calculator. This
+Only a simple lexical analyzer is needed for the @acronym{RPN}
+calculator. This
lexical analyzer skips blanks and tabs, then reads in numbers as
@code{double} and returns them as @code{NUM} tokens. Any other character
that isn't part of a number is a separate token. Note that the token-code
represents a token type. The same text used in Bison rules to stand for
this token type is also a C expression for the numeric code for the type.
This works in two ways. If the token type is a character literal, then its
-numeric code is the ASCII code for that character; you can use the same
+numeric code is that of the character; you can use the same
character literal in the lexical analyzer to express the number. If the
token type is an identifier, that identifier is defined by Bison as a C
macro whose definition is the appropriate number. In this example,
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
- number on the stack and the token NUM, or the ASCII
- character read if not a number. Skips all blanks
- and tabs, returns 0 for EOF. */
+/* The lexical analyzer returns a double floating point
+ number on the stack and the token NUM, or the numeric code
+ of the character read if not a number. It skips all blanks
+ and tabs, and returns 0 for end-of-input. */
#include <ctype.h>
@end group
@{
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);
@}
@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
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:
@group
#include <stdio.h>
+/* Called by yyparse on error. */
void
-yyerror (const char *s) /* Called by yyparse on error */
+yyerror (char const *s)
@{
- printf ("%s\n", s);
+ fprintf (stderr, "%s\n", s);
@}
@end group
@end example
@noindent
In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
-CALCulator''). Bison produces a file named @file{@var{file_name}.tab.c},
-removing the @samp{.y} from the original file name. The file output by
+@sc{calc}ulator''). Bison produces a file named @file{@var{file_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.
@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
@file{calc.y}, an infix desk-top calculator.
@example
-/* Infix notation calculator--calc */
+/* Infix notation calculator. */
%@{
-#define YYSTYPE double
-#include <math.h>
+ #define YYSTYPE double
+ #include <math.h>
+ #include <stdio.h>
+ 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
;
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus (@code{NEG}) is next, followed
-by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator Precedence}.
+by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator
+Precedence}.
-The other important new feature is the @code{%prec} in the grammar section
-for the unary minus operator. The @code{%prec} simply instructs Bison that
-the rule @samp{| '-' exp} has the same precedence as @code{NEG}---in this
-case the next-to-highest. @xref{Contextual Precedence, ,Context-Dependent Precedence}.
+The other important new feature is the @code{%prec} in the grammar
+section for the unary minus operator. The @code{%prec} simply instructs
+Bison that the rule @samp{| '-' exp} has the same precedence as
+@code{NEG}---in this case the next-to-highest. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.
Here is a sample run of @file{calc.y}:
@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
@code{yyerrok}, a macro defined automatically by Bison; its meaning is
that error recovery is complete (@pxref{Error Recovery}). Note the
difference between @code{yyerrok} and @code{yyerror}; neither one is a
-misprint.@refill
+misprint.
This form of error recovery deals with syntax errors. There are other
kinds of errors; for example, division by zero, which raises an exception
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.
/* Location tracking calculator. */
%@{
-#define YYSTYPE int
-#include <math.h>
+ #define YYSTYPE int
+ #include <math.h>
+ int yylex (void);
+ void yyerror (char const *);
%@}
/* Bison declarations. */
%left NEG
%right '^'
-%% /* Grammar follows */
+%% /* The grammar follows. */
@end example
@noindent
@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.
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';
@}
@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;
@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:
Here are the C and Bison declarations for the multi-function calculator.
@smallexample
+@group
%@{
-#include <math.h> /* For math functions, cos(), sin(), etc. */
-#include "calc.h" /* Contains definition of `symrec' */
+ #include <math.h> /* 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 <val> NUM /* Simple double precision number */
-%token <tptr> VAR FNCT /* Variable and Function */
+@end group
+%token <val> NUM /* Simple double precision number. */
+%token <tptr> VAR FNCT /* Variable and Function. */
%type <val> 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.
declarations are augmented with information about their data type (placed
between angle brackets).
-The Bison construct @code{%type} is used for declaring nonterminal symbols,
-just as @code{%token} is used for declaring token types. We have not used
-@code{%type} before because nonterminal symbols are normally declared
-implicitly by the rules that define them. But @code{exp} must be declared
-explicitly so we can specify its value type. @xref{Type Decl, ,Nonterminal Symbols}.
+The Bison construct @code{%type} is used for declaring nonterminal
+symbols, just as @code{%token} is used for declaring token types. We
+have not used @code{%type} before because nonterminal symbols are
+normally declared implicitly by the rules that define them. But
+@code{exp} must be declared explicitly so we can specify its value type.
+@xref{Type Decl, ,Nonterminal Symbols}.
@node Mfcalc Rules
@subsection Grammar Rules for @code{mfcalc}
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 @{ $$ = $3; $1->value.var = $3; @}
| exp '^' exp @{ $$ = pow ($1, $3); @}
| '(' exp ')' @{ $$ = $2; @}
;
-/* End of grammar */
+@end group
+/* End of grammar. */
%%
@end smallexample
@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
@code{init_table} as well:
@smallexample
-@group
#include <stdio.h>
-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,
"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)
@{
@}
@}
@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
@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;
(@code{VAR} or @code{FNCT}) is returned to @code{yyparse}. If it is not
already in the table, then it is installed as a @code{VAR} using
@code{putsym}. Again, a pointer and its type (which must be @code{VAR}) is
-returned to @code{yyparse}.@refill
+returned to @code{yyparse}.
No change is needed in the handling of numeric values and arithmetic
operators in @code{yylex}.
@smallexample
@group
#include <ctype.h>
+@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)
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;
@}
@end group
@group
- while (c != EOF && isalnum (c));
+ while (isalnum (c));
ungetc (c, stdin);
symbuf[i] = '\0';
@end group
@end smallexample
-This program is both powerful and flexible. You may easily add new
-functions, and it is a simple job to modify this code to install predefined
-variables such as @code{pi} or @code{e} as well.
+This program is both powerful and flexible. You may easily add new
+functions, and it is a simple job to modify this code to install
+predefined variables such as @code{pi} or @code{e} as well.
@node Exercises
@section Exercises
@example
%@{
-@var{Prologue}
+ @var{Prologue}
%@}
@var{Bison declarations}
@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.
* 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
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 <stdio.h>
+ #include "ptypes.h"
+%@}
+
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%@{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+%@}
+
+@dots{}
+@end smallexample
+
@node Bison Declarations
@subsection The Bison Declarations Section
@cindex Bison declarations (introduction)
@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.
@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}.
All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
-character literal because its ASCII code, zero, is the code @code{yylex}
-returns for end-of-input (@pxref{Calling Convention, ,Calling Convention
-for @code{yylex}}).
+character literal because its numeric code, zero, signifies
+end-of-input (@pxref{Calling Convention, ,Calling Convention
+for @code{yylex}}). Also, unlike standard C, trigraphs have no
+special meaning in Bison character literals, nor is backslash-newline
+allowed.
@item
@cindex string token
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
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, except that you must not use a null character within a
+string literal. Also, unlike Standard C, trigraphs have no special
+meaning in Bison string literals, nor is backslash-newline allowed. A
+literal string token must contain two or more characters; for a token
+containing just one character, use a character token (see above).
@end itemize
How you choose to write a terminal symbol has no effect on its
grammatical meaning. That depends only on where it appears in rules and
on when the parser function returns that symbol.
-The value returned by @code{yylex} is always one of the terminal symbols
-(or 0 for end-of-input). Whichever way you write the token type in the
-grammar rules, you write it the same way in the definition of @code{yylex}.
-The numeric code for a character token type is simply the ASCII code for
-the character, so @code{yylex} can use the identical character constant to
-generate the requisite code. Each named token type becomes a C macro in
+The value returned by @code{yylex} is always one of the terminal
+symbols, except that a zero or negative value signifies end-of-input.
+Whichever way you write the token type in the grammar rules, you write
+it the same way in the definition of @code{yylex}. The numeric code
+for a character token type is simply the positive numeric code of the
+character, so @code{yylex} can use the identical value to generate the
+requisite code, though you may need to convert it to @code{unsigned
+char} to avoid sign-extension on hosts where @code{char} is signed.
+Each named token type becomes a C macro in
the parser file, so @code{yylex} can use the name to stand for the code.
(This is why periods don't make sense in terminal symbols.)
@xref{Calling Convention, ,Calling Convention for @code{yylex}}.
into a separate header file @file{@var{name}.tab.h} which you can include
in the other source files that need it. @xref{Invocation, ,Invoking Bison}.
+If you want to write a grammar that is portable to any Standard C
+host, you must use only 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
+@acronym{EBCDIC}, the resulting program may not work because the
+tables generated by Bison will assume @acronym{ASCII} numeric values for
+character tokens. It is standard
+practice for software distributions to contain C source files that
+were generated by Bison in an @acronym{ASCII} environment, so installers on
+platforms that are incompatible with @acronym{ASCII} must rebuild those
+files before compiling them.
+
The symbol @code{error} is a terminal symbol reserved for error recovery
(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
-In particular, @code{yylex} should never return this value.
+In particular, @code{yylex} should never return this value. The default
+value of the error token is 256, unless you explicitly assigned 256 to
+one of your tokens with a @code{%token} declaration.
@node Rules
@section Syntax of Grammar Rules
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:
@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
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
In most programs, you will need different data types for different kinds
of tokens and groupings. For example, a numeric constant may need type
-@code{int} or @code{long}, while a string constant needs type @code{char *},
+@code{int} or @code{long int}, while a string constant needs type @code{char *},
and an identifier might need a pointer to an entry in the symbol table.
To use more than one data type for semantic values in one parser, Bison
@itemize @bullet
@item
Specify the entire collection of possible data types, with the
-@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of Value Types}).
+@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
+Value Types}).
@item
Choose one of those types for each symbol (terminal or nonterminal) for
semantic values associated with tokens or smaller groupings.
An action consists of C statements surrounded by braces, much like a
-compound statement in C. It can be placed at any position in the rule; it
-is executed at that position. Most rules have just one action at the end
-of the rule, following all the components. Actions in the middle of a rule
-are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}).
+compound statement in C@. 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
+Actions, ,Actions in Mid-Rule}).
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. (Bison translates both of these constructs
-into array element references when it copies the actions into the parser
-file.)
+being constructed is @code{$$}. Bison translates both of these
+constructs into expressions of the appropriate type when it copies the
+actions into the parser file. @code{$$} is translated to a modifiable
+lvalue, so it can be assigned to.
Here is a typical example:
The sum is stored into @code{$$} so that it becomes the semantic value of
the addition-expression just recognized by the rule. If there were a
useful semantic value associated with the @samp{+} token, it could be
-referred to as @code{$2}.@refill
+referred to as @code{$2}.
-@cindex default action
-If you don't specify an action for a rule, Bison supplies a default:
-@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule becomes
-the value of the whole rule. Of course, the default rule is valid only
-if the two data types match. There is no meaningful default action for
-an empty rule; every empty rule must have an explicit action unless the
-rule's value does not matter.
+Note that the vertical-bar character @samp{|} is really a rule
+separator, and actions are attached to a single rule. This is a
+difference with tools like Flex, for which @samp{|} stands for either
+``or'', or ``the same action as that of the next rule''. In the
+following example, the action is triggered only when @samp{b} is found:
-@code{$@var{n}} with @var{n} zero or negative is allowed for reference
-to tokens and groupings on the stack @emph{before} those that match the
-current rule. This is a very risky practice, and to use it reliably
+@example
+@group
+a-or-b: 'a'|'b' @{ a_or_b_found = 1; @};
+@end group
+@end example
+
+@cindex default action
+If you don't specify an action for a rule, Bison supplies a default:
+@w{@code{$$ = $1}.} Thus, the value of the first symbol in the rule
+becomes the value of the whole rule. Of course, the default action is
+valid only if the two data types match. There is no meaningful default
+action for an empty rule; every empty rule must have an explicit action
+unless the rule's value does not matter.
+
+@code{$@var{n}} with @var{n} zero or negative is allowed for reference
+to tokens and groupings on the stack @emph{before} those that match the
+current rule. This is a very risky practice, and to use it reliably
you must be certain of the context in which the rule is applied. Here
is a case in which you can use this reliably:
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value. Then each time you use @code{$$} or
@code{$@var{n}}, its data type is determined by which symbol it refers to
-in the rule. In this example,@refill
+in the rule. In this example,
@example
@group
@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
have the data type declared for the nonterminal symbol @code{exp}. If
@code{$2} were used, it would have the data type declared for the
-terminal symbol @code{'+'}, whatever that might be.@refill
+terminal symbol @code{'+'}, whatever that might be.
Alternatively, you can specify the data type when you refer to the value,
by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
@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) ?
-
-The way locations are handled is defined by providing a data type, and actions
-to take when rules are matched.
+The way locations are handled is defined by providing a data type, and
+actions to take when rules are matched.
@menu
* Location Type:: Specifying a data type for locations.
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
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
else
@{
$$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
@}
@}
@end group
@end example
As for semantic values, there is a default action for locations that is
-run each time a rule is matched. It sets the beginning of @code{@@$} to the
+run each time a rule is matched. It sets the beginning of @code{@@$} to the
beginning of the first symbol, and the end of @code{@@$} to the end of the
last symbol.
-With this default action, the location tracking can be fully automatic. The
+With this default action, the location tracking can be fully automatic. The
example above simply rewrites this way:
@example
else
@{
$$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
@}
@}
@end group
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
-Actually, actions are not the best place to compute locations. Since locations
-are much more general than semantic values, there is room in the output parser
-to redefine the default action to take for each rule. The
-@code{YYLLOC_DEFAULT} macro is called each time a rule is matched, before the
-associated action is run.
+Actually, actions are not the best place to compute locations. Since
+locations are much more general than semantic values, there is room in
+the output parser to redefine the default action to take for each
+rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
+matched, before the associated action is run. 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 identifies locations of
+all right hand side elements of the rule being matched, and the third
+parameter is the size of the rule's right hand side. When processing
+a syntax error, the second parameter identifies locations of
+the symbols that were discarded during error processing, and the third
+parameter is the number of discarded symbols.
-By default, it is defined this way:
+By default, @code{YYLLOC_DEFAULT} is defined this way:
-@example
+@smallexample
@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) \
+ do \
+ if (N) \
+ @{ \
+ (Current).first_line = YYRHSLOC(Rhs, 1).first_line; \
+ (Current).first_column = YYRHSLOC(Rhs, 1).first_column; \
+ (Current).last_line = YYRHSLOC(Rhs, N).last_line; \
+ (Current).last_column = YYRHSLOC(Rhs, N).last_column; \
+ @} \
+ else \
+ @{ \
+ (Current).first_line = (Current).last_line = \
+ YYRHSLOC(Rhs, 0).last_line; \
+ (Current).first_column = (Current).last_column = \
+ YYRHSLOC(Rhs, 0).last_column; \
+ @} \
+ while (0)
@end group
-@end example
+@end smallexample
+
+where @code{YYRHSLOC (rhs, k)} is the location of the @var{k}th symbol
+in @var{rhs} when @var{k} is positive, and the location of the symbol
+just before the reduction when @var{k} and @var{n} are both zero.
When defining @code{YYLLOC_DEFAULT}, you should consider that:
@itemize @bullet
@item
-All arguments are free of side-effects. However, only the first one (the
+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}.
+For consistency with semantic actions, valid indexes within the
+right hand side range from 1 to @var{n}. When @var{n} is zero, only 0 is a
+valid index, and it refers to the symbol just before the reduction.
+During error processing @var{n} is always positive.
@item
-For consistency with semantic actions, valid indexes for the location array
-range from 1 to @var{n}.
+Your macro should parenthesize its arguments, if need be, since the
+actual arguments may not be surrounded by parentheses. Also, your
+macro should expand to something that can be used as a single
+statement when it is followed by a semicolon.
@end itemize
@node Declarations
The first rule in the file also specifies the start symbol, by default.
If you want some other symbol to be the start symbol, you must declare
-it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars}).
+it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free
+Grammars}).
@menu
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Initial Action Decl:: Code run before parsing starts.
+* Destructor Decl:: Declaring how symbols are freed.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
Precedence}.
You can explicitly specify the numeric code for a token type by appending
-an integer value in the field immediately following the token name:
+a decimal or hexadecimal integer value in the field immediately
+following the token name:
@example
%token NUM 300
+%token XNUM 0x12d // a GNU extension
@end example
@noindent
It is generally best, however, to let Bison choose the numeric codes for
all token types. Bison will automatically select codes that don't conflict
-with each other or with ASCII characters.
+with each other or with normal characters.
In the event that the stack type is a union, you must augment the
@code{%token} or other token declaration to include the data type
-alternative delimited by angle-brackets (@pxref{Multiple Types, ,More Than One Value Type}).
+alternative delimited by angle-brackets (@pxref{Multiple Types, ,More
+Than One Value Type}).
For example:
Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to
declare a token and specify its precedence and associativity, all at
once. These are called @dfn{precedence declarations}.
-@xref{Precedence, ,Operator Precedence}, for general information on operator precedence.
+@xref{Precedence, ,Operator Precedence}, for general information on
+operator precedence.
The syntax of a precedence declaration is the same as that of
@code{%token}: either
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
@end example
@noindent
-Here @var{nonterminal} is the name of a nonterminal symbol, and @var{type}
-is the name given in the @code{%union} to the alternative that you want
-(@pxref{Union Decl, ,The Collection of Value Types}). You can give any number of nonterminal symbols in
-the same @code{%type} declaration, if they have the same value type. Use
-spaces to separate the symbol names.
+Here @var{nonterminal} is the name of a nonterminal symbol, and
+@var{type} is the name given in the @code{%union} to the alternative
+that you want (@pxref{Union Decl, ,The Collection of Value Types}). You
+can give any number of nonterminal symbols in the same @code{%type}
+declaration, if they have the same value type. Use spaces to separate
+the symbol names.
You can also declare the value type of a terminal symbol. To do this,
use the same @code{<@var{type}>} construction in a declaration for the
terminal symbol. All kinds of token declarations allow
@code{<@var{type}>}.
+@node Initial Action Decl
+@subsection Performing Actions before Parsing
+@findex %initial-action
+
+Sometimes your parser needs to perform some initializations before
+parsing. The @code{%initial-action} directive allows for such arbitrary
+code.
+
+@deffn {Directive} %initial-action @{ @var{code} @}
+@findex %initial-action
+Declare that the @var{code} must be invoked before parsing each time
+@code{yyparse} is called. The @var{code} may use @code{$$} and
+@code{@@$} --- initial value and location of the look-ahead --- and the
+@code{%parse-param}.
+@end deffn
+
+For instance, if your locations use a file name, you may use
+
+@example
+%parse-param @{ const char *filename @};
+%initial-action
+@{
+ @@$.begin.filename = @@$.end.filename = filename;
+@};
+@end example
+
+
+@node Destructor Decl
+@subsection Freeing Discarded Symbols
+@cindex freeing discarded symbols
+@findex %destructor
+
+Some symbols can be discarded by the parser. For instance, during error
+recovery (@pxref{Error Recovery}), embarrassing symbols already pushed
+on the stack, and embarrassing tokens coming from the rest of the file
+are thrown away until the parser falls on its feet. If these symbols
+convey heap based information, this memory is lost. While this behavior
+can be tolerable for batch parsers, such as in compilers, it is not for
+possibly ``never ending'' parsers such as shells, or implementations of
+communication protocols.
+
+The @code{%destructor} directive allows for the definition of code that
+is called when a symbol is thrown away.
+
+@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
+@findex %destructor
+Declare that the @var{code} must be invoked for each of the
+@var{symbols} that will be discarded by the parser. The @var{code}
+should use @code{$$} to designate the semantic value associated to the
+@var{symbols}. The additional parser parameters are also available
+(@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> STRING
+%type <string> 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
+
+@sp 1
+
+@cindex discarded symbols
+@dfn{Discarded symbols} are the following:
+
+@itemize
+@item
+stacked symbols popped during the first phase of error recovery,
+@item
+incoming terminals during the second phase of error recovery,
+@item
+the current look-ahead when the parser aborts (either via an explicit
+call to @code{YYABORT}, or as a consequence of a failed error recovery).
+@end itemize
+
+
@node Expect Decl
@subsection Suppressing Conflict Warnings
@cindex suppressing conflict warnings
@cindex warnings, preventing
@cindex conflicts, suppressing warnings of
@findex %expect
+@findex %expect-rr
Bison normally warns if there are any conflicts in the grammar
(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
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.
+For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more serious,
+and should be eliminated entirely. Bison will always report
+reduce/reduce conflicts for these parsers. With @acronym{GLR} parsers, however,
+both shift/reduce and reduce/reduce are routine (otherwise, there
+would be no need to use @acronym{GLR} parsing). Therefore, it is also possible
+to specify an expected number of reduce/reduce conflicts in @acronym{GLR}
+parsers, using the declaration:
+
+@example
+%expect-rr @var{n}
+@end example
+
In general, using @code{%expect} involves these steps:
@itemize @bullet
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
@subsection A Pure (Reentrant) Parser
@cindex reentrant parser
@cindex pure parser
-@findex %pure_parser
+@findex %pure-parser
A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
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}.)
Alternatively, you can generate a pure, reentrant parser. The Bison
-declaration @code{%pure_parser} says that you want the parser to be
+declaration @code{%pure-parser} says that you want the parser to be
reentrant. It looks like this:
@example
-%pure_parser
+%pure-parser
@end example
The result is that the communication variables @code{yylval} and
Here is a summary of the declarations used to define a grammar:
-@table @code
-@item %union
+@deffn {Directive} %union
Declare the collection of data types that semantic values may have
(@pxref{Union Decl, ,The Collection of Value Types}).
+@end deffn
-@item %token
+@deffn {Directive} %token
Declare a terminal symbol (token type name) with no precedence
or associativity specified (@pxref{Token Decl, ,Token Type Names}).
+@end deffn
-@item %right
+@deffn {Directive} %right
Declare a terminal symbol (token type name) that is right-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
-@item %left
+@deffn {Directive} %left
Declare a terminal symbol (token type name) that is left-associative
(@pxref{Precedence Decl, ,Operator Precedence}).
+@end deffn
-@item %nonassoc
+@deffn {Directive} %nonassoc
Declare a terminal symbol (token type name) that is nonassociative
-(using it in a way that would be associative is a syntax error)
(@pxref{Precedence Decl, ,Operator Precedence}).
+Using it in a way that would be associative is a syntax error.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules lacking an explicit @code{%prec} modifier
+(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
+@end deffn
+@end ifset
-@item %type
+@deffn {Directive} %type
Declare the type of semantic values for a nonterminal symbol
(@pxref{Type Decl, ,Nonterminal Symbols}).
+@end deffn
-@item %start
+@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
-Output a definition of the macro @code{YYDEBUG} into the parser file, so
-that the debugging facilities are compiled. @xref{Debugging, ,Debugging
-Your Parser}.
-
-@item %defines
-Write an extra output file containing macro definitions for the token
-type names defined in the grammar and the semantic value type
-@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
+@deffn {Directive} %debug
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@end deffn
+@xref{Tracing, ,Tracing Your Parser}.
+@deffn {Directive} %defines
+Write a header file containing macro definitions for the token type
+names defined in the grammar as well as a few other declarations.
If the parser output file is named @file{@var{name}.c} then this file
-is named @file{@var{name}.h}.@refill
-
-This output file is essential if you wish to put the definition of
-@code{yylex} in a separate source file, because @code{yylex} needs to
-be able to refer to token type codes and the variable
-@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill
-
-@item %file-prefix="@var{prefix}"
+is named @file{@var{name}.h}.
+
+Unless @code{YYSTYPE} is already defined as a macro, the output header
+declares @code{YYSTYPE}. Therefore, if you are using a @code{%union}
+(@pxref{Multiple Types, ,More Than One Value Type}) with components
+that require other definitions, or if you have defined a
+@code{YYSTYPE} macro (@pxref{Value Type, ,Data Types of Semantic
+Values}), you need to arrange for these definitions to be propagated to
+all modules, e.g., by putting them in a
+prerequisite header that is included both by your parser and by any
+other module that needs @code{YYSTYPE}.
+
+Unless your parser is pure, the output header declares @code{yylval}
+as an external variable. @xref{Pure Decl, ,A Pure (Reentrant)
+Parser}.
+
+If you have also used locations, the output header declares
+@code{YYLTYPE} and @code{yylloc} using a protocol similar to that of
+@code{YYSTYPE} and @code{yylval}. @xref{Locations, ,Tracking
+Locations}.
+
+This output file is normally essential if you wish to put the
+definition of @code{yylex} in a separate source file, because
+@code{yylex} typically needs to be able to refer to the
+above-mentioned declarations and to the token type codes.
+@xref{Token Values, ,Semantic Values of Tokens}.
+@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} %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} and @code{yydebug}. 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}.
+@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
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules lacking an explicit @code{%prec}
+modifier (@pxref{Contextual Precedence, ,Context-Dependent
+Precedence}).
+@end deffn
+@end ifset
-@item %no-parser
+@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.
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
contains @samp{"*"*"}. (In C, that would be written as
@code{"\"*\"*\""}).
-When you specify @code{%token_table}, Bison also generates macro
+When you specify @code{%token-table}, Bison also generates macro
definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
@code{YYNRULES}, and @code{YYNSTATES}:
@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.
+that state. @xref{Understanding, , Understanding Your Parser}, for more
+information.
+@end deffn
-This file also describes all the conflicts, both those resolved by
-operator precedence and the unresolved ones.
-
-The file's name is made by removing @samp{.tab.c} or @samp{.c} from
-the parser output file name, and adding @samp{.output} instead.@refill
-
-Therefore, if the input file is @file{foo.y}, then the parser file is
-called @file{foo.tab.c} by default. As a consequence, the verbose
-output file is called @file{foo.output}.@refill
-
-@item %yacc
-@itemx %fixed-output-files
+@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
between different definitions of @code{yyparse}, @code{yylval}, and so on.
The easy way to do this is to use the option @samp{-p @var{prefix}}
-(@pxref{Invocation, ,Invoking Bison}). This renames the interface functions and
-variables of the Bison parser to start with @var{prefix} instead of
-@samp{yy}. You can use this to give each parser distinct names that do
-not conflict.
+(@pxref{Invocation, ,Invoking Bison}). This renames the interface
+functions and variables of the Bison parser to start with @var{prefix}
+instead of @samp{yy}. You can use this to give each parser distinct
+names that do not conflict.
The precise list of symbols renamed is @code{yyparse}, @code{yylex},
-@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yychar} and
-@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
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
+
+If you use a reentrant parser, you can optionally pass additional
+parameter information to it in a reentrant way. To do so, use the
+declaration @code{%parse-param}:
+
+@deffn {Directive} %parse-param @{@var{argument-declaration}@}
+@findex %parse-param
+Declare that an argument declared by @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
+
+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}
To do this, use the @samp{-d} option when you run Bison, so that it will
write these macro definitions into a separate header file
@file{@var{name}.tab.h} which you can include in the other source files
-that need it. @xref{Invocation, ,Invoking Bison}.@refill
+that need it. @xref{Invocation, ,Invoking Bison}.
@menu
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
* Token Values:: How @code{yylex} must return the semantic value
of the token it has read.
-* Token Positions:: How @code{yylex} must return the text position
+* 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
@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
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:
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
@{
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;
@end smallexample
The @code{yytname} table is generated only if you use the
-@code{%token_table} declaration. @xref{Decl Summary}.
+@code{%token-table} declaration. @xref{Decl Summary}.
@end itemize
@node Token Values
@example
@group
@dots{}
- yylval = value; /* Put value onto Bison stack. */
- return INT; /* Return the type of the token. */
+ yylval = value; /* Put value onto Bison stack. */
+ return INT; /* Return the type of the token. */
@dots{}
@end group
@end example
When you are using multiple data types, @code{yylval}'s type is a union
-made from the @code{%union} declaration (@pxref{Union Decl, ,The Collection of Value Types}). So when
-you store a token's value, you must use the proper member of the union.
-If the @code{%union} declaration looks like this:
+made from the @code{%union} declaration (@pxref{Union Decl, ,The
+Collection of Value Types}). So when you store a token's value, you
+must use the proper member of the union. If the @code{%union}
+declaration looks like this:
@example
@group
@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, ,
@node Pure Calling
@subsection Calling Conventions for Pure Parsers
-When you use the Bison declaration @code{%pure_parser} to request a
+When you use the Bison declaration @code{%pure-parser} to request a
pure, reentrant parser, the global communication variables @code{yylval}
and @code{yylloc} cannot be used. (@xref{Pure Decl, ,A Pure (Reentrant)
Parser}.) In such parsers the two global variables are replaced by
@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}
@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
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
@example
@group
void
-yyerror (char *s)
+yyerror (char const *s)
@{
@end group
@group
(@pxref{Error Recovery}). If recovery is impossible, @code{yyparse} will
immediately return 1.
+Obviously, in location tracking pure parsers, @code{yyerror} should have
+an access to the current location.
+This is indeed the case for the @acronym{GLR}
+parsers, but not for the Yacc parser, for historical reasons. I.e., if
+@samp{%locations %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, @acronym{GLR} and Yacc parsers share the same @code{yyerror} calling
+convention for absolutely pure parsers, i.e., when the calling
+convention of @code{yylex} @emph{and} the calling convention of
+@code{%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
-request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}) then it is a local variable
-which only the actions can access.
+request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
+then it is a local variable which only the actions can access.
@node Action Features
@section Special Features for Use in Actions
Here is a table of Bison constructs, variables and macros that
are useful in actions.
-@table @samp
-@item $$
+@deffn {Variable} $$
Acts like a variable that contains the semantic value for the
grouping made by the current rule. @xref{Actions}.
+@end deffn
-@item $@var{n}
+@deffn {Variable} $@var{n}
Acts like a variable that contains the semantic value for the
@var{n}th component of the current rule. @xref{Actions}.
+@end deffn
-@item $<@var{typealt}>$
+@deffn {Variable} $<@var{typealt}>$
Like @code{$$} but specifies alternative @var{typealt} in the union
-specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}.
+specified by the @code{%union} declaration. @xref{Action Types, ,Data
+Types of Values in Actions}.
+@end deffn
-@item $<@var{typealt}>@var{n}
+@deffn {Variable} $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies alternative @var{typealt} in the
union specified by the @code{%union} declaration.
-@xref{Action Types, ,Data Types of Values in Actions}.@refill
+@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.
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}.
@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
* 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
The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared. The second effect is to assign
-precedence levels to certain rules: each rule gets its precedence from the
-last terminal symbol mentioned in the components. (You can also specify
-explicitly the precedence of a rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}.)
-
-Finally, the resolution of conflicts works by comparing the
-precedence of the rule being considered with that of the
-look-ahead token. If the token's precedence is higher, the
-choice is to shift. If the rule's precedence is higher, the
-choice is to reduce. If they have equal precedence, the choice
-is made based on the associativity of that precedence level. The
-verbose output file made by @samp{-v} (@pxref{Invocation, ,Invoking Bison}) says
-how each conflict was resolved.
+precedence levels to certain rules: each rule gets its precedence from
+the last terminal symbol mentioned in the components. (You can also
+specify explicitly the precedence of a rule. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.)
+
+Finally, the resolution of conflicts works by comparing the precedence
+of the rule being considered with that of the look-ahead token. If the
+token's precedence is higher, the choice is to shift. If the rule's
+precedence is higher, the choice is to reduce. If they have equal
+precedence, the choice is made based on the associativity of that
+precedence level. The verbose output file made by @samp{-v}
+(@pxref{Invocation, ,Invoking Bison}) says how each conflict was
+resolved.
Not all rules and not all tokens have precedence. If either the rule or
the look-ahead token has no precedence, then the default is to shift.
@code{%nonassoc}, can only be used once for a given token; so a token has
only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the @code{%prec}
-modifier for rules.@refill
+modifier for rules.
The @code{%prec} modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
@end group
@end example
+@ifset defaultprec
+If you forget to append @code{%prec UMINUS} to the rule for unary
+minus, Bison silently assumes that minus has its usual precedence.
+This kind of problem can be tricky to debug, since one typically
+discovers the mistake only by testing the code.
+
+The @code{%no-default-prec;} declaration makes it easier to discover
+this kind of problem systematically. It causes rules that lack a
+@code{%prec} modifier to have no precedence, even if the last terminal
+symbol mentioned in their components has a declared precedence.
+
+If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
+for all rules that participate in precedence conflict resolution.
+Then you will see any shift/reduce conflict until you tell Bison how
+to resolve it, either by changing your grammar or by adding an
+explicit precedence. This will probably add declarations to the
+grammar, but it helps to protect against incorrect rule precedences.
+
+The effect of @code{%no-default-prec;} can be reversed by giving
+@code{%default-prec;}, which is the default.
+@end ifset
+
@node Parser States
@section Parser States
@cindex finite-state machine
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
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.
When the problem arises, you can often fix it by identifying the two
-parser states that are being confused (using @samp{-v}
-(@pxref{Invocation, ,Invoking Bison}) can help), and adding something to
-make them look distinct. In the above example, adding one rule to
+parser states that are being confused, and adding something to make them
+look distinct. In the above example, adding one rule to
@code{return_spec} as follows makes the problem go away:
@example
;
@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 look-ahead.
+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
+look-ahead, 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 @acronym{GLR} parsers, please see: Elizabeth
+Scott, Adrian Johnstone and Shamsa Sadaf Hussain, Tomita-Style
+Generalised @acronym{LR} Parsers, Royal Holloway, University of
+London, Department of Computer Science, TR-00-12,
+@uref{http://www.cs.rhul.ac.uk/research/languages/publications/tomita_style_1.ps},
+(2000-12-24).
+
@node Stack Overflow
@section Stack Overflow, and How to Avoid It
@cindex stack overflow
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
macro with a value that is an integer. This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.
-It must be a constant expression whose value is known at compile time.
The stack space allowed is not necessarily allocated. If you specify a
large value for @code{YYMAXDEPTH}, the parser actually allocates a small
you do not need to make @code{YYMAXDEPTH} painfully small merely to save
space for ordinary inputs that do not need much stack.
+However, do not allow @code{YYMAXDEPTH} to be a value so large that
+arithmetic overflow could occur when calculating the size of the stack
+space. Also, do not allow @code{YYMAXDEPTH} to be less than
+@code{YYINITDEPTH}.
+
@cindex default stack limit
The default value of @code{YYMAXDEPTH}, if you do not define it, is
10000.
@vindex YYINITDEPTH
You can control how much stack is allocated initially by defining the
-macro @code{YYINITDEPTH}. This value too must be a compile-time
-constant integer. The default is 200.
+macro @code{YYINITDEPTH} to a positive integer. For the C
+@acronym{LALR}(1) parser, this value must be a compile-time constant
+unless you are assuming C99 or some other target language or compiler
+that allows variable-length arrays. The default is 200.
+
+Do not allow @code{YYINITDEPTH} to be a value so large that arithmetic
+overflow would occur when calculating the size of the stack space.
+Also, do not allow @code{YYINITDEPTH} to be greater than
+@code{YYMAXDEPTH}.
+
+@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.
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
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
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
@example
@group
%@{
-int hexflag;
+ int hexflag;
+ int yylex (void);
+ void yyerror (char const *);
%@}
%%
@dots{}
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
-@findex YYDEBUG
-@findex yydebug
-@cindex debugging
-@cindex tracing the parser
-If a Bison grammar compiles properly but doesn't do what you want when it
-runs, the @code{yydebug} parser-trace feature can help you figure out why.
+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}).
-To enable compilation of trace facilities, you must define the macro
-@code{YYDEBUG} when you compile the parser. 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}). Alternatively, use the
-@samp{-t} option when you run Bison (@pxref{Invocation, ,Invoking Bison}).
-We always define @code{YYDEBUG} so that debugging is always possible.
+@menu
+* Understanding:: Understanding the structure of your parser.
+* Tracing:: Tracing the execution of your parser.
+@end menu
-The trace facility uses @code{stderr}, so you must add
-@w{@code{#include <stdio.h>}} to the prologue unless it is already there.
+@node Understanding
+@section Understanding Your Parser
-Once you have compiled the program with trace facilities, the way to
-request a trace is to store a nonzero value in the variable @code{yydebug}.
-You can do this by making the C code do it (in @code{main}, perhaps), or
-you can alter the value with a C debugger.
+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).
-Each step taken by the parser when @code{yydebug} is nonzero produces a
-line or two of trace information, written on @code{stderr}. The trace
-messages tell you these things:
+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}.
-@itemize @bullet
-@item
-Each time the parser calls @code{yylex}, what kind of token was read.
+The following grammar file, @file{calc.y}, will be used in the sequel:
-@item
-Each time a token is shifted, the depth and complete contents of the
-state stack (@pxref{Parser States}).
+@example
+%token NUM STR
+%left '+' '-'
+%left '*'
+%%
+exp: exp '+' exp
+ | exp '-' exp
+ | exp '*' exp
+ | exp '/' exp
+ | NUM
+ ;
+useless: STR;
+%%
+@end example
-@item
-Each time a rule is reduced, which rule it is, and the complete contents
-of the state stack afterward.
-@end itemize
+@command{bison} reports:
-To make sense of this information, it helps to refer to the listing file
-produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking Bison}). This file
-shows the meaning of each state in terms of positions in various rules, and
-also what each state will do with each possible input token. As you read
-the successive trace messages, you can see that the parser is functioning
-according to its specification in the listing file. Eventually you will
-arrive at the place where something undesirable happens, and you will see
-which parts of the grammar are to blame.
+@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
-The parser file is a C program and you can use C debuggers on it, but it's
-not easy to interpret what it is doing. The parser function is a
-finite-state machine interpreter, and aside from the actions it executes
-the same code over and over. Only the values of variables show where in
-the grammar it is working.
+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.
-@findex YYPRINT
-The debugging information normally gives the token type of each token
-read, but not its semantic value. You can optionally define a macro
-named @code{YYPRINT} to provide a way to print the value. If you define
-@code{YYPRINT}, it should take three arguments. The parser will pass a
-standard I/O stream, the numeric code for the token type, and the token
-value (from @code{yylval}).
+The first section includes details on conflicts that were solved thanks
+to precedence and/or associativity:
-Here is an example of @code{YYPRINT} suitable for the multi-function
-calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}):
+@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
-@smallexample
-#define YYPRINT(file, type, value) yyprint (file, type, value)
+@noindent
+The next section lists states that still have conflicts.
-static void
-yyprint (FILE *file, int type, YYSTYPE value)
-@{
- if (type == VAR)
- fprintf (file, " %s", value.tptr->name);
- else if (type == NUM)
- fprintf (file, " %d", value.val);
-@}
-@end smallexample
+@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
-@node Invocation
-@chapter Invoking Bison
-@cindex invoking Bison
-@cindex Bison invocation
-@cindex options for invoking Bison
+@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):
-The usual way to invoke Bison is as follows:
+@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
-bison @var{infile}
+Grammar
+
+ Number, Line, Rule
+ 0 5 $accept -> exp $end
+ 1 5 exp -> exp '+' exp
+ 2 6 exp -> exp '-' exp
+ 3 7 exp -> exp '*' exp
+ 4 8 exp -> exp '/' exp
+ 5 9 exp -> NUM
@end example
-Here @var{infile} is the grammar file name, which usually ends in
+@noindent
+and reports the uses of the symbols:
+
+@example
+Terminals, with rules where they appear
+
+$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 look-ahead is a @code{NUM}, then this token is shifted on
+the parse stack, and the control flow jumps to state 1. Any other
+look-ahead 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 look-ahead token because @code{NUM} can be
+at the beginning of any rule deriving an @code{exp}. By default Bison
+reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
+you want to see more detail you can invoke @command{bison} with
+@option{--report=itemset} to list all the items, include those that can
+be derived:
+
+@example
+state 0
+
+ $accept -> . exp $ (rule 0)
+ exp -> . exp '+' exp (rule 1)
+ exp -> . exp '-' exp (rule 2)
+ exp -> . exp '*' exp (rule 3)
+ exp -> . exp '/' exp (rule 4)
+ exp -> . NUM (rule 5)
+
+ NUM shift, and go to state 1
+
+ exp go to state 2
+@end example
+
+@noindent
+In the state 1...
+
+@example
+state 1
+
+ exp -> NUM . (rule 5)
+
+ $default reduce using rule 5 (exp)
+@end example
+
+@noindent
+the rule 5, @samp{exp: NUM;}, is completed. Whatever the look-ahead token
+(@samp{$default}), the parser will reduce it. If it was coming from
+state 0, then, after this reduction it will return to state 0, and will
+jump to state 2 (@samp{exp: go to state 2}).
+
+@example
+state 2
+
+ $accept -> exp . $ (rule 0)
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ $ shift, and go to state 3
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+@end example
+
+@noindent
+In state 2, the automaton can only shift a symbol. For instance,
+because of the item @samp{exp -> exp . '+' exp}, if the look-ahead 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)]
+ $default reduce using rule 1 (exp)
+@end example
+
+Indeed, there are two actions associated to the look-ahead @samp{/}:
+either shifting (and going to state 7), or reducing rule 1. The
+conflict means that either the grammar is ambiguous, or the parser lacks
+information to make the right decision. Indeed the grammar is
+ambiguous, as, since we did not specify the precedence of @samp{/}, the
+sentence @samp{NUM + NUM / NUM} can be parsed as @samp{NUM + (NUM /
+NUM)}, which corresponds to shifting @samp{/}, or as @samp{(NUM + NUM) /
+NUM}, which corresponds to reducing rule 1.
+
+Because in @acronym{LALR}(1) parsing a single decision can be made, Bison
+arbitrarily chose to disable the reduction, see @ref{Shift/Reduce, ,
+Shift/Reduce Conflicts}. Discarded actions are reported in between
+square brackets.
+
+Note that all the previous states had a single possible action: either
+shifting the next token and going to the corresponding state, or
+reducing a single rule. In the other cases, i.e., when shifting
+@emph{and} reducing is possible or when @emph{several} reductions are
+possible, the look-ahead is required to select the action. State 8 is
+one such state: if the look-ahead is @samp{*} or @samp{/} then the action
+is shifting, otherwise the action is reducing rule 1. In other words,
+the first two items, corresponding to rule 1, are not eligible when the
+look-ahead token is @samp{*}, since we specified that @samp{*} has higher
+precedence than @samp{+}. More generally, some items are eligible only
+with some set of possible look-ahead tokens. When run with
+@option{--report=look-ahead}, Bison specifies these look-ahead tokens:
+
+@example
+state 8
+
+ exp -> exp . '+' exp [$, '+', '-', '/'] (rule 1)
+ exp -> exp '+' exp . [$, '+', '-', '/'] (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 1 (exp)]
+ $default reduce using rule 1 (exp)
+@end example
+
+The remaining states are similar:
+
+@example
+state 9
+
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp '-' exp . (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 2 (exp)]
+ $default reduce using rule 2 (exp)
+
+state 10
+
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp '*' exp . (rule 3)
+ exp -> exp . '/' exp (rule 4)
+
+ '/' shift, and go to state 7
+
+ '/' [reduce using rule 3 (exp)]
+ $default reduce using rule 3 (exp)
+
+state 11
+
+ exp -> exp . '+' exp (rule 1)
+ exp -> exp . '-' exp (rule 2)
+ exp -> exp . '*' exp (rule 3)
+ exp -> exp . '/' exp (rule 4)
+ exp -> exp '/' exp . (rule 4)
+
+ '+' shift, and go to state 4
+ '-' shift, and go to state 5
+ '*' shift, and go to state 6
+ '/' shift, and go to state 7
+
+ '+' [reduce using rule 4 (exp)]
+ '-' [reduce using rule 4 (exp)]
+ '*' [reduce using rule 4 (exp)]
+ '/' [reduce using rule 4 (exp)]
+ $default reduce using rule 4 (exp)
+@end example
+
+@noindent
+Observe that state 11 contains conflicts not only due to the lack of
+precedence of @samp{/} with respect to @samp{+}, @samp{-}, and
+@samp{*}, but also because the
+associativity of @samp{/} is not specified.
+
+
+@node Tracing
+@section Tracing Your Parser
+@findex yydebug
+@cindex debugging
+@cindex tracing the parser
+
+If a Bison grammar compiles properly but doesn't do what you want when it
+runs, the @code{yydebug} parser-trace feature can help you figure out why.
+
+There are several means to enable compilation of trace facilities:
+
+@table @asis
+@item the macro @code{YYDEBUG}
+@findex YYDEBUG
+Define the macro @code{YYDEBUG} to a nonzero value when you compile the
+parser. This is compliant with @acronym{POSIX} Yacc. You could use
+@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
+YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The
+Prologue}).
+
+@item the option @option{-t}, @option{--debug}
+Use the @samp{-t} option when you run Bison (@pxref{Invocation,
+,Invoking Bison}). This is @acronym{POSIX} compliant too.
+
+@item the directive @samp{%debug}
+@findex %debug
+Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison
+Declaration Summary}). This is a Bison extension, which will prove
+useful when Bison will output parsers for languages that don't use a
+preprocessor. Unless @acronym{POSIX} and Yacc portability matter to
+you, this is
+the preferred solution.
+@end table
+
+We suggest that you always enable the debug option so that debugging is
+always possible.
+
+The trace facility outputs messages with macro calls of the form
+@code{YYFPRINTF (stderr, @var{format}, @var{args})} where
+@var{format} and @var{args} are the usual @code{printf} format and
+arguments. If you define @code{YYDEBUG} to a nonzero value but do not
+define @code{YYFPRINTF}, @code{<stdio.h>} is automatically included
+and @code{YYPRINTF} is defined to @code{fprintf}.
+
+Once you have compiled the program with trace facilities, the way to
+request a trace is to store a nonzero value in the variable @code{yydebug}.
+You can do this by making the C code do it (in @code{main}, perhaps), or
+you can alter the value with a C debugger.
+
+Each step taken by the parser when @code{yydebug} is nonzero produces a
+line or two of trace information, written on @code{stderr}. The trace
+messages tell you these things:
+
+@itemize @bullet
+@item
+Each time the parser calls @code{yylex}, what kind of token was read.
+
+@item
+Each time a token is shifted, the depth and complete contents of the
+state stack (@pxref{Parser States}).
+
+@item
+Each time a rule is reduced, which rule it is, and the complete contents
+of the state stack afterward.
+@end itemize
+
+To make sense of this information, it helps to refer to the listing file
+produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking
+Bison}). This file shows the meaning of each state in terms of
+positions in various rules, and also what each state will do with each
+possible input token. As you read the successive trace messages, you
+can see that the parser is functioning according to its specification in
+the listing file. Eventually you will arrive at the place where
+something undesirable happens, and you will see which parts of the
+grammar are to blame.
+
+The parser file is a C program and you can use C debuggers on it, but it's
+not easy to interpret what it is doing. The parser function is a
+finite-state machine interpreter, and aside from the actions it executes
+the same code over and over. Only the values of variables show where in
+the grammar it is working.
+
+@findex YYPRINT
+The debugging information normally gives the token type of each token
+read, but not its semantic value. You can optionally define a macro
+named @code{YYPRINT} to provide a way to print the value. If you define
+@code{YYPRINT}, it should take three arguments. The parser will pass a
+standard I/O stream, the numeric code for the token type, and the token
+value (from @code{yylval}).
+
+Here is an example of @code{YYPRINT} suitable for the multi-function
+calculator (@pxref{Mfcalc Decl, ,Declarations for @code{mfcalc}}):
+
+@smallexample
+%@{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(file, type, value) print_token_value (file, type, value)
+%@}
+
+@dots{} %% @dots{} %% @dots{}
+
+static void
+print_token_value (FILE *file, int type, YYSTYPE value)
+@{
+ if (type == VAR)
+ fprintf (file, "%s", value.tptr->name);
+ else if (type == NUM)
+ fprintf (file, "%d", value.val);
+@}
+@end smallexample
+
+@c ================================================= Invoking Bison
+
+@node Invocation
+@chapter Invoking Bison
+@cindex invoking Bison
+@cindex Bison invocation
+@cindex options for invoking Bison
+
+The usual way to invoke Bison is as follows:
+
+@example
+bison @var{infile}
+@end example
+
+Here @var{infile} is the grammar file name, which usually ends in
@samp{.y}. The parser file's name is made by replacing the @samp{.y}
with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields
@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields
-@file{hack/foo.tab.c}. It's is also possible, in case you are writting
+@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}.
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
@need 1750
@item -y
@itemx --yacc
-@itemx --fixed-output-files
Equivalent to @samp{-o y.tab.c}; the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
@file{y.tab.h}. The purpose of this option is to imitate Yacc's output
file name conventions. Thus, the following shell script can substitute
-for Yacc:@refill
+for Yacc, and the Bison distribution contains such a script for
+compatibility with @acronym{POSIX}:
@example
-bison -y $*
+#! /bin/sh
+bison -y "$@@"
@end example
@end table
@item -t
@itemx --debug
-Output a definition of the macro @code{YYDEBUG} into the parser file, so
-that the debugging facilities are compiled. @xref{Debugging, ,Debugging
-Your Parser}.
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@xref{Tracing, ,Tracing Your Parser}.
@item --locations
Pretend that @code{%locations} was specified. @xref{Decl Summary}.
@itemx --defines
Pretend that @code{%defines} was specified, i.e., write an extra output
file containing macro definitions for the token type names defined in
-the grammar and the semantic value type @code{YYSTYPE}, as well as a few
-@code{extern} variable declarations. @xref{Decl Summary}.
+the grammar, as well as a few other declarations. @xref{Decl Summary}.
@item --defines=@var{defines-file}
Same as above, but save in the file @var{defines-file}.
@item -b @var{file-prefix}
@itemx --file-prefix=@var{prefix}
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 look-ahead
+Implies @code{state} and augments the description of the automaton with
+each rule's look-ahead 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}
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
\line{ --debug \leaderfill -t}
\line{ --defines \leaderfill -d}
\line{ --file-prefix \leaderfill -b}
-\line{ --fixed-output-files \leaderfill -y}
\line{ --graph \leaderfill -g}
\line{ --help \leaderfill -h}
\line{ --name-prefix \leaderfill -p}
--debug -t
--defines=@var{defines-file} -d
--file-prefix=@var{prefix} -b @var{file-prefix}
---fixed-output-files --yacc -y
--graph=@var{graph-file} -d
--help -h
--name-prefix=@var{prefix} -p @var{name-prefix}
--token-table -k
--verbose -v
--version -V
+--yacc -y
@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 command line syntax for Bison on VMS is a variant of the usual
-Bison command syntax---adapted to fit VMS conventions.
+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}).
-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:
+If you use the Yacc library's @code{yyerror} function, you should
+declare @code{yyerror} as follows:
@example
-bison /debug/name_prefix=bar foo.y
+int yyerror (char const *);
@end example
+Bison ignores the @code{int} value returned by this @code{yyerror}.
+If you use the Yacc library's @code{main} function, your
+@code{yyparse} function should have the following type signature:
+
+@example
+int yyparse (void);
+@end example
+
+@c ================================================= 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 the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* C++ Parsers:: Compiling Parsers with C++ Compilers
+* Implementing Gotos/Loops:: Control Flow in the Calculator
+@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 the Parser
+@section How Can I Reset the Parser
+
+The following phenomenon has several symptoms, resulting in the
+following typical questions:
+
+@display
+I invoke @code{yyparse} several times, and on correct input it works
+properly; but when a parse error is found, all the other calls fail
+too. How can I reset the error flag of @code{yyparse}?
+@end display
+
@noindent
-is equivalent to the following command under POSIX.
+or
+
+@display
+My parser includes support for an @samp{#include}-like feature, in
+which case I run @code{yyparse} from @code{yyparse}. This fails
+although I did specify I needed a @code{%pure-parser}.
+@end display
+
+These problems typically come not from Bison itself, but from
+Lex-generated scanners. Because these scanners use large buffers for
+speed, they might not notice a change of input file. As a
+demonstration, consider the following source file,
+@file{first-line.l}:
+
+@verbatim
+%{
+#include <stdio.h>
+#include <stdlib.h>
+%}
+%%
+.*\n ECHO; return 1;
+%%
+int
+yyparse (char const *file)
+{
+ yyin = fopen (file, "r");
+ if (!yyin)
+ exit (2);
+ /* One token only. */
+ yylex ();
+ if (fclose (yyin) != 0)
+ exit (3);
+ return 0;
+}
+
+int
+main (void)
+{
+ yyparse ("input");
+ yyparse ("input");
+ return 0;
+}
+@end verbatim
+
+@noindent
+If the file @file{input} contains
+
+@verbatim
+input:1: Hello,
+input:2: World!
+@end verbatim
+
+@noindent
+then instead of getting the first line twice, you get:
@example
-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 switch to the
+new one. This depends upon your implementation of Lex; see its
+documentation for more. For Flex, it suffices to call
+@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}. If your
+Flex-generated scanner needs to read from several input streams to
+handle features like include files, you might consider using Flex
+functions like @samp{yy_switch_to_buffer} that manipulate multiple
+input buffers.
+
+If your Flex-generated scanner uses start conditions (@pxref{Start
+conditions, , Start conditions, flex, The Flex Manual}), you might
+also want to reset the scanner's state, i.e., go back to the initial
+start condition, through a call to @samp{BEGIN (0)}.
+
+@node Strings are Destroyed
+@section Strings are Destroyed
+
+@display
+My parser seems to destroy old strings, or maybe it loses track of
+them. Instead of reporting @samp{"foo", "bar"}, it reports
+@samp{"bar", "bar"}, or even @samp{"foo\nbar", "bar"}.
+@end display
+
+This error is probably the single most frequent ``bug report'' sent to
+Bison lists, but is only concerned with a misunderstanding of the role
+of scanner. Consider the following Lex code:
+
+@verbatim
+%{
+#include <stdio.h>
+char *yylval = NULL;
+%}
+%%
+.* yylval = yytext; return 1;
+\n /* IGNORE */
+%%
+int
+main ()
+{
+ /* Similar to using $1, $2 in a Bison action. */
+ char *fst = (yylex (), yylval);
+ char *snd = (yylex (), yylval);
+ printf ("\"%s\", \"%s\"\n", fst, snd);
+ return 0;
+}
+@end verbatim
-@node Table of Symbols
-@appendix Bison Symbols
-@cindex Bison symbols, table of
-@cindex symbols in Bison, table of
+If you compile and run this code, you get:
-@table @code
-@item error
-A token name reserved for error recovery. This token may be used in
-grammar rules so as to allow the Bison parser to recognize an error in
-the grammar without halting the process. In effect, a sentence
-containing an error may be recognized as valid. On a parse error, the
-token @code{error} becomes the current look-ahead token. Actions
-corresponding to @code{error} are then executed, and the look-ahead
-token is reset to the token that originally caused the violation.
-@xref{Error Recovery}.
+@example
+$ @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
-@item YYABORT
-Macro to pretend that an unrecoverable syntax error has occurred, by
-making @code{yyparse} return 1 immediately. The error reporting
-function @code{yyerror} is not called. @xref{Parser Function, ,The
-Parser Function @code{yyparse}}.
+@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:
-@item YYACCEPT
-Macro to pretend that a complete utterance of the language has been
-read, by making @code{yyparse} return 0 immediately.
-@xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@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
-@item YYBACKUP
-Macro to discard a value from the parser stack and fake a look-ahead
-token. @xref{Action Features, ,Special Features for Use in Actions}.
-@item 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}.
+@node C++ Parsers
+@section C++ Parsers
-@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.
+@display
+How can I generate parsers in C++?
+@end display
-@item YYINITDEPTH
-Macro for specifying the initial size of the parser stack.
-@xref{Stack Overflow}.
+We are working on a C++ output for Bison, but unfortunately, for lack of
+time, the skeleton is not finished. It is functional, but in numerous
+respects, it will require additional work which @emph{might} break
+backward compatibility. Since the skeleton for C++ is not documented,
+we do not consider ourselves bound to this interface, nevertheless, as
+much as possible we will try to keep compatibility.
-@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}.
+Another possibility is to use the regular C parsers, and to compile them
+with a C++ compiler. This works properly, provided that you bear some
+simple C++ rules in mind, such as not including ``real classes'' (i.e.,
+structure with constructors) in unions. Therefore, in the
+@code{%union}, use pointers to classes.
-@item YYLTYPE
-Macro for the data type of @code{yylloc}; a structure with four
-members. @xref{Location Type, , Data Types of Locations}.
-@item yyltype
-Default value for YYLTYPE.
+@node Implementing Gotos/Loops
+@section Implementing Gotos/Loops
-@item YYMAXDEPTH
-Macro for specifying the maximum size of the parser stack.
-@xref{Stack Overflow}.
+@display
+My simple calculator supports variables, assignments, and functions,
+but how can I implement gotos, or loops?
+@end display
-@item YYPARSE_PARAM
-Macro for specifying the name of a parameter that @code{yyparse} should
-accept. @xref{Pure Calling,, Calling Conventions for Pure Parsers}.
+Although very pedagogical, the examples included in the document blur
+the distinction to make between the parser---whose job is to recover
+the structure of a text and to transmit it to subsequent modules of
+the program---and the processing (such as the execution) of this
+structure. This works well with so called straight line programs,
+i.e., precisely those that have a straightforward execution model:
+execute simple instructions one after the others.
-@item YYRECOVERING
-Macro whose value indicates whether the parser is recovering from a
-syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
+@cindex abstract syntax tree
+@cindex @acronym{AST}
+If you want a richer model, you will probably need to use the parser
+to construct a tree that does represent the structure it has
+recovered; this tree is usually called the @dfn{abstract syntax tree},
+or @dfn{@acronym{AST}} for short. Then, walking through this tree,
+traversing it in various ways, will enable treatments such as its
+execution or its translation, which will result in an interpreter or a
+compiler.
-@item YYSTACK_USE_ALLOCA
-Macro used to control the use of @code{alloca}. If defined to @samp{0},
-the parser will not use @code{alloca} but @code{malloc} when trying to
-grow its internal stacks. Do @emph{not} define @code{YYSTACK_USE_ALLOCA}
-to anything else.
+This topic is way beyond the scope of this manual, and the reader is
+invited to consult the dedicated literature.
-@item YYSTYPE
-Macro for the data type of semantic values; @code{int} by default.
-@xref{Value Type, ,Data Types of Semantic Values}.
-@item yychar
-External integer variable that contains the integer value of the current
-look-ahead token. (In a pure parser, it is a local variable within
-@code{yyparse}.) Error-recovery rule actions may examine this variable.
-@xref{Action Features, ,Special Features for Use in Actions}.
-@item yyclearin
-Macro used in error-recovery rule actions. It clears the previous
-look-ahead token. @xref{Error Recovery}.
+@c ================================================= Table of Symbols
-@item yydebug
-External integer variable set to zero by default. If @code{yydebug}
-is given a nonzero value, the parser will output information on input
-symbols and parser action. @xref{Debugging, ,Debugging Your Parser}.
+@node Table of Symbols
+@appendix Bison Symbols
+@cindex Bison symbols, table of
+@cindex symbols in Bison, table of
-@item yyerrok
-Macro to cause parser to recover immediately to its normal mode
-after a parse error. @xref{Error Recovery}.
+@deffn {Variable} @@$
+In an action, the location of the left-hand side of the rule.
+@xref{Locations, , Locations Overview}.
+@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
-Reporting Function @code{yyerror}}.
+@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 yylex
-User-supplied lexical analyzer function, called with no arguments
-to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+@deffn {Variable} $$
+In an action, the semantic value of the left-hand side of the rule.
+@xref{Actions}.
+@end deffn
-@item yylval
-External variable in which @code{yylex} should place the semantic
-value associated with a token. (In a pure parser, it is a local
-variable within @code{yyparse}, and its address is passed to
-@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
+@deffn {Variable} $@var{n}
+In an action, the semantic value of the @var{n}-th symbol of the
+right-hand side of the rule. @xref{Actions}.
+@end deffn
-@item yylloc
-External variable in which @code{yylex} should place the line and column
-numbers associated with a token. (In a pure parser, it is a local
-variable within @code{yyparse}, and its address is passed to
-@code{yylex}.) You can ignore this variable if you don't use the
-@samp{@@} feature in the grammar actions. @xref{Token Positions,
-,Textual Positions of Tokens}.
+@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 yynerrs
-Global variable which Bison increments each time there is a parse error.
-(In a pure parser, it is a local variable within @code{yyparse}.)
-@xref{Error Reporting, ,The Error Reporting Function @code{yyerror}}.
+@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 yyparse
-The parser function produced by Bison; call this function to start
-parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@deffn {Construct} /*@dots{}*/
+Comment delimiters, as in C.
+@end deffn
+
+@deffn {Delimiter} :
+Separates a rule's result from its components. @xref{Rules, ,Syntax of
+Grammar Rules}.
+@end deffn
+
+@deffn {Delimiter} ;
+Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
-@item %debug
+@deffn {Delimiter} |
+Separates alternate rules for the same result nonterminal.
+@xref{Rules, ,Syntax of Grammar Rules}.
+@end deffn
+
+@deffn {Symbol} $accept
+The predefined nonterminal whose only rule is @samp{$accept: @var{start}
+$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
+Start-Symbol}. It cannot be used in the grammar.
+@end deffn
+
+@deffn {Directive} %debug
Equip the parser for debugging. @xref{Decl Summary}.
+@end deffn
-@item %defines
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
+
+@deffn {Directive} %defines
Bison declaration to create a header file meant for the scanner.
@xref{Decl Summary}.
+@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 {Symbol} $end
+The predefined token marking the end of the token stream. It cannot be
+used in the grammar.
+@end deffn
+
+@deffn {Symbol} error
+A token name reserved for error recovery. This token may be used in
+grammar rules so as to allow the Bison parser to recognize an error in
+the grammar without halting the process. In effect, a sentence
+containing an error may be recognized as valid. On a syntax error, the
+token @code{error} becomes the current 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
+
+@deffn {Directive} %error-verbose
+Bison declaration to request verbose, specific error message strings
+when @code{yyerror} is called.
+@end deffn
-@item %file-prefix="@var{prefix}"
-Bison declaration to set tge prefix of the output files. @xref{Decl
+@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} %initial-action
+Run user code before parsing. @xref{Initial Action Decl, , Performing Actions before Parsing}.
+@end deffn
+
+@deffn {Directive} %left
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
+@end deffn
+
+@deffn {Directive} %lex-param @{@var{argument-declaration}@}
+Bison declaration to specifying an additional parameter that
+@code{yylex} should accept. @xref{Pure Calling,, Calling Conventions
+for Pure Parsers}.
+@end deffn
+
+@deffn {Directive} %merge
+Bison declaration to assign a merging function to a rule. If there is a
+reduce/reduce conflict with a rule having the same merging function, the
+function is applied to the two semantic values to get a single result.
+@xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}.
+@end deffn
+
+@deffn {Directive} %name-prefix="@var{prefix}"
+Bison declaration to rename the external symbols. @xref{Decl Summary}.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
-@item %name-prefix="@var{prefix}"
-Bison declaration to rename the external symbols. @xref{Decl Summary}.
-
-@item %no-lines
+@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
+
+@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
-@item %prec
+@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
-Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}.
+@deffn {Directive} %start
+Bison declaration to specify the start symbol. @xref{Start Decl, ,The
+Start-Symbol}.
+@end deffn
-@item %token
+@deffn {Directive} %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Token Decl, ,Token Type Names}.
+@end deffn
-@item %token-table
+@deffn {Directive} %token-table
Bison declaration to include a token name table in the parser file.
@xref{Decl Summary}.
+@end deffn
+
+@deffn {Directive} %type
+Bison declaration to declare nonterminals. @xref{Type Decl,
+,Nonterminal Symbols}.
+@end deffn
-@item %type
-Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}.
+@deffn {Symbol} $undefined
+The predefined token onto which all undefined values returned by
+@code{yylex} are mapped. It cannot be used in the grammar, rather, use
+@code{error}.
+@end deffn
-@item %union
+@deffn {Directive} %union
Bison declaration to specify several possible data types for semantic
values. @xref{Union Decl, ,The Collection of Value Types}.
-@end table
+@end deffn
-These are the punctuation and delimiters used in Bison input:
+@deffn {Macro} YYABORT
+Macro to pretend that an unrecoverable syntax error has occurred, by
+making @code{yyparse} return 1 immediately. The error reporting
+function @code{yyerror} is not called. @xref{Parser Function, ,The
+Parser Function @code{yyparse}}.
+@end deffn
-@table @samp
-@item %%
-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}.
+@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 %@{ %@}
-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}.
+@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 /*@dots{}*/
-Comment delimiters, as in C.
+@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 :
-Separates a rule's result from its components. @xref{Rules, ,Syntax of
-Grammar Rules}.
+@deffn {Variable} yyclearin
+Macro used in error-recovery rule actions. It clears the previous
+look-ahead token. @xref{Error Recovery}.
+@end deffn
-@item ;
-Terminates a rule. @xref{Rules, ,Syntax of Grammar Rules}.
+@deffn {Macro} YYDEBUG
+Macro to define to equip the parser with tracing code. @xref{Tracing,
+,Tracing Your Parser}.
+@end deffn
-@item |
-Separates alternate rules for the same result nonterminal.
-@xref{Rules, ,Syntax of Grammar Rules}.
-@end table
+@deffn {Variable} yydebug
+External integer variable set to zero by default. If @code{yydebug}
+is given a nonzero value, the parser will output information on input
+symbols and parser action. @xref{Tracing, ,Tracing Your Parser}.
+@end deffn
+
+@deffn {Macro} yyerrok
+Macro to cause parser to recover immediately to its normal mode
+after a syntax error. @xref{Error Recovery}.
+@end deffn
+
+@deffn {Macro} YYERROR
+Macro to pretend that a syntax error has just been detected: call
+@code{yyerror} and then perform normal error recovery if possible
+(@pxref{Error Recovery}), or (if recovery is impossible) make
+@code{yyparse} return 1. @xref{Error Recovery}.
+@end deffn
+
+@deffn {Function} yyerror
+User-supplied function to be called by @code{yyparse} on error.
+@xref{Error Reporting, ,The Error
+Reporting Function @code{yyerror}}.
+@end deffn
+
+@deffn {Macro} YYERROR_VERBOSE
+An obsolete macro that you define with @code{#define} in the prologue
+to request verbose, specific error message strings
+when @code{yyerror} is called. It doesn't matter what definition you
+use for @code{YYERROR_VERBOSE}, just whether you define it. Using
+@code{%error-verbose} is preferred.
+@end deffn
+
+@deffn {Macro} YYINITDEPTH
+Macro for specifying the initial size of the parser stack.
+@xref{Stack Overflow}.
+@end deffn
+
+@deffn {Function} yylex
+User-supplied lexical analyzer function, called with no arguments to get
+the next token. @xref{Lexical, ,The Lexical Analyzer Function
+@code{yylex}}.
+@end deffn
+
+@deffn {Macro} YYLEX_PARAM
+An obsolete macro for specifying an extra argument (or list of extra
+arguments) for @code{yyparse} to pass to @code{yylex}. 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
+
+@deffn {Variable} yylloc
+External variable in which @code{yylex} should place the line and column
+numbers associated with a token. (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.) You can ignore this variable if you don't use the
+@samp{@@} feature in the grammar actions. @xref{Token Locations,
+,Textual Locations of Tokens}.
+@end deffn
+
+@deffn {Type} YYLTYPE
+Data type of @code{yylloc}; by default, a structure with four
+members. @xref{Location Type, , Data Types of Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+External variable in which @code{yylex} should place the semantic
+value associated with a token. (In a pure parser, it is a local
+variable within @code{yyparse}, and its address is passed to
+@code{yylex}.) @xref{Token Values, ,Semantic Values of Tokens}.
+@end deffn
+
+@deffn {Macro} YYMAXDEPTH
+Macro for specifying the maximum size of the parser stack. @xref{Stack
+Overflow}.
+@end deffn
+
+@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
+
+@deffn {Function} yyparse
+The parser function produced by Bison; call this function to start
+parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@end deffn
+
+@deffn {Macro} YYPARSE_PARAM
+An obsolete macro for specifying the name of a parameter that
+@code{yyparse} should accept. The use of this macro is deprecated, and
+is supported only for Yacc like parsers. @xref{Pure Calling,, Calling
+Conventions for Pure Parsers}.
+@end deffn
+
+@deffn {Macro} YYRECOVERING
+Macro whose value indicates whether the parser is recovering from a
+syntax error. @xref{Action Features, ,Special Features for Use in Actions}.
+@end deffn
+
+@deffn {Macro} YYSTACK_USE_ALLOCA
+Macro used to control the use of @code{alloca} when the C
+@acronym{LALR}(1) parser needs to extend its stacks. If defined to 0,
+the parser will use @code{malloc} to extend its stacks. If defined to
+1, the parser will use @code{alloca}. Values other than 0 and 1 are
+reserved for future Bison extensions. If not defined,
+@code{YYSTACK_USE_ALLOCA} defaults to 0.
+
+If you define @code{YYSTACK_USE_ALLOCA} to 1, it is your
+responsibility to make sure that @code{alloca} is visible, e.g., by
+using @acronym{GCC} or by including @code{<stdlib.h>}. Furthermore,
+in the all-too-common case where your code may run on a host with a
+limited stack and with unreliable stack-overflow checking, you should
+set @code{YYMAXDEPTH} to a value that cannot possibly result in
+unchecked stack overflow on any of your target hosts when
+@code{alloca} is called. You can inspect the code that Bison
+generates in order to determine the proper numeric values. This will
+require some expertise in low-level implementation details.
+@end deffn
+
+@deffn {Type} YYSTYPE
+Data type of semantic values; @code{int} by default.
+@xref{Value Type, ,Data Types of Semantic Values}.
+@end deffn
@node Glossary
@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.
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
@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.
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.
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
@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
@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.
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.
@printindex cp
@bye
+
+@c LocalWords: texinfo setfilename settitle setchapternewpage finalout
+@c LocalWords: ifinfo smallbook shorttitlepage titlepage GPL FIXME iftex
+@c LocalWords: akim fn cp syncodeindex vr tp synindex dircategory direntry
+@c LocalWords: ifset vskip pt filll insertcopying sp ISBN Etienne Suvasa
+@c LocalWords: ifnottex yyparse detailmenu GLR RPN Calc var Decls Rpcalc
+@c LocalWords: rpcalc Lexer Gen Comp Expr ltcalc mfcalc Decl Symtab yylex
+@c LocalWords: yyerror pxref LR yylval cindex dfn LALR samp gpl BNF xref
+@c LocalWords: const int paren ifnotinfo AC noindent emph expr stmt findex
+@c LocalWords: glr YYSTYPE TYPENAME prog dprec printf decl init stmtMerge
+@c LocalWords: pre STDC GNUC endif yy YY alloca lf stddef stdlib YYDEBUG
+@c LocalWords: NUM exp subsubsection kbd Ctrl ctype EOF getchar isdigit
+@c LocalWords: ungetc stdin scanf sc calc ulator ls lm cc NEG prec yyerrok
+@c LocalWords: longjmp fprintf stderr preg yylloc YYLTYPE cos ln
+@c LocalWords: smallexample symrec val tptr FNCT fnctptr func struct sym
+@c LocalWords: fnct putsym getsym fname arith fncts atan ptr malloc sizeof
+@c LocalWords: strlen strcpy fctn strcmp isalpha symbuf realloc isalnum
+@c LocalWords: ptypes itype YYPRINT trigraphs yytname expseq vindex dtype
+@c LocalWords: Rhs YYRHSLOC LE nonassoc op deffn typeless typefull yynerrs
+@c LocalWords: yychar yydebug msg YYNTOKENS YYNNTS YYNRULES YYNSTATES
+@c LocalWords: cparse clex deftypefun NE defmac YYACCEPT YYABORT param
+@c LocalWords: strncmp intval tindex lvalp locp llocp typealt YYBACKUP
+@c LocalWords: YYEMPTY YYRECOVERING yyclearin GE def UMINUS maybeword
+@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
+@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype
+@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args
+@c LocalWords: YYPRINTF infile ypp yxx outfile itemx vcg tex leaderfill
+@c LocalWords: hbox hss hfill tt ly yyin fopen fclose ofirst gcc ll
+@c LocalWords: yyrestart nbar yytext fst snd osplit ntwo strdup AST
+@c LocalWords: YYSTACK DVI fdl printindex
projects / bison.git / blobdiff