@c the smallbook format.
@c @smallbook
-@c Set following if you have the new `shorttitlepage' command
-@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
+@ifnotinfo
@syncodeindex fn cp
@syncodeindex vr cp
@syncodeindex tp cp
-@end iftex
+@end ifnotinfo
@ifinfo
@synindex fn cp
@synindex vr cp
@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.
+1999, 2000, 2001, 2002, 2003, 2004, 2005, 2006 Free Software Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
@end quotation
@end copying
-@dircategory GNU programming tools
+@dircategory Software development
@direntry
* bison: (bison). @acronym{GNU} parser generator (Yacc replacement).
@end direntry
-@ifset shorttitlepage-enabled
-@shorttitlepage Bison
-@end ifset
@titlepage
@title Bison
@subtitle The Yacc-compatible Parser Generator
@insertcopying
@sp 2
Published by the Free Software Foundation @*
-59 Temple Place, Suite 330 @*
-Boston, MA 02111-1307 USA @*
+51 Franklin Street, Fifth Floor @*
+Boston, MA 02110-1301 USA @*
Printed copies are available from the Free Software Foundation.@*
@acronym{ISBN} 1-882114-44-2
@sp 2
messy for Bison to handle straightforwardly.
* Debugging:: Understanding or debugging Bison parsers.
* Invocation:: How to run Bison (to produce the parser source file).
+* C++ Language Interface:: Creating C++ parser objects.
+* FAQ:: Frequently Asked Questions
* 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.
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
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
+* GLR Semantic Actions:: Deferred semantic actions have special concerns.
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
Examples
Outline of a Bison Grammar
* Prologue:: Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations:: Syntax and usage of the Bison declarations section.
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
Bison Declarations
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
which reads tokens.
* Error Reporting:: You must supply a function @code{yyerror}.
* Action Features:: Special features for use in actions.
+* Internationalization:: How to let the parser speak in the user's
+ native language.
The Lexical Analyzer Function @code{yylex}
The Bison Parser Algorithm
-* Look-Ahead:: Parser looks one token ahead when deciding what to do.
+* Lookahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator's precedence depends on context.
* 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.
+* Memory Management:: What happens when memory is exhausted. How to avoid it.
Operator Precedence
* Option Cross Key:: Alphabetical list of long options.
* Yacc Library:: Yacc-compatible @code{yylex} and @code{main}.
+C++ Language Interface
+
+* C++ Parsers:: The interface to generate C++ parser classes
+* A Complete C++ Example:: Demonstrating their use
+
+C++ Parsers
+
+* C++ Bison Interface:: Asking for C++ parser generation
+* C++ Semantic Values:: %union vs. C++
+* C++ Location Values:: The position and location classes
+* C++ Parser Interface:: Instantiating and running the parser
+* C++ Scanner Interface:: Exchanges between yylex and parse
+
+A Complete C++ Example
+
+* Calc++ --- C++ Calculator:: The specifications
+* Calc++ Parsing Driver:: An active parsing context
+* Calc++ Parser:: A parser class
+* Calc++ Scanner:: A pure C++ Flex scanner
+* Calc++ Top Level:: Conducting the band
+
Frequently Asked Questions
-* Parser Stack Overflow:: Breaking the Stack Limits
+* Memory Exhausted:: Breaking the Stack Limits
* How Can I Reset the Parser:: @code{yyparse} Keeps some State
* Strings are Destroyed:: @code{yylval} Loses Track of Strings
-* C++ Parsers:: Compiling Parsers with C++ Compilers
* Implementing Gotos/Loops:: Control Flow in the Calculator
+* Multiple start-symbols:: Factoring closely related grammars
+* Secure? Conform?:: Is Bison @acronym{POSIX} safe?
+* I can't build Bison:: Troubleshooting
+* Where can I find help?:: Troubleshouting
+* Bug Reports:: Troublereporting
+* Other Languages:: Parsers in Java and others
+* Beta Testing:: Experimenting development versions
+* Mailing Lists:: Meeting other Bison users
Copying This Manual
@unnumbered Introduction
@cindex introduction
-@dfn{Bison} is a general-purpose parser generator that converts a
-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
+@dfn{Bison} is a general-purpose parser generator that converts an
+annotated context-free grammar into an @acronym{LALR}(1) or
+@acronym{GLR} parser for that grammar. Once you are proficient with
+Bison, you can use it to develop a wide range of language parsers, from those
used in simple desk calculators to complex programming languages.
Bison is upward compatible with Yacc: all properly-written Yacc grammars
ought to work with Bison with no change. Anyone familiar with Yacc
should be able to use Bison with little trouble. You need to be fluent in
-C programming in order to use Bison or to understand this manual.
+C or C++ programming in order to use Bison or to understand this manual.
We begin with tutorial chapters that explain the basic concepts of using
Bison and show three explained examples, each building on the last. If you
@node Conditions
@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 when
-Bison is generating C code for @acronym{LALR}(1) parsers. Formerly, these
+The distribution terms for Bison-generated parsers permit using the
+parsers in nonfree programs. Before Bison version 2.2, these extra
+permissions applied only when Bison was generating @acronym{LALR}(1)
+parsers in C@. And before Bison version 1.24, Bison-generated
parsers could be used only in programs that were free software.
The other @acronym{GNU} programming tools, such as the @acronym{GNU} C
The output of the Bison utility---the Bison parser file---contains a
verbatim copy of a sizable piece of Bison, which is the code for the
-@code{yyparse} function. (The actions from your grammar are inserted
-into this function at one point, but the rest of the function is not
-changed.) When we applied the @acronym{GPL} terms to the code for
-@code{yyparse},
+parser's implementation. (The actions from your grammar are inserted
+into this implementation at one point, but most of the rest of the
+implementation is not changed.) When we applied the @acronym{GPL}
+terms to the skeleton code for the parser's implementation,
the effect was to restrict the use of Bison output to free software.
We didn't change the terms because of sympathy for people who want to
practical conditions for using Bison match the practical conditions for
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.''
+This exception applies when Bison is generating code for a parser.
+You can tell whether the exception applies to a Bison output file by
+inspecting the file for text beginning with ``As a special
+exception@dots{}''. The text spells out the exact terms of the
+exception.
@include gpl.texi
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
+token of lookahead. Strictly speaking, that is a description of an
@acronym{LR}(1) grammar, and @acronym{LALR}(1) involves additional
restrictions that are
hard to explain simply; but it is rare in actual practice to find an
@cindex @acronym{GLR} parsing
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
-@cindex non-deterministic parsing
+@cindex nondeterministic parsing
Parsers for @acronym{LALR}(1) grammars are @dfn{deterministic}, meaning
roughly that the next grammar rule to apply at any point in the input is
uniquely determined by the preceding input and a fixed, finite portion
-(called a @dfn{look-ahead}) of the remaining input. A context-free
+(called a @dfn{lookahead}) of the remaining input. A context-free
grammar can be @dfn{ambiguous}, meaning that there are multiple ways to
-apply the grammar rules to get the 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.
+apply the grammar rules to get the same inputs. Even unambiguous
+grammars can be @dfn{nondeterministic}, meaning that no fixed
+lookahead always suffices to determine the next grammar rule to apply.
With the proper declarations, Bison is also able to parse these more
general context-free grammars, using a technique known as @acronym{GLR}
parsing (for Generalized @acronym{LR}). Bison's @acronym{GLR} parsers
square (int x) /* @r{identifier, open-paren, keyword `int',}
@r{identifier, close-paren} */
@{ /* @r{open-brace} */
- return x * x; /* @r{keyword `return', identifier, asterisk,
- identifier, semicolon} */
+ return x * x; /* @r{keyword `return', identifier, asterisk,}
+ @r{identifier, semicolon} */
@} /* @r{close-brace} */
@end example
@end ifinfo
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
+* Simple GLR Parsers:: Using @acronym{GLR} parsers on unambiguous grammars.
+* Merging GLR Parses:: Using @acronym{GLR} parsers to resolve ambiguities.
+* GLR Semantic Actions:: Deferred semantic actions have special concerns.
+* Compiler Requirements:: @acronym{GLR} parsers require a modern C compiler.
@end menu
@node Simple GLR Parsers
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,
+Such grammars typically require more than one symbol of lookahead,
or (in rare cases) fall into the category of grammars in which the
@acronym{LALR}(1) algorithm throws away too much information (they are in
@acronym{LR}(1), but not @acronym{LALR}(1), @ref{Mystery Conflicts}).
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
+With normal @acronym{LALR}(1) one-token lookahead it is not
possible to decide between the two forms when the identifier
@samp{a} is parsed. It is, however, desirable
for a parser to decide this, since in the latter case
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
+lookahead than the underlying @acronym{LALR}(1) algorithm actually allows
for. In this example, @acronym{LALR}(2) would suffice, but also some cases
that are not @acronym{LALR}(@math{k}) for any @math{k} can be handled this way.
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.
+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
and the parser will report an error during any parse that results in
the offending merge.
+@node GLR Semantic Actions
+@subsection GLR Semantic Actions
+
+@cindex deferred semantic actions
+By definition, a deferred semantic action is not performed at the same time as
+the associated reduction.
+This raises caveats for several Bison features you might use in a semantic
+action in a @acronym{GLR} parser.
+
+@vindex yychar
+@cindex @acronym{GLR} parsers and @code{yychar}
+@vindex yylval
+@cindex @acronym{GLR} parsers and @code{yylval}
+@vindex yylloc
+@cindex @acronym{GLR} parsers and @code{yylloc}
+In any semantic action, you can examine @code{yychar} to determine the type of
+the lookahead token present at the time of the associated reduction.
+After checking that @code{yychar} is not set to @code{YYEMPTY} or @code{YYEOF},
+you can then examine @code{yylval} and @code{yylloc} to determine the
+lookahead token's semantic value and location, if any.
+In a nondeferred semantic action, you can also modify any of these variables to
+influence syntax analysis.
+@xref{Lookahead, ,Lookahead Tokens}.
+
+@findex yyclearin
+@cindex @acronym{GLR} parsers and @code{yyclearin}
+In a deferred semantic action, it's too late to influence syntax analysis.
+In this case, @code{yychar}, @code{yylval}, and @code{yylloc} are set to
+shallow copies of the values they had at the time of the associated reduction.
+For this reason alone, modifying them is dangerous.
+Moreover, the result of modifying them is undefined and subject to change with
+future versions of Bison.
+For example, if a semantic action might be deferred, you should never write it
+to invoke @code{yyclearin} (@pxref{Action Features}) or to attempt to free
+memory referenced by @code{yylval}.
+
+@findex YYERROR
+@cindex @acronym{GLR} parsers and @code{YYERROR}
+Another Bison feature requiring special consideration is @code{YYERROR}
+(@pxref{Action Features}), which you can invoke in a semantic action to
+initiate error recovery.
+During deterministic @acronym{GLR} operation, the effect of @code{YYERROR} is
+the same as its effect in an @acronym{LALR}(1) parser.
+In a deferred semantic action, its effect is undefined.
+@c The effect is probably a syntax error at the split point.
+
+Also, see @ref{Location Default Action, ,Default Action for Locations}, which
+describes a special usage of @code{YYLLOC_DEFAULT} in @acronym{GLR} parsers.
+
@node Compiler Requirements
@subsection Considerations when Compiling @acronym{GLR} Parsers
@cindex @code{inline}
This also includes numerous identifiers used for internal purposes.
Therefore, you should avoid using C identifiers starting with @samp{yy}
or @samp{YY} in the Bison grammar file except for the ones defined in
-this manual.
+this manual. Also, you should avoid using the C identifiers
+@samp{malloc} and @samp{free} for anything other than their usual
+meanings.
In some cases the Bison parser file includes system headers, and in
those cases your code should respect the identifiers reserved by those
-headers. On some non-@acronym{GNU} hosts, @code{<alloca.h>},
+headers. On some non-@acronym{GNU} hosts, @code{<alloca.h>}, @code{<malloc.h>},
@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
-declare memory allocators and related types. Other system headers may
+declare memory allocators and related types. @code{<libintl.h>} is
+included if message translation is in use
+(@pxref{Internationalization}). Other system headers may
be included if you define @code{YYDEBUG} to a nonzero value
(@pxref{Tracing, ,Tracing Your Parser}).
desk-top calculator.
These examples are simple, but Bison grammars for real programming
-languages are written the same way.
-@ifinfo
-You can copy these examples out of the Info file and into a source file
-to try them.
-@end ifinfo
+languages are written the same way. You can copy these examples into a
+source file to try them.
@menu
* RPN Calc:: Reverse polish notation calculator;
The groupings of the rpcalc ``language'' defined here are the expression
(given the name @code{exp}), the line of input (@code{line}), and the
complete input transcript (@code{input}). Each of these nonterminal
-symbols has several alternate rules, joined by the @samp{|} punctuator
+symbols has several alternate rules, joined by the vertical bar @samp{|}
which is read as ``or''. The following sections explain what these rules
mean.
convert it into a parser file:
@example
-bison @var{file_name}.y
+bison @var{file}.y
@end example
@noindent
In this example the file was called @file{rpcalc.y} (for ``Reverse Polish
-@sc{calc}ulator''). Bison produces a file named @file{@var{file_name}.tab.c},
+@sc{calc}ulator''). Bison produces a file named @file{@var{file}.tab.c},
removing the @samp{.y} from the original file name. The file output by
Bison contains the source code for @code{yyparse}. The additional
functions in the input file (@code{yylex}, @code{yyerror} and @code{main})
by default (@pxref{Location Type, ,Data Types of Locations}), which is a
four member structure with the following integer fields:
@code{first_line}, @code{first_column}, @code{last_line} and
-@code{last_column}.
+@code{last_column}. By conventions, and in accordance with the GNU
+Coding Standards and common practice, the line and column count both
+start at 1.
@node Ltcalc Rules
@subsection Grammar Rules for @code{ltcalc}
It is easy to add new operators to the infix calculator as long as they are
only single-character literals. The lexical analyzer @code{yylex} passes
-back all nonnumber characters as tokens, so new grammar rules suffice for
+back all nonnumeric characters as tokens, so new grammar rules suffice for
adding a new operator. But we want something more flexible: built-in
functions whose syntax has this form:
/* The symbol table: a chain of `struct symrec'. */
extern symrec *sym_table;
-symrec *putsym (char const *, func_t);
+symrec *putsym (char const *, int);
symrec *getsym (char const *);
@end group
@end smallexample
The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
-characters with a leading non-digit are recognized as either variables or
+characters with a leading letter are recognized as either variables or
functions depending on what the symbol table says about them.
The string is passed to @code{getsym} for look up in the symbol table. If
@menu
* Prologue:: Syntax and usage of the prologue.
+* Prologue Alternatives:: Syntax and usage of alternatives to the prologue.
* Bison Declarations:: Syntax and usage of the Bison declarations section.
* Grammar Rules:: Syntax and usage of the grammar rules section.
* Epilogue:: Syntax and usage of the epilogue.
@cindex Prologue
@cindex declarations
-The @var{Prologue} section contains macro definitions and
-declarations of functions and variables that are used in the actions in the
-grammar rules. These are copied to the beginning of the parser file so
-that they precede the definition of @code{yyparse}. You can use
-@samp{#include} to get the declarations from a header file. If you don't
-need any C declarations, you may omit the @samp{%@{} and @samp{%@}}
-delimiters that bracket this section.
+The @var{Prologue} section contains macro definitions and declarations
+of functions and variables that are used in the actions in the grammar
+rules. These are copied to the beginning of the parser file so that
+they precede the definition of @code{yyparse}. You can use
+@samp{#include} to get the declarations from a header file. If you
+don't need any C declarations, you may omit the @samp{%@{} and
+@samp{%@}} delimiters that bracket this section.
+
+The @var{Prologue} section is terminated by the first occurrence
+of @samp{%@}} that is outside a comment, a string literal, or a
+character constant.
You may have more than one @var{Prologue} section, intermixed with the
@var{Bison declarations}. This allows you to have C and Bison
@smallexample
%@{
+ #define _GNU_SOURCE
+ #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
+
+When in doubt, it is usually safer to put prologue code before all
+Bison declarations, rather than after. For example, any definitions
+of feature test macros like @code{_GNU_SOURCE} or
+@code{_POSIX_C_SOURCE} should appear before all Bison declarations, as
+feature test macros can affect the behavior of Bison-generated
+@code{#include} directives.
+
+@node Prologue Alternatives
+@subsection Prologue Alternatives
+@cindex Prologue Alternatives
+
+@findex %code
+@findex %requires
+@findex %provides
+@findex %code-top
+The functionality of @var{Prologue} sections can often be subtle and
+inflexible.
+As an alternative, Bison provides a set of more explicit directives:
+@code{%code}, @code{%requires}, @code{%provides}, and @code{%code-top}.
+@xref{Table of Symbols,,Bison Symbols}.
+
+Look again at the example of the previous section:
+
+@smallexample
+%@{
+ #define _GNU_SOURCE
#include <stdio.h>
#include "ptypes.h"
%@}
@dots{}
@end smallexample
+@noindent
+Notice that there are two @var{Prologue} sections here, but there's a subtle
+distinction between their functionality.
+For example, if you decide to override Bison's default definition for
+@code{YYLTYPE}, in which @var{Prologue} section should you write your new
+definition?
+You should write it in the first since Bison will insert that code into the
+parser code file @emph{before} the default @code{YYLTYPE} definition.
+In which @var{Prologue} section should you prototype an internal function,
+@code{trace_token}, that accepts @code{YYLTYPE} and @code{yytokentype} as
+arguments?
+You should prototype it in the second since Bison will insert that code
+@emph{after} the @code{YYLTYPE} and @code{yytokentype} definitions.
+
+This distinction in functionality between the two @var{Prologue} sections is
+established by the appearance of the @code{%union} between them.
+This behavior raises a few questions.
+First, why should the position of a @code{%union} affect definitions related to
+@code{YYLTYPE} and @code{yytokentype}?
+Second, what if there is no @code{%union}?
+In that case, the second kind of @var{Prologue} section is not available.
+This behavior is not intuitive.
+
+To avoid this subtle @code{%union} dependency, rewrite the example using
+@code{%code-top} and @code{%code}.
+Let's go ahead and add the new @code{YYLTYPE} definition and the
+@code{trace_token} prototype at the same time:
+
+@smallexample
+%code-top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+ /* The following code really belongs in a %requires; see below. */
+ #include "ptypes.h"
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+ static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+In this way, @code{%code-top} and @code{%code} achieve the same functionality
+as the two kinds of @var{Prologue} sections, but it's always explicit which
+kind you intend.
+Moreover, both kinds are always available even in the absence of @code{%union}.
+
+The @code{%code-top} block above logically contains two parts.
+The first two lines need to appear in the parser code file.
+The fourth line is required by @code{YYSTYPE} and thus also needs to appear in
+the parser code file.
+However, if you've instructed Bison to generate a parser header file
+(@pxref{Table of Symbols, ,%defines}), you probably want the fourth line to
+appear before the @code{YYSTYPE} definition in that header file as well.
+Also, the @code{YYLTYPE} definition should appear in the parser header file to
+override the default @code{YYLTYPE} definition there.
+
+In other words, in the @code{%code-top} block above, all but the first two
+lines are dependency code for externally exposed definitions (@code{YYSTYPE}
+and @code{YYLTYPE}) required by Bison.
+Thus, they belong in one or more @code{%requires}:
+
+@smallexample
+%code-top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+
+%requires @{
+ #include "ptypes.h"
+@}
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%requires @{
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
+%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+ static void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+Now Bison will insert @code{#include "ptypes.h"} and the new @code{YYLTYPE}
+definition before the Bison-generated @code{YYSTYPE} and @code{YYLTYPE}
+definitions in both the parser code file and the parser header file.
+(By the same reasoning, @code{%requires} would also be the appropriate place to
+write your own definition for @code{YYSTYPE}.)
+
+When you are writing dependency code for @code{YYSTYPE} and @code{YYLTYPE}, you
+should prefer @code{%requires} over @code{%code-top} regardless of whether you
+instruct Bison to generate a parser header file.
+When you are writing code that you need Bison to insert only into the parser
+code file and that has no special need to appear at the top of the code file,
+you should prefer @code{%code} over @code{%code-top}.
+These practices will make the purpose of each block of your code explicit to
+Bison and to other developers reading your grammar file.
+Following these practices, we expect @code{%code} and @code{%requires} to be
+the most important of the four @var{Prologue} alternative directives discussed
+in this section.
+
+At some point while developing your parser, you might decide to provide
+@code{trace_token} to modules that are external to your parser.
+Thus, you might wish for Bison to insert the prototype into both the parser
+header file and the parser code file.
+Since this function is not a dependency of any Bison-required definition (such
+as @code{YYSTYPE}), it doesn't make sense to move its prototype to a
+@code{%requires}.
+More importantly, since it depends upon @code{YYLTYPE} and @code{yytokentype},
+@code{%requires} is not sufficient.
+Instead, move its prototype from the @code{%code} to a @code{%provides}:
+
+@smallexample
+%code-top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+
+%requires @{
+ #include "ptypes.h"
+@}
+%union @{
+ long int n;
+ tree t; /* @r{@code{tree} is defined in @file{ptypes.h}.} */
+@}
+
+%requires @{
+ #define YYLTYPE YYLTYPE
+ typedef struct YYLTYPE
+ @{
+ int first_line;
+ int first_column;
+ int last_line;
+ int last_column;
+ char *filename;
+ @} YYLTYPE;
+@}
+
+%provides @{
+ void trace_token (enum yytokentype token, YYLTYPE loc);
+@}
+
+%code @{
+ static void print_token_value (FILE *, int, YYSTYPE);
+ #define YYPRINT(F, N, L) print_token_value (F, N, L)
+@}
+
+@dots{}
+@end smallexample
+
+@noindent
+Bison will insert the @code{trace_token} prototype into both the parser header
+file and the parser code file after the definitions for @code{yytokentype},
+@code{YYLTYPE}, and @code{YYSTYPE}.
+
+The above examples are careful to write directives in an order that reflects
+the layout of the generated parser code and header files:
+@code{%code-top}, @code{%requires}, @code{%provides}, and then @code{%code}.
+While your grammar files may generally be easier to read if you also follow
+this order, Bison does not require it.
+Instead, Bison lets you choose an organization that makes sense to you.
+
+You may declare any of these directives multiple times in the grammar file.
+In that case, Bison concatenates the contained code in declaration order.
+This is the only way in which the position of one of these directives within
+the grammar file affects its functionality.
+
+The result of the previous two properties is greater flexibility in how you may
+organize your grammar file.
+For example, you may organize semantic-type-related directives by semantic
+type:
+
+@smallexample
+%requires @{ #include "type1.h" @}
+%union @{ type1 field1; @}
+%destructor @{ type1_free ($$); @} <field1>
+%printer @{ type1_print ($$); @} <field1>
+
+%requires @{ #include "type2.h" @}
+%union @{ type2 field2; @}
+%destructor @{ type2_free ($$); @} <field2>
+%printer @{ type2_print ($$); @} <field2>
+@end smallexample
+
+@noindent
+You could even place each of the above directive groups in the rules section of
+the grammar file next to the set of rules that uses the associated semantic
+type.
+And you don't have to worry that some directive (like a @code{%union}) in the
+definitions section is going to adversely affect their functionality in some
+counter-intuitive manner just because it comes first.
+Such an organization is not possible using @var{Prologue} sections.
+
+This section has been concerned with explaining the advantages of the four
+@var{Prologue} alternative directives over the original Yacc @var{Prologue}.
+However, in most cases when using these directives, you shouldn't need to
+think about all the low-level ordering issues discussed here.
+Instead, you should simply use these directives to label each block of your
+code according to its purpose and let Bison handle the ordering.
+@code{%code} is the most generic label.
+Move code to @code{%requires}, @code{%provides}, or @code{%code-top} as needed.
+
@node Bison Declarations
@subsection The Bison Declarations Section
@cindex Bison declarations (introduction)
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.
+even if you define them in the Epilogue.
@xref{Interface, ,Parser C-Language Interface}.
If the last section is empty, you may omit the @samp{%%} that separates it
from the grammar rules.
-The Bison parser itself contains many 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.
+The Bison parser itself contains many macros and identifiers whose names
+start with @samp{yy} or @samp{YY}, so it is a good idea to avoid using
+any such names (except those documented in this manual) in the epilogue
+of the grammar file.
@node Symbols
@section Symbols, Terminal and Nonterminal
class of syntactically equivalent tokens. You use the symbol in grammar
rules to mean that a token in that class is allowed. The symbol is
represented in the Bison parser by a numeric code, and the @code{yylex}
-function returns a token type code to indicate what kind of token has been
-read. You don't need to know what the code value is; you can use the
-symbol to stand for it.
+function returns a token type code to indicate what kind of token has
+been read. You don't need to know what the code value is; you can use
+the symbol to stand for it.
-A @dfn{nonterminal symbol} stands for a class of syntactically equivalent
-groupings. The symbol name is used in writing grammar rules. By convention,
-it should be all lower case.
+A @dfn{nonterminal symbol} stands for a class of syntactically
+equivalent groupings. The symbol name is used in writing grammar rules.
+By convention, it should be all lower case.
Symbol names can contain letters, digits (not at the beginning),
underscores and periods. Periods make sense only in nonterminals.
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
+host, you must use only nonnull character tokens taken from the basic
execution character set of Standard C@. This set consists of the ten
digits, the 52 lower- and upper-case English letters, and the
characters in the following C-language string:
"\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 @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.
@end example
@noindent
+@cindex braced code
+This is an example of @dfn{braced code}, that is, C code surrounded by
+braces, much like a compound statement in C@. Braced code can contain
+any sequence of C tokens, so long as its braces are balanced. Bison
+does not check the braced code for correctness directly; it merely
+copies the code to the output file, where the C compiler can check it.
+
+Within braced code, the balanced-brace count is not affected by braces
+within comments, string literals, or character constants, but it is
+affected by the C digraphs @samp{<%} and @samp{%>} that represent
+braces. At the top level braced code must be terminated by @samp{@}}
+and not by a digraph. Bison does not look for trigraphs, so if braced
+code uses trigraphs you should ensure that they do not affect the
+nesting of braces or the boundaries of comments, string literals, or
+character constants.
+
Usually there is only one action and it follows the components.
@xref{Actions}.
Multiple rules for the same @var{result} can be written separately or can
be joined with the vertical-bar character @samp{|} as follows:
-@ifinfo
-@example
-@var{result}: @var{rule1-components}@dots{}
- | @var{rule2-components}@dots{}
- @dots{}
- ;
-@end example
-@end ifinfo
-@iftex
@example
@group
@var{result}: @var{rule1-components}@dots{}
;
@end group
@end example
-@end iftex
@noindent
They are still considered distinct rules even when joined in this way.
@section Recursive Rules
@cindex recursive rule
-A rule is called @dfn{recursive} when its @var{result} nonterminal appears
-also on its right hand side. Nearly all Bison grammars need to use
-recursion, because that is the only way to define a sequence of any number
-of a particular thing. Consider this recursive definition of a
+A rule is called @dfn{recursive} when its @var{result} nonterminal
+appears also on its right hand side. Nearly all Bison grammars need to
+use recursion, because that is the only way to define a sequence of any
+number of a particular thing. Consider this recursive definition of a
comma-separated sequence of one or more expressions:
@example
@acronym{RPN} and infix calculator examples (@pxref{RPN Calc, ,Reverse Polish
Notation Calculator}).
-Bison's default is to use type @code{int} for all semantic values. To
+Bison normally uses the type @code{int} for semantic values if your
+program uses the same data type for all language constructs. To
specify some other type, define @code{YYSTYPE} as a macro, like this:
@example
@end example
@noindent
+@code{YYSTYPE}'s replacement list should be a type name
+that does not contain parentheses or square brackets.
This macro definition must go in the prologue of the grammar file
(@pxref{Grammar Outline, ,Outline of a Bison Grammar}).
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 int}, while a string constant needs type @code{char *},
-and an identifier might need a pointer to an entry in the symbol table.
+@code{int} or @code{long int}, while a string constant needs type
+@code{char *}, and an identifier might need a pointer to an entry in the
+symbol table.
To use more than one data type for semantic values in one parser, Bison
requires you to do two things:
@itemize @bullet
@item
-Specify the entire collection of possible data types, with the
+Specify the entire collection of possible data types, either by using the
@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
-Value Types}).
+Value Types}), or by using a @code{typedef} or a @code{#define} to
+define @code{YYSTYPE} to be a union type whose member names are
+the type tags.
@item
Choose one of those types for each symbol (terminal or nonterminal) for
is to compute a semantic value for the grouping built by the rule from the
semantic values associated with tokens or smaller groupings.
-An action consists of C statements surrounded by braces, much like a
-compound statement in C@. 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;
+An action consists of braced code containing C statements, and can be
+placed at any position in the rule;
it is executed at that position. Most rules have just one action at the
end of the rule, following all the components. Actions in the middle of
a rule are tricky and used only for special purposes (@pxref{Mid-Rule
always refers to the @code{expr} which precedes @code{bar} in the
definition of @code{foo}.
+@vindex yylval
+It is also possible to access the semantic value of the lookahead token, if
+any, from a semantic action.
+This semantic value is stored in @code{yylval}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
@node Action Types
@subsection Data Types of Values in Actions
@cindex action data types
removes the temporary @code{let}-variable from the list so that it won't
appear to exist while the rest of the program is parsed.
+@findex %destructor
+@cindex discarded symbols, mid-rule actions
+@cindex error recovery, mid-rule actions
+In the above example, if the parser initiates error recovery (@pxref{Error
+Recovery}) while parsing the tokens in the embedded statement @code{stmt},
+it might discard the previous semantic context @code{$<context>5} without
+restoring it.
+Thus, @code{$<context>5} needs a destructor (@pxref{Destructor Decl, , Freeing
+Discarded Symbols}).
+However, Bison currently provides no means to declare a destructor specific to
+a particular mid-rule action's semantic value.
+
+One solution is to bury the mid-rule action inside a nonterminal symbol and to
+declare a destructor for that symbol:
+
+@example
+@group
+%type <context> let
+%destructor @{ pop_context ($$); @} let
+
+%%
+
+stmt: let stmt
+ @{ $$ = $2;
+ pop_context ($1); @}
+ ;
+
+let: LET '(' var ')'
+ @{ $$ = push_context ();
+ declare_variable ($3); @}
+ ;
+
+@end group
+@end example
+
+@noindent
+Note that the action is now at the end of its rule.
+Any mid-rule action can be converted to an end-of-rule action in this way, and
+this is what Bison actually does to implement mid-rule actions.
+
Taking action before a rule is completely recognized often leads to
conflicts since the parser must commit to a parse in order to execute the
action. For example, the following two rules, without mid-rule actions,
when it has read no farther than the open-brace. In other words, it
must commit to using one rule or the other, without sufficient
information to do it correctly. (The open-brace token is what is called
-the @dfn{look-ahead} token at this time, since the parser is still
-deciding what to do about it. @xref{Look-Ahead, ,Look-Ahead Tokens}.)
+the @dfn{lookahead} token at this time, since the parser is still
+deciding what to do about it. @xref{Lookahead, ,Lookahead Tokens}.)
You might think that you could correct the problem by putting identical
actions into the two rules, like this:
@noindent
Now Bison can execute the action in the rule for @code{subroutine} without
-deciding which rule for @code{compound} it will eventually use. Note that
-the action is now at the end of its rule. Any mid-rule action can be
-converted to an end-of-rule action in this way, and this is what Bison
-actually does to implement mid-rule actions.
+deciding which rule for @code{compound} it will eventually use.
@node Locations
@section Tracking Locations
Defining a data type for locations is much simpler than for semantic values,
since all tokens and groupings always use the same type.
-The type of locations is specified by defining a macro called @code{YYLTYPE}.
+You can specify the type of locations by defining a macro called
+@code{YYLTYPE}, just as you can specify the semantic value type by
+defining a @code{YYSTYPE} macro (@pxref{Value Type}).
When @code{YYLTYPE} is not defined, Bison uses a default structure type with
four members:
@} YYLTYPE;
@end example
+At the beginning of the parsing, Bison initializes all these fields to 1
+for @code{yylloc}.
+
@node Actions and Locations
@subsection Actions and Locations
@cindex location actions
@end group
@end example
+@vindex yylloc
+It is also possible to access the location of the lookahead token, if any,
+from a semantic action.
+This location is stored in @code{yylloc}.
+@xref{Action Features, ,Special Features for Use in Actions}.
+
@node Location Default Action
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
+@cindex @acronym{GLR} parsers and @code{YYLLOC_DEFAULT}
Actually, actions are not the best place to compute locations. Since
locations are much more general than semantic values, there is room in
rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
matched, before the associated action is run. It is also invoked
while processing a syntax error, to compute the error's location.
+Before reporting an unresolvable syntactic ambiguity, a @acronym{GLR}
+parser invokes @code{YYLLOC_DEFAULT} recursively to compute the location
+of that ambiguity.
Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
the 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 size of the rule's right hand side.
+When a @acronym{GLR} parser reports an ambiguity, which of multiple candidate
+right hand sides it passes to @code{YYLLOC_DEFAULT} is undefined.
+When processing a syntax error, the second parameter identifies locations
+of the symbols that were discarded during error processing, and the third
parameter is the number of discarded symbols.
By default, @code{YYLLOC_DEFAULT} is defined this way:
Grammars}).
@menu
+* Require Decl:: Requiring a Bison version.
* Token Decl:: Declaring terminal symbols.
* Precedence Decl:: Declaring terminals with precedence and associativity.
* Union Decl:: Declaring the set of all semantic value types.
* Decl Summary:: Table of all Bison declarations.
@end menu
+@node Require Decl
+@subsection Require a Version of Bison
+@cindex version requirement
+@cindex requiring a version of Bison
+@findex %require
+
+You may require the minimum version of Bison to process the grammar. If
+the requirement is not met, @command{bison} exits with an error (exit
+status 63).
+
+@example
+%require "@var{version}"
+@end example
+
@node Token Decl
@subsection Token Type Names
@cindex declaring token type names
@cindex value types, declaring
@findex %union
-The @code{%union} declaration specifies the entire collection of possible
-data types for semantic values. The keyword @code{%union} is followed by a
-pair of braces containing the same thing that goes inside a @code{union} in
-C.
+The @code{%union} declaration specifies the entire collection of
+possible data types for semantic values. The keyword @code{%union} is
+followed by braced code containing the same thing that goes inside a
+@code{union} in C@.
For example:
@end group
@end example
+@noindent
specifies the union tag @code{value}, so the corresponding C type is
@code{union value}. If you do not specify a tag, it defaults to
@code{YYSTYPE}.
+As another extension to @acronym{POSIX}, you may specify multiple
+@code{%union} declarations; their contents are concatenated. However,
+only the first @code{%union} declaration can specify a tag.
+
Note that, unlike making a @code{union} declaration in C, you need not write
a semicolon after the closing brace.
+Instead of @code{%union}, you can define and use your own union type
+@code{YYSTYPE} if your grammar contains at least one
+@samp{<@var{type}>} tag. For example, you can put the following into
+a header file @file{parser.h}:
+
+@example
+@group
+union YYSTYPE @{
+ double val;
+ symrec *tptr;
+@};
+typedef union YYSTYPE YYSTYPE;
+@end group
+@end example
+
+@noindent
+and then your grammar can use the following
+instead of @code{%union}:
+
+@example
+@group
+%@{
+#include "parser.h"
+%@}
+%type <val> expr
+%token <tptr> ID
+@end group
+@end example
+
@node Type Decl
@subsection Nonterminal Symbols
@cindex declaring value types, nonterminals
@deffn {Directive} %initial-action @{ @var{code} @}
@findex %initial-action
-Declare that the @var{code} must be invoked before parsing each time
+Declare that the braced @var{code} must be invoked before parsing each time
@code{yyparse} is called. The @var{code} may use @code{$$} and
-@code{@@$} --- initial value and location of the look-ahead --- and the
+@code{@@$} --- initial value and location of the lookahead --- and the
@code{%parse-param}.
@end deffn
For instance, if your locations use a file name, you may use
@example
-%parse-param @{ const char *filename @};
+%parse-param @{ char const *file_name @};
%initial-action
@{
- @@$.begin.filename = @@$.end.filename = filename;
+ @@$.initialize (file_name);
@};
@end example
@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.
+@findex <*>
+@findex <>
+During error recovery (@pxref{Error Recovery}), symbols already pushed
+on the stack and tokens coming from the rest of the file are discarded
+until the parser falls on its feet. If the parser runs out of memory,
+or if it returns via @code{YYABORT} or @code{YYACCEPT}, all the
+symbols on the stack must be discarded. Even if the parser succeeds, it
+must discard the start symbol.
+
+When discarded symbols convey heap based information, this memory is
+lost. While this behavior can be tolerable for batch parsers, such as
+in traditional compilers, it is unacceptable for programs like shells or
+protocol implementations that may parse and execute indefinitely.
+
+The @code{%destructor} directive defines code that is called when a
+symbol is automatically discarded.
@deffn {Directive} %destructor @{ @var{code} @} @var{symbols}
@findex %destructor
-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{$$} to designate the semantic value associated to the
-@var{symbols}. The additional parser parameters are also available
-(@pxref{Parser Function, , The Parser Function @code{yyparse}}).
-
-@strong{Warning:} as of Bison 1.875, this feature is still considered as
-experimental, as there was not enough user feedback. In particular,
-the syntax might still change.
+Invoke the braced @var{code} whenever the parser discards one of the
+@var{symbols}.
+Within @var{code}, @code{$$} designates the semantic value associated
+with the discarded symbol, and @code{@@$} designates its location.
+The additional parser parameters are also available (@pxref{Parser Function, ,
+The Parser Function @code{yyparse}}).
+
+When a symbol is listed among @var{symbols}, its @code{%destructor} is called a
+per-symbol @code{%destructor}.
+You may also define a per-type @code{%destructor} by listing a semantic type
+tag among @var{symbols}.
+In that case, the parser will invoke this @var{code} whenever it discards any
+grammar symbol that has that semantic type tag unless that symbol has its own
+per-symbol @code{%destructor}.
+
+Finally, you can define two different kinds of default @code{%destructor}s.
+You can place each of @code{<*>} and @code{<>} in the @var{symbols} list of
+exactly one @code{%destructor} declaration in your grammar file.
+The parser will invoke the @var{code} associated with one of these whenever it
+discards any user-defined grammar symbol that has no per-symbol and no per-type
+@code{%destructor}.
+The parser uses the @var{code} for @code{<*>} in the case of such a grammar
+symbol for which you have formally declared a semantic type tag (@code{%type}
+counts as such a declaration, but @code{$<tag>$} does not).
+The parser uses the @var{code} for @code{<>} in the case of such a grammar
+symbol that has no declared semantic type tag.
@end deffn
-For instance:
+@noindent
+For example:
@smallexample
-%union
-@{
- char *string;
-@}
-%token <string> STRING
-%type <string> string
-%destructor @{ free ($$); @} STRING string
+%union @{ char *string; @}
+%token <string> STRING1
+%token <string> STRING2
+%type <string> string1
+%type <string> string2
+%union @{ char character; @}
+%token <character> CHR
+%type <character> chr
+%token TAGLESS
+
+%destructor @{ @} <character>
+%destructor @{ free ($$); @} <*>
+%destructor @{ free ($$); printf ("%d", @@$.first_line); @} STRING1 string1
+%destructor @{ printf ("Discarding tagless symbol.\n"); @} <>
@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:
+guarantees that, when the parser discards any user-defined symbol that has a
+semantic type tag other than @code{<character>}, it passes its semantic value
+to @code{free} by default.
+However, when the parser discards a @code{STRING1} or a @code{string1}, it also
+prints its line number to @code{stdout}.
+It performs only the second @code{%destructor} in this case, so it invokes
+@code{free} only once.
+Finally, the parser merely prints a message whenever it discards any symbol,
+such as @code{TAGLESS}, that has no semantic type tag.
+
+A Bison-generated parser invokes the default @code{%destructor}s only for
+user-defined as opposed to Bison-defined symbols.
+For example, the parser will not invoke either kind of default
+@code{%destructor} for the special Bison-defined symbols @code{$accept},
+@code{$undefined}, or @code{$end} (@pxref{Table of Symbols, ,Bison Symbols}),
+none of which you can reference in your grammar.
+It also will not invoke either for the @code{error} token (@pxref{Table of
+Symbols, ,error}), which is always defined by Bison regardless of whether you
+reference it in your grammar.
+However, it may invoke one of them for the end token (token 0) if you
+redefine it from @code{$end} to, for example, @code{END}:
@smallexample
-comment: "/*" STRING "*/";
+%token END 0
@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:
+@cindex actions in mid-rule
+@cindex mid-rule actions
+Finally, Bison will never invoke a @code{%destructor} for an unreferenced
+mid-rule semantic value (@pxref{Mid-Rule Actions,,Actions in Mid-Rule}).
+That is, Bison does not consider a mid-rule to have a semantic value if you do
+not reference @code{$$} in the mid-rule's action or @code{$@var{n}} (where
+@var{n} is the RHS symbol position of the mid-rule) in any later action in that
+rule.
+However, if you do reference either, the Bison-generated parser will invoke the
+@code{<>} @code{%destructor} whenever it discards the mid-rule symbol.
-@smallexample
-typeless: string; // $$ = $1 does not apply; $1 is destroyed.
-typefull: string; // $$ = $1 applies, $1 is not destroyed.
-@end smallexample
+@ignore
+@noindent
+In the future, it may be possible to redefine the @code{error} token as a
+nonterminal that captures the discarded symbols.
+In that case, the parser will invoke the default destructor for it as well.
+@end ignore
@sp 1
@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).
+the current lookahead and the entire stack (except the current
+right-hand side symbols) when the parser returns immediately, and
+@item
+the start symbol, when the parser succeeds.
@end itemize
+The parser can @dfn{return immediately} because of an explicit call to
+@code{YYABORT} or @code{YYACCEPT}, or failed error recovery, or memory
+exhaustion.
+
+Right-hand size symbols of a rule that explicitly triggers a syntax
+error via @code{YYERROR} are not discarded automatically. As a rule
+of thumb, destructors are invoked only when user actions cannot manage
+the memory.
@node Expect Decl
@subsection Suppressing Conflict Warnings
%expect @var{n}
@end example
-Here @var{n} is a decimal integer. The declaration says there should be
-no warning if there are @var{n} shift/reduce conflicts and no
-reduce/reduce conflicts. The usual warning is
-given if there are either more or fewer conflicts, or if there are any
-reduce/reduce conflicts.
+Here @var{n} is a decimal integer. The declaration says there should
+be @var{n} shift/reduce conflicts and no reduce/reduce conflicts.
+Bison reports an error if the number of shift/reduce conflicts differs
+from @var{n}, or if there are any reduce/reduce conflicts.
-For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more serious,
-and should be eliminated entirely. Bison will always report
-reduce/reduce conflicts for these parsers. With @acronym{GLR} parsers, however,
-both 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:
+For normal @acronym{LALR}(1) parsers, reduce/reduce conflicts are more
+serious, and should be eliminated entirely. Bison will always report
+reduce/reduce conflicts for these parsers. With @acronym{GLR}
+parsers, however, both kinds of conflicts are routine; otherwise,
+there would be no need to use @acronym{GLR} parsing. Therefore, it is
+also possible to specify an expected number of reduce/reduce conflicts
+in @acronym{GLR} parsers, using the declaration:
@example
%expect-rr @var{n}
@item
Add an @code{%expect} declaration, copying the number @var{n} from the
-number which Bison printed.
+number which Bison printed. With @acronym{GLR} parsers, add an
+@code{%expect-rr} declaration as well.
@end itemize
-Now Bison will stop annoying you 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.
+Now Bison will warn you if you introduce an unexpected conflict, but
+will keep silent otherwise.
@node Start Decl
@subsection The Start-Symbol
A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
code. Reentrancy is important whenever asynchronous execution is possible;
-for example, a non-reentrant program may not be safe to call from a signal
-handler. In systems with multiple threads of control, a non-reentrant
+for example, a nonreentrant program may not be safe to call from a signal
+handler. In systems with multiple threads of control, a nonreentrant
program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
If the parser output file is named @file{@var{name}.c} then this file
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}.
+For C parsers, the output header declares @code{YYSTYPE} unless
+@code{YYSTYPE} is already defined as a macro or you have used a
+@code{<@var{type}>} tag without using @code{%union}.
+Therefore, if you are using a @code{%union}
+(@pxref{Multiple Types, ,More Than One Value Type}) with components that
+require other definitions, or if you have defined a @code{YYSTYPE} macro
+or type definition
+(@pxref{Value Type, ,Data Types of Semantic Values}), you need to
+arrange for these definitions to be propagated to all modules, e.g., by
+putting them in a prerequisite header that is included both by your
+parser and by any other module that needs @code{YYSTYPE}.
Unless your parser is pure, the output header declares @code{yylval}
as an external variable. @xref{Pure Decl, ,A Pure (Reentrant)
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
+the @code{YYSTYPE} macro and @code{yylval}. @xref{Locations, ,Tracking
Locations}.
-This output file is normally essential if you wish to put the
-definition of @code{yylex} in a separate source file, because
-@code{yylex} typically needs to be able to refer to the
-above-mentioned declarations and to the token type codes.
-@xref{Token Values, ,Semantic Values of Tokens}.
+This output file is normally essential if you wish to put the definition
+of @code{yylex} in a separate source file, because @code{yylex}
+typically needs to be able to refer to the above-mentioned declarations
+and to the token type codes. @xref{Token Values, ,Semantic Values of
+Tokens}.
+
+@findex %requires
+@findex %provides
+If you have declared @code{%requires} or @code{%provides}, the output
+header also contains their code.
+@xref{Table of Symbols, ,%requires}.
+@end deffn
+
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
@end deffn
@deffn {Directive} %destructor
-Specifying how the parser should reclaim the memory associated to
+Specify how the parser should reclaim the memory associated to
discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
@end deffn
-@deffn {Directive} %file-prefix="@var{prefix}"
+@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
accurate syntax error messages.
@end deffn
-@deffn {Directive} %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
+in C parsers
is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
-@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}.
+@code{yylval}, @code{yychar}, @code{yydebug}, and
+(if locations are used) @code{yylloc}. For example, if you use
+@samp{%name-prefix "c_"}, the names become @code{c_parse}, @code{c_lex},
+and so on. In C++ parsers, it is only the surrounding namespace which is
+named @var{prefix} instead of @samp{yy}.
+@xref{Multiple Parsers, ,Multiple Parsers in the Same Program}.
@end deffn
@ifset defaultprec
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
+into a file named @file{@var{file}.act}, in the form of a
brace-surrounded body fit for a @code{switch} statement.
@end deffn
file in its own right.
@end deffn
-@deffn {Directive} %output="@var{filename}"
-Specify the @var{filename} for the parser file.
+@deffn {Directive} %output "@var{file}"
+Specify @var{file} for the parser file.
@end deffn
@deffn {Directive} %pure-parser
(Reentrant) Parser}).
@end deffn
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
@deffn {Directive} %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
@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
-example, @code{"'+'"} is a single-character literal and @code{"\"<=\""}
-is a literal string token. All the characters of the literal string
-token appear verbatim in the string found in the table; even
-double-quote characters are not escaped. For example, if the token
-consists of three characters @samp{*"*}, its string in @code{yytname}
-contains @samp{"*"*"}. (In C, that would be written as
-@code{"\"*\"*\""}).
+The name in the table includes all the characters needed to represent
+the token in Bison. For single-character literals and literal
+strings, this includes the surrounding quoting characters and any
+escape sequences. For example, the Bison single-character literal
+@code{'+'} corresponds to a three-character name, represented in C as
+@code{"'+'"}; and the Bison two-character literal string @code{"\\/"}
+corresponds to a five-character name, represented in C as
+@code{"\"\\\\/\""}.
When you specify @code{%token-table}, Bison also generates macro
definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
@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
+parser states and what is done for each type of lookahead token in
that state. @xref{Understanding, , Understanding Your Parser}, for more
information.
@end deffn
which reads tokens.
* Error Reporting:: You must supply a function @code{yyerror}.
* Action Features:: Special features for use in actions.
+* Internationalization:: How to let the parser speak in the user's
+ native language.
@end menu
@node Parser Function
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).
+The value is 1 if parsing failed because of invalid input, i.e., input
+that contains a syntax error or that causes @code{YYABORT} to be
+invoked.
+
+The value is 2 if parsing failed due to memory exhaustion.
@end deftypefun
In an action, you can cause immediate return from @code{yyparse} by using
@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.
+Declare that an argument declared by the braced-code
+@var{argument-declaration} is an additional @code{yyparse} argument.
The @var{argument-declaration} is used when declaring
functions or prototypes. The last identifier in
@var{argument-declaration} must be the argument name.
table. The index of the token in the table is the token type's code.
The name of a multicharacter token is recorded in @code{yytname} with a
double-quote, the token's characters, and another double-quote. The
-token's characters are not escaped in any way; they appear verbatim in
-the contents of the string in the table.
+token's characters are escaped as necessary to be suitable as input
+to Bison.
-Here's code for looking up a token in @code{yytname}, assuming that the
-characters of the token are stored in @code{token_buffer}.
+Here's code for looking up a multicharacter token in @code{yytname},
+assuming that the characters of the token are stored in
+@code{token_buffer}, and assuming that the token does not contain any
+characters like @samp{"} that require escaping.
@smallexample
for (i = 0; i < YYNTOKENS; i++)
@subsection Semantic Values of Tokens
@vindex yylval
-In an ordinary (non-reentrant) parser, the semantic value of the token must
+In an ordinary (nonreentrant) parser, the semantic value of the token must
be stored into the global variable @code{yylval}. When you are using
just one data type for semantic values, @code{yylval} has that type.
Thus, if the type is @code{int} (the default), you might write this in
@vindex yylloc
If you are using the @samp{@@@var{n}}-feature (@pxref{Locations, ,
-Tracking Locations}) in actions to keep track of the
-textual locations of tokens and groupings, then you must provide this
-information in @code{yylex}. The function @code{yyparse} expects to
-find the textual location of a token just parsed in the global variable
-@code{yylloc}. So @code{yylex} must store the proper data in that
-variable.
+Tracking Locations}) in actions to keep track of the textual locations
+of tokens and groupings, then you must provide this information in
+@code{yylex}. The function @code{yyparse} expects to find the textual
+location of a token just parsed in the global variable @code{yylloc}.
+So @code{yylex} must store the proper data in that variable.
By default, the value of @code{yylloc} is a structure and you need only
initialize the members that are going to be used by the actions. The
@deffn {Directive} lex-param @{@var{argument-declaration}@}
@findex %lex-param
-Declare that @code{argument-declaration} is an additional @code{yylex}
-argument declaration.
+Declare that the braced-code @var{argument-declaration} is an
+additional @code{yylex} argument declaration.
@end deffn
For instance:
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
+The parser can detect one other kind of error: memory exhaustion. This
+can happen when the input contains constructions that are very deeply
nested. It isn't likely you will encounter this, since the Bison
-parser extends its stack automatically up to a very large limit. But
-if overflow happens, @code{yyparse} calls @code{yyerror} in the usual
-fashion, except that the argument string is @w{@code{"parser stack
-overflow"}}.
+parser normally extends its stack automatically up to a very large limit. But
+if memory is exhausted, @code{yyparse} calls @code{yyerror} in the usual
+fashion, except that the argument string is @w{@code{"memory exhausted"}}.
+
+In some cases diagnostics like @w{@code{"syntax error"}} are
+translated automatically from English to some other language before
+they are passed to @code{yyerror}. @xref{Internationalization}.
The following definition suffices in simple programs:
@vindex yynerrs
The variable @code{yynerrs} contains the number of syntax errors
-encountered so far. Normally this variable is global; but if you
+reported so far. Normally this variable is global; but if you
request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
then it is a local variable which only the actions can access.
@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.
+a single value, and only when there is no lookahead token.
It is also disallowed in @acronym{GLR} parsers.
-It installs a look-ahead token with token type @var{token} and
+It installs a lookahead token with token type @var{token} and
semantic value @var{value}; then it discards the value that was
going to be reduced by this rule.
If the macro is used when it is not valid, such as when there is
-a look-ahead token already, then it reports a syntax error with
+a lookahead token already, then it reports a syntax error with
a message @samp{cannot back up} and performs ordinary error
recovery.
@deffn {Macro} YYEMPTY
@vindex YYEMPTY
-Value stored in @code{yychar} when there is no look-ahead token.
+Value stored in @code{yychar} when there is no lookahead token.
+@end deffn
+
+@deffn {Macro} YYEOF
+@vindex YYEOF
+Value stored in @code{yychar} when the lookahead is the end of the input
+stream.
@end deffn
@deffn {Macro} YYERROR;
@end deffn
@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.
+@findex YYRECOVERING
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
@xref{Error Recovery}.
@end deffn
@deffn {Variable} yychar
-Variable containing 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}.
+Variable containing either the lookahead token, or @code{YYEOF} when the
+lookahead is the end of the input stream, or @code{YYEMPTY} when no lookahead
+has been performed so the next token is not yet known.
+Do not modify @code{yychar} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Lookahead, ,Lookahead Tokens}.
@end deffn
@deffn {Macro} yyclearin;
-Discard the current look-ahead token. This is useful primarily in
-error rules. @xref{Error Recovery}.
+Discard the current lookahead token. This is useful primarily in
+error rules.
+Do not invoke @code{yyclearin} in a deferred semantic action (@pxref{GLR
+Semantic Actions}).
+@xref{Error Recovery}.
@end deffn
@deffn {Macro} yyerrok;
@xref{Error Recovery}.
@end deffn
+@deffn {Variable} yylloc
+Variable containing the lookahead token location when @code{yychar} is not set
+to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylloc} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions and Locations, ,Actions and Locations}.
+@end deffn
+
+@deffn {Variable} yylval
+Variable containing the lookahead token semantic value when @code{yychar} is
+not set to @code{YYEMPTY} or @code{YYEOF}.
+Do not modify @code{yylval} in a deferred semantic action (@pxref{GLR Semantic
+Actions}).
+@xref{Actions, ,Actions}.
+@end deffn
+
@deffn {Value} @@$
@findex @@$
Acts like a structure variable containing information on the textual location
Tracking Locations}.
@end deffn
+@node Internationalization
+@section Parser Internationalization
+@cindex internationalization
+@cindex i18n
+@cindex NLS
+@cindex gettext
+@cindex bison-po
+
+A Bison-generated parser can print diagnostics, including error and
+tracing messages. By default, they appear in English. However, Bison
+also supports outputting diagnostics in the user's native language. To
+make this work, the user should set the usual environment variables.
+@xref{Users, , The User's View, gettext, GNU @code{gettext} utilities}.
+For example, the shell command @samp{export LC_ALL=fr_CA.UTF-8} might
+set the user's locale to French Canadian using the @acronym{UTF}-8
+encoding. The exact set of available locales depends on the user's
+installation.
+
+The maintainer of a package that uses a Bison-generated parser enables
+the internationalization of the parser's output through the following
+steps. Here we assume a package that uses @acronym{GNU} Autoconf and
+@acronym{GNU} Automake.
+
+@enumerate
+@item
+@cindex bison-i18n.m4
+Into the directory containing the @acronym{GNU} Autoconf macros used
+by the package---often called @file{m4}---copy the
+@file{bison-i18n.m4} file installed by Bison under
+@samp{share/aclocal/bison-i18n.m4} in Bison's installation directory.
+For example:
+
+@example
+cp /usr/local/share/aclocal/bison-i18n.m4 m4/bison-i18n.m4
+@end example
+
+@item
+@findex BISON_I18N
+@vindex BISON_LOCALEDIR
+@vindex YYENABLE_NLS
+In the top-level @file{configure.ac}, after the @code{AM_GNU_GETTEXT}
+invocation, add an invocation of @code{BISON_I18N}. This macro is
+defined in the file @file{bison-i18n.m4} that you copied earlier. It
+causes @samp{configure} to find the value of the
+@code{BISON_LOCALEDIR} variable, and it defines the source-language
+symbol @code{YYENABLE_NLS} to enable translations in the
+Bison-generated parser.
+
+@item
+In the @code{main} function of your program, designate the directory
+containing Bison's runtime message catalog, through a call to
+@samp{bindtextdomain} with domain name @samp{bison-runtime}.
+For example:
+
+@example
+bindtextdomain ("bison-runtime", BISON_LOCALEDIR);
+@end example
+
+Typically this appears after any other call @code{bindtextdomain
+(PACKAGE, LOCALEDIR)} that your package already has. Here we rely on
+@samp{BISON_LOCALEDIR} to be defined as a string through the
+@file{Makefile}.
+
+@item
+In the @file{Makefile.am} that controls the compilation of the @code{main}
+function, make @samp{BISON_LOCALEDIR} available as a C preprocessor macro,
+either in @samp{DEFS} or in @samp{AM_CPPFLAGS}. For example:
+
+@example
+DEFS = @@DEFS@@ -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+or:
+
+@example
+AM_CPPFLAGS = -DBISON_LOCALEDIR='"$(BISON_LOCALEDIR)"'
+@end example
+
+@item
+Finally, invoke the command @command{autoreconf} to generate the build
+infrastructure.
+@end enumerate
+
@node Algorithm
@chapter The Bison Parser Algorithm
This kind of parser is known in the literature as a bottom-up parser.
@menu
-* Look-Ahead:: Parser looks one token ahead when deciding what to do.
+* Lookahead:: Parser looks one token ahead when deciding what to do.
* Shift/Reduce:: Conflicts: when either shifting or reduction is valid.
* Precedence:: Operator precedence works by resolving conflicts.
* Contextual Precedence:: When an operator's precedence depends on context.
* 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.
+* Memory Management:: What happens when memory is exhausted. How to avoid it.
@end menu
-@node Look-Ahead
-@section Look-Ahead Tokens
-@cindex look-ahead token
+@node Lookahead
+@section Lookahead Tokens
+@cindex lookahead token
The Bison parser does @emph{not} always reduce immediately as soon as the
last @var{n} tokens and groupings match a rule. This is because such a
token in order to decide what to do.
When a token is read, it is not immediately shifted; first it becomes the
-@dfn{look-ahead token}, which is not on the stack. Now the parser can
+@dfn{lookahead token}, which is not on the stack. Now the parser can
perform one or more reductions of tokens and groupings on the stack, while
-the look-ahead token remains off to the side. When no more reductions
-should take place, the look-ahead token is shifted onto the stack. This
+the lookahead token remains off to the side. When no more reductions
+should take place, the lookahead token is shifted onto the stack. This
does not mean that all possible reductions have been done; depending on the
-token type of the look-ahead token, some rules may choose to delay their
+token type of the lookahead token, some rules may choose to delay their
application.
-Here is a simple case where look-ahead is needed. These three rules define
+Here is a simple case where lookahead is needed. These three rules define
expressions which contain binary addition operators and postfix unary
factorial operators (@samp{!}), and allow parentheses for grouping.
'!'}. No rule allows that sequence.
@vindex yychar
-The current look-ahead token is stored in the variable @code{yychar}.
+@vindex yylval
+@vindex yylloc
+The lookahead token is stored in the variable @code{yychar}.
+Its semantic value and location, if any, are stored in the variables
+@code{yylval} and @code{yylloc}.
@xref{Action Features, ,Special Features for Use in Actions}.
@node Shift/Reduce
Here we assume that @code{IF}, @code{THEN} and @code{ELSE} are
terminal symbols for specific keyword tokens.
-When the @code{ELSE} token is read and becomes the look-ahead token, the
+When the @code{ELSE} token is read and becomes the lookahead token, the
contents of the stack (assuming the input is valid) are just right for
reduction by the first rule. But it is also legitimate to shift the
@code{ELSE}, because that would lead to eventual reduction by the second
The latter alternative, @dfn{right association}, is desirable for
assignment operators. The choice of left or right association is a
matter of whether the parser chooses to shift or reduce when the stack
-contains @w{@samp{1 - 2}} and the look-ahead token is @samp{-}: shifting
+contains @w{@samp{1 - 2}} and the lookahead token is @samp{-}: shifting
makes right-associativity.
@node Using Precedence
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
+of the rule being considered with that of the lookahead token. If the
token's precedence is higher, the choice is to shift. If the rule's
precedence is higher, the choice is to reduce. If they have equal
precedence, the choice is made based on the associativity of that
resolved.
Not all rules and not all tokens have precedence. If either the rule or
-the look-ahead token has no precedence, then the default is to shift.
+the lookahead token has no precedence, then the default is to shift.
@node Contextual Precedence
@section Context-Dependent Precedence
near the top of the stack. The current state collects all the information
about previous input which is relevant to deciding what to do next.
-Each time a look-ahead token is read, the current parser state together
-with the type of look-ahead token are looked up in a table. This table
-entry can say, ``Shift the look-ahead token.'' In this case, it also
+Each time a lookahead token is read, the current parser state together
+with the type of lookahead token are looked up in a table. This table
+entry can say, ``Shift the lookahead token.'' In this case, it also
specifies the new parser state, which is pushed onto the top of the
parser stack. Or it can say, ``Reduce using rule number @var{n}.''
This means that a certain number of tokens or groupings are taken off
that number of states are popped from the stack, and one new state is
pushed.
-There is one other alternative: the table can say that the look-ahead token
+There is one other alternative: the table can say that the lookahead token
is erroneous in the current state. This causes error processing to begin
(@pxref{Error Recovery}).
@end example
It would seem that this grammar can be parsed with only a single token
-of look-ahead: when a @code{param_spec} is being read, an @code{ID} is
+of lookahead: when a @code{param_spec} is being read, an @code{ID} is
a @code{name} if a comma or colon follows, or a @code{type} if another
@code{ID} follows. In other words, this grammar is @acronym{LR}(1).
same. They appear similar because the same set of rules would be
active---the rule for reducing to a @code{name} and that for reducing to
a @code{type}. Bison is unable to determine at that stage of processing
-that the rules would require different look-ahead tokens in the two
+that the rules would require different lookahead tokens in the two
contexts, so it makes a single parser state for them both. Combining
the two contexts causes a conflict later. In parser terminology, this
occurrence means that the grammar is not @acronym{LALR}(1).
;
@end example
+For a more detailed exposition of @acronym{LALR}(1) parsers and parser
+generators, please see:
+Frank DeRemer and Thomas Pennello, Efficient Computation of
+@acronym{LALR}(1) Look-Ahead Sets, @cite{@acronym{ACM} Transactions on
+Programming Languages and Systems}, Vol.@: 4, No.@: 4 (October 1982),
+pp.@: 615--649 @uref{http://doi.acm.org/10.1145/69622.357187}.
+
@node Generalized LR Parsing
@section Generalized @acronym{LR} (@acronym{GLR}) Parsing
@cindex @acronym{GLR} parsing
@cindex generalized @acronym{LR} (@acronym{GLR}) parsing
@cindex ambiguous grammars
-@cindex non-deterministic parsing
+@cindex nondeterministic parsing
Bison produces @emph{deterministic} parsers that choose uniquely
when to reduce and which reduction to apply
-based on a summary of the preceding input and on one extra token of look-ahead.
+based on a summary of the preceding input and on one extra token of lookahead.
As a result, normal Bison handles a proper subset of the family of
context-free languages.
Ambiguous grammars, since they have strings with more than one possible
sequence of reductions cannot have deterministic parsers in this sense.
The same is true of languages that require more than one symbol of
-look-ahead, since the parser lacks the information necessary to make a
+lookahead, since the parser lacks the information necessary to make a
decision at the point it must be made in a shift-reduce parser.
Finally, as previously mentioned (@pxref{Mystery Conflicts}),
there are languages where Bison's particular choice of how to
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
+prefix of the input. Thus, really ambiguous or nondeterministic
grammars can require exponential time and space to process. Such badly
behaving examples, however, are not generally of practical interest.
-Usually, non-determinism in a grammar is local---the parser is ``in
+Usually, nondeterminism in a grammar is local---the parser is ``in
doubt'' only for a few tokens at a time. Therefore, the current data
structure should generally be adequate. On @acronym{LALR}(1) portions of a
grammar, in particular, it is only slightly slower than with the default
@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
+@node Memory Management
+@section Memory Management, and How to Avoid Memory Exhaustion
+@cindex memory exhaustion
+@cindex memory management
@cindex stack overflow
@cindex parser stack overflow
@cindex overflow of parser stack
-The Bison parser stack can overflow if too many tokens are shifted and
+The Bison parser stack can run out of memory if too many tokens are shifted and
not reduced. When this happens, the parser function @code{yyparse}
-returns a nonzero value, pausing only to call @code{yyerror} to report
-the overflow.
+calls @code{yyerror} and then returns 2.
Because Bison parsers have growing stacks, hitting the upper limit
usually results from using a right recursion instead of a left
@vindex YYMAXDEPTH
By defining the macro @code{YYMAXDEPTH}, you can control how deep the
-parser stack can become before a stack overflow occurs. Define the
+parser stack can become before memory is exhausted. Define the
macro with a value that is an integer. This value is the maximum number
of tokens that can be shifted (and not reduced) before overflow.
The stack space allowed is not necessarily allocated. If you specify a
-large value for @code{YYMAXDEPTH}, the parser actually allocates a small
+large value for @code{YYMAXDEPTH}, the parser normally allocates a small
stack at first, and then makes it bigger by stages as needed. This
increasing allocation happens automatically and silently. Therefore,
you do not need to make @code{YYMAXDEPTH} painfully small merely to save
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}.
+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.
+@acronym{LALR}(1) parsers in C produced by Bison cannot grow when compiled
+by C++ compilers. In this precise case (compiling a C parser as C++) you are
+suggested to grow @code{YYINITDEPTH}. The Bison maintainers hope to fix
+this deficiency in a future release.
@node Error Recovery
@chapter Error Recovery
@code{error} token is acceptable. (This means that the subexpressions
already parsed are discarded, back to the last complete @code{stmnts}.)
At this point the @code{error} token can be shifted. Then, if the old
-look-ahead token is not acceptable to be shifted next, the parser reads
+lookahead token is not acceptable to be shifted next, the parser reads
tokens and discards them until it finds a token which is acceptable. In
this example, Bison reads and discards input until the next newline so
that the fourth rule can apply. Note that discarded symbols are
@samp{yyerrok;} is a valid C statement.
@findex yyclearin
-The previous look-ahead token is reanalyzed immediately after an error. If
+The previous lookahead token is reanalyzed immediately after an error. If
this is unacceptable, then the macro @code{yyclearin} may be used to clear
this token. Write the statement @samp{yyclearin;} in the error rule's
action.
+@xref{Action Features, ,Special Features for Use in Actions}.
For example, suppose that on a syntax error, an error handling routine is
called that advances the input stream to some point where parsing should
once again commence. The next symbol returned by the lexical scanner is
-probably correct. The previous look-ahead token ought to be discarded
+probably correct. The previous lookahead token ought to be discarded
with @samp{yyclearin;}.
@vindex YYRECOVERING
-The macro @code{YYRECOVERING} stands for an expression that has the
-value 1 when the parser is recovering from a syntax error, and 0 the
-rest of the time. A value of 1 indicates that error messages are
-currently suppressed for new syntax errors.
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+Syntax error diagnostics are suppressed while recovering from a syntax
+error.
@node Context Dependency
@chapter Handling Context Dependencies
earlier:
@example
-typedef int foo, bar, lose;
-static foo (bar); /* @r{redeclare @code{bar} as static variable} */
-static int foo (lose); /* @r{redeclare @code{foo} as function} */
+typedef int foo, bar;
+int baz (void)
+@{
+ static bar (bar); /* @r{redeclare @code{bar} as static variable} */
+ extern foo foo (foo); /* @r{redeclare @code{foo} as function} */
+ return foo (bar);
+@}
@end example
Unfortunately, the name being declared is separated from the declaration
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).
+representation of it, either textually or graphically (as a DOT file).
The textual file is generated when the options @option{--report} or
@option{--verbose} are specified, see @xref{Invocation, , Invoking
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
+symbol, and the lookahead is a @code{NUM}, then this token is shifted on
the parse stack, and the control flow jumps to state 1. Any other
-look-ahead triggers a syntax error.''
+lookahead triggers a syntax error.''
@cindex core, item set
@cindex item set core
@cindex kernel, item set
@cindex item set core
Even though the only active rule in state 0 seems to be rule 0, the
-report lists @code{NUM} as a look-ahead token because @code{NUM} can be
+report lists @code{NUM} as a lookahead token because @code{NUM} can be
at the beginning of any rule deriving an @code{exp}. By default Bison
reports the so-called @dfn{core} or @dfn{kernel} of the item set, but if
you want to see more detail you can invoke @command{bison} with
@end example
@noindent
-the rule 5, @samp{exp: NUM;}, is completed. Whatever the look-ahead token
+the rule 5, @samp{exp: NUM;}, is completed. Whatever the lookahead token
(@samp{$default}), the parser will reduce it. If it was coming from
state 0, then, after this reduction it will return to state 0, and will
jump to state 2 (@samp{exp: go to state 2}).
@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
+because of the item @samp{exp -> exp . '+' exp}, if the lookahead if
@samp{+}, it will be shifted on the parse stack, and the automaton
control will jump to state 4, corresponding to the item @samp{exp -> exp
'+' . exp}. Since there is no default action, any other token than
$default reduce using rule 1 (exp)
@end example
-Indeed, there are two actions associated to the look-ahead @samp{/}:
+Indeed, there are two actions associated to the lookahead @samp{/}:
either shifting (and going to state 7), or reducing rule 1. The
conflict means that either the grammar is ambiguous, or the parser lacks
information to make the right decision. Indeed the grammar is
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
+possible, the lookahead is required to select the action. State 8 is
+one such state: if the lookahead is @samp{*} or @samp{/} then the action
is shifting, otherwise the action is reducing rule 1. In other words,
the first two items, corresponding to rule 1, are not eligible when the
-look-ahead token is @samp{*}, since we specified that @samp{*} has higher
+lookahead token is @samp{*}, since we specified that @samp{*} has higher
precedence than @samp{+}. More generally, some items are eligible only
-with some set of possible look-ahead tokens. When run with
-@option{--report=look-ahead}, Bison specifies these look-ahead tokens:
+with some set of possible lookahead tokens. When run with
+@option{--report=lookahead}, Bison specifies these lookahead tokens:
@example
state 8
@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}.
+and @code{YYFPRINTF} is defined to @code{fprintf}.
Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable @code{yydebug}.
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 also possible, in case you are writing
+with @samp{.tab.c} and removing any leading directory. Thus, the
+@samp{bison foo.y} file name yields
+@file{foo.tab.c}, and the @samp{bison hack/foo.y} file name yields
+@file{foo.tab.c}. It's also possible, in case you are writing
C++ code instead of C in your grammar file, to name it @file{foo.ypp}
or @file{foo.y++}. Then, the output files will take an extension like
the given one as input (respectively @file{foo.tab.cpp} and
@file{foo.tab.c++}).
-This feature takes effect with all options that manipulate filenames like
+This feature takes effect with all options that manipulate file names like
@samp{-o} or @samp{-d}.
For example :
@itemx --version
Print the version number of Bison and exit.
-@need 1750
+@item --print-localedir
+Print the name of the directory containing locale-dependent data.
+
@item -y
@itemx --yacc
-Equivalent to @samp{-o y.tab.c}; the parser output file is called
+Act more like the traditional Yacc command. This can cause
+different diagnostics to be generated, and may change behavior in
+other minor ways. Most importantly, imitate Yacc's output
+file name conventions, so that the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
-@file{y.tab.h}. The purpose of this option is to imitate Yacc's output
-file name conventions. Thus, the following shell script can substitute
-for Yacc, and the Bison distribution contains such a script for
-compatibility with @acronym{POSIX}:
+@file{y.tab.h}.
+Also, if generating an @acronym{LALR}(1) parser in C, generate @code{#define}
+statements in addition to an @code{enum} to associate token numbers with token
+names.
+Thus, the following shell script can substitute for Yacc, and the Bison
+distribution contains such a script for compatibility with @acronym{POSIX}:
@example
#! /bin/sh
bison -y "$@@"
@end example
+
+The @option{-y}/@option{--yacc} option is intended for use with
+traditional Yacc grammars. If your grammar uses a Bison extension
+like @samp{%glr-parser}, Bison might not be Yacc-compatible even if
+this option is specified.
+
@end table
@noindent
@item -p @var{prefix}
@itemx --name-prefix=@var{prefix}
-Pretend that @code{%name-prefix="@var{prefix}"} was specified.
+Pretend that @code{%name-prefix "@var{prefix}"} was specified.
@xref{Decl Summary}.
@item -l
@item -b @var{file-prefix}
@itemx --file-prefix=@var{prefix}
-Pretend that @code{%verbose} was specified, i.e, specify prefix to use
+Pretend that @code{%file-prefix} was specified, i.e., specify prefix to use
for all Bison output file names. @xref{Decl Summary}.
@item -r @var{things}
Description of the grammar, conflicts (resolved and unresolved), and
@acronym{LALR} automaton.
-@item look-ahead
+@item lookahead
Implies @code{state} and augments the description of the automaton with
-each rule's look-ahead set.
+each rule's lookahead set.
@item itemset
Implies @code{state} and augments the description of the automaton with
the full set of items for each state, instead of its core only.
@end table
-For instance, on the following grammar
-
@item -v
@itemx --verbose
-Pretend that @code{%verbose} was specified, i.e, write an extra output
+Pretend that @code{%verbose} was specified, i.e., write an extra output
file containing verbose descriptions of the grammar and
parser. @xref{Decl Summary}.
-@item -o @var{filename}
-@itemx --output=@var{filename}
-Specify the @var{filename} for the parser file.
+@item -o @var{file}
+@itemx --output=@var{file}
+Specify the @var{file} for the parser file.
-The other output files' names are constructed from @var{filename} as
+The other output files' names are constructed from @var{file} as
described under the @samp{-v} and @samp{-d} options.
@item -g
-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}.
+Output a graphical representation of the @acronym{LALR}(1) grammar
+automaton computed by Bison, in @uref{http://www.graphviz.org/, Graphviz}
+@uref{http://www.graphviz.org/doc/info/lang.html, @acronym{DOT}} format.
+If the grammar file is @file{foo.y}, the output file will
+be @file{foo.dot}.
@item --graph=@var{graph-file}
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.
+the output graph file.
@end table
@node Option Cross Key
@section Option Cross Key
+@c FIXME: How about putting the directives too?
Here is a list of options, alphabetized by long option, to help you find
the corresponding short option.
-@tex
-\def\leaderfill{\leaders\hbox to 1em{\hss.\hss}\hfill}
-
-{\tt
-\line{ --debug \leaderfill -t}
-\line{ --defines \leaderfill -d}
-\line{ --file-prefix \leaderfill -b}
-\line{ --graph \leaderfill -g}
-\line{ --help \leaderfill -h}
-\line{ --name-prefix \leaderfill -p}
-\line{ --no-lines \leaderfill -l}
-\line{ --no-parser \leaderfill -n}
-\line{ --output \leaderfill -o}
-\line{ --token-table \leaderfill -k}
-\line{ --verbose \leaderfill -v}
-\line{ --version \leaderfill -V}
-\line{ --yacc \leaderfill -y}
-}
-@end tex
-
-@ifinfo
-@example
---debug -t
---defines=@var{defines-file} -d
---file-prefix=@var{prefix} -b @var{file-prefix}
---graph=@var{graph-file} -d
---help -h
---name-prefix=@var{prefix} -p @var{name-prefix}
---no-lines -l
---no-parser -n
---output=@var{outfile} -o @var{outfile}
---token-table -k
---verbose -v
---version -V
---yacc -y
-@end example
-@end ifinfo
+@multitable {@option{--defines=@var{defines-file}}} {@option{-b @var{file-prefix}XXX}}
+@headitem Long Option @tab Short Option
+@item @option{--debug} @tab @option{-t}
+@item @option{--defines=@var{defines-file}} @tab @option{-d}
+@item @option{--file-prefix=@var{prefix}} @tab @option{-b @var{file-prefix}}
+@item @option{--graph=@var{graph-file}} @tab @option{-d}
+@item @option{--help} @tab @option{-h}
+@item @option{--name-prefix=@var{prefix}} @tab @option{-p @var{name-prefix}}
+@item @option{--no-lines} @tab @option{-l}
+@item @option{--no-parser} @tab @option{-n}
+@item @option{--output=@var{outfile}} @tab @option{-o @var{outfile}}
+@item @option{--print-localedir} @tab
+@item @option{--token-table} @tab @option{-k}
+@item @option{--verbose} @tab @option{-v}
+@item @option{--version} @tab @option{-V}
+@item @option{--yacc} @tab @option{-y}
+@end multitable
@node Yacc Library
@section Yacc Library
int yyparse (void);
@end example
-@c ================================================= Invoking Bison
+@c ================================================= C++ Bison
+
+@node C++ Language Interface
+@chapter C++ Language Interface
+
+@menu
+* C++ Parsers:: The interface to generate C++ parser classes
+* A Complete C++ Example:: Demonstrating their use
+@end menu
+
+@node C++ Parsers
+@section C++ Parsers
+
+@menu
+* C++ Bison Interface:: Asking for C++ parser generation
+* C++ Semantic Values:: %union vs. C++
+* C++ Location Values:: The position and location classes
+* C++ Parser Interface:: Instantiating and running the parser
+* C++ Scanner Interface:: Exchanges between yylex and parse
+@end menu
+
+@node C++ Bison Interface
+@subsection C++ Bison Interface
+@c - %skeleton "lalr1.cc"
+@c - Always pure
+@c - initial action
+
+The C++ parser @acronym{LALR}(1) skeleton is named @file{lalr1.cc}. To
+select it, you may either pass the option @option{--skeleton=lalr1.cc}
+to Bison, or include the directive @samp{%skeleton "lalr1.cc"} in the
+grammar preamble. When run, @command{bison} will create several
+entities in the @samp{yy} namespace. Use the @samp{%name-prefix}
+directive to change the namespace name, see @ref{Decl Summary}. The
+various classes are generated in the following files:
+
+@table @file
+@item position.hh
+@itemx location.hh
+The definition of the classes @code{position} and @code{location},
+used for location tracking. @xref{C++ Location Values}.
+
+@item stack.hh
+An auxiliary class @code{stack} used by the parser.
+
+@item @var{file}.hh
+@itemx @var{file}.cc
+(Assuming the extension of the input file was @samp{.yy}.) The
+declaration and implementation of the C++ parser class. The basename
+and extension of these two files follow the same rules as with regular C
+parsers (@pxref{Invocation}).
+
+The header is @emph{mandatory}; you must either pass
+@option{-d}/@option{--defines} to @command{bison}, or use the
+@samp{%defines} directive.
+@end table
+
+All these files are documented using Doxygen; run @command{doxygen}
+for a complete and accurate documentation.
+
+@node C++ Semantic Values
+@subsection C++ Semantic Values
+@c - No objects in unions
+@c - YSTYPE
+@c - Printer and destructor
+
+The @code{%union} directive works as for C, see @ref{Union Decl, ,The
+Collection of Value Types}. In particular it produces a genuine
+@code{union}@footnote{In the future techniques to allow complex types
+within pseudo-unions (similar to Boost variants) might be implemented to
+alleviate these issues.}, which have a few specific features in C++.
+@itemize @minus
+@item
+The type @code{YYSTYPE} is defined but its use is discouraged: rather
+you should refer to the parser's encapsulated type
+@code{yy::parser::semantic_type}.
+@item
+Non POD (Plain Old Data) types cannot be used. C++ forbids any
+instance of classes with constructors in unions: only @emph{pointers}
+to such objects are allowed.
+@end itemize
+
+Because objects have to be stored via pointers, memory is not
+reclaimed automatically: using the @code{%destructor} directive is the
+only means to avoid leaks. @xref{Destructor Decl, , Freeing Discarded
+Symbols}.
+
+
+@node C++ Location Values
+@subsection C++ Location Values
+@c - %locations
+@c - class Position
+@c - class Location
+@c - %define "filename_type" "const symbol::Symbol"
+
+When the directive @code{%locations} is used, the C++ parser supports
+location tracking, see @ref{Locations, , Locations Overview}. Two
+auxiliary classes define a @code{position}, a single point in a file,
+and a @code{location}, a range composed of a pair of
+@code{position}s (possibly spanning several files).
+
+@deftypemethod {position} {std::string*} file
+The name of the file. It will always be handled as a pointer, the
+parser will never duplicate nor deallocate it. As an experimental
+feature you may change it to @samp{@var{type}*} using @samp{%define
+"filename_type" "@var{type}"}.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} line
+The line, starting at 1.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} lines (int @var{height} = 1)
+Advance by @var{height} lines, resetting the column number.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} column
+The column, starting at 0.
+@end deftypemethod
+
+@deftypemethod {position} {unsigned int} columns (int @var{width} = 1)
+Advance by @var{width} columns, without changing the line number.
+@end deftypemethod
+
+@deftypemethod {position} {position&} operator+= (position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position} operator+ (const position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position&} operator-= (const position& @var{pos}, int @var{width})
+@deftypemethodx {position} {position} operator- (position& @var{pos}, int @var{width})
+Various forms of syntactic sugar for @code{columns}.
+@end deftypemethod
+
+@deftypemethod {position} {position} operator<< (std::ostream @var{o}, const position& @var{p})
+Report @var{p} on @var{o} like this:
+@samp{@var{file}:@var{line}.@var{column}}, or
+@samp{@var{line}.@var{column}} if @var{file} is null.
+@end deftypemethod
+
+@deftypemethod {location} {position} begin
+@deftypemethodx {location} {position} end
+The first, inclusive, position of the range, and the first beyond.
+@end deftypemethod
+
+@deftypemethod {location} {unsigned int} columns (int @var{width} = 1)
+@deftypemethodx {location} {unsigned int} lines (int @var{height} = 1)
+Advance the @code{end} position.
+@end deftypemethod
+
+@deftypemethod {location} {location} operator+ (const location& @var{begin}, const location& @var{end})
+@deftypemethodx {location} {location} operator+ (const location& @var{begin}, int @var{width})
+@deftypemethodx {location} {location} operator+= (const location& @var{loc}, int @var{width})
+Various forms of syntactic sugar.
+@end deftypemethod
+
+@deftypemethod {location} {void} step ()
+Move @code{begin} onto @code{end}.
+@end deftypemethod
+
+
+@node C++ Parser Interface
+@subsection C++ Parser Interface
+@c - define parser_class_name
+@c - Ctor
+@c - parse, error, set_debug_level, debug_level, set_debug_stream,
+@c debug_stream.
+@c - Reporting errors
+
+The output files @file{@var{output}.hh} and @file{@var{output}.cc}
+declare and define the parser class in the namespace @code{yy}. The
+class name defaults to @code{parser}, but may be changed using
+@samp{%define "parser_class_name" "@var{name}"}. The interface of
+this class is detailed below. It can be extended using the
+@code{%parse-param} feature: its semantics is slightly changed since
+it describes an additional member of the parser class, and an
+additional argument for its constructor.
+
+@defcv {Type} {parser} {semantic_value_type}
+@defcvx {Type} {parser} {location_value_type}
+The types for semantics value and locations.
+@end defcv
+
+@deftypemethod {parser} {} parser (@var{type1} @var{arg1}, ...)
+Build a new parser object. There are no arguments by default, unless
+@samp{%parse-param @{@var{type1} @var{arg1}@}} was used.
+@end deftypemethod
+
+@deftypemethod {parser} {int} parse ()
+Run the syntactic analysis, and return 0 on success, 1 otherwise.
+@end deftypemethod
+
+@deftypemethod {parser} {std::ostream&} debug_stream ()
+@deftypemethodx {parser} {void} set_debug_stream (std::ostream& @var{o})
+Get or set the stream used for tracing the parsing. It defaults to
+@code{std::cerr}.
+@end deftypemethod
+
+@deftypemethod {parser} {debug_level_type} debug_level ()
+@deftypemethodx {parser} {void} set_debug_level (debug_level @var{l})
+Get or set the tracing level. Currently its value is either 0, no trace,
+or nonzero, full tracing.
+@end deftypemethod
+
+@deftypemethod {parser} {void} error (const location_type& @var{l}, const std::string& @var{m})
+The definition for this member function must be supplied by the user:
+the parser uses it to report a parser error occurring at @var{l},
+described by @var{m}.
+@end deftypemethod
+
+
+@node C++ Scanner Interface
+@subsection C++ Scanner Interface
+@c - prefix for yylex.
+@c - Pure interface to yylex
+@c - %lex-param
+
+The parser invokes the scanner by calling @code{yylex}. Contrary to C
+parsers, C++ parsers are always pure: there is no point in using the
+@code{%pure-parser} directive. Therefore the interface is as follows.
+
+@deftypemethod {parser} {int} yylex (semantic_value_type& @var{yylval}, location_type& @var{yylloc}, @var{type1} @var{arg1}, ...)
+Return the next token. Its type is the return value, its semantic
+value and location being @var{yylval} and @var{yylloc}. Invocations of
+@samp{%lex-param @{@var{type1} @var{arg1}@}} yield additional arguments.
+@end deftypemethod
+
+
+@node A Complete C++ Example
+@section A Complete C++ Example
+
+This section demonstrates the use of a C++ parser with a simple but
+complete example. This example should be available on your system,
+ready to compile, in the directory @dfn{../bison/examples/calc++}. It
+focuses on the use of Bison, therefore the design of the various C++
+classes is very naive: no accessors, no encapsulation of members etc.
+We will use a Lex scanner, and more precisely, a Flex scanner, to
+demonstrate the various interaction. A hand written scanner is
+actually easier to interface with.
+
+@menu
+* Calc++ --- C++ Calculator:: The specifications
+* Calc++ Parsing Driver:: An active parsing context
+* Calc++ Parser:: A parser class
+* Calc++ Scanner:: A pure C++ Flex scanner
+* Calc++ Top Level:: Conducting the band
+@end menu
+
+@node Calc++ --- C++ Calculator
+@subsection Calc++ --- C++ Calculator
+
+Of course the grammar is dedicated to arithmetics, a single
+expression, possibly preceded by variable assignments. An
+environment containing possibly predefined variables such as
+@code{one} and @code{two}, is exchanged with the parser. An example
+of valid input follows.
+
+@example
+three := 3
+seven := one + two * three
+seven * seven
+@end example
+
+@node Calc++ Parsing Driver
+@subsection Calc++ Parsing Driver
+@c - An env
+@c - A place to store error messages
+@c - A place for the result
+
+To support a pure interface with the parser (and the scanner) the
+technique of the ``parsing context'' is convenient: a structure
+containing all the data to exchange. Since, in addition to simply
+launch the parsing, there are several auxiliary tasks to execute (open
+the file for parsing, instantiate the parser etc.), we recommend
+transforming the simple parsing context structure into a fully blown
+@dfn{parsing driver} class.
+
+The declaration of this driver class, @file{calc++-driver.hh}, is as
+follows. The first part includes the CPP guard and imports the
+required standard library components, and the declaration of the parser
+class.
+
+@comment file: calc++-driver.hh
+@example
+#ifndef CALCXX_DRIVER_HH
+# define CALCXX_DRIVER_HH
+# include <string>
+# include <map>
+# include "calc++-parser.hh"
+@end example
+
+
+@noindent
+Then comes the declaration of the scanning function. Flex expects
+the signature of @code{yylex} to be defined in the macro
+@code{YY_DECL}, and the C++ parser expects it to be declared. We can
+factor both as follows.
+
+@comment file: calc++-driver.hh
+@example
+// Tell Flex the lexer's prototype ...
+# define YY_DECL \
+ yy::calcxx_parser::token_type \
+ yylex (yy::calcxx_parser::semantic_type* yylval, \
+ yy::calcxx_parser::location_type* yylloc, \
+ calcxx_driver& driver)
+// ... and declare it for the parser's sake.
+YY_DECL;
+@end example
+
+@noindent
+The @code{calcxx_driver} class is then declared with its most obvious
+members.
+
+@comment file: calc++-driver.hh
+@example
+// Conducting the whole scanning and parsing of Calc++.
+class calcxx_driver
+@{
+public:
+ calcxx_driver ();
+ virtual ~calcxx_driver ();
+
+ std::map<std::string, int> variables;
+
+ int result;
+@end example
+
+@noindent
+To encapsulate the coordination with the Flex scanner, it is useful to
+have two members function to open and close the scanning phase.
+
+@comment file: calc++-driver.hh
+@example
+ // Handling the scanner.
+ void scan_begin ();
+ void scan_end ();
+ bool trace_scanning;
+@end example
+
+@noindent
+Similarly for the parser itself.
+
+@comment file: calc++-driver.hh
+@example
+ // Handling the parser.
+ void parse (const std::string& f);
+ std::string file;
+ bool trace_parsing;
+@end example
+
+@noindent
+To demonstrate pure handling of parse errors, instead of simply
+dumping them on the standard error output, we will pass them to the
+compiler driver using the following two member functions. Finally, we
+close the class declaration and CPP guard.
+
+@comment file: calc++-driver.hh
+@example
+ // Error handling.
+ void error (const yy::location& l, const std::string& m);
+ void error (const std::string& m);
+@};
+#endif // ! CALCXX_DRIVER_HH
+@end example
+
+The implementation of the driver is straightforward. The @code{parse}
+member function deserves some attention. The @code{error} functions
+are simple stubs, they should actually register the located error
+messages and set error state.
+
+@comment file: calc++-driver.cc
+@example
+#include "calc++-driver.hh"
+#include "calc++-parser.hh"
+
+calcxx_driver::calcxx_driver ()
+ : trace_scanning (false), trace_parsing (false)
+@{
+ variables["one"] = 1;
+ variables["two"] = 2;
+@}
+
+calcxx_driver::~calcxx_driver ()
+@{
+@}
+
+void
+calcxx_driver::parse (const std::string &f)
+@{
+ file = f;
+ scan_begin ();
+ yy::calcxx_parser parser (*this);
+ parser.set_debug_level (trace_parsing);
+ parser.parse ();
+ scan_end ();
+@}
+
+void
+calcxx_driver::error (const yy::location& l, const std::string& m)
+@{
+ std::cerr << l << ": " << m << std::endl;
+@}
+
+void
+calcxx_driver::error (const std::string& m)
+@{
+ std::cerr << m << std::endl;
+@}
+@end example
+
+@node Calc++ Parser
+@subsection Calc++ Parser
+
+The parser definition file @file{calc++-parser.yy} starts by asking for
+the C++ LALR(1) skeleton, the creation of the parser header file, and
+specifies the name of the parser class. Because the C++ skeleton
+changed several times, it is safer to require the version you designed
+the grammar for.
+
+@comment file: calc++-parser.yy
+@example
+%skeleton "lalr1.cc" /* -*- C++ -*- */
+%require "2.1a"
+%defines
+%define "parser_class_name" "calcxx_parser"
+@end example
+
+@noindent
+@findex %requires
+Then come the declarations/inclusions needed to define the
+@code{%union}. Because the parser uses the parsing driver and
+reciprocally, both cannot include the header of the other. Because the
+driver's header needs detailed knowledge about the parser class (in
+particular its inner types), it is the parser's header which will simply
+use a forward declaration of the driver.
+@xref{Table of Symbols, ,%requires}.
+
+@comment file: calc++-parser.yy
+@example
+%requires @{
+# include <string>
+class calcxx_driver;
+@}
+@end example
+
+@noindent
+The driver is passed by reference to the parser and to the scanner.
+This provides a simple but effective pure interface, not relying on
+global variables.
+
+@comment file: calc++-parser.yy
+@example
+// The parsing context.
+%parse-param @{ calcxx_driver& driver @}
+%lex-param @{ calcxx_driver& driver @}
+@end example
+
+@noindent
+Then we request the location tracking feature, and initialize the
+first location's file name. Afterwards new locations are computed
+relatively to the previous locations: the file name will be
+automatically propagated.
+
+@comment file: calc++-parser.yy
+@example
+%locations
+%initial-action
+@{
+ // Initialize the initial location.
+ @@$.begin.filename = @@$.end.filename = &driver.file;
+@};
+@end example
+
+@noindent
+Use the two following directives to enable parser tracing and verbose
+error messages.
+
+@comment file: calc++-parser.yy
+@example
+%debug
+%error-verbose
+@end example
+
+@noindent
+Semantic values cannot use ``real'' objects, but only pointers to
+them.
+
+@comment file: calc++-parser.yy
+@example
+// Symbols.
+%union
+@{
+ int ival;
+ std::string *sval;
+@};
+@end example
+
+@noindent
+@findex %code
+The code between @samp{%code @{} and @samp{@}} is output in the
+@file{*.cc} file; it needs detailed knowledge about the driver.
+
+@comment file: calc++-parser.yy
+@example
+%code @{
+# include "calc++-driver.hh"
+@}
+@end example
+
+
+@noindent
+The token numbered as 0 corresponds to end of file; the following line
+allows for nicer error messages referring to ``end of file'' instead
+of ``$end''. Similarly user friendly named are provided for each
+symbol. Note that the tokens names are prefixed by @code{TOKEN_} to
+avoid name clashes.
+
+@comment file: calc++-parser.yy
+@example
+%token END 0 "end of file"
+%token ASSIGN ":="
+%token <sval> IDENTIFIER "identifier"
+%token <ival> NUMBER "number"
+%type <ival> exp "expression"
+@end example
+
+@noindent
+To enable memory deallocation during error recovery, use
+@code{%destructor}.
+
+@c FIXME: Document %printer, and mention that it takes a braced-code operand.
+@comment file: calc++-parser.yy
+@example
+%printer @{ debug_stream () << *$$; @} "identifier"
+%destructor @{ delete $$; @} "identifier"
+
+%printer @{ debug_stream () << $$; @} "number" "expression"
+@end example
+
+@noindent
+The grammar itself is straightforward.
+
+@comment file: calc++-parser.yy
+@example
+%%
+%start unit;
+unit: assignments exp @{ driver.result = $2; @};
+
+assignments: assignments assignment @{@}
+ | /* Nothing. */ @{@};
+
+assignment:
+ "identifier" ":=" exp
+ @{ driver.variables[*$1] = $3; delete $1; @};
+
+%left '+' '-';
+%left '*' '/';
+exp: exp '+' exp @{ $$ = $1 + $3; @}
+ | exp '-' exp @{ $$ = $1 - $3; @}
+ | exp '*' exp @{ $$ = $1 * $3; @}
+ | exp '/' exp @{ $$ = $1 / $3; @}
+ | "identifier" @{ $$ = driver.variables[*$1]; delete $1; @}
+ | "number" @{ $$ = $1; @};
+%%
+@end example
+
+@noindent
+Finally the @code{error} member function registers the errors to the
+driver.
+
+@comment file: calc++-parser.yy
+@example
+void
+yy::calcxx_parser::error (const yy::calcxx_parser::location_type& l,
+ const std::string& m)
+@{
+ driver.error (l, m);
+@}
+@end example
+
+@node Calc++ Scanner
+@subsection Calc++ Scanner
+
+The Flex scanner first includes the driver declaration, then the
+parser's to get the set of defined tokens.
+
+@comment file: calc++-scanner.ll
+@example
+%@{ /* -*- C++ -*- */
+# include <cstdlib>
+# include <errno.h>
+# include <limits.h>
+# include <string>
+# include "calc++-driver.hh"
+# include "calc++-parser.hh"
+
+/* Work around an incompatibility in flex (at least versions
+ 2.5.31 through 2.5.33): it generates code that does
+ not conform to C89. See Debian bug 333231
+ <http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=333231>. */
+# undef yywrap
+# define yywrap() 1
+
+/* By default yylex returns int, we use token_type.
+ Unfortunately yyterminate by default returns 0, which is
+ not of token_type. */
+#define yyterminate() return token::END
+%@}
+@end example
+
+@noindent
+Because there is no @code{#include}-like feature we don't need
+@code{yywrap}, we don't need @code{unput} either, and we parse an
+actual file, this is not an interactive session with the user.
+Finally we enable the scanner tracing features.
+
+@comment file: calc++-scanner.ll
+@example
+%option noyywrap nounput batch debug
+@end example
+
+@noindent
+Abbreviations allow for more readable rules.
+
+@comment file: calc++-scanner.ll
+@example
+id [a-zA-Z][a-zA-Z_0-9]*
+int [0-9]+
+blank [ \t]
+@end example
+
+@noindent
+The following paragraph suffices to track locations accurately. Each
+time @code{yylex} is invoked, the begin position is moved onto the end
+position. Then when a pattern is matched, the end position is
+advanced of its width. In case it matched ends of lines, the end
+cursor is adjusted, and each time blanks are matched, the begin cursor
+is moved onto the end cursor to effectively ignore the blanks
+preceding tokens. Comments would be treated equally.
+
+@comment file: calc++-scanner.ll
+@example
+%@{
+# define YY_USER_ACTION yylloc->columns (yyleng);
+%@}
+%%
+%@{
+ yylloc->step ();
+%@}
+@{blank@}+ yylloc->step ();
+[\n]+ yylloc->lines (yyleng); yylloc->step ();
+@end example
+
+@noindent
+The rules are simple, just note the use of the driver to report errors.
+It is convenient to use a typedef to shorten
+@code{yy::calcxx_parser::token::identifier} into
+@code{token::identifier} for instance.
+
+@comment file: calc++-scanner.ll
+@example
+%@{
+ typedef yy::calcxx_parser::token token;
+%@}
+ /* Convert ints to the actual type of tokens. */
+[-+*/] return yy::calcxx_parser::token_type (yytext[0]);
+":=" return token::ASSIGN;
+@{int@} @{
+ errno = 0;
+ long n = strtol (yytext, NULL, 10);
+ if (! (INT_MIN <= n && n <= INT_MAX && errno != ERANGE))
+ driver.error (*yylloc, "integer is out of range");
+ yylval->ival = n;
+ return token::NUMBER;
+@}
+@{id@} yylval->sval = new std::string (yytext); return token::IDENTIFIER;
+. driver.error (*yylloc, "invalid character");
+%%
+@end example
+
+@noindent
+Finally, because the scanner related driver's member function depend
+on the scanner's data, it is simpler to implement them in this file.
+
+@comment file: calc++-scanner.ll
+@example
+void
+calcxx_driver::scan_begin ()
+@{
+ yy_flex_debug = trace_scanning;
+ if (!(yyin = fopen (file.c_str (), "r")))
+ error (std::string ("cannot open ") + file);
+@}
+
+void
+calcxx_driver::scan_end ()
+@{
+ fclose (yyin);
+@}
+@end example
+
+@node Calc++ Top Level
+@subsection Calc++ Top Level
+
+The top level file, @file{calc++.cc}, poses no problem.
+
+@comment file: calc++.cc
+@example
+#include <iostream>
+#include "calc++-driver.hh"
+
+int
+main (int argc, char *argv[])
+@{
+ calcxx_driver driver;
+ for (++argv; argv[0]; ++argv)
+ if (*argv == std::string ("-p"))
+ driver.trace_parsing = true;
+ else if (*argv == std::string ("-s"))
+ driver.trace_scanning = true;
+ else
+ @{
+ driver.parse (*argv);
+ std::cout << driver.result << std::endl;
+ @}
+@}
+@end example
+
+@c ================================================= FAQ
@node FAQ
@chapter Frequently Asked Questions
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
+* Memory Exhausted:: Breaking the Stack Limits
+* How Can I Reset the Parser:: @code{yyparse} Keeps some State
+* Strings are Destroyed:: @code{yylval} Loses Track of Strings
+* Implementing Gotos/Loops:: Control Flow in the Calculator
+* Multiple start-symbols:: Factoring closely related grammars
+* Secure? Conform?:: Is Bison @acronym{POSIX} safe?
+* I can't build Bison:: Troubleshooting
+* Where can I find help?:: Troubleshouting
+* Bug Reports:: Troublereporting
+* Other Languages:: Parsers in Java and others
+* Beta Testing:: Experimenting development versions
+* Mailing Lists:: Meeting other Bison users
@end menu
-@node Parser Stack Overflow
-@section Parser Stack Overflow
+@node Memory Exhausted
+@section Memory Exhausted
@display
-My parser returns with error with a @samp{parser stack overflow}
+My parser returns with error with a @samp{memory exhausted}
message. What can I do?
@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:
+of the scanner. Consider the following Lex code:
@verbatim
%{
@end example
-@node C++ Parsers
-@section C++ Parsers
-
-@display
-How can I generate parsers in C++?
-@end display
-
-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.
-
-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.
-
-
@node Implementing Gotos/Loops
@section Implementing Gotos/Loops
invited to consult the dedicated literature.
+@node Multiple start-symbols
+@section Multiple start-symbols
+
+@display
+I have several closely related grammars, and I would like to share their
+implementations. In fact, I could use a single grammar but with
+multiple entry points.
+@end display
+
+Bison does not support multiple start-symbols, but there is a very
+simple means to simulate them. If @code{foo} and @code{bar} are the two
+pseudo start-symbols, then introduce two new tokens, say
+@code{START_FOO} and @code{START_BAR}, and use them as switches from the
+real start-symbol:
+
+@example
+%token START_FOO START_BAR;
+%start start;
+start: START_FOO foo
+ | START_BAR bar;
+@end example
+
+These tokens prevents the introduction of new conflicts. As far as the
+parser goes, that is all that is needed.
+
+Now the difficult part is ensuring that the scanner will send these
+tokens first. If your scanner is hand-written, that should be
+straightforward. If your scanner is generated by Lex, them there is
+simple means to do it: recall that anything between @samp{%@{ ... %@}}
+after the first @code{%%} is copied verbatim in the top of the generated
+@code{yylex} function. Make sure a variable @code{start_token} is
+available in the scanner (e.g., a global variable or using
+@code{%lex-param} etc.), and use the following:
+
+@example
+ /* @r{Prologue.} */
+%%
+%@{
+ if (start_token)
+ @{
+ int t = start_token;
+ start_token = 0;
+ return t;
+ @}
+%@}
+ /* @r{The rules.} */
+@end example
+
+
+@node Secure? Conform?
+@section Secure? Conform?
+
+@display
+Is Bison secure? Does it conform to POSIX?
+@end display
+
+If you're looking for a guarantee or certification, we don't provide it.
+However, Bison is intended to be a reliable program that conforms to the
+@acronym{POSIX} specification for Yacc. If you run into problems,
+please send us a bug report.
+
+@node I can't build Bison
+@section I can't build Bison
+
+@display
+I can't build Bison because @command{make} complains that
+@code{msgfmt} is not found.
+What should I do?
+@end display
+
+Like most GNU packages with internationalization support, that feature
+is turned on by default. If you have problems building in the @file{po}
+subdirectory, it indicates that your system's internationalization
+support is lacking. You can re-configure Bison with
+@option{--disable-nls} to turn off this support, or you can install GNU
+gettext from @url{ftp://ftp.gnu.org/gnu/gettext/} and re-configure
+Bison. See the file @file{ABOUT-NLS} for more information.
+
+
+@node Where can I find help?
+@section Where can I find help?
+
+@display
+I'm having trouble using Bison. Where can I find help?
+@end display
+
+First, read this fine manual. Beyond that, you can send mail to
+@email{help-bison@@gnu.org}. This mailing list is intended to be
+populated with people who are willing to answer questions about using
+and installing Bison. Please keep in mind that (most of) the people on
+the list have aspects of their lives which are not related to Bison (!),
+so you may not receive an answer to your question right away. This can
+be frustrating, but please try not to honk them off; remember that any
+help they provide is purely voluntary and out of the kindness of their
+hearts.
+
+@node Bug Reports
+@section Bug Reports
+
+@display
+I found a bug. What should I include in the bug report?
+@end display
+
+Before you send a bug report, make sure you are using the latest
+version. Check @url{ftp://ftp.gnu.org/pub/gnu/bison/} or one of its
+mirrors. Be sure to include the version number in your bug report. If
+the bug is present in the latest version but not in a previous version,
+try to determine the most recent version which did not contain the bug.
+
+If the bug is parser-related, you should include the smallest grammar
+you can which demonstrates the bug. The grammar file should also be
+complete (i.e., I should be able to run it through Bison without having
+to edit or add anything). The smaller and simpler the grammar, the
+easier it will be to fix the bug.
+
+Include information about your compilation environment, including your
+operating system's name and version and your compiler's name and
+version. If you have trouble compiling, you should also include a
+transcript of the build session, starting with the invocation of
+`configure'. Depending on the nature of the bug, you may be asked to
+send additional files as well (such as `config.h' or `config.cache').
+
+Patches are most welcome, but not required. That is, do not hesitate to
+send a bug report just because you can not provide a fix.
+
+Send bug reports to @email{bug-bison@@gnu.org}.
+
+@node Other Languages
+@section Other Languages
+
+@display
+Will Bison ever have C++ support? How about Java or @var{insert your
+favorite language here}?
+@end display
+
+C++ support is there now, and is documented. We'd love to add other
+languages; contributions are welcome.
+
+@node Beta Testing
+@section Beta Testing
+
+@display
+What is involved in being a beta tester?
+@end display
+
+It's not terribly involved. Basically, you would download a test
+release, compile it, and use it to build and run a parser or two. After
+that, you would submit either a bug report or a message saying that
+everything is okay. It is important to report successes as well as
+failures because test releases eventually become mainstream releases,
+but only if they are adequately tested. If no one tests, development is
+essentially halted.
+
+Beta testers are particularly needed for operating systems to which the
+developers do not have easy access. They currently have easy access to
+recent GNU/Linux and Solaris versions. Reports about other operating
+systems are especially welcome.
+
+@node Mailing Lists
+@section Mailing Lists
+
+@display
+How do I join the help-bison and bug-bison mailing lists?
+@end display
+
+See @url{http://lists.gnu.org/}.
@c ================================================= Table of Symbols
@xref{Rules, ,Syntax of Grammar Rules}.
@end deffn
+@deffn {Directive} <*>
+Used to define a default tagged @code{%destructor} or default tagged
+@code{%printer}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
+@deffn {Directive} <>
+Used to define a default tagless @code{%destructor} or default tagless
+@code{%printer}.
+@xref{Destructor Decl, , Freeing Discarded Symbols}.
+@end deffn
+
@deffn {Symbol} $accept
The predefined nonterminal whose only rule is @samp{$accept: @var{start}
$end}, where @var{start} is the start symbol. @xref{Start Decl, , The
Start-Symbol}. It cannot be used in the grammar.
@end deffn
+@deffn {Directive} %code @{@var{code}@}
+Other than semantic actions, this is probably the most common place you should
+write verbatim code for the parser implementation.
+For C/C++, it replaces the traditional Yacc prologue,
+@code{%@{@var{code}%@}}, for most purposes.
+For Java, it inserts code into the parser class.
+
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} (@pxref{Prologue, ,The Prologue})
+appearing after the first @code{%union @{@var{code}@}} in a C/C++ based grammar
+file.
+While Bison will continue to support @code{%@{@var{code}%@}} for backward
+compatibility, @code{%code @{@var{code}@}} is cleaner as its functionality does
+not depend on its position in the grammar file relative to any
+@code{%union @{@var{code}@}}.
+Specifically, @code{%code @{@var{code}@}} always inserts your @var{code} into
+the parser code file after the usual contents of the parser header file.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
+@deffn {Directive} %code-top @{@var{code}@}
+Occasionally for C/C++ it is desirable to insert code near the top of the
+parser code file.
+For example:
+
+@smallexample
+%code-top @{
+ #define _GNU_SOURCE
+ #include <stdio.h>
+@}
+@end smallexample
+
+@noindent
+For Java, @code{%code-top @{@var{code}@}} is currently unused.
+
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} appearing before the first
+@code{%union @{@var{code}@}} in a C/C++ based grammar file.
+@code{%code-top @{@var{code}@}} is cleaner as its functionality does not depend
+on its position in the grammar file relative to any
+@code{%union @{@var{code}@}}.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
+@deffn {Directive} %debug
+Equip the parser for debugging. @xref{Decl Summary}.
+@end deffn
+
@deffn {Directive} %debug
Equip the parser for debugging. @xref{Decl Summary}.
@end deffn
@xref{Decl Summary}.
@end deffn
+@deffn {Directive} %defines @var{defines-file}
+Same as above, but save in the file @var{defines-file}.
+@xref{Decl Summary}.
+@end deffn
+
@deffn {Directive} %destructor
-Specifying how the parser should reclaim the memory associated to
+Specify how the parser should reclaim the memory associated to
discarded symbols. @xref{Destructor Decl, , Freeing Discarded Symbols}.
@end deffn
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 @code{error} becomes the current lookahead token. Actions
+corresponding to @code{error} are then executed, and the lookahead
token is reset to the token that originally caused the violation.
@xref{Error Recovery}.
@end deffn
when @code{yyerror} is called.
@end deffn
-@deffn {Directive} %file-prefix="@var{prefix}"
+@deffn {Directive} %file-prefix "@var{prefix}"
Bison declaration to set the prefix of the output files. @xref{Decl
Summary}.
@end deffn
@xref{GLR Parsers, ,Writing @acronym{GLR} Parsers}.
@end deffn
-@deffn {Directive} %name-prefix="@var{prefix}"
+@deffn {Directive} %name-prefix "@var{prefix}"
Bison declaration to rename the external symbols. @xref{Decl Summary}.
@end deffn
@end deffn
@deffn {Directive} %nonassoc
-Bison declaration to assign non-associativity to token(s).
+Bison declaration to assign nonassociativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn
-@deffn {Directive} %output="@var{filename}"
+@deffn {Directive} %output "@var{file}"
Bison declaration to set the name of the parser file. @xref{Decl
Summary}.
@end deffn
@xref{Contextual Precedence, ,Context-Dependent Precedence}.
@end deffn
+@deffn {Directive} %provides @{@var{code}@}
+This is the right place to write additional definitions you would like Bison to
+expose externally.
+For C/C++, this directive inserts your @var{code} both into the parser header
+file (if generated; @pxref{Table of Symbols, ,%defines}) and into the parser
+code file after Bison's required definitions.
+For Java, it inserts your @var{code} into the parser java file after the parser
+class.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
@deffn {Directive} %pure-parser
Bison declaration to request a pure (reentrant) parser.
@xref{Pure Decl, ,A Pure (Reentrant) Parser}.
@end deffn
+@deffn {Directive} %require "@var{version}"
+Require version @var{version} or higher of Bison. @xref{Require Decl, ,
+Require a Version of Bison}.
+@end deffn
+
+@deffn {Directive} %requires @{@var{code}@}
+This is the right place to write dependency code for externally exposed
+definitions required by Bison.
+For C/C++, such exposed definitions are those usually appearing in the parser
+header file.
+Thus, this is the right place to define types referenced in
+@code{%union @{@var{code}@}} directives, and it is the right place to override
+Bison's default @code{YYSTYPE} and @code{YYLTYPE} definitions.
+For Java, this is the right place to write import directives.
+
+@cindex Prologue
+@findex %union
+Compare with @code{%@{@var{code}%@}} (@pxref{Prologue, ,The Prologue})
+appearing before the first @code{%union @{@var{code}@}} in a C/C++ based
+grammar file.
+Unlike @code{%@{@var{code}%@}}, @code{%requires @{@var{code}@}} inserts your
+@var{code} both into the parser code file and into the parser header file (if
+generated; @pxref{Table of Symbols, ,%defines}) since Bison's required
+definitions should depend on it in both places.
+
+@xref{Prologue Alternatives}.
+@end deffn
+
@deffn {Directive} %right
Bison declaration to assign right associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@end deffn
@deffn {Macro} YYBACKUP
-Macro to discard a value from the parser stack and fake a look-ahead
+Macro to discard a value from the parser stack and fake a lookahead
token. @xref{Action Features, ,Special Features for Use in Actions}.
@end deffn
@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
+External integer variable that contains the integer value of the
+lookahead token. (In a pure parser, it is a local variable within
@code{yyparse}.) Error-recovery rule actions may examine this variable.
@xref{Action Features, ,Special Features for Use in Actions}.
@end deffn
@deffn {Variable} yyclearin
Macro used in error-recovery rule actions. It clears the previous
-look-ahead token. @xref{Error Recovery}.
+lookahead token. @xref{Error Recovery}.
@end deffn
@deffn {Macro} YYDEBUG
@deffn {Macro} YYINITDEPTH
Macro for specifying the initial size of the parser stack.
-@xref{Stack Overflow}.
+@xref{Memory Management}.
@end deffn
@deffn {Function} yylex
@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
+arguments) for @code{yyparse} to pass to @code{yylex}. The use of this
macro is deprecated, and is supported only for Yacc like parsers.
@xref{Pure Calling,, Calling Conventions for Pure Parsers}.
@end deffn
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}.
+@code{yylex}.)
+You can ignore this variable if you don't use the @samp{@@} feature in the
+grammar actions.
+@xref{Token Locations, ,Textual Locations of Tokens}.
+In semantic actions, it stores the location of the lookahead token.
+@xref{Actions and Locations, ,Actions and Locations}.
@end deffn
@deffn {Type} YYLTYPE
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}.
+@code{yylex}.)
+@xref{Token Values, ,Semantic Values of Tokens}.
+In semantic actions, it stores the semantic value of the lookahead token.
+@xref{Actions, ,Actions}.
@end deffn
@deffn {Macro} YYMAXDEPTH
-Macro for specifying the maximum size of the parser stack. @xref{Stack
-Overflow}.
+Macro for specifying the maximum size of the parser stack. @xref{Memory
+Management}.
@end deffn
@deffn {Variable} yynerrs
-Global variable which Bison increments each time there is a syntax error.
+Global variable which Bison increments each time it reports 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
@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}.
+The expression @code{YYRECOVERING ()} yields 1 when the parser
+is recovering from a syntax error, and 0 otherwise.
+@xref{Action Features, ,Special Features for Use in Actions}.
@end deffn
@deffn {Macro} YYSTACK_USE_ALLOCA
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
+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
@item Literal string token
A token which consists of two or more fixed characters. @xref{Symbols}.
-@item Look-ahead token
-A token already read but not yet shifted. @xref{Look-Ahead, ,Look-Ahead
+@item Lookahead token
+A token already read but not yet shifted. @xref{Lookahead, ,Lookahead
Tokens}.
@item @acronym{LALR}(1)
@item @acronym{LR}(1)
The class of context-free grammars in which at most one token of
-look-ahead is needed to disambiguate the parsing of any piece of input.
+lookahead is needed to disambiguate the parsing of any piece of input.
@item Nonterminal symbol
A grammar symbol standing for a grammatical construct that can
@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: YYEMPTY YYEOF YYRECOVERING yyclearin GE def UMINUS maybeword
@c LocalWords: Johnstone Shamsa Sadaf Hussain Tomita TR uref YYMAXDEPTH
-@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm VCG notype
+@c LocalWords: YYINITDEPTH stmnts ref stmnt initdcl maybeasm notype
@c LocalWords: hexflag STR exdent itemset asis DYYDEBUG YYFPRINTF args
-@c LocalWords: YYPRINTF infile ypp yxx outfile itemx vcg tex leaderfill
+@c LocalWords: infile ypp yxx outfile itemx 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