This file documents the Bison parser generator.
Copyright (C) 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998, 1999,
-2000, 2001
+2000, 2001, 2002
Free Software Foundation, Inc.
Permission is granted to make and distribute verbatim copies of
@page
@vskip 0pt plus 1filll
Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001
+1999, 2000, 2001, 2002
Free Software Foundation, Inc.
@sp 2
@dfn{nonterminal symbols}; those which can't be subdivided are called
@dfn{terminal symbols} or @dfn{token types}. We call a piece of input
corresponding to a single terminal symbol a @dfn{token}, and a piece
-corresponding to a single nonterminal symbol a @dfn{grouping}.@refill
+corresponding to a single nonterminal symbol a @dfn{grouping}.
We can use the C language as an example of what symbols, terminal and
nonterminal, mean. The tokens of C are identifiers, constants (numeric and
@emph{any} integer constant is grammatically valid in that position. The
precise value of the constant is irrelevant to how to parse the input: if
@samp{x+4} is grammatical then @samp{x+1} or @samp{x+3989} is equally
-grammatical.@refill
+grammatical.
But the precise value is very important for what the input means once it is
parsed. A compiler is useless if it fails to distinguish between 4, 1 and
@code{INTEGER}, @code{IDENTIFIER} or @code{','}. It tells everything
you need to know to decide where the token may validly appear and how to
group it with other tokens. The grammar rules know nothing about tokens
-except their types.@refill
+except their types.
The semantic value has all the rest of the information about the
meaning of the token, such as the value of an integer, or the name of an
expressions. As it does this, it runs the actions for the grammar rules it
uses.
-The tokens come from a function called the @dfn{lexical analyzer} that you
-must supply in some fashion (such as by writing it in C). The Bison parser
-calls the lexical analyzer each time it wants a new token. It doesn't know
-what is ``inside'' the tokens (though their semantic values may reflect
-this). Typically the lexical analyzer makes the tokens by parsing
-characters of text, but Bison does not depend on this. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The tokens come from a function called the @dfn{lexical analyzer} that
+you must supply in some fashion (such as by writing it in C). The Bison
+parser calls the lexical analyzer each time it wants a new token. It
+doesn't know what is ``inside'' the tokens (though their semantic values
+may reflect this). Typically the lexical analyzer makes the tokens by
+parsing characters of text, but Bison does not depend on this.
+@xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
The Bison parser file is C code which defines a function named
@code{yyparse} which implements that grammar. This function does not make
those cases your code should respect the identifiers reserved by those
headers. On some non-@sc{gnu} hosts, @code{<alloca.h>},
@code{<stddef.h>}, and @code{<stdlib.h>} are included as needed to
-declare memory allocators and related types. On all hosts,
-@code{<stdio.h>} is included if you define @code{YYDEBUG}
-(@pxref{Debugging, ,Debugging Your Parser}).
+declare memory allocators and related types.
+Other system headers may be included if you define @code{YYDEBUG} to a
+nonzero value (@pxref{Debugging, ,Debugging Your Parser}).
@node Stages
@section Stages in Using Bison
@enumerate
@item
Formally specify the grammar in a form recognized by Bison
-(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule in the language,
-describe the action that is to be taken when an instance of that rule
-is recognized. The action is described by a sequence of C statements.
+(@pxref{Grammar File, ,Bison Grammar Files}). For each grammatical rule
+in the language, describe the action that is to be taken when an
+instance of that rule is recognized. The action is described by a
+sequence of C statements.
@item
-Write a lexical analyzer to process input and pass tokens to the
-parser. The lexical analyzer may be written by hand in C
-(@pxref{Lexical, ,The Lexical Analyzer Function @code{yylex}}). It could also be produced using Lex, but the use
-of Lex is not discussed in this manual.
+Write a lexical analyzer to process input and pass tokens to the parser.
+The lexical analyzer may be written by hand in C (@pxref{Lexical, ,The
+Lexical Analyzer Function @code{yylex}}). It could also be produced
+using Lex, but the use of Lex is not discussed in this manual.
@item
Write a controlling function that calls the Bison-produced parser.
The @code{#include} directive is used to declare the exponentiation
function @code{pow}.
-The second section, Bison declarations, provides information to Bison about
-the token types (@pxref{Bison Declarations, ,The Bison Declarations Section}). Each terminal symbol that is
-not a single-character literal must be declared here. (Single-character
+The second section, Bison declarations, provides information to Bison
+about the token types (@pxref{Bison Declarations, ,The Bison
+Declarations Section}). Each terminal symbol that is not a
+single-character literal must be declared here. (Single-character
literals normally don't need to be declared.) In this example, all the
arithmetic operators are designated by single-character literals, so the
only terminal symbol that needs to be declared is @code{NUM}, the token
@cindex writing a lexical analyzer
@cindex lexical analyzer, writing
-The lexical analyzer's job is low-level parsing: converting characters or
-sequences of characters into tokens. The Bison parser gets its tokens by
-calling the lexical analyzer. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+The lexical analyzer's job is low-level parsing: converting characters
+or sequences of characters into tokens. The Bison parser gets its
+tokens by calling the lexical analyzer. @xref{Lexical, ,The Lexical
+Analyzer Function @code{yylex}}.
Only a simple lexical analyzer is needed for the RPN calculator. This
lexical analyzer skips blanks and tabs, then reads in numbers as
represents a token type. The same text used in Bison rules to stand for
this token type is also a C expression for the numeric code for the type.
This works in two ways. If the token type is a character literal, then its
-numeric code is the ASCII code for that character; you can use the same
+numeric code is that of the character; you can use the same
character literal in the lexical analyzer to express the number. If the
token type is an identifier, that identifier is defined by Bison as a C
macro whose definition is the appropriate number. In this example,
@example
@group
/* Lexical analyzer returns a double floating point
- number on the stack and the token NUM, or the ASCII
- character read if not a number. Skips all blanks
+ number on the stack and the token NUM, or the numeric code
+ of the character read if not a number. Skips all blanks
and tabs, returns 0 for EOF. */
#include <ctype.h>
declarations; the higher the line number of the declaration (lower on
the page or screen), the higher the precedence. Hence, exponentiation
has the highest precedence, unary minus (@code{NEG}) is next, followed
-by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator Precedence}.
+by @samp{*} and @samp{/}, and so on. @xref{Precedence, ,Operator
+Precedence}.
-The other important new feature is the @code{%prec} in the grammar section
-for the unary minus operator. The @code{%prec} simply instructs Bison that
-the rule @samp{| '-' exp} has the same precedence as @code{NEG}---in this
-case the next-to-highest. @xref{Contextual Precedence, ,Context-Dependent Precedence}.
+The other important new feature is the @code{%prec} in the grammar
+section for the unary minus operator. The @code{%prec} simply instructs
+Bison that the rule @samp{| '-' exp} has the same precedence as
+@code{NEG}---in this case the next-to-highest. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.
Here is a sample run of @file{calc.y}:
@code{yyerrok}, a macro defined automatically by Bison; its meaning is
that error recovery is complete (@pxref{Error Recovery}). Note the
difference between @code{yyerrok} and @code{yyerror}; neither one is a
-misprint.@refill
+misprint.
This form of error recovery deals with syntax errors. There are other
kinds of errors; for example, division by zero, which raises an exception
declarations are augmented with information about their data type (placed
between angle brackets).
-The Bison construct @code{%type} is used for declaring nonterminal symbols,
-just as @code{%token} is used for declaring token types. We have not used
-@code{%type} before because nonterminal symbols are normally declared
-implicitly by the rules that define them. But @code{exp} must be declared
-explicitly so we can specify its value type. @xref{Type Decl, ,Nonterminal Symbols}.
+The Bison construct @code{%type} is used for declaring nonterminal
+symbols, just as @code{%token} is used for declaring token types. We
+have not used @code{%type} before because nonterminal symbols are
+normally declared implicitly by the rules that define them. But
+@code{exp} must be declared explicitly so we can specify its value type.
+@xref{Type Decl, ,Nonterminal Symbols}.
@node Mfcalc Rules
@subsection Grammar Rules for @code{mfcalc}
(@code{VAR} or @code{FNCT}) is returned to @code{yyparse}. If it is not
already in the table, then it is installed as a @code{VAR} using
@code{putsym}. Again, a pointer and its type (which must be @code{VAR}) is
-returned to @code{yyparse}.@refill
+returned to @code{yyparse}.
No change is needed in the handling of numeric values and arithmetic
operators in @code{yylex}.
@end smallexample
This program is both powerful and flexible. You may easily add new
-functions, and it is a simple job to modify this code to install predefined
-variables such as @code{pi} or @code{e} as well.
+functions, and it is a simple job to modify this code to install
+predefined variables such as @code{pi} or @code{e} as well.
@node Exercises
@section Exercises
All the usual escape sequences used in character literals in C can be
used in Bison as well, but you must not use the null character as a
-character literal because its ASCII code, zero, is the code @code{yylex}
+character literal because its numeric code, zero, is the code @code{yylex}
returns for end-of-input (@pxref{Calling Convention, ,Calling Convention
for @code{yylex}}).
The value returned by @code{yylex} is always one of the terminal symbols
(or 0 for end-of-input). Whichever way you write the token type in the
grammar rules, you write it the same way in the definition of @code{yylex}.
-The numeric code for a character token type is simply the ASCII code for
+The numeric code for a character token type is simply the numeric code of
the character, so @code{yylex} can use the identical character constant to
generate the requisite code. Each named token type becomes a C macro in
the parser file, so @code{yylex} can use the name to stand for the code.
into a separate header file @file{@var{name}.tab.h} which you can include
in the other source files that need it. @xref{Invocation, ,Invoking Bison}.
+The @code{yylex} function must use the same character set and encoding
+that was used by Bison. For example, if you run Bison in an
+@sc{ascii} environment, but then compile and run the resulting program
+in an environment that uses an incompatible character set like
+@sc{ebcdic}, the resulting program will probably not work because the
+tables generated by Bison will assume @sc{ascii} numeric values for
+character tokens. Portable grammars should avoid non-@sc{ascii}
+character tokens, as implementations in practice often use different
+and incompatible extensions in this area. However, it is standard
+practice for software distributions to contain C source files that
+were generated by Bison in an @sc{ascii} environment, so installers on
+platforms that are incompatible with @sc{ascii} must rebuild those
+files before compiling them.
+
The symbol @code{error} is a terminal symbol reserved for error recovery
(@pxref{Error Recovery}); you shouldn't use it for any other purpose.
-In particular, @code{yylex} should never return this value.
+In particular, @code{yylex} should never return this value. The default
+value of the error token is 256, unless you explicitly assigned 256 to
+one of your tokens with a @code{%token} declaration.
@node Rules
@section Syntax of Grammar Rules
@itemize @bullet
@item
Specify the entire collection of possible data types, with the
-@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of Value Types}).
+@code{%union} Bison declaration (@pxref{Union Decl, ,The Collection of
+Value Types}).
@item
Choose one of those types for each symbol (terminal or nonterminal) for
semantic values associated with tokens or smaller groupings.
An action consists of C statements surrounded by braces, much like a
-compound statement in C. It can be placed at any position in the rule; it
-is executed at that position. Most rules have just one action at the end
-of the rule, following all the components. Actions in the middle of a rule
-are tricky and used only for special purposes (@pxref{Mid-Rule Actions, ,Actions in Mid-Rule}).
+compound statement in C. It can be placed at any position in the rule;
+it is executed at that position. Most rules have just one action at the
+end of the rule, following all the components. Actions in the middle of
+a rule are tricky and used only for special purposes (@pxref{Mid-Rule
+Actions, ,Actions in Mid-Rule}).
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
The sum is stored into @code{$$} so that it becomes the semantic value of
the addition-expression just recognized by the rule. If there were a
useful semantic value associated with the @samp{+} token, it could be
-referred to as @code{$2}.@refill
+referred to as @code{$2}.
+
+Note that the vertical-bar character @samp{|} is really a rule
+separator, and actions are attached to a single rule. This is a
+difference with tools like Flex, for which @samp{|} stands for either
+``or'', or ``the same action as that of the next rule''. In the
+following example, the action is triggered only when @samp{b} is found:
+
+@example
+@group
+a-or-b: 'a'|'b' @{ a_or_b_found = 1; @};
+@end group
+@end example
@cindex default action
If you don't specify an action for a rule, Bison supplies a default:
must declare a choice among these types for each terminal or nonterminal
symbol that can have a semantic value. Then each time you use @code{$$} or
@code{$@var{n}}, its data type is determined by which symbol it refers to
-in the rule. In this example,@refill
+in the rule. In this example,
@example
@group
@code{$1} and @code{$3} refer to instances of @code{exp}, so they all
have the data type declared for the nonterminal symbol @code{exp}. If
@code{$2} were used, it would have the data type declared for the
-terminal symbol @code{'+'}, whatever that might be.@refill
+terminal symbol @code{'+'}, whatever that might be.
Alternatively, you can specify the data type when you refer to the value,
by inserting @samp{<@var{type}>} after the @samp{$} at the beginning of the
@c (terminal or not) ?
-The way locations are handled is defined by providing a data type, and actions
-to take when rules are matched.
+The way locations are handled is defined by providing a data type, and
+actions to take when rules are matched.
@menu
* Location Type:: Specifying a data type for locations.
@subsection Default Action for Locations
@vindex YYLLOC_DEFAULT
-Actually, actions are not the best place to compute locations. Since locations
-are much more general than semantic values, there is room in the output parser
-to redefine the default action to take for each rule. The
-@code{YYLLOC_DEFAULT} macro is called each time a rule is matched, before the
-associated action is run.
+Actually, actions are not the best place to compute locations. Since
+locations are much more general than semantic values, there is room in
+the output parser to redefine the default action to take for each
+rule. The @code{YYLLOC_DEFAULT} macro is invoked each time a rule is
+matched, before the associated action is run.
Most of the time, this macro is general enough to suppress location
dedicated code from semantic actions.
The first rule in the file also specifies the start symbol, by default.
If you want some other symbol to be the start symbol, you must declare
-it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free Grammars}).
+it explicitly (@pxref{Language and Grammar, ,Languages and Context-Free
+Grammars}).
@menu
* Token Decl:: Declaring terminal symbols.
@noindent
It is generally best, however, to let Bison choose the numeric codes for
all token types. Bison will automatically select codes that don't conflict
-with each other or with ASCII characters.
+with each other or with normal characters.
In the event that the stack type is a union, you must augment the
@code{%token} or other token declaration to include the data type
-alternative delimited by angle-brackets (@pxref{Multiple Types, ,More Than One Value Type}).
+alternative delimited by angle-brackets (@pxref{Multiple Types, ,More
+Than One Value Type}).
For example:
Use the @code{%left}, @code{%right} or @code{%nonassoc} declaration to
declare a token and specify its precedence and associativity, all at
once. These are called @dfn{precedence declarations}.
-@xref{Precedence, ,Operator Precedence}, for general information on operator precedence.
+@xref{Precedence, ,Operator Precedence}, for general information on
+operator precedence.
The syntax of a precedence declaration is the same as that of
@code{%token}: either
@end example
@noindent
-Here @var{nonterminal} is the name of a nonterminal symbol, and @var{type}
-is the name given in the @code{%union} to the alternative that you want
-(@pxref{Union Decl, ,The Collection of Value Types}). You can give any number of nonterminal symbols in
-the same @code{%type} declaration, if they have the same value type. Use
-spaces to separate the symbol names.
+Here @var{nonterminal} is the name of a nonterminal symbol, and
+@var{type} is the name given in the @code{%union} to the alternative
+that you want (@pxref{Union Decl, ,The Collection of Value Types}). You
+can give any number of nonterminal symbols in the same @code{%type}
+declaration, if they have the same value type. Use spaces to separate
+the symbol names.
You can also declare the value type of a terminal symbol. To do this,
use the same @code{<@var{type}>} construction in a declaration for the
@subsection A Pure (Reentrant) Parser
@cindex reentrant parser
@cindex pure parser
-@findex %pure_parser
+@findex %pure-parser
A @dfn{reentrant} program is one which does not alter in the course of
execution; in other words, it consists entirely of @dfn{pure} (read-only)
including @code{yylval} and @code{yylloc}.)
Alternatively, you can generate a pure, reentrant parser. The Bison
-declaration @code{%pure_parser} says that you want the parser to be
+declaration @code{%pure-parser} says that you want the parser to be
reentrant. It looks like this:
@example
-%pure_parser
+%pure-parser
@end example
The result is that the communication variables @code{yylval} and
@table @code
@item %debug
-Output a definition of the macro @code{YYDEBUG} into the parser file, so
-that the debugging facilities are compiled. @xref{Debugging, ,Debugging
-Your Parser}.
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@xref{Debugging, ,Debugging Your Parser}.
@item %defines
Write an extra output file containing macro definitions for the token
@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
If the parser output file is named @file{@var{name}.c} then this file
-is named @file{@var{name}.h}.@refill
+is named @file{@var{name}.h}.
This output file is essential if you wish to put the definition of
@code{yylex} in a separate source file, because @code{yylex} needs to
be able to refer to token type codes and the variable
-@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill
+@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.
@item %file-prefix="@var{prefix}"
Specify a prefix to use for all Bison output file names. The names are
chosen as if the input file were named @file{@var{prefix}.y}.
-@c @item %header_extension
+@c @item %header-extension
@c Specify the extension of the parser header file generated when
@c @code{%define} or @samp{-d} are used.
@c
@c For example, a grammar file named @file{foo.ypp} and containing a
-@c @code{%header_extension .hh} directive will produce a header file
+@c @code{%header-extension .hh} directive will produce a header file
@c named @file{foo.tab.hh}
@item %locations
Rename the external symbols used in the parser so that they start with
@var{prefix} instead of @samp{yy}. The precise list of symbols renamed
is @code{yyparse}, @code{yylex}, @code{yyerror}, @code{yynerrs},
-@code{yylval}, @code{yychar} and @code{yydebug}. For example, if you
-use @samp{%name-prefix="c_"}, the names become @code{c_parse},
-@code{c_lex}, and so on. @xref{Multiple Parsers, ,Multiple Parsers in
-the Same Program}.
+@code{yylval}, @code{yychar}, @code{yydebug}, and possible
+@code{yylloc}. For example, if you use @samp{%name-prefix="c_"}, the
+names become @code{c_parse}, @code{c_lex}, and so on. @xref{Multiple
+Parsers, ,Multiple Parsers in the Same Program}.
@item %no-parser
Do not include any C code in the parser file; generate tables only. The
Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure
(Reentrant) Parser}).
-@c @item %source_extension
+@c @item %source-extension
@c Specify the extension of the parser output file.
@c
@c For example, a grammar file named @file{foo.yy} and containing a
-@c @code{%source_extension .cpp} directive will produce a parser file
+@c @code{%source-extension .cpp} directive will produce a parser file
@c named @file{foo.tab.cpp}
-@item %token_table
+@item %token-table
Generate an array of token names in the parser file. The name of the
array is @code{yytname}; @code{yytname[@var{i}]} is the name of the
token whose internal Bison token code number is @var{i}. The first three
contains @samp{"*"*"}. (In C, that would be written as
@code{"\"*\"*\""}).
-When you specify @code{%token_table}, Bison also generates macro
+When you specify @code{%token-table}, Bison also generates macro
definitions for macros @code{YYNTOKENS}, @code{YYNNTS}, and
@code{YYNRULES}, and @code{YYNSTATES}:
operator precedence and the unresolved ones.
The file's name is made by removing @samp{.tab.c} or @samp{.c} from
-the parser output file name, and adding @samp{.output} instead.@refill
+the parser output file name, and adding @samp{.output} instead.
Therefore, if the input file is @file{foo.y}, then the parser file is
called @file{foo.tab.c} by default. As a consequence, the verbose
-output file is called @file{foo.output}.@refill
+output file is called @file{foo.output}.
@item %yacc
-@itemx %fixed-output-files
Pretend the option @option{--yacc} was given, i.e., imitate Yacc,
including its naming conventions. @xref{Bison Options}, for more.
@end table
between different definitions of @code{yyparse}, @code{yylval}, and so on.
The easy way to do this is to use the option @samp{-p @var{prefix}}
-(@pxref{Invocation, ,Invoking Bison}). This renames the interface functions and
-variables of the Bison parser to start with @var{prefix} instead of
-@samp{yy}. You can use this to give each parser distinct names that do
-not conflict.
+(@pxref{Invocation, ,Invoking Bison}). This renames the interface
+functions and variables of the Bison parser to start with @var{prefix}
+instead of @samp{yy}. You can use this to give each parser distinct
+names that do not conflict.
The precise list of symbols renamed is @code{yyparse}, @code{yylex},
@code{yyerror}, @code{yynerrs}, @code{yylval}, @code{yychar} and
To do this, use the @samp{-d} option when you run Bison, so that it will
write these macro definitions into a separate header file
@file{@var{name}.tab.h} which you can include in the other source files
-that need it. @xref{Invocation, ,Invoking Bison}.@refill
+that need it. @xref{Invocation, ,Invoking Bison}.
@menu
* Calling Convention:: How @code{yyparse} calls @code{yylex}.
@end smallexample
The @code{yytname} table is generated only if you use the
-@code{%token_table} declaration. @xref{Decl Summary}.
+@code{%token-table} declaration. @xref{Decl Summary}.
@end itemize
@node Token Values
@end example
When you are using multiple data types, @code{yylval}'s type is a union
-made from the @code{%union} declaration (@pxref{Union Decl, ,The Collection of Value Types}). So when
-you store a token's value, you must use the proper member of the union.
-If the @code{%union} declaration looks like this:
+made from the @code{%union} declaration (@pxref{Union Decl, ,The
+Collection of Value Types}). So when you store a token's value, you
+must use the proper member of the union. If the @code{%union}
+declaration looks like this:
@example
@group
@node Pure Calling
@subsection Calling Conventions for Pure Parsers
-When you use the Bison declaration @code{%pure_parser} to request a
+When you use the Bison declaration @code{%pure-parser} to request a
pure, reentrant parser, the global communication variables @code{yylval}
and @code{yylloc} cannot be used. (@xref{Pure Decl, ,A Pure (Reentrant)
Parser}.) In such parsers the two global variables are replaced by
the proper object type, or you can declare it as @code{void *} and
access the contents as shown above.
-You can use @samp{%pure_parser} to request a reentrant parser without
+You can use @samp{%pure-parser} to request a reentrant parser without
also using @code{YYPARSE_PARAM}. Then you should call @code{yyparse}
with no arguments, as usual.
@vindex yynerrs
The variable @code{yynerrs} contains the number of syntax errors
encountered so far. Normally this variable is global; but if you
-request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser}) then it is a local variable
-which only the actions can access.
+request a pure parser (@pxref{Pure Decl, ,A Pure (Reentrant) Parser})
+then it is a local variable which only the actions can access.
@node Action Features
@section Special Features for Use in Actions
@item $<@var{typealt}>$
Like @code{$$} but specifies alternative @var{typealt} in the union
-specified by the @code{%union} declaration. @xref{Action Types, ,Data Types of Values in Actions}.
+specified by the @code{%union} declaration. @xref{Action Types, ,Data
+Types of Values in Actions}.
@item $<@var{typealt}>@var{n}
Like @code{$@var{n}} but specifies alternative @var{typealt} in the
union specified by the @code{%union} declaration.
-@xref{Action Types, ,Data Types of Values in Actions}.@refill
+@xref{Action Types, ,Data Types of Values in Actions}.
@item YYABORT;
Return immediately from @code{yyparse}, indicating failure.
The first effect of the precedence declarations is to assign precedence
levels to the terminal symbols declared. The second effect is to assign
-precedence levels to certain rules: each rule gets its precedence from the
-last terminal symbol mentioned in the components. (You can also specify
-explicitly the precedence of a rule. @xref{Contextual Precedence, ,Context-Dependent Precedence}.)
-
-Finally, the resolution of conflicts works by comparing the
-precedence of the rule being considered with that of the
-look-ahead token. If the token's precedence is higher, the
-choice is to shift. If the rule's precedence is higher, the
-choice is to reduce. If they have equal precedence, the choice
-is made based on the associativity of that precedence level. The
-verbose output file made by @samp{-v} (@pxref{Invocation, ,Invoking Bison}) says
-how each conflict was resolved.
+precedence levels to certain rules: each rule gets its precedence from
+the last terminal symbol mentioned in the components. (You can also
+specify explicitly the precedence of a rule. @xref{Contextual
+Precedence, ,Context-Dependent Precedence}.)
+
+Finally, the resolution of conflicts works by comparing the precedence
+of the rule being considered with that of the look-ahead token. If the
+token's precedence is higher, the choice is to shift. If the rule's
+precedence is higher, the choice is to reduce. If they have equal
+precedence, the choice is made based on the associativity of that
+precedence level. The verbose output file made by @samp{-v}
+(@pxref{Invocation, ,Invoking Bison}) says how each conflict was
+resolved.
Not all rules and not all tokens have precedence. If either the rule or
the look-ahead token has no precedence, then the default is to shift.
@code{%nonassoc}, can only be used once for a given token; so a token has
only one precedence declared in this way. For context-dependent
precedence, you need to use an additional mechanism: the @code{%prec}
-modifier for rules.@refill
+modifier for rules.
The @code{%prec} modifier declares the precedence of a particular rule by
specifying a terminal symbol whose precedence should be used for that rule.
@node Debugging
@chapter Debugging Your Parser
-@findex YYDEBUG
@findex yydebug
@cindex debugging
@cindex tracing the parser
If a Bison grammar compiles properly but doesn't do what you want when it
runs, the @code{yydebug} parser-trace feature can help you figure out why.
-To enable compilation of trace facilities, you must define the macro
-@code{YYDEBUG} when you compile the parser. You could use @samp{-DYYDEBUG=1}
-as a compiler option or you could put @samp{#define YYDEBUG 1} in the prologue
-of the grammar file (@pxref{Prologue, , The Prologue}). Alternatively, use the
-@samp{-t} option when you run Bison (@pxref{Invocation, ,Invoking Bison}).
-We always define @code{YYDEBUG} so that debugging is always possible.
+There are several means to enable compilation of trace facilities:
+
+@table @asis
+@item the macro @code{YYDEBUG}
+@findex YYDEBUG
+Define the macro @code{YYDEBUG} to a nonzero value when you compile the
+parser. This is compliant with POSIX Yacc. You could use
+@samp{-DYYDEBUG=1} as a compiler option or you could put @samp{#define
+YYDEBUG 1} in the prologue of the grammar file (@pxref{Prologue, , The
+Prologue}).
+
+@item the option @option{-t}, @option{--debug}
+Use the @samp{-t} option when you run Bison (@pxref{Invocation,
+,Invoking Bison}). This is POSIX compliant too.
+
+@item the directive @samp{%debug}
+@findex %debug
+Add the @code{%debug} directive (@pxref{Decl Summary, ,Bison
+Declaration Summary}). This is a Bison extension, which will prove
+useful when Bison will output parsers for languages that don't use a
+preprocessor. Useless POSIX and Yacc portability matter to you, this is
+the preferred solution.
+@end table
+
+We suggest that you always enable the debug option so that debugging is
+always possible.
-The trace facility uses @code{stderr}, so you must add
-@w{@code{#include <stdio.h>}} to the prologue unless it is already there.
+The trace facility outputs messages with macro calls of the form
+@code{YYFPRINTF (stderr, @var{format}, @var{args})} where
+@var{format} and @var{args} are the usual @code{printf} format and
+arguments. If you define @code{YYDEBUG} to a nonzero value but do not
+define @code{YYFPRINTF}, @code{<stdio.h>} is automatically included
+and @code{YYPRINTF} is defined to @code{fprintf}.
Once you have compiled the program with trace facilities, the way to
request a trace is to store a nonzero value in the variable @code{yydebug}.
@end itemize
To make sense of this information, it helps to refer to the listing file
-produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking Bison}). This file
-shows the meaning of each state in terms of positions in various rules, and
-also what each state will do with each possible input token. As you read
-the successive trace messages, you can see that the parser is functioning
-according to its specification in the listing file. Eventually you will
-arrive at the place where something undesirable happens, and you will see
-which parts of the grammar are to blame.
+produced by the Bison @samp{-v} option (@pxref{Invocation, ,Invoking
+Bison}). This file shows the meaning of each state in terms of
+positions in various rules, and also what each state will do with each
+possible input token. As you read the successive trace messages, you
+can see that the parser is functioning according to its specification in
+the listing file. Eventually you will arrive at the place where
+something undesirable happens, and you will see which parts of the
+grammar are to blame.
The parser file is a C program and you can use C debuggers on it, but it's
not easy to interpret what it is doing. The parser function is a
@samp{.y}. The parser file's name is made by replacing the @samp{.y}
with @samp{.tab.c}. Thus, the @samp{bison foo.y} filename yields
@file{foo.tab.c}, and the @samp{bison hack/foo.y} filename yields
-@file{hack/foo.tab.c}. It's is also possible, in case you are writting
+@file{hack/foo.tab.c}. It's is also possible, in case you are writing
C++ code instead of C in your grammar file, to name it @file{foo.ypp}
or @file{foo.y++}. Then, the output files will take an extention like
the given one as input (repectively @file{foo.tab.cpp} and @file{foo.tab.c++}).
@need 1750
@item -y
@itemx --yacc
-@itemx --fixed-output-files
Equivalent to @samp{-o y.tab.c}; the parser output file is called
@file{y.tab.c}, and the other outputs are called @file{y.output} and
@file{y.tab.h}. The purpose of this option is to imitate Yacc's output
file name conventions. Thus, the following shell script can substitute
-for Yacc:@refill
+for Yacc:
@example
bison -y $*
@item -t
@itemx --debug
-Output a definition of the macro @code{YYDEBUG} into the parser file, so
-that the debugging facilities are compiled. @xref{Debugging, ,Debugging
-Your Parser}.
+In the parser file, define the macro @code{YYDEBUG} to 1 if it is not
+already defined, so that the debugging facilities are compiled.
+@xref{Debugging, ,Debugging Your Parser}.
@item --locations
Pretend that @code{%locations} was specified. @xref{Decl Summary}.
environment variable @code{BISON_SIMPLE} to the path of the file will
cause Bison to use that copy instead.
-When the @samp{%semantic_parser} declaration is used, Bison copies from
+When the @samp{%semantic-parser} declaration is used, Bison copies from
a file called @file{bison.hairy} instead. The location of this file can
also be specified or overridden in a similar fashion, with the
@code{BISON_HAIRY} environment variable.
\line{ --debug \leaderfill -t}
\line{ --defines \leaderfill -d}
\line{ --file-prefix \leaderfill -b}
-\line{ --fixed-output-files \leaderfill -y}
\line{ --graph \leaderfill -g}
\line{ --help \leaderfill -h}
\line{ --name-prefix \leaderfill -p}
--debug -t
--defines=@var{defines-file} -d
--file-prefix=@var{prefix} -b @var{file-prefix}
---fixed-output-files --yacc -y
--graph=@var{graph-file} -d
--help -h
--name-prefix=@var{prefix} -p @var{name-prefix}
--token-table -k
--verbose -v
--version -V
+--yacc -y
@end example
@end ifinfo
@cindex symbols in Bison, table of
@table @code
+@item @@$
+In an action, the location of the left-hand side of the rule.
+ @xref{Locations, , Locations Overview}.
+
+@item @@@var{n}
+In an action, the location of the @var{n}-th symbol of the right-hand
+side of the rule. @xref{Locations, , Locations Overview}.
+
+@item $$
+In an action, the semantic value of the left-hand side of the rule.
+@xref{Actions}.
+
+@item $@var{n}
+In an action, the semantic value of the @var{n}-th symbol of the
+right-hand side of the rule. @xref{Actions}.
+
@item error
A token name reserved for error recovery. This token may be used in
grammar rules so as to allow the Bison parser to recognize an error in
Macro to discard a value from the parser stack and fake a look-ahead
token. @xref{Action Features, ,Special Features for Use in Actions}.
+@item YYDEBUG
+Macro to define to equip the parser with tracing code. @xref{Debugging,
+,Debugging Your Parser}.
+
@item YYERROR
Macro to pretend that a syntax error has just been detected: call
@code{yyerror} and then perform normal error recovery if possible
Reporting Function @code{yyerror}}.
@item yylex
-User-supplied lexical analyzer function, called with no arguments
-to get the next token. @xref{Lexical, ,The Lexical Analyzer Function @code{yylex}}.
+User-supplied lexical analyzer function, called with no arguments to get
+the next token. @xref{Lexical, ,The Lexical Analyzer Function
+@code{yylex}}.
@item yylval
External variable in which @code{yylex} should place the semantic
Bison declaration to set tge prefix of the output files. @xref{Decl
Summary}.
-@c @item %source_extension
+@c @item %source-extension
@c Bison declaration to specify the generated parser output file extension.
@c @xref{Decl Summary}.
@c
-@c @item %header_extension
+@c @item %header-extension
@c Bison declaration to specify the generated parser header file extension
@c if required. @xref{Decl Summary}.
@xref{Precedence Decl, ,Operator Precedence}.
@item %start
-Bison declaration to specify the start symbol. @xref{Start Decl, ,The Start-Symbol}.
+Bison declaration to specify the start symbol. @xref{Start Decl, ,The
+Start-Symbol}.
@item %token
Bison declaration to declare token(s) without specifying precedence.
@xref{Decl Summary}.
@item %type
-Bison declaration to declare nonterminals. @xref{Type Decl, ,Nonterminal Symbols}.
+Bison declaration to declare nonterminals. @xref{Type Decl,
+,Nonterminal Symbols}.
@item %union
Bison declaration to specify several possible data types for semantic
values. @xref{Union Decl, ,The Collection of Value Types}.
@end table
+@sp 1
+
These are the punctuation and delimiters used in Bison input:
@table @samp
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