@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
@value{UPDATED}), the @acronym{GNU} parser generator.
Copyright @copyright{} 1988, 1989, 1990, 1991, 1992, 1993, 1995, 1998,
-1999, 2000, 2001, 2002, 2003 Free Software Foundation, Inc.
+1999, 2000, 2001, 2002, 2003, 2004 Free Software Foundation, Inc.
@quotation
Permission is granted to copy, distribute and/or modify this document
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Destructor Decl:: Declaring how symbols are freed.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
Frequently Asked Questions
* Parser Stack Overflow:: Breaking the Stack Limits
-* How Can I Reset @code{yyparse}:: @code{yyparse} Keeps some State
+* 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 Loops:: Control Flow in the Calculator
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 a
+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
void
yyerror (char const *s)
@{
- printf ("%s\n", s);
+ fprintf (stderr, "%s\n", s);
@}
@end group
@end example
read your program will be confused.
All the escape sequences used in string literals in C can be used in
-Bison as well. However, unlike Standard C, trigraphs have no special
+Bison as well, except that you must not use a null character within a
+string literal. Also, unlike Standard C, trigraphs have no special
meaning in Bison string literals, nor is backslash-newline allowed. A
literal string token must contain two or more characters; for a token
containing just one character, use a character token (see above).
The C code in an action can refer to the semantic values of the components
matched by the rule with the construct @code{$@var{n}}, which stands for
the value of the @var{n}th component. The semantic value for the grouping
-being constructed is @code{$$}. (Bison translates both of these constructs
-into array element references when it copies the actions into the parser
-file.)
+being constructed is @code{$$}. Bison translates both of these
+constructs into expressions of the appropriate type when it copies the
+actions into the parser file. @code{$$} is translated to a modifiable
+lvalue, so it can be assigned to.
Here is a typical example:
functional parser, it can be useful to process some additional information,
especially symbol locations.
-@c (terminal or not) ?
-
The way locations are handled is defined by providing a data type, and
actions to take when rules are matched.
else
@{
$$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
@}
@}
@end group
else
@{
$$ = 1;
- printf("Division by zero, l%d,c%d-l%d,c%d",
- @@3.first_line, @@3.first_column,
- @@3.last_line, @@3.last_column);
+ fprintf (stderr,
+ "Division by zero, l%d,c%d-l%d,c%d",
+ @@3.first_line, @@3.first_column,
+ @@3.last_line, @@3.last_column);
@}
@}
@end group
@example
@group
-#define YYLLOC_DEFAULT(Current, Rhs, N) \
- Current.first_line = Rhs[1].first_line; \
- Current.first_column = Rhs[1].first_column; \
- Current.last_line = Rhs[N].last_line; \
- Current.last_column = Rhs[N].last_column;
+# define YYLLOC_DEFAULT(Current, Rhs, N) \
+ ((Current).first_line = (Rhs)[1].first_line, \
+ (Current).first_column = (Rhs)[1].first_column, \
+ (Current).last_line = (Rhs)[N].last_line, \
+ (Current).last_column = (Rhs)[N].last_column)
@end group
@end example
@example
@group
-#define YYLLOC_DEFAULT(Current, Rhs, N) \
- Current.first_line = YYRHSLOC(Rhs,1).first_line; \
- Current.first_column = YYRHSLOC(Rhs,1).first_column; \
- Current.last_line = YYRHSLOC(Rhs,N).last_line; \
- Current.last_column = YYRHSLOC(Rhs,N).last_column;
+# define YYLLOC_DEFAULT(yyCurrent, yyRhs, YYN) \
+ ((yyCurrent).first_line = YYRHSLOC(yyRhs, 1).first_line, \
+ (yyCurrent).first_column = YYRHSLOC(yyRhs, 1).first_column, \
+ (yyCurrent).last_line = YYRHSLOC(yyRhs, YYN).last_line, \
+ (yyCurrent).last_column = YYRHSLOC(yyRhs, YYN).last_column)
@end group
@end example
@item
For consistency with semantic actions, valid indexes for the location
array range from 1 to @var{n}.
+
+@item
+Your macro should parenthesize its arguments, if need be, since the
+actual arguments may not be surrounded by parentheses. Also, your
+macro should expand to something that can be used as a single
+statement when it is followed by a semicolon.
@end itemize
@node Declarations
* Union Decl:: Declaring the set of all semantic value types.
* Type Decl:: Declaring the choice of type for a nonterminal symbol.
* Destructor Decl:: Declaring how symbols are freed.
-* Expect Decl:: Suppressing warnings about shift/reduce conflicts.
+* Expect Decl:: Suppressing warnings about parsing conflicts.
* Start Decl:: Specifying the start symbol.
* Pure Decl:: Requesting a reentrant parser.
* Decl Summary:: Table of all Bison declarations.
Precedence}.
You can explicitly specify the numeric code for a token type by appending
-an integer value in the field immediately following the token name:
+a decimal or hexadecimal integer value in the field immediately
+following the token name:
@example
%token NUM 300
+%token XNUM 0x12d // a GNU extension
@end example
@noindent
@cindex warnings, preventing
@cindex conflicts, suppressing warnings of
@findex %expect
+@findex %expect-rr
Bison normally warns if there are any conflicts in the grammar
(@pxref{Shift/Reduce, ,Shift/Reduce Conflicts}), but most real grammars
given if there are either more or fewer conflicts, or if there are any
reduce/reduce conflicts.
+For normal 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 GLR parsers, however,
+both shift/reduce and reduce/reduce are routine (otherwise, there
+would be no need to use GLR parsing). Therefore, it is also possible
+to specify an expected number of reduce/reduce conflicts in GLR
+parsers, using the declaration:
+
+@example
+%expect-rr @var{n}
+@end example
+
In general, using @code{%expect} involves these steps:
@itemize @bullet
@deffn {Directive} %nonassoc
Declare a terminal symbol (token type name) that is nonassociative
-(using it in a way that would be associative is a syntax error)
-@end deffn
(@pxref{Precedence Decl, ,Operator Precedence}).
+Using it in a way that would be associative is a syntax error.
+@end deffn
+
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules lacking an explicit @code{%prec} modifier
+(@pxref{Contextual Precedence, ,Context-Dependent Precedence}).
+@end deffn
+@end ifset
@deffn {Directive} %type
Declare the type of semantic values for a nonterminal symbol
Program}.
@end deffn
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules lacking an explicit @code{%prec}
+modifier (@pxref{Contextual Precedence, ,Context-Dependent
+Precedence}).
+@end deffn
+@end ifset
+
@deffn {Directive} %no-parser
Do not include any C code in the parser file; generate tables only. The
parser file contains just @code{#define} directives and static variable
@end group
@end example
+@ifset defaultprec
+If you forget to append @code{%prec UMINUS} to the rule for unary
+minus, Bison silently assumes that minus has its usual precedence.
+This kind of problem can be tricky to debug, since one typically
+discovers the mistake only by testing the code.
+
+The @code{%no-default-prec;} declaration makes it easier to discover
+this kind of problem systematically. It causes rules that lack a
+@code{%prec} modifier to have no precedence, even if the last terminal
+symbol mentioned in their components has a declared precedence.
+
+If @code{%no-default-prec;} is in effect, you must specify @code{%prec}
+for all rules that participate in precedence conflict resolution.
+Then you will see any shift/reduce conflict until you tell Bison how
+to resolve it, either by changing your grammar or by adding an
+explicit precedence. This will probably add declarations to the
+grammar, but it helps to protect against incorrect rule precedences.
+
+The effect of @code{%no-default-prec;} can be reversed by giving
+@code{%default-prec;}, which is the default.
+@end ifset
+
@node Parser States
@section Parser States
@cindex finite-state machine
@example
#! /bin/sh
-bison -y "$@"
+bison -y "$@@"
@end example
@end table
@menu
* Parser Stack Overflow:: Breaking the Stack Limits
-* How Can I Reset @code{yyparse}:: @code{yyparse} Keeps some State
+* 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 Loops:: Control Flow in the Calculator
This question is already addressed elsewhere, @xref{Recursion,
,Recursive Rules}.
-@node How Can I Reset @code{yyparse}
-@section How Can I Reset @code{yyparse}
+@node How Can I Reset the Parser
+@section How Can I Reset the Parser
-The following phenomenon gives raise to several incarnations,
-resulting in the following typical questions:
+The following phenomenon has several symptoms, resulting in the
+following typical questions:
@display
I invoke @code{yyparse} several times, and on correct input it works
properly; but when a parse error is found, all the other calls fail
-too. How can I reset @code{yyparse}'s error flag?
+too. How can I reset the error flag of @code{yyparse}?
@end display
@noindent
or
@display
-My parser includes support for a @samp{#include} like feature, in
+My parser includes support for an @samp{#include}-like feature, in
which case I run @code{yyparse} from @code{yyparse}. This fails
although I did specify I needed a @code{%pure-parser}.
@end display
-These problems are not related to Bison itself, but with the Lex
-generated scanners. Because these scanners use large buffers for
+These problems typically come not from Bison itself, but from
+Lex-generated scanners. Because these scanners use large buffers for
speed, they might not notice a change of input file. As a
demonstration, consider the following source file,
@file{first-line.l}:
.*\n ECHO; return 1;
%%
int
-yyparse (const char *file)
+yyparse (char const *file)
{
yyin = fopen (file, "r");
if (!yyin)
exit (2);
/* One token only. */
yylex ();
- if (!fclose (yyin))
+ if (fclose (yyin) != 0)
exit (3);
return 0;
}
int
-main ()
+main (void)
{
yyparse ("input");
yyparse ("input");
@end verbatim
@noindent
-then instead of getting twice the first line, you get:
+then instead of getting the first line twice, you get:
@example
$ @kbd{flex -ofirst-line.c first-line.l}
input:2: World!
@end example
-Therefore, whenever you change @code{yyin}, you must tell the Lex
-generated scanner to discard its current buffer, and to switch to the
-new one. This depends upon your implementation of Lex, see its
-documentation for more. For instance, in the case of Flex, a simple
-call @samp{yyrestart (yyin)} suffices after each change to
-@code{yyin}.
+Therefore, whenever you change @code{yyin}, you must tell the
+Lex-generated scanner to discard its current buffer and switch to the
+new one. This depends upon your implementation of Lex; see its
+documentation for more. For Flex, it suffices to call
+@samp{YY_FLUSH_BUFFER} after each change to @code{yyin}. If your
+Flex-generated scanner needs to read from several input streams to
+handle features like include files, you might consider using Flex
+functions like @samp{yy_switch_to_buffer} that manipulate multiple
+input buffers.
+
+If your Flex-generated scanner uses start conditions (@pxref{Start
+conditions, , Start conditions, flex, The Flex Manual}), you might
+also want to reset the scanner's state, i.e., go back to the initial
+start condition, through a call to @samp{BEGIN (0)}.
@node Strings are Destroyed
@section Strings are Destroyed
@end display
Although very pedagogical, the examples included in the document blur
-the distinction to make between the parser ---whose job is to recover
+the distinction to make between the parser---whose job is to recover
the structure of a text and to transmit it to subsequent modules of
-the program--- and the processing (such as the execution) of this
+the program---and the processing (such as the execution) of this
structure. This works well with so called straight line programs,
i.e., precisely those that have a straightforward execution model:
execute simple instructions one after the others.
Equip the parser for debugging. @xref{Decl Summary}.
@end deffn
+@ifset defaultprec
+@deffn {Directive} %default-prec
+Assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
+
@deffn {Directive} %defines
Bison declaration to create a header file meant for the scanner.
@xref{Decl Summary}.
Bison declaration to rename the external symbols. @xref{Decl Summary}.
@end deffn
+@ifset defaultprec
+@deffn {Directive} %no-default-prec
+Do not assign a precedence to rules that lack an explicit @samp{%prec}
+modifier. @xref{Contextual Precedence, ,Context-Dependent
+Precedence}.
+@end deffn
+@end ifset
+
@deffn {Directive} %no-lines
Bison declaration to avoid generating @code{#line} directives in the
parser file. @xref{Decl Summary}.
projects / bison.git / blobdiff