Bison was written primarily by Robert Corbett; Richard Stallman made it
Yacc-compatible. Wilfred Hansen of Carnegie Mellon University added
-multicharacter string literals and other features.
+multi-character string literals and other features.
This edition corresponds to version @value{VERSION} of Bison.
The function @code{yylex} must now recognize variables, numeric values, and
the single-character arithmetic operators. Strings of alphanumeric
-characters with a leading nondigit are recognized as either variables or
+characters with a leading non-digit 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
A @dfn{literal string token} is written like a C string constant; for
example, @code{"<="} is a literal string token. A literal string token
doesn't need to be declared unless you need to specify its semantic
-value data type (@pxref{Value Type}), associativity, precedence
+value data type (@pxref{Value Type}), associativity, or precedence
(@pxref{Precedence}).
You can associate the literal string token with a symbolic name as an
@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 which semantic values are used. This is done for tokens with the
-@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names}) and for groupings
-with the @code{%type} Bison declaration (@pxref{Type Decl, ,Nonterminal Symbols}).
+Choose one of those types for each symbol (terminal or nonterminal) for
+which semantic values are used. This is done for tokens with the
+@code{%token} Bison declaration (@pxref{Token Decl, ,Token Type Names})
+and for groupings with the @code{%type} Bison declaration (@pxref{Type
+Decl, ,Nonterminal Symbols}).
@end itemize
@node Actions, Action Types, Multiple Types, Semantics
the parser, so that the function @code{yylex} (if it is in this file)
can use the name @var{name} to stand for this token type's code.
-Alternatively, you can use @code{%left}, @code{%right}, or @code{%nonassoc}
-instead of @code{%token}, if you wish to specify associativity and precedence.
-@xref{Precedence Decl, ,Operator Precedence}.
+Alternatively, you can use @code{%left}, @code{%right}, or
+@code{%nonassoc} instead of @code{%token}, if you wish to specify
+associativity and precedence. @xref{Precedence Decl, ,Operator
+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 @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 nonreentrant program may not be safe to call from a signal
-handler. In systems with multiple threads of control, a nonreentrant
+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
program must be called only within interlocks.
Normally, Bison generates a parser which is not reentrant. This is
Declare the expected number of shift-reduce conflicts
(@pxref{Expect Decl, ,Suppressing Conflict Warnings}).
+@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.
+
@item %locations
Generate the code processing the locations (@pxref{Action Features,
,Special Features for Use in Actions}). This mode is enabled as soon as
Request a pure (reentrant) parser program (@pxref{Pure Decl, ,A Pure
(Reentrant) Parser}).
+@item %no_parser
+Do not include any C code in the parser file; generate tables only. The
+parser file contains just @code{#define} directives and static variable
+declarations.
+
+This option also tells Bison to write the C code for the grammar actions
+into a file named @file{@var{filename}.act}, in the form of a
+brace-surrounded body fit for a @code{switch} statement.
+
@item %no_lines
Don't generate any @code{#line} preprocessor commands in the parser
file. Ordinarily Bison writes these commands in the parser file so that
associate errors with the parser file, treating it an independent source
file in its own right.
+@item %debug
+Output a definition of the macro @code{YYDEBUG} into the parser file, so
+that the debugging facilities are compiled. @xref{Debugging, ,Debugging
+Your Parser}.
+
+@item %defines
+Write an extra output file containing macro definitions for the token
+type names defined in the grammar and the semantic value type
+@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
+
+If the parser output file is named @file{@var{name}.c} then this file
+is named @file{@var{name}.h}.@refill
+
+This output file is essential if you wish to put the definition of
+@code{yylex} in a separate source file, because @code{yylex} needs to
+be able to refer to token type codes and the variable
+@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill
+
+@item %verbose
+Write an extra output file containing verbose descriptions of the
+parser states and what is done for each type of look-ahead token in
+that state.
+
+This file also describes all the conflicts, both those resolved by
+operator precedence and the unresolved ones.
+
+The file's name is made by removing @samp{.tab.c} or @samp{.c} from
+the parser output file name, and adding @samp{.output} instead.@refill
+
+Therefore, if the input file is @file{foo.y}, then the parser file is
+called @file{foo.tab.c} by default. As a consequence, the verbose
+output file is called @file{foo.output}.@refill
+
@item %raw
The output file @file{@var{name}.h} normally defines the tokens with
Yacc-compatible token numbers. If this option is specified, the
You call the function @code{yyparse} to cause parsing to occur. This
function reads tokens, executes actions, and ultimately returns when it
encounters end-of-input or an unrecoverable syntax error. You can also
-write an action which directs @code{yyparse} to return immediately without
-reading further.
+write an action which directs @code{yyparse} to return immediately
+without reading further.
The value returned by @code{yyparse} is 0 if parsing was successful (return
is due to end-of-input).
@subsection Semantic Values of Tokens
@vindex yylval
-In an ordinary (nonreentrant) parser, the semantic value of the token must
+In an ordinary (non-reentrant) 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
@noindent
Suppose the parser has seen the tokens @samp{1}, @samp{-} and @samp{2};
-should it reduce them via the rule for the subtraction operator? It depends
-on the next token. Of course, if the next token is @samp{)}, we must
-reduce; shifting is invalid because no single rule can reduce the token
-sequence @w{@samp{- 2 )}} or anything starting with that. But if the next
-token is @samp{*} or @samp{<}, we have a choice: either shifting or
-reduction would allow the parse to complete, but with different
-results.
-
-To decide which one Bison should do, we must consider the
-results. If the next operator token @var{op} is shifted, then it
-must be reduced first in order to permit another opportunity to
-reduce the difference. The result is (in effect) @w{@samp{1 - (2
-@var{op} 3)}}. On the other hand, if the subtraction is reduced
-before shifting @var{op}, the result is @w{@samp{(1 - 2) @var{op}
-3}}. Clearly, then, the choice of shift or reduce should depend
-on the relative precedence of the operators @samp{-} and
-@var{op}: @samp{*} should be shifted first, but not @samp{<}.
+should it reduce them via the rule for the subtraction operator? It
+depends on the next token. Of course, if the next token is @samp{)}, we
+must reduce; shifting is invalid because no single rule can reduce the
+token sequence @w{@samp{- 2 )}} or anything starting with that. But if
+the next token is @samp{*} or @samp{<}, we have a choice: either
+shifting or reduction would allow the parse to complete, but with
+different results.
+
+To decide which one Bison should do, we must consider the results. If
+the next operator token @var{op} is shifted, then it must be reduced
+first in order to permit another opportunity to reduce the difference.
+The result is (in effect) @w{@samp{1 - (2 @var{op} 3)}}. On the other
+hand, if the subtraction is reduced before shifting @var{op}, the result
+is @w{@samp{(1 - 2) @var{op} 3}}. Clearly, then, the choice of shift or
+reduce should depend on the relative precedence of the operators
+@samp{-} and @var{op}: @samp{*} should be shifted first, but not
+@samp{<}.
@cindex associativity
What about input such as @w{@samp{1 - 2 - 5}}; should this be
-@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}? For
-most operators we prefer the former, which is called @dfn{left
-association}. 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 makes right-associativity.
+@w{@samp{(1 - 2) - 5}} or should it be @w{@samp{1 - (2 - 5)}}? For most
+operators we prefer the former, which is called @dfn{left association}.
+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
+makes right-associativity.
@node Using Precedence, Precedence Examples, Why Precedence, Precedence
@subsection Specifying Operator Precedence
construct itself by a complicated syntactic structure---the ``declarator''.
As a result, part of the Bison parser for C needs to be duplicated, with
-all the nonterminal names changed: once for parsing a declaration in which
-a typedef name can be redefined, and once for parsing a declaration in
-which that can't be done. Here is a part of the duplication, with actions
-omitted for brevity:
+all the nonterminal names changed: once for parsing a declaration in
+which a typedef name can be redefined, and once for parsing a
+declaration in which that can't be done. Here is a part of the
+duplication, with actions omitted for brevity:
@example
initdcl:
@table @option
@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}.
+Output a definition of the macro @code{YYDEBUG} into the parser file, so
+that the debugging facilities are compiled. @xref{Debugging, ,Debugging
+Your Parser}.
@item --locations
Pretend that @code{%locactions} was specified. @xref{Decl Summary}.
@item -n
@itemx --no-parser
-Do not include any C code in the parser file; generate tables only. The
-parser file contains just @code{#define} directives and static variable
-declarations.
-
-This option also tells Bison to write the C code for the grammar actions
-into a file named @file{@var{filename}.act}, in the form of a
-brace-surrounded body fit for a @code{switch} statement.
+Pretend that @code{%no_parser} was specified. @xref{Decl Summary}.
@item -r
@itemx --raw
@table @option
@item -d
@itemx --defines
-Write an extra output file containing macro definitions for the token
-type names defined in the grammar and the semantic value type
-@code{YYSTYPE}, as well as a few @code{extern} variable declarations.
-
-If the parser output file is named @file{@var{name}.c} then this file
-is named @file{@var{name}.h}.@refill
-
-This output file is essential if you wish to put the definition of
-@code{yylex} in a separate source file, because @code{yylex} needs to
-be able to refer to token type codes and the variable
-@code{yylval}. @xref{Token Values, ,Semantic Values of Tokens}.@refill
+Pretend that @code{%verbose} was specified, i.e., write an extra output
+file containing macro definitions for the token type names defined in
+the grammar and the semantic value type @code{YYSTYPE}, as well as a few
+@code{extern} variable declarations. @xref{Decl Summary}.
@item -b @var{file-prefix}
@itemx --file-prefix=@var{prefix}
@item -v
@itemx --verbose
-Write an extra output file containing verbose descriptions of the
-parser states and what is done for each type of look-ahead token in
-that state.
-
-This file also describes all the conflicts, both those resolved by
-operator precedence and the unresolved ones.
-
-The file's name is made by removing @samp{.tab.c} or @samp{.c} from
-the parser output file name, and adding @samp{.output} instead.@refill
-
-Therefore, if the input file is @file{foo.y}, then the parser file is
-called @file{foo.tab.c} by default. As a consequence, the verbose
-output file is called @file{foo.output}.@refill
+Pretend that @code{%verbose} was specified, i.e, write an extra output
+file containing verbose descriptions of the grammar and
+parser. @xref{Decl Summary}, for more.
@item -o @var{outfile}
@itemx --output-file=@var{outfile}
The parser function produced by Bison; call this function to start
parsing. @xref{Parser Function, ,The Parser Function @code{yyparse}}.
+@item %debug
+Equip the parser for debugging. @xref{Decl Summary}.
+
+@item %defines
+Bison declaration to create a header file meant for the scanner.
+@xref{Decl Summary}.
+
@item %left
Bison declaration to assign left associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
parser file. @xref{Decl Summary}.
@item %nonassoc
-Bison declaration to assign nonassociativity to token(s).
+Bison declaration to assign non-associativity to token(s).
@xref{Precedence Decl, ,Operator Precedence}.
@item %prec
tokens are parsed. @xref{Lexical Tie-ins}.
@item Literal string token
-A token which consists of two or more fixed characters.
-@xref{Symbols}.
+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
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