YACC(1)                 NetBSD General Commands Manual                 YACC(1)

     yacc -- an LALR(1) parser generator

     yacc [-BdgilLPrtvVy] [-b file_prefix] [-o output_file] [-p symbol_prefix]

     yacc reads the grammar specification in the file filename and generates
     an LALR(1) parser for it.  The parsers consist of a set of LALR(1) pars-
     ing tables and a driver routine written in the C programming language.
     yacc normally writes the parse tables and the driver routine to the file

     The following options are available:

     -b file_prefix    The -b option changes the prefix prepended to the out-
                       put file names to the string denoted by file_prefix.
                       The default prefix is the character y.

     -B                Create a backtracking parser (compile-type configura-
                       tion for yacc).

     -d                The -d option causes the header file y.tab.h to be
                       written.  It contains #define's for the token identi-

     -g                The -g option causes a graphical description of the
                       generated LALR(1) parser to be written to the file
                       y.dot in graphviz format, ready to be processed by

     -i                The -i option causes a supplementary header file
                       y.tab.i to be written.  It contains extern declarations
                       and supplementary #define's as needed to map the con-
                       ventional yacc yy-prefixed names to whatever the -p
                       option may specify.  The code file, e.g., y.tab.c is
                       modified to #include this file as well as the y.tab.h
                       file, enforcing consistent usage of the symbols defined
                       in those files.  The supplementary header file makes it
                       simpler to separate compilation of lex- and yacc-files.

     -l                If the -l option is not specified, yacc will insert
                       #line directives in the generated code.  The #line
                       directives let the C compiler relate errors in the gen-
                       erated code to the user's original code.  If the -l
                       option is specified, yacc will not insert the #line
                       directives.  #line directives specified by the user
                       will be retained.

     -L                Enable position processing, e.g., ``%locations'' (com-
                       pile-type configuration for yacc).

     -o output_file    Specify the filename for the parser file.  If this
                       option is not given, the output filename is the file
                       prefix concatenated with the file suffix, e.g.
                       y.tab.c.  This overrides the -b option.

     -P                The -P options instructs yacc to create a reentrant
                       parser, like ``%pure-parser'' does.

     -p symbol_prefix  The -p option changes the prefix prepended to yacc-gen-
                       erated symbols to the string denoted by symbol_prefix.
                       The default prefix is the string yy.

     -r                The -r option causes yacc to produce separate files for
                       code and tables.  The code file is named y.code.c, and
                       the tables file is named y.tab.c.  The prefix ``y''.
                       can be overridden using the -b option.

     -s                Suppress ``#define'' statements generated for string
                       literals in a ``%token'' statement, to more closely
                       match original yacc behavior.

                       Normally when yacc sees a line such as ``%token OP_ADD
                       ADD'' it notices that the quoted ``ADD'' is a valid C
                       identifier, and generates a #define not only for
                       OP_ADD, but for ADD as well, e.g.,

                             #define OP_ADD 257
                             #define ADD 258
                       The original yacc does not generate the second
                       ``#define''.  The -s option suppresses this

                       IEEE Std 1003.1 (``POSIX.1'') documents only names and
                       numbers for ``%token'', though the original yacc and
                       bison(1) also accept string literals.

     -t                The -t option changes the preprocessor directives gen-
                       erated by yacc so that debugging statements will be
                       incorporated in the compiled code.

     -V                The -V option prints the version number to the standard

     -v                The -v option causes a human-readable description of
                       the generated parser to be written to the file

     -y                yacc ignores this option, which bison(1) supports for
                       ostensible POSIX compatibility.

     yacc provides some extensions for compatibility with bison(1) and other
     implementations of yacc.  The ``%destructor'' and ``%locations'' features
     are available only if yacc has been configured and compiled to support
     the back-tracking functionality.  The remaining features are always

     %destructor { code } symbol+

     Defines code that is invoked when a symbol is automatically discarded
     during error recovery.  This code can be used to reclaim dynamically
     allocated memory associated with the corresponding semantic value for
     cases where user actions cannot manage the memory explicitly.

     On encountering a parse error, the generated parser discards symbols on
     the stack and input tokens until it reaches a state that will allow pars-
     ing to continue.  This error recovery approach results in a memory leak
     if the ``YYSTYPE'' value is, or contains, pointers to dynamically allo-
     cated memory.

     The bracketed code is invoked whenever the parser discards one of the
     symbols.  Within code, ``$$'' or ``$<tag>$'' designates the semantic
     value associated with the discarded symbol, and ``@$'' designates its
     location (see ``%locations'' directive).

     A per-symbol destructor is defined by listing a grammar symbol in
     symbol+.  A per-type destructor is defined  by listing a semantic type
     tag (e.g., ``<some_tag>'') in symbol+; in this case, the parser will
     invoke code whenever it discards any grammar symbol that has that seman-
     tic type tag, unless that symbol has its own per-symbol destructor.

     Two categories of default destructor are supported that are invoked when
     discarding any grammar symbol that has no per-symbol and no per-type

     The code for ``<*>'' is used for grammar symbols that have an explicitly
     declared semantic type tag (via ``%type'');

     the code for ``<>'' is used for grammar symbols that have no declared
     semantic type tag.

     %expect number     Tell yacc the expected number of shift/reduce con-
                        flicts.  That makes it only report the number if it
     %expect-rr number  Tell yacc the expected number of reduce/reduce con-
                        flicts.  That makes it only report the number if it
                        differs.  This is (unlike bison(1)) allowable in
                        LALR(1) parsers.
     %locations         Tell yacc to enable  management of position informa-
                        tion associated with each token, provided by the lexer
                        in the global variable yylloc, similar to management
                        of semantic value information provided in yylval.

                        As for semantic values, locations can be referenced
                        within actions using @$ to refer to the location of
                        the left hand side symbol, and @N (N an integer) to
                        refer to the location of one of the right hand side
                        symbols.  Also as for semantic values, when a rule is
                        matched, a default action is used the compute the
                        location represented by @$ as the beginning of the
                        first symbol and the end of the last symbol in the
                        right hand side of the rule.  This default computation
                        can be overridden by explicit assignment to @$ in a
                        rule action.

                        The type of yylloc is YYLTYPE, which is defined by
                        default as:

                              typedef struct YYLTYPE {
                                  int first_line;
                                  int first_column;
                                  int last_line;
                                  int last_column;
                              } YYLTYPE;

                        YYLTYPE can be redefined by the user
                        (YYLTYPE_IS_DEFINED must be defined, to inhibit the
                        default) in the declarations section of the specifica-
                        tion file.  As in bison(1), the macro YYLLOC_DEFAULT
                        is invoked each time a rule is matched to calculate a
                        position for the left hand side of the rule, before
                        the associated action is executed; this macro can be
                        redefined by the user.

                        This directive adds a YYLTYPE parameter to yyerror().
                        If the ``%pure-parser'' directive is present, a
                        YYLTYPE parameter is added to yylex() calls.
     %lex-param { argument-declaration }
                        By default, the lexer accepts no parameters, e.g.,
                        yylex().  Use this directive to add parameter declara-
                        tions for your customized lexer.
     %parse-param { argument-declaration }
                        By default, the parser accepts no parameters, e.g.,
                        yyparse().  Use this directive to add parameter decla-
                        rations for your customized parser.
     %pure-parser       Most variables (other than yydebug and yynerrs) are
                        allocated on the stack within yyparse(), making the
                        parser reasonably reentrant.
     %token-table       Make the parser's names for tokens available in the
                        yytname array.  However, yacc yacc does not predefine
                        ``$end'', ``$error'' or ``$undefined'' in this array.

     According to Robert Corbett:

     Berkeley Yacc is an LALR(1) parser generator.  Berkeley Yacc has been
     made as compatible as possible with AT&T Yacc.  Berkeley Yacc can accept
     any input specification that conforms to the AT&T Yacc documentation.
     Specifications that take advantage of undocumented features of AT&T Yacc
     will probably be rejected.

     The rationale in documents some features of AT&T yacc which are no longer
     required for POSIX compliance.

     That said, you may be interested in reusing grammar files with some other
     implementation which is not strictly compatible with AT&T yacc.  For
     instance, there is bison(1).  Here are a few differences: yacc accepts an
     equals mark preceding the left curly brace of an action (as in the origi-
     nal grammar file ftp.y):

           |       STAT CRLF
                   = {
     yacc and bison(1) emit code in different order, and in particular
     bison(1) makes forward reference to common functions such as yylex(),
     yyparse() and yyerror() without providing prototypes.

     bison(1) support for ``%expect'' is broken in more than one release.  For
     best results using bison(1), delete that directive.

     bison(1) no equivalent for some of 's command-line options, relying on
     directives embedded in the grammar file.

     bison(1) -y option does not affect bison's lack of support for features
     of AT&T yacc which were deemed obsolescent.

     yacc accepts multiple parameters with ``%lex-param'' and ``%parse-param''
     in two forms

           {type1 name1} {type2 name2} ...
           {type1 name1,  type2 name2 ...}

     bison(1) accepts the latter (though undocumented), but depending on the
     release may generate bad code.

     Like bison(1), yacc will add parameters specified via ``%parse-param'' to
     yyparse(), yyerror() and (if configured for back-tracking) +to the
     destructor declared using ``%destructor''.

     bison(1) puts the additional parameters first for yyparse() and yyerror()
     but last for destructors.  yacc matches this behavior.

     The following environment variable is referenced by yacc:

     TMPDIR  If the environment variable TMPDIR is set, the string denoted by
             TMPDIR will be used as the name of the directory where the tempo-
             rary files are created.

     The names of the tables generated by this version of yacc are ``yylhs'',
     ``yylen'', ``yydefred'', ``yydgoto'', ``yysindex'', ``yyrindex'',
     ``yygindex'', ``yytable'', and ``yycheck''.  Two additional tables,
     ``yyname'' and ``yyrule'', are created if YYDEBUG is defined and non-


     If there are rules that are never reduced, the number of such rules is
     written to the standard error.  If there are any LALR(1) conflicts, the
     number of conflicts is also written to the standard error.

     The yacc utility conforms to IEEE Std 1003.2 (``POSIX.2'').

NetBSD 7.0                      October 5, 2014                     NetBSD 7.0

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