Skip site navigation (1)Skip section navigation (2)

FreeBSD Man Pages

Man Page or Keyword Search:
Man Architecture
Apropos Keyword Search (all sections) Output format
home | help
YACC(1)                          User Commands                         YACC(1)

       Yacc - an LALR(1) parser generator

       yacc [ -dgilrtv ] [ -b file_prefix ] [ -p symbol_prefix ] filename

       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)
       parsing 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 output file
            names to the string denoted by file_prefix.  The default prefix is
            the character y.

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

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

       -i   The -i option causes a supplementary header file to be
            written.  It contains extern declarations and supplementary
            #define's as needed to map the conventional yacc yy-prefixed names
            to whatever the -p option may specify.  The code file, e.g.,
   is modified to #include this file as well as the
            file, enforcing consistent usage of the symbols defined in those

            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 generated 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.

       -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.,  This overrides the -p option.

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

       -P   create a reentrant parser, e.g., "%pure-parser".

       -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  The prefix "y." can be overridden using the -b

       -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,

                #define OP_ADD 257
                #define ADD 258

            The original yacc does not generate the second "#define".  The -s
            option suppresses this "#define".

            POSIX (IEEE 1003.1 2004) documents only names and numbers for
            "%token", though original yacc and bison also accept string

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

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

       -V   print the version number to the standard output.

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

       yacc provides some extensions for compatibility with bison and other
       implementations of yacc:

        %expect number
              tell yacc the expected number of shift/reduce conflicts.  That
              makes it only report the number if it differs.

        %expect-rr number
              tell yacc the expected number of reduce/reduce conflicts.  That
              makes it only report the number if it differs.  This is (unlike
              bison) allowable in LALR parsers.

        %lex-param { argument-declaration }
              By default, the lexer accepts no parameters, e.g., yylex().  Use
              this directive to add parameter declarations for your customized

        %parse-param { argument-declaration }
              By default, the parser accepts no parameters, e.g., yyparse().
              Use this directive to add parameter declarations for your
              customized parser.

              Most variables (other than yydebug and yynerrs) are allocated on
              the stack within yyparse, making the parser reasonably

       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

       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 grammary files with some
       other implementation which is not strictly compatible with AT&T yacc.
       For instance, there is bison.  Here are a few differences:

       o   Yacc accepts an equals mark preceding the left curly brace of an
           action (as in the original grammar file ftp.y):

                    |    STAT CRLF
                         = {

       o   Yacc and bison emit code in different order, and in particular
           bison makes forward reference to common functions such as yylex,
           yyparse and yyerror without providing prototypes.

       o   Bison's support for "%expect" is broken in more than one release.
           For best results using bison, delete that directive.

       o   Bison has no equivalent for some of yacc's commmand-line options,
           relying on directives embedded in the grammar file.

       o   Bison's "-y" option does not affect bison's lack of support for
           features of AT&T yacc which were deemed obsolescent.

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

Berkeley Yacc                  September 7, 2011                       YACC(1)


Want to link to this manual page? Use this URL:

home | help