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YACC(1)                          User Commands                         YACC(1)

NAME
       Yacc - an LALR(1) parser generator

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

DESCRIPTION
       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 y.tab.c.

       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.

       -B   create a backtracking parser (compile-type configuration for
            btyacc).

       -d   The -d option causes the header file y.tab.h 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 y.dot in graphviz format,
            ready to be processed by dot(1).

       -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 conventional 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 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.

       -L   enable position processing, e.g., "%locations" (compile-type
            configuration for btyacc).

       -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 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 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 "#define".

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

       -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.

EXTENSIONS
       yacc provides some extensions for compatibility with bison 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 (btyacc) functionality.  The remaining features are
       always available:

        %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 parsing to continue.  This error recovery
              approach results in a memory leak if the YYSTYPE value is, or
              contains, pointers to dynamically allocated 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 semantic 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 destructor:

              o   the code for "<*>" is used for grammar symbols that have an
                  explicitly declared semantic type tag (via "%type");

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

        %expect number
              tells 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.

        %locations
              tells yacc to enable  management of position information
              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 specification file.  As in bison, 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 declarations for your customized
              lexer.

        %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.

        %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 does not predefine "$end", "$error" or
              "$undefined" in this array.

PORTABILITY
       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

           http://pubs.opengroup.org/onlinepubs/9699919799/utilities/yacc.html

       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.  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
                         = {
                              statcmd();
                         }

       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.

       o   Yacc accepts multiple parameters with %lex-param and %parse-param
           in two forms

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

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

       o   Like bison, yacc will add parameters specified via %parse-param to
           yyparse, yyerror and (if configured for back-tracking) to the
           destructor declared using %destructor.  Bison puts the additional
           parameters first for yyparse and yyerror but last for destructors.
           Yacc matches this behavior.

DIAGNOSTICS
       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                   October 5, 2014                        YACC(1)

NAME | SYNOPSIS | DESCRIPTION | EXTENSIONS | PORTABILITY | DIAGNOSTICS

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