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 YACC(1)                        Berkeley Yacc                        YACC(1)
 User Commands                                                 User Commands

                               March 28, 2021



 NAME
      Yacc - an LALR(1) parser generator

 SYNOPSIS
      yacc [ -BdghilLPrtvVy ] [ -b file_prefix ] [ -H defines_file ] [ -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-time configuration for
           btyacc).

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

      -h   print a usage message.

      -H defines_file
           causes #define's for the token identifiers to be written to the
           given defines_file rather than the y.tab.h file used by the -d
           option.

      -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.




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 YACC(1)                        Berkeley Yacc                        YACC(1)
 User Commands                                                 User Commands

                               March 28, 2021



      -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-time
           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.




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 YACC(1)                        Berkeley Yacc                        YACC(1)
 User Commands                                                 User Commands

                               March 28, 2021



      -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.

      The filename parameter is not optional.  However, yacc accepts a
      single ``-'' to read the grammar from the standard input.  A double
      ``--'' marker denotes the end of options.  A single filename parameter
      is expected after a ``--'' marker.

 EXTENSIONS
      Yacc provides some extensions for compatibility with bison and other
      implementations of yacc.  It accepts several long options which have
      equivalents in 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:

       %code keyword { code }
           Adds the indicated source code at a given point in the output
           file.  The optional keyword tells yacc where to insert the code:

             top  just after the version-definition in the generated code-
                  file.

             requires
                  just after the declaration of public parser variables.  If
                  the -d option is given, the code is inserted at the
                  beginning of the defines-file.

             provides
                  just after the declaration of private parser variables.
                  If the -d option is given, the code is inserted at the end
                  of the defines-file.

           If no keyword is given, the code is inserted at the beginning of
           the section of code copied verbatim from the source file.
           Multiple %code directives may be given; yacc inserts those into
           the corresponding code- or defines-file in the order that they
           appear in the source file.

       %destructor { code } symbol+



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 YACC(1)                        Berkeley Yacc                        YACC(1)
 User Commands                                                 User Commands

                               March 28, 2021



           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:

           +   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
           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.




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 YACC(1)                        Berkeley Yacc                        YACC(1)
 User Commands                                                 User Commands

                               March 28, 2021



           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.




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 YACC(1)                        Berkeley Yacc                        YACC(1)
 User Commands                                                 User Commands

                               March 28, 2021



 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:

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

                 |   STAT CRLF
                     = {
                             statcmd();
                     }

      +   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.

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

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

      +   Bison's ``-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 accepts the latter (though undocumented), but depending on



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 YACC(1)                        Berkeley Yacc                        YACC(1)
 User Commands                                                 User Commands

                               March 28, 2021



          the release may generate bad code.

      +   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.








































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