YACC(1)                          User Commands                         YACC(1)

       Yacc - an LALR(1) parser generator

       yacc  [ -BdgilLPrtvVy ] [ -b file_prefix ] [ -o output_file ] [ -p sym-
       bol_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  lan-
       guage.  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.

       -B   create a backtracking parser (compile-type configuration for  bty-

       -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  compi-
            lation of lex- and yacc-files.

       -l   If  the  -l option is not specified, yacc will insert #line direc-
            tives in the generated code.  The #line directives let the C  com-
            piler  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

       -L   enable position processing, e.g., "%locations" (compile-type  con-
            figuration 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.,  This overrides the -b option.

       -p symbol_prefix
            The  -p option changes the prefix prepended to yacc-generated sym-
            bols 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 gen-
            erates 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 liter-

       -t   The  -t  option  changes  the preprocessor directives generated by
            yacc so that debugging statements will be incorporated in the com-
            piled 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.  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 dis-
              carded 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-sym-
              bol 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.

              tells yacc to enable  management of position information associ-
              ated  with each token, provided by the lexer in the global vari-
              able 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 loca-
              tion 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  assign-
              ment 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

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

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

              Make the parser's names for  tokens  available  in  the  yytname
              array.   However,  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

       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.  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                   January 1, 2014                        YACC(1)

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