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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 sym-
       bol_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  lan-
       guage.  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  bty-
            acc).

       -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  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
            retained.

       -L   enable position processing, e.g., "%locations" (compile-time  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., y.tab.c.  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  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 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-
            als.

       -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.


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 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.

        %locations
              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
              lexer.

        %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.

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

        %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                    June 9, 2018                          YACC(1)

byacc 20180609 - Generated Tue Jun 12 05:57:39 CDT 2018
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