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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-type 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-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., 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			October	5, 2014			       YACC(1)

NAME | SYNOPSIS | DESCRIPTION | EXTENSIONS | PORTABILITY | DIAGNOSTICS

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