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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 #de-
	    fine'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, en-
	    forcing 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 op-
	    tion.

       -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 al-
       ways 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  ap-
	      proach results in	a memory leak if the YYSTYPE value is, or con-
	      tains, 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  se-
	      mantic  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  in-
	      voked  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 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  ac-
	      tions  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 ar-
	      ray.  However, yacc does not predefine "$end", "$error" or "$un-
	      defined" 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 bi-
	   son makes forward reference to common functions such	as yylex,  yy-
	   parse 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 de-
	   structor declared using %destructor.	 Bison puts the	additional pa-
	   rameters  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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