Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
YACC(1)				 User Commands			       YACC(1)

NAME
       Yacc - an LALR(1) parser	generator

SYNOPSIS
       yacc [ -BdgilLPrtvVy ] [	-b file_prefix ] [ -H defines_file ] [ -o out-
       put_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  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   causes  the	 header	 file y.tab.h to be written.  It contains #de-
	    fine's for the token identifiers.

       -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 op-
	    tion.

       -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-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 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.

       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.	 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:

	%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	begin-
		   ning	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.	  Mul-
	      tiple %code directives may be given; yacc	inserts	those into the
	      corresponding code- or defines-file in the order that  they  ap-
	      pear in the source file.

	%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 lo-
	      cation 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 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 command-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		       November	19, 2019		       YACC(1)

NAME | SYNOPSIS | DESCRIPTION | EXTENSIONS | PORTABILITY | DIAGNOSTICS

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=byacc&sektion=1&manpath=FreeBSD+13.0-RELEASE+and+Ports>

home | help