Updated: 2021/Apr/14


YACC(1)                     General Commands Manual                    YACC(1)

NAME
     yacc - an LALR(1) parser generator

SYNOPSIS
     yacc [-BdgilLPrtvVy] [-b file_prefix] [-o output_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
     language.  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
                 yacc).

     -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 compilation of lex-
                 and yacc-files.

     -l          If the -l option is not specified, yacc will insert #line
                 directives in the generated code.  The #line directives let
                 the C compiler 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
                 configuration for yacc).

     -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          The -P options instructs yacc to create a reentrant parser,
                 like `%pure-parser' does.

     -p symbol_prefix
                 The -p option changes the prefix prepended to yacc-generated
                 symbols to the string denoted by symbol_prefix.  The default
                 prefix is the string `yy'.

     -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
                 generates 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.

                 IEEE Std 1003.1 ("POSIX.1") documents only names and numbers
                 for `%token', though the original yacc and bison(1) also
                 accept string literals.

     -t          The -t option changes the preprocessor directives generated
                 by yacc so that debugging statements will be incorporated in
                 the compiled code.

     -V          The -V option prints the version number to the standard
                 output.

     -v          The -v option causes a human-readable description of the
                 generated parser to be written to the file y.output.

     -y          yacc ignores this option, which bison(1) supports for
                 ostensible POSIX compatibility.

EXTENSIONS
     yacc provides some extensions for compatibility with bison(1) 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 functionality.  The remaining features are always
     available:

     %destructor { code } symbol+
                 Defines code that is invoked when a symbol is automatically
                 discarded 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 it `$$' 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-
                 symbol and no per-type destructor:

                 The code for `<*>' is used for grammar symbols that have an
                 explicitly declared semantic type tag (via `%type');

                 The code for `<>' is used for grammar symbols that have no
                 declared semantic type tag.

     %expect number
                 Tell 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(1), allowable in LALR(1) parsers.

     %locations  Tell yacc to enable  management of position information
                 associated with each token, provided by the lexer in the
                 global variable yylloc, similar to management of semantic
                 value information provided in yylval.

                 As for semantic values, locations can be referenced within
                 actions using `@$' to refer to the location of the left hand
                 side symbol, and `@N' (N an integer) to refer to the location
                 of one of the right hand side symbols.  Also as for semantic
                 values, when a rule is matched, a default action is used the
                 compute the location represented by `@$' 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(1), 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 customized parser.

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

     %token-table
                 Make the parser's names for tokens available in the yytname
                 array.  However, yacc 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(1).  Here are a few differences:

     yacc accepts an equals mark preceding the left curly brace of an action
     (as in the original grammar file ftp.y):

               |   STAT CRLF
                   = {
                           statcmd();
                   }

     yacc and bison(1) emit code in different order, and in particular
     bison(1) makes forward reference to common functions such as yylex(),
     yyparse() and yyerror() without providing prototypes.

     bison(1) support for `%expect' is broken in more than one release.  For
     best results using bison(1), delete that directive.

     bison(1) has no equivalent for some of yacc's command-line options,
     relying on directives embedded in the grammar file.

     bison(1) -y option does not affect bison's lack of support for features
     of AT&T yacc which were deemed obsolescent.

     yacc accepts multiple parameters with `%lex-param' and `%parse-param' in
     two forms

           {type1 name1} {type2 name2} ...
           {type1 name1,  type2 name2 ...}

     bison(1) accepts the latter (though undocumented), but depending on the
     release may generate bad code.

     Like bison(1), yacc will add parameters specified via `%parse-param' to
     yyparse(), yyerror() and (if configured for back-tracking) to the
     destructor declared using `%destructor'.

     bison(1) puts the additional parameters first for yyparse() and yyerror()
     but last for destructors.  yacc matches this behavior.

ENVIRONMENT
     The following environment variable is referenced by yacc:

     TMPDIR  If the environment variable TMPDIR is set, the string denoted by
             TMPDIR will be used as the name of the directory where the
             temporary files are created.

TABLES
     The names of the tables generated by this version of yacc are yylhs,
     yylen, yydefred, yydgoto, yysindex, yyrindex, yygindex, yytable, and
     yycheck.  Two additional tables, yyname and yyrule, are created if
     YYDEBUG is defined and non-zero.

FILES
     y.code.c
     y.tab.c
     y.tab.h
     y.output
     /tmp/yacc.aXXXXXX
     /tmp/yacc.tXXXXXX
     /tmp/yacc.uXXXXXX

DIAGNOSTICS
     If there are rules that are never reduced, the number of such rules is
     written to the standard error.  If there are any LALR(1) conflicts, the
     number of conflicts is also written to the standard error.

STANDARDS
     The yacc utility conforms to IEEE Std 1003.2 ("POSIX.2").

NetBSD 9.99                     October 5, 2014                    NetBSD 9.99