YACC(1) Berkeley Yacc YACC(1)
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NAME
Yacc - an LALR(1) parser generator
SYNOPSIS
yacc [ -dgilrtv ] [ -b file_prefix ] [ -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.
-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.
-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 -p option.
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-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.
-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
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".
POSIX (IEEE 1003.1 2004) documents only names and numbers for
"%token", though original yacc and bison 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 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:
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%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) allowable in LALR parsers.
%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.
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 grammary files with some
other implementation which is not strictly compatible with AT&T yacc.
For instance, there is bison. Here are a few differences:
+ Yacc accepts an equals mark preceding the left curly brace of an
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action (as in the original grammar file ftp.y):
| STAT CRLF
= {
statcmd();
}
+ 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.
+ Bison's support for "%expect" is broken in more than one release.
For best results using bison, delete that directive.
+ Bison has no equivalent for some of yacc's commmand-line options,
relying on directives embedded in the grammar file.
+ Bison's "-y" option does not affect bison's lack of support for
features of AT&T yacc which were deemed obsolescent.
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.
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