yacc (1) - Linux Manuals

yacc: an LALR(1) parser generator


Yacc - an LALR(1) parser generator


yacc [ -dgilrtv ] [ -b file_prefix ] [ -p symbol_prefix ] filename


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.
The -d option causes the header file y.tab.h to be written. It contains #define's for the token identifiers.
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).
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.
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.
-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.
create a reentrant parser, e.g., "%pure-parser".
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.
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.
The -t option changes the preprocessor directives generated by yacc so that debugging statements will be incorporated in the compiled code.
The -v option causes a human-readable description of the generated parser to be written to the file y.output.
print the version number to the standard output.
yacc ignores this option, which bison supports for ostensible POSIX compatibility.


yacc provides some extensions for compatibility with bison and other implementations of yacc:
%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.
Most variables (other than yydebug and yynerrs) are allocated on the stack within yyparse, making the parser reasonably reentrant.


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

The rationale in


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 action (as in the original grammar file ftp.y):
        |       STAT CRLF
                = {
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.


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.