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YACC(1plan9) YACC(1plan9)


yacc - yet another compiler-compiler


yacc [ option ... ] grammar


Yacc converts a context-free grammar and translation code into a set of tables for an LR(1) parser and translator. The grammar may be ambiguous; specified precedence rules are used to break ambiguities.

The output file,, must be compiled by the C compiler to produce a program This program must be loaded with a lexical analyzer function, yylex(void) (often generated by lex(1)), with a main(int argc, char *argv[]) program, and with an error handling routine, yyerror(char*).

The options are

Direct output to the specified file instead of
Create file y.debug, containing diagnostic messages. To incorporate them in the parser, compile it with preprocessor symbol yydebug defined. The amount of diagnostic output from the parser is regulated by value n. The value 0 reports errors; 1 reports reductions; higher values (up to 4) include more information about state transitions.
Create file y.output, containing a description of the parsing tables and of conflicts arising from ambiguities in the grammar.
Create file, containing #define statements that associate yacc-assigned `token codes' with user-declared `token names'. Include it in source files other than to give access to the token codes.
Change the prefix of the file names,, y.debug, and y.output to stem.
Write a parser that uses Stdio instead of the print routines in libc.
Disable #line directives in the generated parser.
Generate a parser that takes an argument of type Yyarg and passes this argument to each invocation of the lexer function, yylex. Yyarg contains per-instance state and a single user-visible member, arg, of type void*.

The specification of yacc itself is essentially the same as the UNIX version described in the references mentioned below. Besides the -D option, the main relevant differences are:

The interface to the C environment is by default through <libc.h> rather than <stdio.h>; the -S option reverses this.
The parser accepts UTF input text (see utf(7)), which has a couple of effects. First, the return value of yylex() no longer fits in a short; second, the starting value for non-terminals is now 0xE000 rather than 257.
The generated parser can be recursive: actions can call yyparse, for example to implement a sort of #include statement in an interpreter.
Finally, some undocumented inner workings of the parser have been changed, which may affect programs that know too much about its structure.


temporary file
temporary file
parser prototype
parser prototype using stdio




S. C. Johnson and R. Sethi, ``Yacc: A parser generator'', Unix Research System Programmer's Manual, Tenth Edition, Volume 2
B. W. Kernighan and Rob Pike, The UNIX Programming Environment, Prentice Hall, 1984


The parser may not have full information when it writes to y.debug so that the names of the tokens returned by may be missing.