NAME

yacc − yet another compiler-compiler

SYNOPSIS

yacc [ −vdlt ] grammar

DESCRIPTION

Yacc converts a context-free grammar into a set of tables for a simple automaton which executes an LR(1) parsing algorithm.  The grammar may be ambiguous; specified precedence rules are used to break ambiguities. 

The output file, y.tab.c, must be compiled by the C compiler to produce a program yyparse. This program must be loaded with the lexical analyzer program, yylex, as well as main and yyerror, an error handling routine. These routines must be supplied by the user; Lex(1) is useful for creating lexical analyzers usable by yacc.

If the −v flag is given, the file y.output is prepared, which contains a description of the parsing tables and a report on conflicts generated by ambiguities in the grammar. 

If the −d flag is used, the file y.tab.h is generated with the define statements that associate the yacc -assigned ‘token codes’ with the user-declared ‘token names’.  This allows source files other than y.tab.c to access the token codes. 

If the −l flag is specified, the code produced in y.tab.c will not contain any ‘#line’ directives.  This should only be used once the grammar and actions are fully debugged. 

Run-time debugging code is always generated in y.tab.c under conditional compilation control.  The pre-processor symbol YYDEBUG is used to control the inclusion of this code.  If YYDEBUG is defined and non-zero then the code is compiled, otherwise it is not.  Specifying the −t flag reverses the default, so that the code always compiled unless YYDEBUG is explicitly defined as zero.  In any case, the program must also enable debugging output by setting the integer variable yydebug to a non-zero number.  This variable is declared automatically if the debugging code is compiled, and its initial value is zero. 

FILES

y.output
y.tab.c
y.tab.hdefines for token names
yacc.tmp, yacc.actstemporary files
/usr/lib/yaccparparser prototype for C programs

SEE ALSO

lex(1)
LR Parsing by A. V. Aho and S. C. Johnson, Computing Surveys, June, 1974. 
YACC − Yet Another Compiler Compiler by S. C. Johnson. 

DIAGNOSTICS

The number of reduce-reduce and shift-reduce conflicts is reported on the standard output; a more detailed report is found in the y.output file.  Similarly, if some rules are not reachable from the start symbol, this is also reported. 

BUGS

Because file names are fixed, at most one yacc process can be active in a given directory at a time. 

7th Edition  —  Revision 1.3 of 10/10/90