When the user inputs a specification to Yacc, the output is a file of C programs, called y.tab.c on most systems (due to local file system conventions, the names may differ from installation to installation). The function produced by Yacc is called yyparse \|; it is an integer valued function. When it is called, it in turn repeatedly calls yylex , the lexical analyzer supplied by the user (see Section 3) to obtain input tokens. Eventually, either an error is detected, in which case (if no error recovery is possible) yyparse returns the value 1, or the lexical analyzer returns the endmarker token and the parser accepts. In this case, yyparse returns the value 0.

The user must provide a certain amount of environment for this parser in order to obtain a working program. For example, as with every C program, a program called main must be defined, that eventually calls yyparse . In addition, a routine called yyerror prints a message when a syntax error is detected.

These two routines must be supplied in one form or another by the user. To ease the initial effort of using Yacc, a library has been provided with default versions of main and yyerror . The name of this library is system dependent; on many systems the library is accessed by a -ly argument to the loader. To show the triviality of these default programs, the source is given below: main(){ return( yyparse() ); } and # include <stdio.h> yyerror(s) char *s; { fprintf( stderr, "%s\en", s ); } The argument to yyerror is a string containing an error message, usually the string ``syntax error''. The average application will want to do better than this. Ordinarily, the program should keep track of the input line number, and print it along with the message when a syntax error is detected. The external integer variable yychar contains the lookahead token number at the time the error was detected; this may be of some interest in giving better diagnostics. Since the main program is probably supplied by the user (to read arguments, etc.) the Yacc library is useful only in small projects, or in the earliest stages of larger ones.

The external integer variable yydebug is normally set to 0. If it is set to a nonzero value, the parser will output a verbose description of its actions, including a discussion of which input symbols have been read, and what the parser actions are. Depending on the operating environment, it may be possible to set this variable by using a debugging system.