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.SH
8: The Yacc Environment
.PP
When the user inputs a specification
to Yacc, the output is a file of C programs, called
.I 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
.I yyparse \|;
it is an integer valued function.
When it is called, it in turn repeatedly calls
.I 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)
.I yyparse
returns the value 1,
or the lexical analyzer returns the endmarker token
and the parser accepts.
In this case,
.I yyparse
returns the value 0.
.PP
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
.I main
must be defined, that eventually calls
.I yyparse .
In addition, a routine called
.I yyerror
prints a message
when a syntax error is detected.
.PP
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
.I main
and
.I yyerror .
The name of this library is system dependent;
on many systems the library is accessed by a
.B \-ly
argument to the loader.
To show the triviality of these default programs, the source is
given below:
.DS
main(){
	return( yyparse() );
	}
.DE
and
.DS
# include <stdio.h>

yyerror(s) char *s; {
	fprintf( stderr, "%s\en", s );
	}
.DE
The argument to
.I 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
.I 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
.I 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.
.PP
The external integer variable
.I 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.
