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1.1     ! root        1: .\"    @(#)ss8 6.1 (Berkeley) 5/8/86
        !             2: .\"
        !             3: .SH
        !             4: 8: The Yacc Environment
        !             5: .PP
        !             6: When the user inputs a specification
        !             7: to Yacc, the output is a file of C programs, called
        !             8: .I y.tab.c
        !             9: on most
        !            10: systems
        !            11: (due to local file system conventions, the names may differ from
        !            12: installation to installation).
        !            13: The function produced by Yacc is called
        !            14: .I yyparse \|;
        !            15: it is an integer valued function.
        !            16: When it is called, it in turn repeatedly calls
        !            17: .I yylex ,
        !            18: the lexical analyzer
        !            19: supplied by the user (see Section 3)
        !            20: to obtain input tokens.
        !            21: Eventually, either an error is detected, in which case
        !            22: (if no error recovery is possible)
        !            23: .I yyparse
        !            24: returns the value 1,
        !            25: or the lexical analyzer returns the endmarker token
        !            26: and the parser accepts.
        !            27: In this case,
        !            28: .I yyparse
        !            29: returns the value 0.
        !            30: .PP
        !            31: The user must provide a certain amount of environment for this
        !            32: parser in order to obtain a working program.
        !            33: For example, as with every C program, a program called
        !            34: .I main
        !            35: must be defined, that eventually calls
        !            36: .I yyparse .
        !            37: In addition, a routine called
        !            38: .I yyerror
        !            39: prints a message
        !            40: when a syntax error is detected.
        !            41: .PP
        !            42: These two routines must be supplied in one form or another by the
        !            43: user.
        !            44: To ease the initial effort of using Yacc, a library has been
        !            45: provided with default versions of
        !            46: .I main
        !            47: and
        !            48: .I yyerror .
        !            49: The name of this library is system dependent;
        !            50: on many systems the library is accessed by a
        !            51: .B \-ly
        !            52: argument to the loader.
        !            53: To show the triviality of these default programs, the source is
        !            54: given below:
        !            55: .DS
        !            56: main(){
        !            57:        return( yyparse() );
        !            58:        }
        !            59: .DE
        !            60: and
        !            61: .DS
        !            62: # include <stdio.h>
        !            63: 
        !            64: yyerror(s) char *s; {
        !            65:        fprintf( stderr, "%s\en", s );
        !            66:        }
        !            67: .DE
        !            68: The argument to
        !            69: .I yyerror
        !            70: is a string containing an error message, usually
        !            71: the string ``syntax error''.
        !            72: The average application will want to do better than this.
        !            73: Ordinarily, the program should keep track of the input line number, and print it
        !            74: along with the message when a syntax error is detected.
        !            75: The external integer variable
        !            76: .I yychar
        !            77: contains the lookahead token number at the time the error was detected;
        !            78: this may be of some interest in giving better diagnostics.
        !            79: Since the
        !            80: .I main
        !            81: program is probably supplied by the user (to read arguments, etc.)
        !            82: the Yacc library is useful only in small
        !            83: projects, or in the earliest stages of larger ones.
        !            84: .PP
        !            85: The external integer variable
        !            86: .I yydebug
        !            87: is normally set to 0.
        !            88: If it is set to a nonzero value, the parser will output a
        !            89: verbose description of its actions, including
        !            90: a discussion of which input symbols have been read, and
        !            91: what the parser actions are.
        !            92: Depending on the operating environment,
        !            93: it may be possible to set this variable by using a debugging system.

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