Error recovery using flex and yacc

Greetings,


Using Flex 2.5.4 and Berkeley Yacc, I have been trying to create an
error recovery method for my grammar. In a nutshell, I have succeeded
to a certain extent, but I am worried about having to change the
generated lex.yy.c input and yyunput declarations from using static to
extern.


Each line of my source file is identified by a category symbol and
these should occur in certain set groups. In the simple case of
trying to deal with an unknown symbol, I use the rule:


source: ErrGroup Pgroup ErrGroup statement ErrGroup


where ErrGroup is defined as being:


ErrGroup:
                                | error
                                                {
                                                recover();
                                                yyclearin;
                                                yyerrok;
                                                error_handler(11);
                                                }
                                                NEWLINE
                                ;


which recognizes the unknown token and then expects to recover to a
state where a NEWLINE token will be recognized.


Basically, I just plan to throw away the entire line when an error
occurs and hope that recovers to a useful state, otherwise, I'll throw
away the next line, etc.


My recover routine is simply:


recover() {
                int c;


                for (;;) {
                                c = input();
                                if (c == EOF || (char) c == '\n') break;
                }
                yyunput(c, yytext);
                return 1;
}


which works as long as I go into lex.yy.c and redefine the input and
yyunput functions to be extern rather than static functions. There
lies the heart of my problem. When I read O'Reilly, it seems to me
that these functions should be available in my grammar, but my version
of flex keeps creating these as static functions. Should I be using
another function serving as a wrapper for these functions or using
some command line parameter to alter the behavior of flex? The only
option I use on flex now is the -I option, and on yacc I only use the
-d option. TIA for your help.


Francis R. Harvey III
WB303, x3952
harveyf1@westat.com
[Put the recover routine at the end of your lexer. But if you just want
to throw away the input up to a new line, that's easier to do by using
an exclusive start state, e.g.


<RECOVER>.*\n { BEGIN NORMAL; }
...


recover() { BEGIN RECOVER; }


  -John]

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