Xref: utzoo comp.lang.c:30990 comp.unix.questions:24517
Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!uunet!ittc!ptb
From: ptb@ittc.wec.com (Pat Broderick)
Newsgroups: comp.lang.c,comp.unix.questions
Subject: Re: Using Lex (and Yacc) on a string.
Summary: using yacc/lex on memory strings
Message-ID: <174@ittc.wec.com>
Date: 10 Aug 90 14:41:51 GMT
References: <1990Aug10.012927.5558@basho.uucp>
Followup-To: comp.lang.c
Organization: Westinghouse Electric Corp., ITTC, Pgh. PA.
Lines: 42

In article <1990Aug10.012927.5558@basho.uucp>, john@basho.uucp (John Lacey) writes:
> Normally, of course, one wants a scanner (and a parser) to work from 
> a file, perhaps stdin.  Sigh.  Well, I want one that works from a string.
> ...

Recently I had occasion to do something similar.  What we did was
roughly as follows:

   - strings to be parsed are maintained in memory 
   - to parse a string a global pointer known to lex is set to point at
     the beginning of the string
   - the input() macro was redefined in terms of this pointer (standard
     uses getc(yyin))

The things needed might look something like:

LEX:

# define input() (((yytchar=yysptr>yysbuf?U(*--yysptr):getc(yyin))==10?(yylineno++,yytchar):yytchar)==EOF?0:yytchar)   /* standard defn from lex */

# define input() (((yytchar=yysptr>yysbuf?U(*--yysptr):(*yynyy++))==10?(yylineno++,yytchar):yytchar)==EOF?0:yytchar)                   ^^^^^^^^^^

					/* modified defn to use string */

extern char *yynyy;			/* will pt to start of string */


Function invoking parser:

char *yynyy;			/* globally visible */

....

    yynyy = start_of_string;
    yyparse();


This works fine for us, hope it helps.
-- 
Patrick T. Broderick           |ptb@ittc.wec.com |
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