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Internet Message Format | 1996-08-05 | 2.9 KB

Path: cortex.corpus.uni-muenster.de!news
From: [e-mail] (Markus Gutschke)
Newsgroups: comp.lang.c,comp.windows.x.motif
Subject: Re: [Q:] flex input from string?
Date: 14 Jan 1996 13:19:08 GMT
Organization: Markus Gutschke, Schlage 5a, 48268 Greven-Gimbte, Germany
Distribution: inet
Message-ID: <[e-mail]>
References: <[e-mail]>
NNTP-Posting-Host: pppe085.uni-muenster.de
In-reply-to: Joachim Fabini's message of Fri, 12 Jan 1996 14:22:44 +0100

In article <[e-mail]> Joachim Fabini <[e-mail]> writes:
>       My question: (f)lex reads by default from the FILE* yyin,
> defaulting to stdin. How can I read input from a _STRING_ ? Say, I get
> the text field's value and pass it as input to the scanner, that
> returns some token and I know exactly if the input was correct or not.

I would advise you to get hold of a copy of

       John R. Levine, Tony Mason, Doug Brown; Lex&Yacc; UNIX
       Programming Tools; O'Reilly&Associates, Inc; Second Edition;
       October 1992; ISBN 1-56592-000-7.

This book gives an excellent overview on Lex and Yacc related problems
and summarizes differences between commonly available implementations.

It says the following on "Input from Strings" pp. 156:

> INPUT FROM STRINGS
>
> Normally lex reads from a file, but sometimes you want it to read from
> some other source, such as a string in memory. All versions of lex
> make this possible, but the details vary considerably.
>
> ...
>
> FLEX
>
> Although flex provides an input() function, it gets characters using
> optimized in-line code. You can redefine YY_INPUT, the macro it uses
> to read blocks of data. It is called as:
>
>   YY_INPUT(buffer, result, max_size)
>
> where buffer is a character buffer, result is a variable in which to
> store the number of characters actually read, and max_size is the size
> of the buffer. To read from a string, have your version of YY_INPUT
> copy data from your string buffer.
>
> %{
> #undef YY_INPUT
> #define YY_INPUT(b, r, ms) (r = my_yyinput(b, ms))
> %}
> ...
> extern char myinput[];
> extern char *myinputptr; /* current position in myinput */
> extern int myinputlim; /* end of data */
>
> int my_yyinput(char *buf, int max_size)
> {
>   int n = min(max_size, myinputlim - myinputptr);
>
>   if (n > 0) {
>     memcpy(buf, myinputptr, n);
>     myinputptr += n;
>   }
>   return n;
> }

So, as long as you stick to using Flex, it appears that redefining
YY_INPUT will do the trick. Nonetheless, if you want to release your
code under a license other than GPL, you should be aware of copyright
restriction imposed onto Flex. Thus, you might have to get hold of a
different lexical scanner generator; in this case you should have a
look in "Lex&Yacc" on how you need to modify your code...


Markus
--
Markus Gutschke            Internet: [e-mail]
Schlage 5a
D-48268 Greven-Gimbte
Germany