Using Lex (and Yacc) on a string.

[Joseph A. Leggio]

>From article <1990Aug10.012927.5558 at basho.uucp>, by john at basho.uucp (John Lacey):
> 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.
> 

> Has anyone done this, or see a way to do it, or know a way to do it, or ....
> 
> -- 
> John Lacey, 

I have used these "input" and "unput" routines in
many programs where I wanted complete control of the
input stream.  The example here uses fgets to fill
a character array from stdin, but you could fill it
from any source you wish.  You only need point pointer "p"
to the start of the array each time you read a new line.

Only restriction: unput cannot back up past the start of a line.
(I have not found this to be a problem as I do not usually try
to match patterns which span multiple lines.)

I use System V Release 3 AT&T lex, "flex" might work the same, look
for the #defines for "input" and "unput" in your code.
==================================================
%%
	Lex reg-expr's go here
%%
#define BUFFER_SIZE 1024

char *p;
char buf[BUFFER_SIZE];

main(){
    p = buf;        /* point "p" to start of buf for first line     */
    while( fgets(buf, sizeof(buf), stdin) != NULL ) { /* read line  */
        yylex();                                      /* parse line */
        p = buf;    /* point "p" back to start of buf for next line */
    }
exit(0);
}

#ifdef input
#undef input
#endif
#ifdef unput
#undef unput
#endif

/* replacement "input" routine for lex, uses char array "buf"  */
char input()
{
    if ( p < buf + ( BUFFER_SIZE - 1 ) )
            return(*p++);
    else
            return((char)0);
}

/* replacement "unput" routine for lex, uses char array "buf"  */
unput(c)
char c;
{
    if ( p > buf )
        *(--p) = c;     
}

=============================================================
Joe Leggio WB2HOL
AT&T Customer Software Services
Valhalla, NY
att!valha1!jal