How to redirect yyout to char* buffer in GNU flex?

Question

By default yyout is set to stdout and can be redirected to FILE* stream. Is a way to redirect yyout to char*?

Answer 1

There are a number of ways to do it. If you post a bit of your scanner code where you are trying to use yyout may be I can give you a more specific answer:

Typically it's in your action where you'd do this. So instead of using ECHO or fprintf(yyout... you'd use something else like

<token>  { snprintf(buf, sizeof(buf), "%s", yytext); }

earlier you'd have declared:

char buf[BUFSIZE];

addendum I - An alternate approach

There are clever ways of dealing with the situation, but they are prone to maintenance problems in the long run because they are "clever" .. .and the cleverer a solution the shorter its life. (unless the cleverness is well documented with caveats attached)

 int yyprintf(const char *fmt, ...)  {
     va_list ap;
     va_start(ap, fmt);
     if ( some_flag & FLAG_OUTFILE ) {
         vfprintf(yyout, fmt, ap);
     }
     else {
         sprintf(buf, fmt, ap);
     }
     va_end(ap);
 }

where buf is a global buffer.

However if you want to make things a bit local:

Approach 2: Fine-grain control over where things go and when

You want fine grain control over where things go along the way. Sometimes you want output to file, other times to a string, and you don't always know which is which and when and where you can use something like this:

 int myvprintf(void *here, size_t len, const char *fmt, va_list ap) 
      __attribute__((format (gnu_printf, 3, 4), nonnull(1, 3))) {
     
     int rv;

     if ( len > 0 ) { 
        rv = vsnprintf((char *), len, fmt, ap);
     }
     else {
        rv = vfprintf((FILE *)here, fmt, ap);
     }

     return rv;
 }

 int myprintf(void *here, size_t len, const char *fmt, ... ) 
      __attribute__((format (gnu_printf, 3, 4), nonnull(1, 3))) {
     int rv;
     va_list ap;
     va_start(ap, fmt);
     rv = myvprintf(here, len, fmt, ap);
     va_end(ap);
     return rv;
 }

and use myprintf along the way, you will have control over what is here all the time.

Just for Fun

Don't try this at home. But all yy* identifiers and ECHO are not plain variables, they are #define's. So you could do some clever macro rewriting:

For example, if you are using ECHO everywhere then you can redefine it to do whatever you like (just #undef and #define) in the top section:

%{
#undef ECHO
#define ECHO snprintf(buf, sizeof(buf), "%s", yytext)
%}

%%

<token> ECHO;
%% 

just hide it all in convoluted headers and do other cleverness that will make debugging a hell later for a programmer you hate. This can have its own rewards and giggles.