Checking for multiple input sources in FORMS

[slevy]

I'd like to suggest a change to loops that look like:

	while (1) {
	    if (async_fnextc(stdin, 0) != NODATA) {
		obj = LSexpr(lake);
		LEval(obj);
	    }
	    fl_check_forms();
	}

The problem is that, if nothing interesting is happening, this loop
will keep grinding along soaking up CPU cycles.  Anything else running
on the same machine -- e.g. geomview -- will be noticeably slower.

The simplest kind of change is just to add a fixed short delay to the loop,
or at least one that's exercised if there's nothing happening.
sginap() will do this but it's naturally SGI specific.  To write portable
code dealing with short time delays, you're stuck with select(), but
it's fairly tame in this application:

#include <sys/time.h>		/* for declaration of "struct timeval" */

    ...

	while (1) {
	    if (async_fnextc(stdin, 0) != NODATA) {
		obj = LSexpr(lake);
		LEval(obj);
	    } else {
		static struct timeval tenth = { 0, 100000 }; /* 0.1 sec */
		select(0, NULL, NULL, NULL, &tenth);
	    }
	    fl_check_forms();
	}

[You could do a bit better by telling select() about the file descriptors
you really care about, but it's probably not worth the bother here.]