__LINE__ and __FILE__ -- a beginner's question

[Michael Condict]

In article <6257 at arcturus> evil at arcturus.UUCP (Wade Guthrie) writes:
>For what purpose are the __LINE__ and __FILE__ macros used?  Are these
>only useful for the writers of code that, in turn, produces compileable
>code, or is there some use for the applications programmer?

My favorite use in application programs is in the definition of a debugging
macro that, given a Boolean expression, evaluates it and reports an error
message if false.  (It usually also does something drastic like intentionally
dumping core.)  The __LINE__ and __FILE__ vars provide a handy way to
report a uniquely identify which assertion failed.  Example:

	#define ASSERT(b) \
		if (!b) {
			fprintf(stderr,
			   "Assertion violated in %s, line %d.  Goodbye.\n",
			   __FILE__, __LINE__);
			abort();
		}

Michael Condict
Vrije University
Amsterdam