__LINE__ and __FILE__ -- a beginner's question

[Doug Gwyn]

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?

We've found them extremely helpful in identifying "memory leaks"
(allocations without corresponding deallocations) and other such
usage errors in a large project.  The way we did this is typified by:

#ifdef MmLOG
extern pointer	Mm_LQAllo( unsigned nbytes, const char *file, int line );
#define	Mm_QAllo( nbytes )	Mm_LQAllo( nbytes, __FILE__, __LINE__ )
#else
extern pointer	Mm_QAllo( unsigned nbytes );
#endif

The application uses Mm_QAllo() to allocate a chunk of memory and a
similar Mm_QFree() function to deallocate it.  When MmLOG is defined
during compilation of the application, both these functions actually
invoke a different function, Mm_LQAllo() or Mm_LQFree(), passing the
application souce code file name and line number as arguments.  The
implementation of the Mm_L*() functions maintains internal data
structures that keep track of all current allocations, and if an
attempt is made to deallocate a not-currently-allocated block an
error message results, containing the source code information giving
the location of the erroneous Mm_QFree() call.  Other checks are also
made.  At the end of application execution, an atexit()-registered
function inspects the Mm_L*() data structures and lists all blocks
that are considered still allocated.

This should give some indication of how __LINE__ and __FILE__ can be
useful.