(void) casts when function return values are ignored

[Alan Barrett <apb%cequrux.com@localhost>]

On Wed, 24 Aug 2011, Marc Balmer wrote:
> A compiler can detect that I don't use the return value by 
> seeing that I don't use the return value.
>
> Why tell the compiler that I don't want to use the return value?  
> It's a stupid rule, to say the least.

Of course the compiler can tell that you don't use the return 
value.  But neither the compiler nor a human reader can easily tell 
that this is deliberate, as opposed to a mistake.

Functions with return values fall into a few broad categories:

  1. Functions whose return value contains the result of the
     function; e.g. sqrt(3).

  2. Functions whose return value is used to report success or
     failure; e.g. sscanf(3), write(2).

  3. Functions whose return value contains information that is
     uninteresting, or also available elsewhere; e.g. strcpy(3).

Failure to check the return value in cases 1 or 2 is almost always 
a bug, but if you have a good reason to ignore the return value, 
then the (void) convention is useful to tell both human readers 
and code analysis tools that it's deliberate.  (It might still be 
a bug, but at least you thought a little about the fact that you 
are ignoring the return value.)

Failure to check the return value in case 3 is perfectly normal, 
and should not need any special annotation.  However, some 
imperfect code analysis tools complain if you don't have a (void) 
cast, so some people are in the habit of using lots of (void) 
casts to appease the tools.

I personally think that (void) casts in cases 1 and 2 are 
useful, while (void) casts in case 3 are annoying and point to a 
deficiency in the tools.

--apb (Alan Barrett)