Re: build warnings

[Erik Faye-Lund <kusmabite@googlemail.com>]

On Fri, Jan 29, 2010 at 12:02 PM, Jeff King <peff@peff.net> wrote:
> On Fri, Jan 29, 2010 at 08:03:37PM +1100, Michael Wookey wrote:
>
>> With current master (dace5dd1), the following build warnings appear on
>> Ubuntu 9.10 (x86):
>>
>>   run-command.c: In function ‘notify_parent’:
>>   run-command.c:70: warning: ignoring return value of ‘write’,
>> declared with attribute warn_unused_result
>>   run-command.c: In function ‘die_child’:
>>   run-command.c:80: warning: ignoring return value of ‘write’,
>> declared with attribute warn_unused_result
>>   run-command.c:81: warning: ignoring return value of ‘write’,
>> declared with attribute warn_unused_result
>>   run-command.c:82: warning: ignoring return value of ‘write’,
>> declared with attribute warn_unused_result
>
> There is no point in looking at the return value of any of those calls.
> The first one is about notifying the parent process of a child's failure
> to exec while it is dying (the surrounding function is even an atexit
> handler!). If we can't do that, there is really no alternative behavior.
> The latter three are printing fatal error messages. If we fail at that,
> there is not much to do (unless we should print an error...).
>
>>   ~$ gcc --version
>>   gcc (Ubuntu 4.4.1-4ubuntu9) 4.4.1
>
> I have heard that Ubuntu recently switched on unused result warnings by
> default, and I have seen complaints that it is generating a lot of
> uninteresting warnings like these.
>
> Does anybody know if this behavior is here to stay? Can it be worked
> around with -Wno-warn-unused-result or something? There are few enough
> callsites here that I am not entirely opposed to annotating them with
> "(void)write" (does that actually work?), but I worry that this is a
> slippery slope. There are a lot of other calls whose return values are
> also uninteresting (just looking in the vicinity of this code, I see an
> fflush and a close, neither of whose failure would be interesting). I'm
> not excited at the prospect of annotating all of them.
>

In my experience, quieting warn-unused-result globally isn't ideal;
this warning has helped me track down some serious issues many times
in the past. IIRC, gcc requires a specific attribute on a function
prototype in order to generate warnings when the return-value isn't
used. I guess the issue we're seeing here is that the glibc that
Ubuntu ships has recently added this attribute for some CRT-functions.

Personally I think that quieting them ("(void)func(...)" does work
AFAIK) can make sense as long as there's only a few call-sites.
However, if it's so many that it'll generate substantial noise in the
git-sources or Ubuntu-users will get annoyed beyond sense,
"-Wno-warn-unused-result" might be the only choice. But perhaps this
is something Ubuntu-users would put in their config.mak?

-- 
Erik "kusma" Faye-Lund