[PATCH] Warning for main returning a bool.

[Aaron Ballman via cfe-commits]

On Tue, Nov 15, 2016 at 5:22 PM, Hal Finkel <hfinkel at anl.gov> wrote:
> ----- Original Message -----
>> From: "Aaron Ballman via cfe-commits" <cfe-commits at lists.llvm.org>
>> To: "Joshua Hurwitz" <hurwitzj at google.com>
>> Cc: "cfe-commits" <cfe-commits at lists.llvm.org>
>> Sent: Tuesday, November 15, 2016 12:17:28 PM
>> Subject: Re: [PATCH] Warning for main returning a bool.
>>
>> On Fri, Oct 14, 2016 at 1:17 PM, Joshua Hurwitz via cfe-commits
>> <cfe-commits at lists.llvm.org> wrote:
>> > See attached.
>> >
>> > Returning a bool from main is a special case of return type
>> > mismatch. The
>> > common convention when returning a bool is that 'true' (== 1)
>> > indicates
>> > success and 'false' (== 0) failure. But since main expects a return
>> > value of
>> > 0 on success, returning a bool is usually unintended.
>>
>> I am not convinced that this is a high-value diagnostic. Returning a
>> Boolean from main() may or may not be a bug (the returned value is
>> generally a convention more than anything else). Also, why Boolean
>> and
>> not, say, long long or float?
>
> I've seen this error often enough, but I think we need to be careful about false positives here. I recommend that we check only for explicit uses of boolean immediates (i.e. return true; or return false;) -- these are often bugs.

I could get behind that.

> Aaron, I disagree that the return value is just some kind of convention. It has a clear meaning.

For many hosted environments, certainly. Freestanding implementations?
Much less so, but I suppose this behavior is still reasonable enough
for them (not to mention, there may not even *be* a main() for a
freestanding implementation).

> Furthermore, the behavior of the system can be quite different for a non-zero exit code than otherwise, and users who don't understand what's going on can find it very difficult to understand what's going wrong.

That's a fair point, but my question still stands -- why only Boolean
values, and not "return 0x1234567800000000ULL;" or "return 1.2;"?

Combining with your idea above, if the check flagged instances where a
literal of non-integral type (other than Boolean) is returned from
main(), that seems like good value.

~Aaron

>
> Thanks again,
> Hal
>
>>
>> ~Aaron
>>
>> >
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>
> --
> Hal Finkel
> Lead, Compiler Technology and Programming Languages
> Leadership Computing Facility
> Argonne National Laboratory