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From: "Chris M. Thomasson" <chris.m.thomasson.1@gmail.com>
Newsgroups: comp.lang.c
Subject: Re: Casting the return value of ...
Date: Thu, 28 Mar 2024 14:15:54 -0700
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On 3/28/2024 2:07 PM, Keith Thompson wrote:
> bart <bc@freeuk.com> writes:
>> On 28/03/2024 19:38, Keith Thompson wrote:
>>> Kaz Kylheku <433-929-6894@kylheku.com> writes:
>>> [...]
>>>> Conversions between function pointers and data pointers are an
>>>> extension; it is not well-defined behavior in ISO C.
>>>>
>>>> Therefore we can neither say that ISO C doesn't require a cast there (it
>>>> imposes no requirements at all), nor that the conversion is fine with a
>>>> cast.
>>>>
>>>> The cast is /likely/ necessary, in order to correctly trigger the
>>>> extension.
>>> ISO C does require a cast.  The cast is necessary to avoid a
>>> constraint violation and a mandatory diagnostic.  The resulting
>>> behavior is undefined in ISO C, but defined by POSIX.  Assigning a
>>> void* value to a pointer-to-function object without a cast violates
>>> the constraint for simple assignment (N1570 6.5.16.1p1).
>>
>> What would such a cast look like? Since this gives a warning with
>> -Wpedantic even with a cast:
>>
>>      void* p;
>>      void(*q)(void);
>>
>>      p=(void*)q;
>>      q=(void(*)(void))p;
> 
> The warnings I get from gcc are:
> 
> warning: ISO C forbids conversion of function pointer to object pointer type [-Wpedantic]
> warning: ISO C forbids conversion of object pointer to function pointer type [-Wpedantic]
> 
> With -pedantic-errors, these become fatal errors.
> 
> I disagree with gcc.  ISO C doesn't define the behavior, but it doesn't
> forbid the conversion.  (Anyone who disagrees is invited to cite the
> constraint that it violates.)
> 
> Note that clang doesn't issue this diagnostic.
> 

Casting might prevent a warning in certain compilers...