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From: David Brown <david.brown@hesbynett.no>
Newsgroups: comp.lang.c
Subject: Re: Casting the return value of ...
Date: Fri, 29 Mar 2024 14:06:58 +0100
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On 28/03/2024 22:07, 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.)
> 

I think that the C standards don't forbid the conversion, but the 
description of pointer conversions (6.3.2.3) does not describe such 
conversions.  That makes it, AFAICS, undefined behaviour rather than 
"forbidden" (which I would define as something that mandates a diagnostic).

Dereferencing such converted pointers might be undefined behaviour (if 
you haven't converted back to the original type), or 
implementation-dependent behaviour (if the conversions change the 
bitwise representation of the pointer).

I personally think it's good that gcc has this diagnostic, even if the 
message text is not strictly accurate.


> Note that clang doesn't issue this diagnostic.
>