Re: Casting the return value of ...

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.

--
Keith Thompson (The_Other_Keith) Keith.S.Thompson+u@gmail.com
Working, but not speaking, for Medtronic
void Void(void) { Void(); } /* The recursive call of the void */