Re: Casting the return value of ...

On 28/03/2024 21:30, bart wrote:

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;
 

One method that silences all gcc warnings here is to cast via uintptr_t:
#include <stdint.h>
void* p;
void(*q)(void);
typedef void(*FVoid)(void);
void foo(void) {
     p = (void*) (uintptr_t) q;
}
void bar(void) {
     q = (FVoid) (uintptr_t) p;
}
I think it is good to have the gcc warnings enabled to help catch mistakes, and you then need to make special effort when doing something odd - hence the casts.  (On any POSIX system where casting between pointer-to-function and pointer-to-object types can work, uintptr_t will exist and the cast here will not affect the value of the pointer.)