Is cast function pointers safe to call?

Question

In particular, will the following work as expected:

typedef void(*func_p)(void*); void foo(int* data) { printf("%i\n",*data); } int main(int argc, char** argv) { func_p bar; int x = 42; bar = foo; bar((void*)&x); return 0; } 

those. is it possible to rely on data pointers ( void* , int* , struct baz* , but not necessarily void(*)(void) ), always passed compatible?

Answer 1

The standard does not require this to work. The standard requires that char* and void* have the same representation and alignment requirements, all pointers to structures do this, and all connection pointers do that too. Although undefined behavior calls a casted function call anyway (§6.5.2.2p9), the same presentation and alignment requirements provide a practical guarantee that the call works (§6.2.5p27).

Other types of pointers should not behave this way, although I personally have not seen such a machine. It is theoretically possible that int* has a smaller sizeof than a char* for example (if int has more stringent alignment requirements than char , this might be wise). According to the correct calling procedure on such a machine, it would be almost impossible to call the pointer to the casted function.

Answer 2

If your code does not target strange ancient mainframes, it will work fine for all practical purposes. No modern machine uses different representations for different types of pointers, and I don't think it is realistic that any future machine will ever be. It is simply illogical and vice versa, very similar to one - it complements arithmetic.

If you are still worried or want to satisfy language purists, follow the caf.