C: Why is casting from void pointer to function pointer undefined? [duplicate]

Question

I have read that converting a function pointer to a data pointer and vice versa works on most platforms but is not guaranteed to work. Why is this the case? Shouldn't both be simply addresses into main memory and therefore be compatible?

Answer 1

An architecture doesn't have to store code and data in the same memory. With a Harvard architecture, code and data are stored in completely different memory. Most architectures are Von Neumann architectures with code and data in the same memory but C doesn't limit itself to only certain types of architectures if at all possible.

Answer 2

For those who remember MS-DOS, Windows 3.1 and older the answer is quite easy. All of these used to support several different memory models, with varying combinations of characteristics for code and data pointers.

So for instance for the Compact model (small code, large data):

sizeof(void *) > sizeof(void(*)())

and conversely in the Medium model (large code, small data):

sizeof(void *) < sizeof(void(*)())

In this case you didn't have separate storage for code and date but still couldn't convert between the two pointers (short of using non-standard __near and __far modifiers).

Additionally there's no guarantee that even if the pointers are the same size, that they point to the same thing - in the DOS Small memory model, both code and data used near pointers, but they pointed to different segments. So converting a function pointer to a data pointer wouldn't give you a pointer that had any relationship to the function at all, and hence there was no use for such a conversion.