Why are function pointers and data pointers incomp

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

Pointers to void are supposed to be able to accommodate a pointer to any kind of data -- but not necessarily a pointer to a function. Some systems have different requirements for pointers to functions than pointers to data (e.g, there are DSPs with different addressing for data vs. code, medium model on MS-DOS used 32-bit pointers for code but only 16-bit pointers for data).

Answer 2

On most architectures, pointers to all normal data types have the same representation, so casting between data pointer types is a no-op.

However, it's conceivable that function pointers might require a different representation, perhaps they're larger than other pointers. If void* could hold function pointers, this would mean that void*'s representation would have to be the larger size. And all casts of data pointers to/from void* would have to perform this extra copy.

As someone mentioned, if you need this you can achieve it using a union. But most uses of void* are just for data, so it would be onerous to increase all their memory use just in case a function pointer needs to be stored.