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

Some computers have (had) separate address spaces for code and data. On such hardware it just doesn't work.

The language is designed not only for current desktop applications, but to allow it to be implemented on a large set of hardware.

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It seems like the C language committee never intended void* to be a pointer to function, they just wanted a generic pointer to objects.

The C99 Rationale says:

6.3.2.3 Pointers
C has now been implemented on a wide range of architectures. While some of these architectures feature uniform pointers which are the size of some integer type, maximally portable code cannot assume any necessary correspondence between different pointer types and the integer types. On some implementations, pointers can even be wider than any integer type.

The use of void* (“pointer to void”) as a generic object pointer type is an invention of the C89 Committee. Adoption of this type was stimulated by the desire to specify function prototype arguments that either quietly convert arbitrary pointers (as in fread) or complain if the argument type does not exactly match (as in strcmp). Nothing is said about pointers to functions, which may be incommensurate with object pointers and/or integers.

Note Nothing is said about pointers to functions in the last paragraph. They might be different from other pointers, and the committee is aware of that.

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.