What is guaranteed about the size of a function pointer?

Question

In C, I need to know the size of a struct, which has function pointers in it. Can I be guaranteed that on all platforms and architectures:

• the size of a void* is the same size as a function pointer?
• the size of the function pointer does not differ due to its return type?
• the size of the function pointer does not differ due to its parameter types?

I assume the answer is yes to all of these, but I want to be sure. For context, I'm calling sizeof(struct mystruct) and nothing more.

Solution 1

From C99 spec, section 6.2.5, paragraph 27:

A pointer to void shall have the same representation and alignment requirements as a pointer to a character type. Similarly, pointers to qualified or unqualified versions of compatible types shall have the same representation and alignment requirements. All pointers to structure types shall have the same representation and alignment requirements as each other. All pointers to union types shall have the same representation and alignment requirements as each other. Pointers to other types need not have the same representation or alignment requirements.

So no; no guarantee that a void * can hold a function pointer.

And section 6.3.2.3, paragraph 8:

A pointer to a function of one type may be converted to a pointer to a function of another type and back again; the result shall compare equal to the original pointer.

implying that one function pointer type can hold any other function pointer value. Technically, that's not the same as guaranteeing that function-pointer types can't vary in size, merely that their values occupy the same range as each other.

Solution 2

No, no, no.

C doesn't favour Harvard architectures with different code and data pointer sizes, because ideally when programming for such an architecture you want to store data in program memory (string literals and the like), and to do that you'd need object pointers into the code space. But it doesn't forbid them, so as far as the standard is concerned function pointers can refer to an address space which has a different size from the data address space.

However, any function pointer can be cast to another function pointer type[*] and back without trashing the value, in the same way that any object pointer can be cast to void* and back. So it would be rather surprising for function pointers to vary in size according to their signature. There's no obvious "use" for the extra space, if you have to be able to somehow store the same value in less space and then retrieve it when cast back.

[*] Thanks, schot.

Solution 3

In addition to the other answers, Wikipedia says this:

http://en.wikipedia.org/wiki/Function_pointer

Although function pointers in C and C++ can be implemented as simple addresses, so that typically sizeof(Fx)==sizeof(void *), member pointers in C++ are often implemented as "fat pointers", typically two or three times the size of a simple function pointer, in order to deal with virtual inheritance.

A function pointer is an abstraction. As long as the requirements of the standard are fulfilled, anything is possible. I.e. if you have less than 256 functions in your program, function pointers could be implemented by using a single byte with the value 0 for NULL and the values 1 to 255 as the index into a table with the physical addresses. If you exceed 255 functions, it could be extended to use 2 bytes.

Solution 4

There is a practical example of differing sizes that used to be common. In the MS-DOS & early Windows C programming, in the "medium" memory model you had 16-bit data pointers but 32-bit function pointers, and the "compact" memory model was the other way round.

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