Function pointer conversion in C

Question

As I heard, It is not safe to convert pointer to function to void*.

Okay, is it safe to cast

void (*enumerate) (void (*)(const struct foo *event, void *), void *)

to

void (*enumerate) (void (*)(const void *event, void *), void *)

and then call it with

void (*)(const void *, void *) as first argument, which treats its first void* as struct foo*?

@rkosegi Casting a function pointer to void* is undefined behavior. There's no guarantee it'll fit. It might work though, if you're lucky. — onemasse
– onemasse, Commented Jun 9, 2012 at 12:18
IIRC, it's not just UB; it's plain forbidden, so a strict compiler would not accept a program that tries (gcc -Wall -pedantic -Werror wouldn't). — Fred Foo
– Fred Foo, Commented Jun 9, 2012 at 12:21

Fred Foo · Accepted Answer · 2012-06-09 12:38:32Z

7

No, this is not safe. It's not guaranteed by the C standard that a struct foo * and a void * have the same size and format. In general, casting function pointers to other function pointer types is a recipe for disaster. The safe solution is to insert an extra function that converts the arguments to the right type, just as you'd do when writing a comparison function for qsort that handles non-void * arguments:

static int compare_foo_as_voidp(void const *a, void const *b)
{
    return compare_foo((struct foo const *)a, (struct foo const *)b);
}

(As Oli Charlesworth writes in the comment, the cast itself is not the problem, but calling through the pointer causes UB.)

edited Jun 9, 2012 at 12:38

answered Jun 9, 2012 at 12:22

Fred Foo

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7 Comments

Oliver Charlesworth Over a year ago

+1. However, casting between function pointer types is ok (although silly), so long as you never call the function from the wrong pointer type.

Aaron Digulla Over a year ago

Can you give an example of a version of C which has two different pointer sizes?

Brett Hale Over a year ago

@AaronDigulla - x86 DOS made use of near and far pointers.

Oliver Charlesworth Over a year ago

@AaronDigulla: On a Harvard architecture, code and data live in separate address spaces, so function pointers may well have a different size to object pointers.

Fred Foo Over a year ago

@AaronDigulla: when a function is called through a pointer of the wrong type, the compiler will not convert its arguments.

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