Function pointer conversion in C
c, function, function-pointers, gcc
Solution
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.)
Problem
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*?