fn_cast.c

/*
	If -Wpedantic is enabled, casts between function pointers and void* are forbidden.
	("ISO C forbids conversion of function pointer to object pointer type")
	However, a function pointer can be wrapped in a struct, a pointer to which can in turn be casted to void*.

	This program demonstrates how this trick can be abused to cause undefined behavior.

	Build: cc -Wall -Wextra -Wpedantic -Werror callback.c -o callback
	Run: ./callback

	License: CC BY-SA 4.0
*/
#include <stdio.h>

struct func_pointer_1 {
	void (*ptr)(char *);
};

struct func_pointer_2 {
	void (*ptr)(void *, void *);
};

void my_func(char *str)
{
	printf("%s", str);
}

void call_func(void *fn, void *arg1, void *arg2)
{
	// undefined behavior (but works on my machine)
	((struct func_pointer_2 *) fn)->ptr(arg1, arg2);
}

int main()
{
	call_func(&(struct func_pointer_1) { &my_func }, "hello\n", NULL);
}