Function Pointers & Callbacks

A function's name, like an array's, decays into a pointer to it — letting a function be stored in a variable, passed as an argument, or chosen at runtime.

int add(int a, int b) { return a + b; }
int subtract(int a, int b) { return a - b; }

int compute(int a, int b, int (*operation)(int, int)) {
    return operation(a, b);
}

int main(void) {
    printf("%d\n", compute(5, 3, add));      // 8
    printf("%d\n", compute(5, 3, subtract)); // 2
    return 0;
}

This is exactly how qsort (from <stdlib.h>) accepts a custom comparison function, and how C code implements what other languages call a callback, without any special language feature beyond ordinary pointers.

Common mistakes

• Mismatching a function pointer's declared signature with the actual function assigned to it — the types must match exactly, including parameter and return types.