Function Pointers & Callbacks

Concept

A function pointer stores the address of a function so it can be passed and called later — the basis of callbacks and dispatch tables.

qsort from <stdlib.h> takes a comparison function pointer, making it work for any type.

Read complex declarations with the clockwise/spiral rule: int (*fp)(int, int) is a pointer to a function taking two ints and returning int.

Code (C17)

#include <stdio.h>
#include <stdlib.h>

int add(int a, int b) { return a + b; }
int mul(int a, int b) { return a * b; }

int cmp_desc(const void *a, const void *b) {
    return *(const int *)b - *(const int *)a;
}

int main(void) {
    int (*ops[])(int, int) = {add, mul};
    printf("%d %d\n", ops[0](3, 4), ops[1](3, 4));  // 7 12

    int v[] = {4, 9, 1, 7};
    qsort(v, 4, sizeof v[0], cmp_desc);
    for (int i = 0; i < 4; i++) printf("%d ", v[i]);
    return 0;
}

Remember

• Function name decays to its address.
• qsort comparator returns <0, 0, >0.
• Use typedef to simplify pointer types.

Interview questions