# Function Pointers & Callbacks — C Programming Fundamentals: Pointers, Memory and Files

Source: https://www.geekswithgeeks.com/en/c/c-function-pointers

> Store and pass functions with function pointers to write callbacks and flexible code like qsort comparators.

## Declaring a function pointer

A function pointer stores the address of a function. The declaration repeats the function's return type and parameter types.

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

int apply(int (*op)(int, int), int x, int y) {
    return op(x, y);   // call through the pointer
}

int main(void) {
    printf("%d %d\n", apply(add, 3, 4), apply(mul, 3, 4));   // 7 12
    return 0;
}
```

## Callbacks with qsort

The standard `qsort` takes a comparison function, so one routine sorts any type.

```c
#include <stdlib.h>

int cmp_int(const void *a, const void *b) {
    int x = *(const int *)a, y = *(const int *)b;
    return (x > y) - (x < y);
}

int nums[] = {5, 2, 9, 1};
qsort(nums, 4, sizeof(int), cmp_int);
```

Use a `typedef` to keep function pointer types readable, for example `typedef int (*BinOp)(int, int);`.

Quick check

**Quiz:** What does a function pointer store?

- [x] The address of a function
- [ ] The return value of a function
- [ ] A copy of the function body
- [ ] The number of arguments

*Answer:* The address of a function. You call the function through that address.