Function Pointers and Advanced Pointers in C

Function Pointers

A function pointer stores the address of a function and allows calling it indirectly.

Syntax:

return_type (*pointer_name)(parameter_types);

Example:

#include <stdio.h>

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

int main() {
    int (*op)(int, int);   /* function pointer */
    
    op = add;
    printf("Add: %d\n", op(3, 4));   /* 7 */
    
    op = mul;
    printf("Mul: %d\n", op(3, 4));   /* 12 */
    
    return 0;
}

---

Array of Function Pointers

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

int (*ops[3])(int, int) = {add, sub, mul};
char *names[] = {"Add", "Sub", "Mul"};

for (int i = 0; i < 3; i++) {
    printf("%s(5,3) = %d\n", names[i], ops[i](5, 3));
}

---

Passing Function Pointer to Another Function

void apply(int *arr, int n, int (*transform)(int)) {
    for (int i = 0; i < n; i++) {
        arr[i] = transform(arr[i]);
    }
}

int square(int x) { return x * x; }
int doubleIt(int x) { return x * 2; }

int arr[] = {1, 2, 3, 4};
apply(arr, 4, square);   /* {1, 4, 9, 16} */

---

Pointer to Array vs Array of Pointers

int arr[5] = {1, 2, 3, 4, 5};

int (*ptr)[5] = &arr;     /* pointer to entire array */
printf("%d\n", (*ptr)[2]);  /* 3 */

int *ptrs[5];             /* array of 5 int pointers */
for (int i = 0; i < 5; i++) ptrs[i] = &arr[i];

---

Pointer to Pointer (Double Pointer)

int a = 10;
int *p = &a;
int **pp = &p;

printf("%d\n", **pp);   /* 10 */
*p = 20;
printf("%d\n", a);      /* 20 */

Use case — dynamic 2D array:

int rows = 3, cols = 3;
int **matrix = (int **)malloc(rows * sizeof(int *));
for (int i = 0; i < rows; i++)
    matrix[i] = (int *)malloc(cols * sizeof(int));

matrix[0][0] = 1;
matrix[2][2] = 9;

for (int i = 0; i < rows; i++) free(matrix[i]);
free(matrix);