C Pointers and Functions

Pointers and functions work together in three ways.

1. Pass a pointer to change values

#include <stdio.h>

void increase(int *n) {
    *n = *n + 10;
}

int main(void) {
    int x = 5;
    increase(&x);
    printf("%d\n", x);
    return 0;
}

Output:

2. Get more than one value from a function

A function can return only one value, but with pointers it can fill many variables.

#include <stdio.h>

void minMax(int a[], int n, int *min, int *max) {
    *min = *max = a[0];
    for (int i = 1; i < n; i++) {
        if (a[i] < *min) *min = a[i];
        if (a[i] > *max) *max = a[i];
    }
}

int main(void) {
    int marks[] = {45, 90, 33, 78};
    int lo, hi;

    minMax(marks, 4, &lo, &hi);
    printf("Min = %d, Max = %d\n", lo, hi);
    return 0;
}

Output:

3. Function that returns a pointer

Never return the address of a normal local variable. It is removed when the function ends. Return an address of a static, global or dynamic memory.

#include <stdio.h>

int *bigger(int *a, int *b) {
    return (*a > *b) ? a : b;
}

int main(void) {
    int x = 8, y = 15;
    int *p = bigger(&x, &y);

    printf("%d\n", *p);
    return 0;
}

Output:

Function pointer

A pointer can also store the address of a function. Then you can call the function through it.

#include <stdio.h>

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

int main(void) {
    int (*op)(int, int);

    op = add;
    printf("%d\n", op(3, 4));
    op = mul;
    printf("%d\n", op(3, 4));
    return 0;
}

Output:

Next: C Dynamic Memory Allocation.

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