Pointers

Every variable lives somewhere in memory, and that location has an address. A pointer is a variable that stores an address. Pointers are what make C powerful: they let functions change their caller's variables, and they are the basis of arrays and dynamic memory.

& and *

• &x gives the address of x.
• int *p declares p as a pointer to an int.
• *p (called dereferencing) means "the value at the address stored in p".

#include <stdio.h>

int main(void) {
    int score = 50;
    int *p = &score;     /* p now points to score */

    printf("score = %d\n", score);
    printf("*p = %d\n", *p);

    *p = 75;             /* change score through the pointer */
    printf("score is now %d\n", score);
    return 0;
}

score = 50
*p = 50
score is now 75

You can print an address with printf("%p\n", (void *)p);, but the number will be different every time you run the program.

Changing a variable inside a function

C passes arguments by value, so a function normally gets a copy. Pass a pointer and the function can change the original.

#include <stdio.h>

void swap(int *a, int *b) {
    int tmp = *a;
    *a = *b;
    *b = tmp;
}

int main(void) {
    int x = 3, y = 8;
    swap(&x, &y);
    printf("x = %d, y = %d\n", x, y);
    return 0;
}

x = 8, y = 3

This is exactly why scanf("%d", &n) needs the &: scanf has to write into your variable.

Returning more than one result

#include <stdio.h>

void divide(int a, int b, int *quotient, int *remainder) {
    *quotient = a / b;
    *remainder = a % b;
}

int main(void) {
    int q, r;
    divide(47, 6, &q, &r);
    printf("47 = 6 * %d + %d\n", q, r);
    return 0;
}

47 = 6 * 7 + 5

NULL pointers

A pointer that points to nothing should be set to NULL. Always check before dereferencing a pointer that might be NULL.

#include <stdio.h>

int main(void) {
    int *p = NULL;
    if (p == NULL) {
        printf("p does not point anywhere yet\n");
    }
    return 0;
}

p does not point anywhere yet

GATE-style practice questions

#include <stdio.h>
int main(void) {
    int a = 5, b = 10;
    int *p = &a, *q = &b;
    *p = *q + 1;
    p = q;
    *p = *p * 2;
    printf("%d %d\n", a, b);
    return 0;
}

#include <stdio.h>
void update(int *x, int y) {
    *x = *x + y;
    y = y * 2;
}
int main(void) {
    int a = 3, b = 4;
    update(&a, b);
    update(&b, a);
    printf("%d %d\n", a, b);
    return 0;
}