Pointers in C

A pointer is a variable that stores the memory address of another variable.

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Pointer Basics

int a = 10;
int *p;       /* pointer declaration */
p = &a;       /* p holds address of a */

printf("%d\n", a);    /* value: 10 */
printf("%p\n", p);    /* address of a */
printf("%d\n", *p);   /* dereference: value at address = 10 */

Operators:

• & (address-of) — gives the address of a variable
• * (dereference) — gives the value at the address

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Pointer Arithmetic

int arr[5] = {10, 20, 30, 40, 50};
int *p = arr;   /* points to arr[0] */

printf("%d\n", *p);       /* 10 */
printf("%d\n", *(p+1));   /* 20 */
printf("%d\n", *(p+2));   /* 30 */

p++;   /* now points to arr[1] */

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Pointer and Arrays

int arr[3] = {1, 2, 3};
int *p = arr;

/* Both are equivalent: */
printf("%d\n", arr[1]);   /* using array notation */
printf("%d\n", *(p+1));   /* using pointer arithmetic */

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Pointer to Pointer

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

printf("%d\n", **pp);   /* 10 */

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NULL Pointer

int *p = NULL;   /* pointer pointing to nothing */
if (p == NULL) {
    printf("Pointer is null\n");
}

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Pointer as Function Argument (Call by Reference)

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

int main() {
    int x = 5, y = 10;
    swap(&x, &y);
    printf("x=%d y=%d\n", x, y);  /* x=10 y=5 */
}

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Dynamic Memory Allocation

#include <stdlib.h>

int *p = (int *)malloc(5 * sizeof(int));  /* allocate 5 ints */
if (p == NULL) {
    printf("Memory allocation failed\n");
}

p[0] = 10;
p[1] = 20;

free(p);   /* always free allocated memory */