Unit 8: Pointers

Introduction and importance of pointers

A pointer is a variable that stores the address of another variable. Pointers are important in C because they:

• Allow efficient access and manipulation of memory.
• Enable functions to modify variables passed to them (pass by reference).
• Are used to handle arrays, strings, and structures effectively.
• Are required for dynamic memory allocation.

Declaration:

data_type *pointer_name;

int *p;       /* pointer to int    */
float *f;     /* pointer to float  */
char *c;      /* pointer to char   */

Reference and dereference operator

• Reference operator (&) — returns the address of a variable.
• Dereference operator (*) — accesses the value stored at a pointer’s address.

#include <stdio.h>

int main(void)
{
    int a = 10;
    int *p = &a;          /* p holds address of a   */

    printf("Address of a : %p\n", (void *)&a);
    printf("Value of p   : %p\n", (void *)p);
    printf("Value of *p  : %d\n", *p);   /* 10  */

    *p = 20;              /* modifies a through p   */
    printf("Value of a   : %d\n", a);    /* 20  */
    return 0;
}

Pointer arithmetic

Arithmetic on pointers is performed in units of the pointed-to data type.

Operation	Meaning
p + 1	Address of the next element
p - 1	Address of the previous element
p1 - p2	Number of elements between p1 and p2
++p, --p	Move to next / previous element

#include <stdio.h>

int main(void)
{
    int arr[3] = {10, 20, 30};
    int *p = arr;
    printf("%d\n", *p);       /* 10 */
    p++;
    printf("%d\n", *p);       /* 20 */
    p++;
    printf("%d\n", *p);       /* 30 */
    return 0;
}

Pointer and array

The name of an array acts as a pointer to its first element. arr[i] is equivalent to *(arr + i).

#include <stdio.h>

int main(void)
{
    int arr[5] = {1, 2, 3, 4, 5};
    int *p = arr;

    for (int i = 0; i < 5; i++)
        printf("%d ", *(p + i));   /* 1 2 3 4 5 */
    return 0;
}

Pointer with multidimensional array

In a 2D array, arr[i][j] is equivalent to *(*(arr + i) + j).

#include <stdio.h>

int main(void)
{
    int matrix[2][3] = {{1, 2, 3}, {4, 5, 6}};

    for (int i = 0; i < 2; i++) {
        for (int j = 0; j < 3; j++)
            printf("%d ", *(*(matrix + i) + j));
        printf("\n");
    }
    return 0;
}

Pointer and strings

Strings can be accessed and manipulated using character pointers.

#include <stdio.h>

int main(void)
{
    char *s = "Hello";
    while (*s != '\0') {
        printf("%c", *s);
        s++;
    }
    printf("\n");
    return 0;
}

Dynamic memory allocation

C provides functions in <stdlib.h> to allocate and release memory at runtime.

Function	Purpose
malloc(n)	Allocates n bytes; returns a pointer (memory uninitialized).
calloc(n, size)	Allocates memory for n elements; initializes to 0.
realloc(p, n)	Resizes the previously allocated block.
free(p)	Releases previously allocated memory.

Example — using malloc:

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
    int n, *arr;

    printf("Enter size: ");
    scanf("%d", &n);

    arr = (int *) malloc(n * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed\n");
        return 1;
    }

    for (int i = 0; i < n; i++)
        arr[i] = i + 1;

    for (int i = 0; i < n; i++)
        printf("%d ", arr[i]);

    free(arr);
    return 0;
}

Example — using calloc:

int *arr = (int *) calloc(5, sizeof(int));
/* All 5 elements initialized to 0 */
free(arr);