C Pointers

What is a Pointer?

A pointer is a variable that stores the memory address of another variable. Pointers are one of C's most powerful features — they enable dynamic memory allocation, efficient array handling, and passing variables by reference.

& — Address-of operator: gets the memory address of a variable
* — Dereference operator: accesses the value at the address stored in a pointer

int x = 10;
int *ptr = &x;   // ptr holds the address of x

printf("%d\n", x);    // 10  — value of x
printf("%p\n", ptr);  // address of x (e.g., 0x7fff...)
printf("%d\n", *ptr); // 10  — value at the address (dereference)

Pointer Arithmetic

You can perform arithmetic on pointers. When you increment a pointer, it moves by the size of the type it points to.

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

p++;            // now points to arr[1] (moves 4 bytes for int)
printf("%d", *p);  // 20

NULL Pointer

A NULL pointer is a pointer that doesn't point to any valid memory location. Always initialize pointers to NULL if not assigning an address immediately, and check before dereferencing.

Pointer Basics — Declare, Assign, Dereference

#include <stdio.h>

int main() {
    int x = 42;
    int *ptr = &x;  // ptr stores address of x

    printf("Value of x:       %d\n",  x);
    printf("Address of x:     %p\n",  (void*)&x);
    printf("Value of ptr:     %p\n",  (void*)ptr);   // same as &x
    printf("Dereference *ptr: %d\n",  *ptr);         // 42

    // Modify x through pointer
    *ptr = 100;
    printf("x after *ptr=100: %d\n", x);  // 100

    // Pointer to pointer
    int **pptr = &ptr;
    printf("\nPointer to pointer:\n");
    printf("**pptr = %d\n", **pptr);  // 100

    // Size of pointer (same regardless of type on 64-bit)
    printf("\nsizeof(int*):    %zu\n", sizeof(int*));
    printf("sizeof(double*): %zu\n", sizeof(double*));
    printf("sizeof(char*):   %zu\n", sizeof(char*));

    return 0;
}

Pointer Arithmetic and Pointer to Array

#include <stdio.h>

int main() {
    int arr[] = {10, 20, 30, 40, 50};
    int *p = arr;  // array name = pointer to first element

    // Traverse array using pointer arithmetic
    printf("Array using pointer arithmetic:\n");
    for (int i = 0; i < 5; i++) {
        printf("arr[%d] = %d  (address: %p)\n", i, *(p + i), (void*)(p + i));
    }

    // Pointer increment
    printf("\nUsing p++:\n");
    p = arr;  // reset to start
    while (p < arr + 5) {
        printf("%d ", *p);
        p++;
    }
    printf("\n");

    // Pointer difference
    int *start = arr;
    int *end   = arr + 4;
    printf("\nPointer difference: %td elements\n", end - start);  // 4

    return 0;
}

Pointer to Function and NULL Pointer Check

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

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 main() {
    // Pointer to function: return_type (*name)(param_types)
    int (*operation)(int, int);

    operation = add;
    printf("add(5, 3) = %d\n", operation(5, 3));  // 8

    operation = sub;
    printf("sub(5, 3) = %d\n", operation(5, 3));  // 2

    operation = mul;
    printf("mul(5, 3) = %d\n", operation(5, 3));  // 15

    // NULL pointer — always check before dereferencing
    int *ptr = NULL;
    if (ptr != NULL) {
        printf("Value: %d\n", *ptr);
    } else {
        printf("Pointer is NULL — safe to skip dereference\n");
    }

    // Dynamic allocation returns NULL on failure
    int *arr = (int*)malloc(5 * sizeof(int));
    if (arr == NULL) {
        printf("Memory allocation failed!\n");
        return 1;
    }
    arr[0] = 42;
    printf("arr[0] = %d\n", arr[0]);
    free(arr);

    return 0;
}

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C Pointers

What is a Pointer?

Pointer Arithmetic

NULL Pointer

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