C Null Pointer

A NULL pointer in C is a pointer that doesn’t point to any valid memory location. It acts as a special value to show that a pointer is either not initialized yet or is pointing to an invalid or unavailable memory address.

Syntax

In C, the NULL pointer is defined in the <stddef.h> header file. It is typically assigned to a pointer to indicate that it is not pointing to any valid memory.

#include <stddef.h>

pointer_type *pointer_name = NULL;

• pointer_type: The data type of the pointer .
• pointer_name: The name of the pointer.

Example

In the example, we declare an integer pointer ptr and initialize it to NULL, indicating that it doesn’t point to any valid memory location:

#include <stdio.h>
#include <stddef.h>

int main() {
  int *ptr = NULL;  // Initialize pointer to NULL

  if (ptr == NULL) {
    printf("Pointer is NULL\n");
  }

  return 0;
}

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The if statement checks if ptr is NULL, if true, it prints “Pointer is NULL” to the screen. This ensures the pointer is not used before properly initialising to a valid memory address.

Usage

1. Initializing Pointers

It is a common practice to initialize pointers to NULL to avoid them pointing to random or undefined memory locations. This helps prevent undefined behavior caused by dereferencing uninitialized or dangling pointers.

int *ptr = NULL;

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2. Pointer Checking

Before dereferencing a pointer, it is a good practice to check whether it is NULL. Dereferencing a NULL pointer will cause a segmentation fault, leading to runtime errors.

if (ptr != NULL) {
  // Safe to dereference ptr
  printf("%d", *ptr);
} else {
  printf("Pointer is NULL\n");
}

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3. Return Value for Failure

Functions that involve memory allocation or searching operations often return a NULL pointer to indicate failure. For example, when using .malloc() to allocate memory, if the allocation fails, it returns NULL.

int *ptr = malloc(sizeof(int));
if (ptr == NULL) {
  printf("Memory allocation failed!\n");
}

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4. NULL vs. 0

In C, NULL is often defined as ((void \*)0). While NULL and 0 can be used interchangeably in many contexts, using NULL improves code readability, making it clear that the value is a pointer rather than an integer.

Best Practices

1. Always initialize pointers to NULL to avoid pointing to garbage memory.
2. Always check for NULL before dereferencing pointers.
3. Use NULL as a return value for functions that encounter errors or failure in memory allocation.
4. Avoid using pointer arithmetic on NULL.