NULL Pointer in C

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A NULL pointer in C is a pointer that doesn’t point to any of the memory locations. The NULL constant is defined in the header files stdio.h, stddef.h as well as stdlib.h.

A pointer is initialized to NULL to avoid the unpredicted behavior of a program or to prevent segmentation fault errors.

Syntax

This is how you would initialize a NULL pointer −

type *ptr = NULL;

Or, you can use this syntax too −

type *ptr = 0;

Applications of NULL Pointer

Following are some of the applications of a NULL pointer −

• To initialize a pointer variable when that pointer variable isn’t assigned any valid memory address yet.
• To pass a null pointer to a function argument when we don’t want to pass any valid memory address.
• To check for a null pointer before accessing any pointer variable so that we can perform error handling in pointer-related code. For example, dereference a pointer variable only if it’s not NULL.

A NULL pointer is always used to detect the endpoint of trees, linked lists, and other dynamic data structures.

Check Whether a Pointer is NULL

It is always recommended to check whether a pointer is NULL before dereferencing it to fetch the value of its target variable.

Example

Take a look at the following example −

#include <stdio.h> 

int main(){ 

   int *ptr = NULL;   // null pointer

   if (ptr == NULL) { 
      printf("Pointer is a NULL pointer"); 
   } 
   else { 
      printf("Value stored in the address referred by the pointer: %d", *ptr); 
   } 
   
   return 0; 
}

Output

When you run this code, it will produce the following output −

Pointer is a NULL pointer

Check Memory Allocation Using NULL Pointer

The malloc() and calloc() functions are used to dynamically allocate a block of memory. On success, these functions return the pointer to the allocated block; whereas on failure, they return NULL.

Example

The following example shows how you can use the NULL pointer to check whether memory allocation was successful or not −

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

int main(){

   int* ptr = (int*)malloc(sizeof(int));


   if (ptr == NULL){
      printf("Memory Allocation Failed");
      exit(0);
   }
   else{
      printf("Memory Allocated successfully");
   }

   return 0;
}

Output

Run the code and check its output −

Memory Allocated successfully

NULL File Pointer

Checking if the FILE pointer returned by the fopen() function is NULL is always a recommended approach to avoid runtime errors in file-related processing.

Example

The following example shows how you can use the NULL file pointer to ensure that a file is accessible or not −

#include <stdio.h>
#include <string.h>

int main(){

   FILE *fp;
   char *s;
   int i, a;  
   float p;

   fp = fopen ("file3.txt", "r");

   if (fp == NULL){
      puts ("Cannot open file"); return 0;
   }

   while (fscanf(fp, "%d %f %s", &a, &p, s) != EOF)
      printf ("Name: %s Age: %d Percent: %f\n", s, a, p);

   fclose(fp);
   
   return 0;
}

You should always initialize a pointer variable to NULL when the target variable hasn’t been assigned any valid memory address yet.