NULL Pointer

NULL Pointer in C

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.

Declare and Initialize a NULL Pointer

This is how you would declare and initialize a NULL pointer −

type *ptr =NULL;

Or, you can use this syntax too −

type *ptr =0;

Example of a NULL Pointer

The following example demonstrates how to declare and initialize a NULL pointer −

#include <stdio.h> intmain(){int*p=NULL;//initialize the pointer as null.printf("The value of pointer is %u",p);return0;}

Output

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

The value of pointer is 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> intmain(){int*ptr =NULL;// null pointerif(ptr ==NULL){printf("Pointer is a NULL pointer");}else{printf("Value stored in the address referred by the pointer: %d",*ptr);}return0;}

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>intmain(){int* ptr =(int*)malloc(sizeof(int));if(ptr ==NULL){printf("Memory Allocation Failed");exit(0);}else{printf("Memory Allocated successfully");}return0;}

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>intmain(){

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

   fp =fopen("file3.txt","r");if(fp ==NULL){puts("Cannot open file");return0;}while(fscanf(fp,"%d %f %s",&a,&p, s)!=EOF)printf("Name: %s Age: %d Percent: %f\n", s, a, p);fclose(fp);return0;}

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