NULL Pointer in C

The Null Pointer is the pointer that does not point to any location but NULL. According to C11 standard:

“An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.”

Syntax of Null Pointer Declaration in C

type pointer_name = NULL;
type pointer_name = 0;

We just have to assign the NULL value. Strictly speaking, NULL expands to an implementation-defined null pointer constant which is defined in many header files such as “stdio.h”, “stddef.h”, “stdlib.h” etc.

Uses of NULL Pointer in C

Following are some most common uses of the NULL pointer in C:

1. To initialize a pointer variable when that pointer variable hasn't been assigned any valid memory address yet.
2. To check for a null pointer before accessing any pointer variable. By doing so, we can perform error handling in pointer-related code, e.g., dereference a pointer variable only if it’s not NULL.
3. To pass a null pointer to a function argument when we don’t want to pass any valid memory address.
4. A NULL pointer is used in data structures like trees, linked lists, etc. to indicate the end.

Check if the pointer is NULL

It is a valid operation in pointer arithmetic to check whether the pointer is NULL. We just have to use isequal to operator ( == ) as shown below:

ptr == NULL;

The above equation will be true if the pointer is NULL, otherwise, it will be false.

Examples of NULL Pointer in C

Example 1: C Program to avoid segmentation fault while accessing the value of pointer using NULL pointer

Pointer does not point to anything

By specifically mentioning the NULL pointer, the C standard gives a mechanism using which a C programmer can check whether a given pointer is legitimate or not.

Example 2: C Program to check successful memory allocation using malloc()

The malloc() function returns the NULL pointer when the memory allocation is failed. We will use this property to check if the memory allocation is successful.

Example 3: Dereferencing a NULL Pointer in C

What’s going to happen if we use the following C code:

On some machines, the above would compile successfully but crash when the program is run. Though that doesn't mean it would show the same behavior across all the machines. As the value of NULL in predefined libraries is 0 and the pointer (that's pointing to NULL) is not pointing to any memory location, this behavior occurs.

Example 4: C Program to demonstrate how to pass NULL to a function

We can pass a NULL value for pointers that should not contain any value to the function in C.

NULL Pointer Passed

Difference Between NULL Pointer and Void Pointer in C

The following table list the differences between a null pointer and a void pointer in C: