What is a Pointer to a Null pointer

Difficulty Level : Basic
Last Updated : 28 Aug, 2019

NULL pointer in C
At the very high level, we can think of NULL as a null pointer which is used in C for various purposes. Some of the most common use cases for NULL are

To initialize a pointer variable when that pointer variable isn’t assigned any valid memory address yet.
int* pInt = NULL;
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 pointer variable only if it’s not NULL.
if (pInt != NULL) /*We could use if(pInt) as well*/
{ /*Some code*/
}
else { /*Some code*/
}
To pass a null pointer to a function argument when we don’t want to pass any valid memory address.
int fun(int* ptr)
{
/*Fun specific stuff is done with ptr here*/
return 10;
}
fun(NULL);

Pointer to a Null Pointer
As Null pointer always points to null, one would think that Pointer to a Null Pointer is invalid and won’t be compiled by the compiler. But it is not the case.

Consider the following example:

#include <stdio.h>
  
int main()
{
  
    // Null pointer
    char* np = NULL;
  
    // Pointer to null pointer
    char** pnp = &np;
  
    if (*pnp == NULL) {
        printf("Pointer to a null pointer is valid\n");
    }
    else {
        printf("Pointer to a null pointer is invalid\n");
    }
  
    return 0;
}

Not only this program compiles but executes successfully to give the output as
Output:

Pointer to a null pointer is valid

Explanation:
What happens here is that when a Null pointer is created, it points to null, without any doubt. But the variable of Null pointer takes some memory. Hence when a pointer to a null pointer is created, it points to an actual memory space, which in turn points to null.

Hence Pointer to a null pointer is not only valid but important concept.

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