What is a Pointer to a Null pointer

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

1. To initialize a pointer variable when that pointer variable isn’t assigned any valid memory address yet. 

C

int* pInt = NULL;

1. 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. 

C

if (pInt != NULL) /*We could use if(pInt) as well*/
{ /*Some code*/
}
else { /*Some code*/
}

1. To pass a null pointer to a function argument when we don’t want to pass any valid memory address. 

C

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: 

C++

#include <iostream>
using namespace std;

int main()
{

    // Null pointer
    char* np = NULL;

    // Pointer to null pointer
    char** pnp = &np;

    if (*pnp == NULL) {
        cout << "Pointer to a null pointer is valid" << endl;
    }
    else {
        cout << "Pointer to a null pointer is invalid" << endl;
    }

    return 0;
}

// This code is contributed by sarajadhav12052009

C

#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.