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

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.

My Personal Notes

Recommended Posts: