In C, can an address of a pointer be 0?

Question

I'm reading the solution to a problem in the book Cracking the Coding interview (Q 1.2). The objective there is to implement a function void revers(char* str) in C that reverses a null terminating string.

The solution code looks something like this:

void reverse(char *str)
{
   char* end=str;
   char tmp;
   if(str)
   {
       while(*end)
       {
          end++;
       }
       end--;
       //code to reverse
   }
}

Here, str contains an address right? And if(str) will only evaluate to false if str is 0, right?

So what I'm saying is, is there no chance that str will contain the address 0x0000, thus evaluating if(str) to false?

Answer 1

str does indeed contain an address (it's a pointer to char), and if(str) will evaluate to false iff the str is equal to the null pointer.

Note that the null pointer is not required by the standard to refer to address 0; however, the standard mandates that a literal value of 0 when used in a pointer context must be interpreted by the compiler as the address of the null pointer -- whatever that might be.

This means that the test if(p == 0) is guaranteed to always be the same as if(p == NULL). Also, the conditional if(p) is guaranteed to always be the same as if(p != 0).

Conclusion: your code will always detect a null pointer, but that's not technically the same as a pointer that points to address zero (even though in practice you are going to find that it usually is).