FAQ 32.10 When is zero not necessarily zero?

When dealing with pointers.

When the token 0 appears in the source code at a place where a pointer should be, the compiler interprets the token 0 as the NULL pointer. However, the bit pattern for the NULL pointer is not guaranteed to be all zeros. More specifically, setting a pointer to NULL may set some of the bits of that pointer to 1.

Depending on the hardware, the operating system, or the compiler, a pointer whose bits are all zeros may not be the same as the NULL pointer. For example, using memset() to set all the bits of a pointer to zero may not make that pointer equal to NULL.

In the following program, all conforming C++ compilers produce code that prints 0 is NULL, then NULL is NULL, but some may produce code that prints memsetPtr is not NULL.

 #include <iostream> #include <string> using namespace std; void checkForNull(const string& nameOfPointer, char* p) {   cout << nameOfPointer;   if (p == NULL)     cout << " is NULL\n";   else     cout << " is not NULL\n"; } int main() {   checkForNull("0", 0);                              <-- 1   checkForNull("NULL", NULL);                        <-- 2   char* memsetPtr;   memset(&memsetPtr, '\0', sizeof(memsetPtr));       <-- 3   checkForNull("memsetPtr", memsetPtr); } 

(1) OK: NULL pointer

(2) OK: NULL pointer

(3) BAD: Undefined pointer!

Another common way to generate a pointer whose bits are all zero that is equally dangerous is with unions. For example, the following is wrong on two levels. First, it accesses the char* member of the union even though it was the unsigned long member that was set most recently. Second, it assumes that a pointer whose bits are all zero is the same as a NULL pointer the output may be unionPtr is not NULL on some machines.

 union Fred {   unsigned long n;   char* p; }; void badForm() {   Fred x;   x.n = 0;                                           <-- 1   char* unionPtr = x.p;                              <-- 2   checkForNull("unionPtr", unionPtr); } 

(1) OK: Set the unsigned long member to 0

(2) BAD: The char* member isn't defined!