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Is NULL always equal to 0(zero)?

The answer depends on what you mean by "equal to." If you mean "compares equal to," such as

if ( /* ... */ )
     p = NULL;
     p = /* something else */;
/* ... */
if ( p == 0 )

then yes, NULL is always equal to 0. That's the whole point of the definition of a null pointer.

If you mean "is stored the same way as an integer zero," the answer is no, not necessarily. That's the most common way to store a null pointer. On some machines, a different representation is used.

The only way you're likely to tell that a null pointer isn't stored the same way as zero is by displaying a pointer in a debugger, or printing it. (If you cast a null pointer to an integer type, that might also show a nonzero value.)