Is NULL always false?

Question

Is it safe to assume that NULL always translates to false in C?

void *somePtr = NULL;

if (!somePtr) {
  /* This will always be executed? */
}
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Answer 1

Yes, if(!p) is valid and guaranteed to work.

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant. If a null pointer constant is converted to a pointer type, the resulting pointer, called a null pointer, is guaranteed to compare unequal to a pointer to any object or function.

https://port70.net/~nsz/c/c11/n1570.html#6.3.2.3p3

This means that (void*)0 is a null pointer. It also means that if p is a pointer, then p==0 is equivalent to p==(void*)0.

It also means that if p is a not a null pointer, then p==(void*)0 will evaluate to 0.

So far, so good.

Conversion of a null pointer to another pointer type yields a null pointer of that type. Any two null pointers shall compare equal.

http://port70.net/~nsz/c/c11/n1570.html#6.3.2.3p4

Note that "Any two null pointers shall compare equal." This means that if p is a null pointer, then p==0 will evaluate to true, because 0 will be promoted to (void*)0 which is a null pointer. It also mean that no non-null pointer can be equal to a null pointer.

Let's have a look at the negation operator.

The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0. The result has type int. The expression !E is equivalent to (0==E).

http://port70.net/~nsz/c/c11/n1570.html#6.5.3.3p5

This tells us that !p is the same as p==0 by definition, which is the same as p==(void*)0 as mentioned above.

And taking the fact that all null pointers are equal into consideration, this means that p==(void*)0 can only evaluate to true if p is a null pointer and and only false if p is not a null pointer.

So yes, if(!p) is a perfectly safe way to check if p is a null pointer or not.