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? */
}
<         


        

Answer 1

My copy of ISO/IEC 9899:TC3 (Committee Draft — Septermber 7, 2007) says:

6.3 Conversions

1 Several operators convert operand values from one type to another automatically.

6.3.2.3 Pointers

3 An integer constant expression with the value 0 [...] 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.

So far, ptr!=0 is true (1) for every non-null ptr, but it's still open, how two null pointers compare.

6.5.9 Equality operators

5 [...] If one operand is a pointer and the other is a null pointer constant, the null pointer constant is converted to the type of the pointer.

6 Two pointers compare equal if and only if both are null pointers, both are [...]

Hence, ptr==0 is 1 (and ptr!=0 is 0), if and only if ptr is a null pointer.

6.5.3.3 Unary arithmetic operators

5 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).

So the same holds for !ptr.

6.8.4.1 The if statement

1 The controlling expression of an if statement shall have scalar type.

2 In both forms, the first substatement is executed if the expression compares unequal to 0.

Note, that a scalar type is an arithmetic type or a pointer type (see "6.2.5 Types", clause 21). Putting it together, we have:

• if (ptr) succeeds ⇔ ptr!=0 is 1 ⇔ ptr is not a null pointer.
• if (!ptr) succeeds ⇔ ptr==0 is 1 ⇔ ptr is a null pointer.