Difference between if (!file) and if (file == NULL)

Question

Is there any difference between these 2 versions of checking if file is actually opened:

FILE *file = fopen(fname, "rb");
if (!file)
{
    exit(1);
}

And

FILE *file = fopen(fname, "rb");
if (file == NULL)
{
    exit(1);
}

Answer 1

Both of these are equivalent.

The logical NOT operator ! is defined as follows in section 6.5.3.3p5 of the C standard:

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 !file is the same as 0 == file. The value 0 is considered a null pointer constant, defined in section 6.3.2.3p3:

An integer constant expression with the value 0, or such an expression cast to type void *, is called a null pointer constant.⁶⁶⁾ 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

66 ) The macro NULL is defined in <stddef.h> (and other headers) as a null pointer constant; see 7.19

This means that comparing a pointer to 0 is the same as comparing it to NULL. So !file and file == NULL are the same.