！0保证在C89中为1？

Question

I've searched the standard but didn't notice the mentioned part.

Is it just "anything but 0" and 1 or it is compiler-dependent?

Answer 1

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 .

Appears in C89/C90, C99, and C11.

Answer 2

As hobbs said in his answer, section 6.5.3.3.5 of the C standard states that !0 evaluates to 1 .

Additionally, this behavior can be used to normalize an integer to a boolean value (ie either 0 or 1 ) with the expression !!x .

• When x = 0 , !!x = !!0 = !1 = 0 .
• When x != 0 , !x = 0 , so !!x = !0 = 1 .