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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?
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Zero is used to represent false, and One is used to represent true. For interpretation, Zero is interpreted as false and anything non-zero is interpreted as true. To make life easier, C Programmers typically define the terms "true" and "false" to have values 1 and 0 respectively.
Because 0 is a whole number. The smallest natural number is 1, not 0 because natural numbers start from 1 and the whole numbers start from 0.
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 typeint.
Appears in C89/C90, C99, and C11.
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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 (i.e. either 0 or 1) with the expression !!x.
x = 0, !!x = !!0 = !1 = 0.x != 0, !x = 0, so !!x = !0 = 1.
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