double negation in C : is it guaranteed to return 0/1?

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Yes, in C99, see §6.5.3.3/4:

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 !x and !!y can only yield 0 or 1, as ints.

For other operators, in C99, see also Is the “true” result of >, <, !, &&, || or == defined?

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