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cgccbooleanboolean-expression

What is "!!" in C?

I have encountered the following snippet:

pt->aa[!!(ts->flags & MASK)] = -val;
  1. What does !! (double exclamation marks/ exclamation points/ two NOT operators) stand for in c?
  2. Doesn't (!!NULL) == NULL?
Solution

  • ! is negation. So !! is negation of negation. What is important is the fact that the result will be an int.

    • !!x if x == 0 is !!0, that is !1, that is 0.
    • !!x if x != 0 is !!(!0), that is !!1, that is !0, that is 1.

    !! is used commonly if you want to convert any non-zero value to 1 while being certain that 0 remains a 0.

    And indeed, !!NULL == NULL, since !!NULL == !!0 and !!0 == !1 and finally !1 == 0.

    Consequently, in the short piece of code you cited the array subscript will be either 0 if the value of the expression in parenthesis is NULL, and 1 otherwise.