What is “!!” in C? [duplicate]

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! 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.

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