What is "!!" in C? [duplicate]
! is negation. So !! is negation of negation. What is important is the fact that the result will be an int.
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!!xifx == 0is!!0, that is!1, that is0. -
!!xifx != 0is!!(!0), that is!!1, that is!0, that is1.
!! 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.
It is commonly (ab)used to convert any value into the ints 0 or 1 by repeated application of the boolean not operator, !.
For instance: !56 is 0, since 56 is "true" when viewed as a boolean. This means that !!56 is 1, since !0 is 1.