What is “!!” in C? [duplicate]

Answer 1

In C99 you can replace it by

#include 


pt->aa[(bool)(ts->flags & MASK)] = -val;

Of course if your code is to be portable to C89 then you'd be better off doing the !! trick or

pt->aa[(ts->flags & MASK)!=0] = -val;

or

pt->aa[(ts->flags & MASK)?1:0] = -val;

The generated code will be certainly identical.