What does !!(x) mean in C (esp. the Linux kernel)?

Question

I\'ve been reading through the Linux kernel (specifically, 2.6.11). I came across the following definition:

#define unlikely(x)     __builtin_expect(!!(x), 0)         


        

Answer 1

It's not so much a language syntax but a common shorthand for converting a char or int into quasi-boolean.

In C logical operations such as == && ! and so on can act on int, char etc, as there is no boolean type, however according to the standard they are guaranteed to return 0 for False and 1 for true.

So for example if you have

int x = 5;

you can force it to convert to a "boolean" type (there is no boolean type in C hence the quotes) you do

x = !x; /* !5 which gives 0 always */
x = !x; /* which gives 1 always */