C Language – What Does !!(x) Mean in C (Linux Kernel)?

It is just two ! boolean not operators sitting next to each other.

The reason to use this idiom is to make sure that you receive a 1 or a 0. Actually it returns an empty string which numifys to 0. It's usually only used in numeric, or boolean context though.

You will often see this in Code Golf competitions, because it is shorter than using the ternary ? : operator with 1 and 0 _{($test ? 1 : 0)}.

!! undef  == 0
!! 0      == 0
!! 1      == 1
!! $obj   == 1
!! 100    == 1

undef ? 1 : 0  == 0
0     ? 1 : 0  == 0
1     ? 1 : 0  == 1
$obj  ? 1 : 0  == 1
100   ? 1 : 0  == 1

Boolean/logical operators in C are required to yield either 0 or 1.

From section 6.5.3.3/5 of the ISO C99 standard:

The result of the logical negation operator ! is 0 if the value of its operand compares unequal to 0, 1 if the value of its operand compares equal to 0.

In fact, !!x is a common idiom for forcing a value to be either 0 or 1 (I personally prefer x != 0, though).

Also see Q9.2 from the comp.lang.c FAQ.