what does it use two "!!"

[Valdis.Kletnieks at vt.edu]

On Mon, 01 Apr 2013 15:10:46 +0800, Ben Wu said:

> 1> I found some placeuse two "!!", what's means 
>    if(button->gpio != INVALID_GPIO)
>        state = !!((gpio_get_value(button->gpio) ? 1 : 0) ^ button->active_low);
>    else

Gaah. That line of code fell out of the ugly tree and hit every branch
on the way down.

Use of "!!"  *and*  "? 1 :0 " in the same line of code to do the same thing.
Ouch.
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