using "!!" in "c"

Hello,
I saw in some open source projects a use of "!!" in "C" code;
for example:
in some header file
#define event_pending(v) \
(!!(v)->vcpu_info->evtchn_upcall_pending & \
!(v)->vcpu_info->evtchn_upcall_mask)

whereas evtchn_upcall_pending is of type unsigned char
(and also evtchn_upcall_mask is of type unsigned char).

What does "!!" operator do in this case ? Any ideas?
MR

.