Actually this function returns the number of data items in
the queue minus the number of waiting threads, so a negative
value means waiting threads, and a positive value means available
entries in the queue. A return value of 0 could mean n entries
in the queue and n threads waiting. This can happen due to locking
of the queue or due to scheduling.
This function must be called while holding the queue's lock.
Returns the length of the queue.
Actually this function returns the number of data items in the queue minus the number of waiting threads, so a negative value means waiting threads, and a positive value means available entries in the queue. A return value of 0 could mean n entries in the queue and n threads waiting. This can happen due to locking of the queue or due to scheduling.
This function must be called while holding the queue's lock.