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.
Return: the length of the @queue.