Description

   The sem_getvalue function updates a location referenced by the
   sval argument with the value of semaphore sem. The updated value
   represents an actual semaphore value that occurred during the
   call, but may not be the actual value of the semaphore at the
   time that the value is returned to the calling process.

   If the semaphore is locked, the value returned will either be
   zero or a negative number indicating the number of processes
   waiting for the semaphore at some time during the call.