Argument

 

semid

   A semaphore set identifier, a positive integer. It is created
   by the semget function and used to identify the semaphore set on
   which to perform the control operation.
 

semnum

   Semaphore number, a non-negative integer. It identifies a
   semaphore within the semaphore set on which to perform the
   control operation.
 

cmd

   The control operation to perform on the semaphore.
 

 . . .

   Optional fourth argument of type union semun, which depends on
   the control operation requested in cmd.