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.