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(ipc.info)semctl

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semctl
------

     int semctl (int semid, int semnum, int cmd, union semun arg);

   * semid : id obtained by a call to semget.

   * cmd :
          GETPID  return pid for the process that executed the last
          semop.

          GETVAL  return semval of semaphore with index semnum.

          GETNCNT return number of processes waiting for semval to
          increase.

          GETZCNT return number of processes waiting for semval to
          become 0

          SETVAL  set semval = arg.val.

          GETALL  read all semval's into arg.array.

          SETALL  set all semval's with values given in arg.array.

   * returns : 0 on success or as given above. -1 on failure.

   The first 4 operate on the semaphore with index semnum in the set.
The last two operate on all semaphores in the set.

   `arg' is a union :
     union semun
         int val;               value for SETVAL.
         struct semid_ds *buf;  buffer for IPC_STAT and IPC_SET.
         ushort *array;         array for GETALL and SETALL

   * IPC_SET, SETVAL, SETALL : sem_ctime is updated.

   * SETVAL, SETALL : Undo entries are cleared for altered semaphores in
     all processes. Processes sleeping on the wait queues are awakened
     if a semval becomes 0 or increases.

   * IPC_SET : sem_perm.uid, sem_perm.gid, sem_perm.mode are updated
     from user supplied values.

Errors:

EACCES : do not have permission for specified access.
EFAULT : arg is not accessible.
EIDRM  : The resource was removed.
EINVAL : semid < 0 or semnum < 0 or semnum >= nsems.
EPERM  : IPC_RMID, IPC_SET ... not creator, owner or super-user.
ERANGE : arg.array[i].semval > SEMVMX or < 0 for some i.