semctl(2)                                                            semctl(2)


NAME
     semctl - semaphore control operations

SYNOPSIS
     #include <sys/types.h>
     #include <sys/ipc.h>
     #include <sys/sem.h>

     union semun {
          int val;
          struct semid_ds *buf;
          ushort *array;
     };

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

DESCRIPTION
     semctl provides a variety of semaphore control operations as specified by
     cmd.

     The following cmds are executed with respect to the semaphore specified
     by semid and semnum:

          GETVAL     Return the value of semval [see intro(2)].  {READ}

          SETVAL     Set the value of semval to arg.val.  {ALTER}.  When this
                     command is successfully executed, the semadj value
                     corresponding to the specified semaphore in all processes
                     is cleared.

          GETPID     Return the value of (int) sempid.  {READ}

          GETNCNT    Return the value of semncnt.  {READ}

          GETZCNT    Return the value of semzcnt.  {READ}

     The following cmds return and set, respectively, every semval in the set
     of semaphores.

          GETALL     Place semvals into array pointed to by arg.array.  {READ}

          SETALL     Set semvals according to the array pointed to by
                     arg.array.  {ALTER}.  When this cmd is successfully
                     executed, the semadj values corresponding to each
                     specified semaphore in all processes are cleared.

     The following cmds are also available:

          IPC_STAT   Place the current value of each member of the data
                     structure associated with semid into the structure
                     pointed to by arg.buf.  The contents of this structure
                     are defined in intro(2).  {READ}


          IPC_SET    Set the value of the following members of the data
                     structure associated with semid to the corresponding
                     value found in the structure pointed to by arg.buf:

                         sem_perm.uid
                         sem_perm.gid
                         sem_perm.mode /* only access permission bits */

                     This command can be executed only by a process that has
                     an effective user ID equal to the value of sem_perm.cuid
                     or sem_perm.uid in the data structure associated with
                     semid or to a process that has the super-user privilege.

          IPC_RMID   Remove the semaphore identifier specified by semid from
                     the system and destroy the set of semaphores and data
                     structure associated with it.  This command can be
                     executed only by a process that has an effective user ID
                     equal to the value of sem_perm.cuid or sem_perm.uid in
                     the data structure associated with semid or to a process
                     that has the super-user privilege.

     semctl fails if one or more of the following are true:

     EACCES         Operation permission is denied to the calling process [see
                    intro(2)].

     EINVAL         semid is not a valid semaphore identifier.

     EINVAL         semnum is less than 0 or greater than sem_nsems.

     EINVAL         cmd is not a valid command.

     EINVAL         cmd is IPC_SET and sem_perm.uid or sem_perm.gid is not
                    valid.

     EOVERFLOW      cmd is IPC_STAT and uid or gid is too large to be stored
                    in the structure pointed to by arg.buf.

     ERANGE         cmd is SETVAL or SETALL and the value to which semval is
                    to be set is greater than the system imposed maximum.

     EPERM          cmd is equal to IPC_RMID or IPC_SET and the effective user
                    ID of the calling process is not equal to the value of
                    sem_perm.cuid or sem_perm.uid in the data structure
                    associated with semid and the calling process does not
                    have super-user privilege.

     EFAULT         arg.buf points to an illegal address.


SEE ALSO
     intro(2), semget(2), semop(2).

DIAGNOSTICS
     Upon successful completion, the value returned depends on cmd as follows:
          GETVAL    the value of semval
          GETPID    the value of (int) sempid
          GETNCNT   the value of semncnt
          GETZCNT   the value of semzcnt
          all others
                    a value of 0
     Otherwise, a value of -1 is returned and errno is set to indicate the
     error.


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