semctl(2)                                               semctl(2)



     NAME
          semctl - semaphore control operations

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

          int semctl(semid, semnum, cmd, arg)
          int semid, cmd;
          int semnum;
          union semun {
              int val;
              struct semid_ds *buf;
              ushort *array;
          } arg;

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

          The following cmds are executed with respect to the sema-
          phore specified by semid and semnum (see intro(2) for re-
          quired permissions and structure declarations):

          GETVAL     Return the value of semval (see intro(2)).

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

          GETPID     Return the value of sempid.

          GETNCNT    Return the value of semncnt.

          GETZCNT    Return the value of semzcnt.

          The following cmds return and set, respectively, every sem-
          val in the set of semaphores.

          GETALL     Place semvals into array pointed to by arg.array.

          SETALL     Set semvals according to the array pointed to by
                     arg.array.  When this command is successfully ex-
                     ecuted, 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



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     semctl(2)                                               semctl(2)



                     structure pointed to by arg.buf.  The contents of
                     this structure are defined in intro(2).

          IPC_SET    Set the value of the following members of the
                     data structure associated with semid to the
                     corresponding value found in the structure point-
                     ed to by arg.buf:
                     sem_perm.uid
                     sem_perm.gid
                     sem_perm.mode /* only low 9 bits */

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

          IPC_RMID   Remove the semaphore identifier specified by sem-
                     id from the system and destroy the set of sema-
                     phores and data structure associated with it.
                     This command can only be executed by a process
                     that has an effective user ID equal to either
                     that of superuser or to the value of sem_perm.uid
                     in the data structure associated with semid.  The
                     identifier and its associated data structure are
                     not actually removed until there are no more re-
                     ferencing processes.  See ipcrm(1), and ipcs(1).

     RETURN VALUE
          Upon successful completion, the value returned depends on
          cmd as follows:

               GETVAL    The value of semval.

               GETPID    The value of 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 in-
          dicate the error.

     ERRORS
          semctl will fail if one or more of the following are true:

          [EINVAL]       semid is not a valid semaphore identifier.

          [EINVAL]       semnum is less than zero or greater than
                         sem_nsems.



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     semctl(2)                                               semctl(2)



          [EINVAL]       cmd is not a valid command.

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

          [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 that of superuser and it is not
                         equal to the value of sem_perm.uid in the
                         data structure associated with semid.

          [EFAULT]       arg.buf points to an illegal address.

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




































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