SEMCTL(2-SVR4)      RISC/os Reference Manual       SEMCTL(2-SVR4)



NAME
     semctl - semaphore control operations

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

     union semun {
          int val;
          struct semidds *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 sema-
     phore 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 sem-
     val 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.





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     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 struc-
                     ture 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 pro-
                     cess that has an effective user ID equal to
                     either that of superuser, or to the value of
                     sem_perm.cuid or sem_perm.uid in the data
                     structure associated with semid.

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

     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



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                    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, or to the
                    value of sem_perm.cuid or 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).

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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