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SEMCTL(2)                     System Calls Manual                    SEMCTL(2)



NAME
       semctl - semaphore control operations

SYNOPSIS
       #include <&lt;sys/types.h>&gt;
       #include <&lt;sys/ipc.h>&gt;
       #include <&lt;sys/sem.h>&gt;

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

DESCRIPTION
       semctl()  provides  a variety of semaphore control operations as speci-
       fied 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
                      cmd is successfully executed, the  semadj  value  corre-
                      sponding  to the specified semaphore in all processes is
                      cleared.

       GETPID         Return the value of 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  the 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 exe-
                      cuted 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 low 9 bits */

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

       In  the semop(2) and semctl(2) system call descriptions, the permission
       required for an operation is given as "[token]", where "token"  is  the
       type of permission needed interpreted as follows:

              00400                       Read by user
              00200                       Alter by user
              00060                       Read, Alter by group
              00006                       Read, Alter by others

       Read  and  Alter permissions on a semid are granted to a process if one
       or more of the following are true:

              The effective user ID of the process is super-user.

              The effective user ID of the process matches sem_perm.[c]uid  in
              the data structure associated with semid and the appropriate bit
              of the "user" portion (0600) of sem_perm.mode is set.

              The  effective  user  ID  of  the   process   does   not   match
              sem_perm.[c]uid  and  the  effective  group  ID  of  the process
              matches sem_perm.[c]gid and the appropriate bit of  the  "group"
              portion (060) of sem_perm.mode is set.

              The   effective   user   ID   of  the  process  does  not  match
              sem_perm.[c]uid and the effective group ID of the  process  does
              not match sem_perm.[c]gid and the appropriate bit of the "other"
              portion (06) of sem_perm.mode is set.

       Otherwise, the corresponding permissions are denied.

RETURN VALUES
       On success, the value returned by semctl() 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     0.

       On failure, semctl() returns -1 and sets errno to indicate the error.

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

       EFAULT         arg.buf points to an illegal address.

       EINVAL         semid is not a valid semaphore identifier.

                      semnum is less than zero or greater than sem_nsems.

                      cmd is not a valid command.

       EPERM          cmd  is IPC_RMID or IPC_SET and the effective user ID of
                      the calling process is not super-user.

                      cmd is IPC_RMID or IPC_SET and the effective user ID  of
                      the calling process is not the value of sem_perm.cuid or
                      sem_perm.uid  in  the  data  structure  associated  with
                      semid.

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

SEE ALSO
       intro(2), semget(2), semop(2), ipcrm(1), ipcs(1)



                                21 January 1990                      SEMCTL(2)