semctl(2)							System Calls Manual							 semctl(2)

Name
       semctl - semaphore control operations

Syntax
       #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
       The  system  call  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.  For further information, see

       SETVAL	      Set the value of semval to arg.val.  When this command is successfully executed, the semadj value corresponding to the spec-
		      ified 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 every semval 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 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

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

Return Values
       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 other      A value of 0.

       Otherwise, a value of -1 is returned and errno is set to indicate the error.

Diagnostics
       The system call fails if any of the following is true:

       [EINVAL]       The semid is not a valid semaphore identifier.

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

       [EINVAL]       The cmd is not a valid command.

       [EACCES]       Operation permission is denied to the calling process.  For further information, see

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

       [EPERM]	      The  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
		      nor to the value of sem_perm.uid in the data structure associated with semid.

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

See Also
       errno(2), intro(2), semget(2), semop(2)

																	 semctl(2)