SEMCTL(P)						     POSIX Programmer's Manual							 SEMCTL(P)

NAME

       semctl - XSI semaphore control operations

SYNOPSIS

       #include <sys/sem.h>

       int semctl(int semid, int semnum, int cmd, ...);

DESCRIPTION

       The semctl() function operates on XSI semaphores (see the Base Definitions volume of IEEE Std 1003.1-2001, Section 4.15, Semaphore).  It is
       unspecified whether this function interoperates with the realtime interprocess communication facilities defined in Realtime .

       The semctl() function provides a variety of semaphore control operations as specified by cmd. The fourth argument is optional  and  depends
       upon the operation requested. If required, it is of type union semun, which the application shall explicitly declare:

	      union semun {
		  int val;
		  struct semid_ds *buf;
		  unsigned short  *array;
	      } arg;

       The  following  semaphore  control operations as specified by cmd are executed with respect to the semaphore specified by semid and semnum.
       The level of permission required for each operation is shown with each command; see XSI Interprocess Communication . The symbolic names for
       the values of cmd are defined in the <sys/sem.h> header:

       GETVAL Return the value of semval; see <sys/sem.h>. Requires read permission.

       SETVAL Set  the	value  of  semval to arg.val, where arg is the value of the fourth argument to semctl(). When this command is successfully
	      executed, the semadj value corresponding to the specified semaphore in all processes is cleared. Requires alter permission; see  XSI
	      Interprocess Communication .

       GETPID Return the value of sempid. Requires read permission.

       GETNCNT
	      Return the value of semncnt. Requires read permission.

       GETZCNT
	      Return the value of semzcnt. Requires read permission.

       The following values of cmd operate on each semval in the set of semaphores:

       GETALL Return  the  value  of semval for each semaphore in the semaphore set and place into the array pointed to by arg.array, where arg is
	      the fourth argument to semctl().	Requires read permission.

       SETALL Set the value of semval for each semaphore in the semaphore set according to the array pointed to by arg.array,  where  arg  is  the
	      fourth argument to semctl(). When this command is successfully executed, the semadj values corresponding to each specified semaphore
	      in all processes are cleared.  Requires alter permission.

       The following values of cmd are also available:

       IPC_STAT
	      Place the current value of each member of the semid_ds data structure associated	with  semid  into  the	structure  pointed  to	by
	      arg.buf, where arg is the fourth argument to semctl(). The contents of this structure are defined in <sys/sem.h>. Requires read per-
	      mission.

       IPC_SET
	      Set the value of the following members of the semid_ds data structure associated with semid to the corresponding value found in  the
	      structure pointed to by arg.buf, where arg is the fourth argument to semctl():

	      sem_perm.uid
	      sem_perm.gid
	      sem_perm.mode

       The  mode bits specified in IPC General Description are copied into the corresponding bits of the sem_perm.mode associated with semid.  The
       stored values of any other bits are unspecified.

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

       IPC_RMID
	      Remove  the  semaphore  identifier  specified by semid from the system and destroy the set of semaphores and semid_ds 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	a  process
	      with appropriate privileges or to the value of sem_perm.cuid or sem_perm.uid in the semid_ds data structure associated with semid.

RETURN VALUE

       If successful, 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.

       Otherwise, semctl() shall return -1 and set errno to indicate the error.

ERRORS

       The semctl() function shall fail if:

       EACCES Operation permission is denied to the calling process; see XSI Interprocess Communication .

       EINVAL The value of semid is not a valid semaphore identifier, or the value of semnum is less than 0 or greater than or equal to sem_nsems,
	      or the value of cmd is not a valid command.

       EPERM  The argument cmd is equal to IPC_RMID or IPC_SET and the effective user ID of the calling process is not equal to that of a  process
	      with  appropriate privileges and it is not equal to the value of sem_perm.cuid or sem_perm.uid in the data structure associated with
	      semid.

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

       The following sections are informative.

EXAMPLES

       None.

APPLICATION USAGE

       The fourth parameter in the SYNOPSIS section is now specified as "..."  in order to avoid a clash with the ISO C standard when referring to
       the union semun (as defined in Issue 3) and for backwards-compatibility.

       The  POSIX  Realtime  Extension	defines  alternative interfaces for interprocess communication. Application developers who need to use IPC
       should design their applications so that modules using the IPC routines described in XSI Interprocess Communication can be easily  modified
       to use the alternative interfaces.

RATIONALE

       None.

FUTURE DIRECTIONS

       None.

SEE ALSO

       XSI  Interprocess  Communication , Realtime , semget() , semop() , sem_close() , sem_destroy() , sem_getvalue() , sem_init() , sem_open() ,
       sem_post() , sem_unlink() , sem_wait() , the Base Definitions volume of IEEE Std 1003.1-2001, <sys/sem.h>

COPYRIGHT

       Portions of this text are reprinted and reproduced in electronic form from IEEE Std 1003.1, 2003 Edition, Standard for Information Technol-
       ogy -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6, Copyright (C) 2001-2003 by the Institute of
       Electrical and Electronics Engineers, Inc and The Open Group. In the event of any discrepancy between this version and  the  original  IEEE
       and  The  Open Group Standard, the original IEEE and The Open Group Standard is the referee document. The original Standard can be obtained
       online at http://www.opengroup.org/unix/online.html .

IEEE
/The Open Group						       2003								 SEMCTL(P)