SEMCTL(3P)                                          POSIX Programmer's Manual                                         SEMCTL(3P)



PROLOG
       This  manual  page is part of the POSIX Programmer's Manual.  The Linux implementation of this interface may differ (con-
       sult the corresponding Linux manual page for details of Linux behavior), or the  interface  may  not  be  implemented  on
       Linux.

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  facili-
       ties 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 corre-
              sponding 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 permission.

       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  sys-
              tem-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 Technology -- Portable Operating System Interface (POSIX), The Open Group Base Specifications Issue 6,  Copy-
       right (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.open-
       group.org/unix/online.html .



IEEE/The Open Group                                           2003                                                    SEMCTL(3P)