SHMCTL(3P)                                          POSIX Programmer's Manual                                         SHMCTL(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
       shmctl - XSI shared memory control operations

SYNOPSIS
       #include <sys/shm.h>

       int shmctl(int shmid, int cmd, struct shmid_ds *buf);


DESCRIPTION
       The  shmctl()  function  operates  on XSI shared memory (see the Base Definitions volume of IEEE Std 1003.1-2001, Section
       3.340, Shared Memory Object). It is unspecified whether this function interoperates with the realtime interprocess commu-
       nication facilities defined in Realtime .

       The  shmctl()  function  provides a variety of shared memory control operations as specified by cmd. The following values
       for cmd are available:

       IPC_STAT
              Place the current value of each member of the shmid_ds data structure associated with  shmid  into  the  structure
              pointed to by buf. The contents of the structure are defined in <sys/shm.h>.

       IPC_SET
              Set  the  value of the following members of the shmid_ds data structure associated with shmid to the corresponding
              value found in the structure pointed to by buf:


              shm_perm.uid
              shm_perm.gid
              shm_perm.mode    Low-order nine bits.

       IPC_SET can only be executed by a process that has an effective user ID equal to either that of a process with  appropri-
       ate privileges or to the value of shm_perm.cuid or shm_perm.uid in the shmid_ds data structure associated with shmid.

       IPC_RMID
              Remove  the  shared memory identifier specified by shmid from the system and destroy the shared memory segment and
              shmid_ds data structure associated with it. IPC_RMID 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 shm_perm.cuid or shm_perm.uid
              in the shmid_ds data structure associated with shmid.


RETURN VALUE
       Upon successful completion, shmctl() shall return 0; otherwise, it shall return -1 and set errno to indicate the error.

ERRORS
       The shmctl() function shall fail if:

       EACCES The argument cmd is equal to IPC_STAT and the calling process does not have read permission; see XSI  Interprocess
              Communication .

       EINVAL The value of shmid is not a valid shared memory identifier, 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 shm_perm.cuid or shm_perm.uid in
              the data structure associated with shmid.


       The shmctl() function may fail if:

       EOVERFLOW
              The cmd argument is IPC_STAT and the gid or uid value is too large to be stored in the structure pointed to by the
              buf argument.


       The following sections are informative.

EXAMPLES
       None.

APPLICATION USAGE
       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, shmat(), shmdt(), shmget(), shm_open(), shm_unlink(), the Base Definitions vol-
       ume of IEEE Std 1003.1-2001, <sys/shm.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                                                    SHMCTL(3P)