SHMCTL(3)                  POSIX Programmer's Manual                 SHMCTL(3)



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

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 POSIX.1-2017, Section 3.346, Shared Memory
       Object).  It is unspecified whether this function interoperates with
       the realtime interprocess communication facilities defined in Section
       2.8, 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.

                   Also, the shm__ctime timestamp shall be set to the current
                   time, as described in Section 2.7.1, IPC General
                   Description.

                   IPC_SET 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.

       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 Section 2.7, 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 Section 2.7, XSI Interprocess Communication can be easily
       modified to use the alternative interfaces.

RATIONALE
       None.

FUTURE DIRECTIONS
       None.

SEE ALSO
       Section 2.7, XSI Interprocess Communication, Section 2.8, Realtime,
       shmat(), shmdt(), shmget(), shm__open(), shm__unlink()

       The Base Definitions volume of POSIX.1-2017, Section 3.346, Shared
       Memory Object, <sys_shm.h>

COPYRIGHT
       Portions of this text are reprinted and reproduced in electronic form
       from IEEE Std 1003.1-2017, Standard for Information Technology --
       Portable Operating System Interface (POSIX), The Open Group Base
       Specifications Issue 7, 2018 Edition, Copyright (C) 2018 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 .

       Any typographical or formatting errors that appear in this page are
       most likely to have been introduced during the conversion of the source
       files to man page format. To report such errors, see
       https://www.kernel.org/doc/man-pages/reporting_bugs.html .



IEEE/The Open Group                  2017                            SHMCTL(3)
