This manual page is part of the POSIX Programmer's Manual. The Linux
implementation of this interface may differ (consult the
corresponding Linux manual page for details of Linux behavior), or
the interface may not be implemented on Linux.
The shmctl() function operates on XSI shared memory (see the Base
Definitions volume of POSIX.1‐2008, Section 3.342, Shared MemoryObject). It is unspecified whether this function interoperates with
the realtime interprocess communication facilities defined in Section2.8, Realtime.
The shmctl() function provides a variety of shared memory control
operations as specified by cmd. The following values for cmd are
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.uidshm_perm.gidshm_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 GeneralDescription.
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
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
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, XSIInterprocess 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:
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
The following sections are informative.
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.
Portions of this text are reprinted and reproduced in electronic form
from IEEE Std 1003.1, 2013 Edition, Standard for Information
Technology -- Portable Operating System Interface (POSIX), The Open
Group Base Specifications Issue 7, Copyright (C) 2013 by the
Institute of Electrical and Electronics Engineers, Inc and The Open
Group. (This is POSIX.1-2008 with the 2013 Technical Corrigendum 1
applied.) 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.unix.org/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
IEEE/The Open Group 2013 SHMCTL(3P)