SHMCTL
Section: POSIX Programmer's Manual (3P)
Updated: 2017
Page Index
PROLOG
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.
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 .
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