shmget — allocates a shared memory segment
#include <sys/ipc.h> #include <sys/shm.h>
int
shmget( |
key_t | key, |
| size_t | size, | |
| int | shmflg); |
shmget() returns the
identifier of the shared memory segment associated with the
value of the argument key. A new shared memory
segment, with size equal to the value of size rounded up to a multiple
of PAGE_SIZE, is created if
key has the value
IPC_PRIVATE or key isn't IPC_PRIVATE, no shared memory segment
corresponding to key
exists, and IPC_CREAT is
specified in shmflg.
If shmflg
specifies both IPC_CREAT and
IPC_EXCL and a shared memory
segment already exists for key, then shmget() fails with errno set to EEXIST. (This is analogous to the effect
of the combination O_CREAT |
O_EXCL for open(2).)
The value shmflg
is composed of:
IPC_CREATto create a new segment. If this flag is not used,
then shmget() will find
the segment associated with key and check to see if
the user has permission to access the segment.
IPC_EXCLused with IPC_CREAT to
ensure failure if the segment already exists.
mode_flags(least significant 9 bits) specifying the
permissions granted to the owner, group, and world.
These bits have the same format, and the same meaning,
as the mode
argument of open(2). Presently,
the execute permissions are not used by the system.
SHM_HUGETLB (since Linux
2.6)Allocate the segment using "huge pages." See the
kernel source file Documentation/vm/hugetlbpage.txt for
further information.
SHM_NORESERVE (since Linux
2.6.15)This flag serves the same purpose as the mmap(2) MAP_NORESERVE flag. Do not reserve
swap space for this segment. When swap space is
reserved, one has the guarantee that it is possible to
modify the segment. When swap space is not reserved one
might get SIGSEGV upon a
write if no physical memory is available. See also the
discussion of the file /proc/sys/vm/overcommit_memory in
proc(5).
When a new shared memory segment is created, its contents
are initialized to zero values, and its associated data
structure, shmid_ds
(see shmctl(2)), is initialized
as follows:
shm_perm.cuid and
shm_perm.uid
are set to the effective user ID of the calling
process.
shm_perm.cgid and
shm_perm.gid
are set to the effective group ID of the calling
process.
The least significant 9 bits of shm_perm.mode are set
to the least significant 9 bit of shmflg.
shm_segsz
is set to the value of size.
shm_lpid,
shm_nattch,
shm_atime and
shm_dtime are
set to 0.
shm_ctime
is set to the current time.
If the shared memory segment already exists, the permissions are verified, and a check is made to see if it is marked for destruction.
On failure, errno is set to
one of the following:
The user does not have permission to access the
shared memory segment, and does not have the
CAP_IPC_OWNER
capability.
IPC_CREAT | IPC_EXCL was specified and the segment exists.
A new segment was to be created and size < SHMMIN or size > SHMMAX, or no new segment was to be
created, a segment with given key existed, but
size is greater
than the size of that segment.
The system limit on the total number of open files has been reached.
No segment exists for the given key, and IPC_CREAT was not specified.
No memory could be allocated for segment overhead.
All possible shared memory IDs have been taken
(SHMMNI), or allocating a
segment of the requested size would cause the
system to exceed the system-wide limit on shared memory
(SHMALL).
The SHM_HUGETLB flag
was specified, but the caller was not privileged (did
not have the CAP_IPC_LOCK
capability).
IPC_PRIVATE isn't a flag
field but a key_t type. If this
special value is used for key, the system call ignores
everything but the least significant 9 bits of shmflg and creates a new shared
memory segment (on success).
The following limits on shared memory segment resources
affect the shmget() call:
SHMALLSystem wide maximum of shared memory pages (on
Linux, this limit can be read and modified via
/proc/sys/kernel/shmall).
SHMMAXMaximum size in bytes for a shared memory segment:
policy dependent (on Linux, this limit can be read and
modified via /proc/sys/kernel/shmmax).
SHMMINMinimum size in bytes for a shared memory segment:
implementation dependent (currently 1 byte, though
PAGE_SIZE is the
effective minimum size).
SHMMNISystem wide maximum number of shared memory
segments: implementation dependent (currently 4096, was
128 before Linux 2.3.99; on Linux, this limit can be
read and modified via /proc/sys/kernel/shmmni).
The implementation has no specific limits for the
per-process maximum number of shared memory segments
(SHMSEG).
The name choice IPC_PRIVATE
was perhaps unfortunate, IPC_NEW would more clearly show its
function.
This page is part of release 2.79 of the Linux man-pages project. A
description of the project, and information about reporting
bugs, can be found at
http://www.kernel.org/doc/man-pages/.
|
Copyright (c) 1993 Luigi P. Bai (lpbsoftint.com) July 28, 1993 Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. Modified Wed Jul 28 10:57:35 1993, Rik Faith <faithcs.unc.edu> Modified Sun Nov 28 16:43:30 1993, Rik Faith <faithcs.unc.edu> with material from Giorgio Ciucci <giorgiocrcc.it> Portions Copyright 1993 Giorgio Ciucci <giorgiocrcc.it> Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond <esrthyrsus.com> Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpagesgmail.com> Removed EIDRM from errors - that can't happen... Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com> Language and formatting clean-ups Added notes on /proc files |