Section (2) shmop
shmat, shmdt — System V shared memory operations
#include <sys/types.h> #include <sys/shm.h>
|const void *shmaddr,|
||const void *shmaddr
shmat() attaches the
System V shared memory segment identified by
shmid to the address space of
the calling process. The attaching address is specified by
shmaddr with one of
the following criteria:
shmaddris NULL, the system chooses a suitable (unused) page-aligned address to attach the segment.
shmaddrisn_zsingle_quotesz_t NULL and
SHM_RNDis specified in
shmflg, the attach occurs at the address equal to
shmaddrrounded down to the nearest multiple of
shmaddrmust be a page-aligned address at which the attach occurs.
In addition to
the following flags may be specified in the
shmflg bit-mask argument:
SHM_EXEC(Linux-specific; since Linux 2.6.9)
Allow the contents of the segment to be executed. The caller must have execute permission on the segment.
Attach the segment for read-only access. The process must have read permission for the segment. If this flag is not specified, the segment is attached for read and write access, and the process must have read and write permission for the segment. There is no notion of a write-only shared memory segment.
This flag specifies that the mapping of the segment should replace any existing mapping in the range starting at
shmaddrand continuing for the size of the segment. (Normally, an EINVAL error would result if a mapping already exists in this address range.) In this case,
shmaddrmust not be NULL.
The brk(2) value of the calling process is not altered by the attach. The segment will automatically be detached at process exit. The same segment may be attached as a read and as a read-write one, and more than once, in the process_zsingle_quotesz_s address space.
updates the members of the
shmid_ds structure (see
with the shared memory segment as follows:
shm_atimeis set to the current time.
shm_lpidis set to the process-ID of the calling process.
shm_nattchis incremented by one.
shmdt() detaches the
shared memory segment located at the address specified by
shmaddr from the
address space of the calling process. The to-be-detached
segment must be currently attached with
shmaddr equal to the value
returned by the attaching
On a successful
call, the system updates the members of the
associated with the shared memory segment as follows:
shm_dtimeis set to the current time.
shm_lpidis set to the process-ID of the calling process.
shm_nattchis decremented by one. If it becomes 0 and the segment is marked for deletion, the segment is deleted.
the address of the attached shared memory segment; on error,
(void *) −1 is
errno is set to
indicate the cause of the error.
0; on error −1 is returned, and
errno is set to indicate the cause of the
errno is set to one of the
The calling process does not have the required permissions for the requested attach type, and does not have the
CAP_IPC_OWNERcapability in the user namespace that governs its IPC namespace.
shmidpoints to a removed identifier.
shmidvalue, unaligned (i.e., not page-aligned and
SHM_RNDwas not specified) or invalid
shmaddrvalue, or can_zsingle_quotesz_t attach segment at
SHM_REMAPwas specified and
Could not allocate memory for the descriptor or for the page tables.
errno is set as follows:
There is no shared memory segment attached at
shmaddris not aligned on a page boundary.
POSIX.1-2001, POSIX.1-2008, SVr4.
In SVID 3 (or perhaps earlier), the type of the
shmaddr argument was changed
from char * into
const void *, and the returned type
shmat() from char * into void *.
After a fork(2), the child inherits the attached shared memory segments.
After an execve(2), all attached shared memory segments are detached from the process.
Upon _exit(2), all attached shared memory segments are detached from the process.
shmaddr equal to NULL
is the preferred, portable way of attaching a shared memory
segment. Be aware that the shared memory segment attached in
this way may be attached at different addresses in different
processes. Therefore, any pointers maintained within the
shared memory must be made relative (typically to the
starting address of the segment), rather than absolute.
On Linux, it is possible to attach a shared memory segment even if it is already marked to be deleted. However, POSIX.1 does not specify this behavior and many other implementations do not support it.
The following system parameter affects
Segment low boundary address multiple. When explicitly specifying an attach address in a call to
shmat(), the caller should ensure that the address is a multiple of this value. This is necessary on some architectures, in order either to ensure good CPU cache performance or to ensure that different attaches of the same segment have consistent views within the CPU cache.
SHMLBAis normally some multiple of the system page size. (On many Linux architectures,
SHMLBAis the same as the system page size.)
The implementation places no intrinsic per-process limit
on the number of shared memory segments (
This page is part of release 5.04 of the Linux
man-pages project. A
description of the project, information about reporting bugs,
and the latest version of this page, can be found at
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it)
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
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Since the Linux kernel and libraries are constantly changing, this
manual page may be incorrect or out-of-date. The author(s) assume no
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the use of the information contained herein. The author(s) may not
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which is licensed free of charge, as they might when working
Formatted or processed versions of this manual, if unaccompanied by
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Modified Sun Nov 28 17:06:19 1993, Rik Faith (faithcs.unc.edu)
with material from Luigi P. Bai (lpbsoftint.com)
Portions Copyright 1993 Luigi P. Bai
Modified Tue Oct 22 22:04:23 1996 by Eric S. Raymond <esrthyrsus.com>
Modified, 5 Jan 2002, Michael Kerrisk <mtk.manpagesgmail.com>
Modified, 19 Sep 2002, Michael Kerrisk <mtk.manpagesgmail.com>
Added SHM_REMAP flag description
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
Changed wording and placement of sentence regarding attachment
of segments marked for destruction
FIXME . Add an example program to this page.