shmat, shmdt — System V shared memory operations
#include <sys/types.h> #include <sys/shm.h>
void
*shmat( |
int shmid, |
| const void *shmaddr, | |
int shmflg); |
int
shmdt( |
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:
If shmaddr
is NULL, the system chooses a suitable (unused)
address at which to attach the segment.
If shmaddr
isn't NULL and SHM_RND
is specified in shmflg, the attach
occurs at the address equal to shmaddr rounded down to
the nearest multiple of SHMLBA.
Otherwise, shmaddr must be a
page-aligned address at which the attach occurs.
In addition to SHM_RND,
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.
SHM_RDONLYAttach 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.
SHM_REMAP
(Linux-specific)This flag specifies that the mapping of the
segment should replace any existing mapping in the
range starting at shmaddr and 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, shmaddr must 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's address space.
A successful shmat() call
updates the members of the shmid_ds structure (see
shmctl(2)) associated
with the shared memory segment as follows:
shm_atime is set to
the current time.
shm_lpid
is set to the process-ID of the calling process.
shm_nattch is
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 shmat() call.
On a successful shmdt()
call, the system updates the members of the shmid_ds structure
associated with the shared memory segment as follows:
shm_dtime is set to
the current time.
shm_lpid
is set to the process-ID of the calling process.
shm_nattch is
decremented by one. If it becomes 0 and the segment
is marked for deletion, the segment is deleted.
On success, shmat() returns
the address of the attached shared memory segment; on error,
(void *) −1 is
returned, and errno is set to
indicate the cause of the error.
On success, shmdt() returns
0; on error −1 is returned, and errno is set to indicate the cause of the
error.
When shmat() fails,
errno is set to one of the
following:
The calling process does not have the required
permissions for the requested attach type, and does not
have the CAP_IPC_OWNER
capability.
shmid points
to a removed identifier.
Invalid shmid value, unaligned
(i.e., not page-aligned and SHM_RND was not specified) or invalid
shmaddr value,
or can't attach segment at shmaddr, or SHM_REMAP was specified and
shmaddr was
NULL.
Could not allocate memory for the descriptor or for the page tables.
When shmdt() fails,
errno is set as follows:
There is no shared memory segment attached at
shmaddr; or,
shmaddr is 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
of 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.
Using shmat() with
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 shmat():
SHMLBASegment 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. SHMLBA is normally some multiple of
the system page size (on many Linux architectures, it
is the same as the system page size).
The implementation places no intrinsic per-process limit
on the number of shared memory segments (SHMSEG).
This page is part of release 4.07 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
https://www.kernel.org/doc/man−pages/.
|
Copyright 1993 Giorgio Ciucci (giorgiocrcc.it) %%%LICENSE_START(VERBATIM) 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. %%%LICENSE_END 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. |