SHMOP(2) System Calls SHMOP(2)
NAME
shmop, shmat, shmdt - shared memory operations
SYNOPSIS
#include <sys/types.h>
#include <sys/shm.h>
void *shmat(
int shmid,
const void *shmaddr,
int shmflg);
int shmdt(
char *shmaddr);
Standard conforming
int shmdt(
const void *shmaddr);
DESCRIPTION
The
shmat() function attaches the shared memory segment associated
with the shared memory identifier specified by
shmid to the data
segment of the calling process.
The permission required for a shared memory control operation is
given as {
token}, where
token is the type of permission needed. The
types of permission are interpreted as follows:
00400 READ by user
00200 WRITE by user
00040 READ by group
00020 WRITE by group
00004 READ by others
00002 WRITE by others
See the
Shared Memory Operation Permissions section of
Intro(2) for
more information.
For shared memory segments created with the
SHM_SHARE_MMU or
SHM_PAGEABLE flags, the default protections cannot be changed so as
to prevent a single process from affecting other processes sharing
the same shared segment.
When (
shmflg&SHM_SHARE_MMU) is true, virtual memory resources in
addition to shared memory itself are shared among processes that use
the same shared memory.
When (
shmflg&SHM_PAGEABLE) is true, virtual memory resources are
shared and the dynamic shared memory (DISM) framework is created. The
dynamic shared memory can be resized dynamically within the specified
size in
shmget(2). The DISM shared memory is pageable unless it is
locked.
The shared memory segment is attached to the data segment of the
calling process at the address specified based on one of the
following criteria:
o If
shmaddr is equal to
(void *) 0, the segment is attached
to the first available address as selected by the system.
o If
shmaddr is equal to
(void *) 0 and (
shmflg&SHM_SHARE_MMU) or (
shmflg&SHM_PAGEABLE) is true,
then the segment is attached to the first available
suitably aligned address. When (
shmflg&SHM_SHARE_MMU) or
(
shmflg&SHM_PAGEABLE) is set, however, the permission
given by
shmget() determines whether the segment is
attached for reading or reading and writing.
o If
shmaddr is not equal to
(void *) 0 and (
shmflg&SHM_RND)
is true, the segment is attached to the address given by
(
shmaddr- (
shmaddr modulus
SHMLBA)).
o If
shmaddr is not equal to
(void *) 0 and (
shmflg&SHM_RND)
is false, the segment is attached to the address given by
shmaddr.
o The segment is attached for reading if (
shmflg&SHM_RDONLY)
is true
{READ}, otherwise it is attached for reading and
writing
{READ/WRITE}.
The
shmdt() function detaches from the calling process's data segment
the shared memory segment located at the address specified by
shmaddr. If the application is standard-conforming (see
standards(7)), the
shmaddr argument is of type
const void *.
Otherwise it is of type
char *.
Shared memory segments must be explicitly removed after the last
reference to them has been removed.
RETURN VALUES
Upon successful completion,
shmat() returns the data segment start
address of the attached shared memory segment;
shmdt() returns
0.
Otherwise,
-1 is returned, the shared memory segment is not attached,
and
errno is set to indicate the error.
ERRORS
The
shmat() function will fail if:
EACCES Operation permission is denied to the calling process (see
Intro(2)).
EINVAL The
shmid argument is not a valid shared memory identifier.
The
shmaddr argument is not equal to 0, and the value of
(
shmaddr- (
shmaddr modulus
SHMLBA)) is an illegal address.
The
shmaddr argument is not equal to 0, is an illegal
address, and (
shmflg&SHM_RND) is false.
The
shmaddr argument is not equal to 0, is not properly
aligned, and (
shmfg&SHM_SHARE_MMU) is true.
SHM_SHARE_MMU is not supported in certain architectures.
Both (
shmflg&SHM_SHARE_MMU) and
(shmflg&SHM_PAGEABLE) are
true.
(
shmflg&SHM_SHARE_MMU) is true and the shared memory
segment specified by
shmid() had previously been attached
by a call to
shmat() in which (
shmflg&SHM_PAGEABLE) was
true.
(
shmflg&SHM_PAGEABLE) is true and the shared memory segment
specified by
shmid() had previously been attached by a call
to
shmat() in which (
shmflg&SHM_SHARE_MMU) was true.
EMFILE The number of shared memory segments attached to the
calling process would exceed the system-imposed limit.
ENOMEM The available data space is not large enough to
accommodate the shared memory segment.
The
shmdt() function will fail if:
EINVAL The
shmaddr argument is not the data segment start address
of a shared memory segment.
ENOMEM (
shmflg&SHM_SHARE_MMU) is true and attaching to the shared
memory segment would exceed a limit or resource control on
locked memory.
WARNINGS
Using a fixed value for the
shmaddr argument can adversely affect
performance on certain platforms due to D-cache aliasing.
ATTRIBUTES
See
attributes(7) for descriptions of the following attributes:
+--------------------+-------------------+
| ATTRIBUTE TYPE | ATTRIBUTE VALUE |
+--------------------+-------------------+
|Interface Stability | Committed |
+--------------------+-------------------+
|MT-Level | Async-Signal-Safe |
+--------------------+-------------------+
|Standard | See
standards(7). |
+--------------------+-------------------+
SEE ALSO
Intro(2),
exec(2),
exit(2),
fork(2),
shmctl(2),
shmget(2),
attributes(7),
standards(7) March 10, 2008 SHMOP(2)
