shmget() - Unix, Linux System Call
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NAME
shmget - allocates a shared memory segment
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTION
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 isnt
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:
Tag | Description |
SHM_HUGETLB |
used for allocating HUGETLB pages for shared memory.
IPC_CREAT to 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_EXCL |
used 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).
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When a new shared memory segment is created,
its contents are initialised to zero values, and
its associated data structure,
shmid_ds (see
shmctl(2)),
is initialised as follows:
Tag | Description |
|
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.
SYSTEM CALLS
Tag | Description |
fork() |
After a
fork() the child inherits the attached shared memory segments.
|
exec() |
After an
exec() all attached shared memory segments are detached (not destroyed).
|
exit() |
Upon
exit() all attached shared memory segments are detached (not destroyed).
|
RETURN VALUE
A valid segment identifier,
shmid, is returned on success, -1 on error.
ERRORS
On failure,
errno is set to one of the following:
Tag | Description |
EACCES |
The user does not have permission to access the
shared memory segment, and does not have the
CAP_IPC_OWNER capability.
|
EEXIST |
IPC_CREAT | IPC_EXCL was specified and the segment exists.
|
EINVAL |
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.
|
ENFILE |
The system limit on the total number of open files has been reached.
|
ENOENT |
No segment exists for the given key, and
IPC_CREAT was not specified.
|
ENOMEM |
No memory could be allocated for segment overhead.
|
ENOSPC |
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). |
EPERM |
The
SHM_HUGETLB flag was specified, but the caller was not privileged (did not have the
CAP_IPC_LOCK capability).
|
NOTES
IPC_PRIVATE isnt 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:
Tag | Description |
SHMALL |
System wide maximum of shared memory pages
(on Linux, this limit can be read and modified via
/proc/sys/kernel/shmall). |
SHMMAX |
Maximum size in bytes for a shared memory segment: policy dependent
(on Linux, this limit can be read and modified via
/proc/sys/kernel/shmmax). |
SHMMIN |
Minimum size in bytes for a shared memory segment: implementation
dependent (currently 1 byte, though
PAGE_SIZE is the effective minimum size).
|
SHMMNI |
System 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). BUGS
The name choice IPC_PRIVATE was perhaps unfortunate, IPC_NEW
would more clearly show its function.
CONFORMING TO
SVr4, POSIX.1-2001.
LINUX NOTES
Until version 2.3.30 Linux would return EIDRM for a
shmget() on a shared memory segment scheduled for deletion.
SHM_HUGETLB is a non-portable Linux extension.
SEE ALSO
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