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_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).
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.
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:
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).
CONFORMING TO
SVr4, POSIX.1-2001.
SHM_HUGETLB
is a non-portable Linux extension.
NOTES
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:
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).
Linux Notes
Until version 2.3.30 Linux would return
EIDRM
for a
shmget()
on a shared memory segment scheduled for deletion.
BUGS
The name choice
IPC_PRIVATE
was perhaps unfortunate,
IPC_NEW
would more clearly show its function.
This page is part of release 3.14 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/.
