shmget - allocates a shared memory segment
int shmget(key_t key, size_t size, int
() 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
, is created if key
has the value IPC_PRIVATE
, no shared memory segment corresponding to
exists, and IPC_CREAT
is specified in shmflg
specifies both IPC_CREAT
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
The value shmflg
is composed of:
- 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.
- used with IPC_CREAT to ensure failure if the segment
- (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
Linux kernel source file Documentation/vm/hugetlbpage.txt for
- 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
When a new shared memory segment is created, its contents are initialized to
zero values, and its associated data structure, shmid_ds
), 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.
A valid segment identifier, shmid
, is returned on success, -1 on error.
On failure, errno
is set to one of the following:
- The user does not have permission to access the shared
memory segment, and does not have the CAP_IPC_OWNER
- IPC_CREAT | IPC_EXCL was specified and the segment
- 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.
- The system limit on the total number of open files has been
- No segment exists for the given key, and
IPC_CREAT was not specified.
- No memory could be allocated for segment overhead.
- 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
- The SHM_HUGETLB flag was specified, but the caller
was not privileged (did not have the CAP_IPC_LOCK capability).
is a nonportable Linux extension.
The inclusion of <sys/types.h>
required on Linux or by any version of POSIX. However, some old
implementations required the inclusion of these header files, and the SVID
also documented their inclusion. Applications intended to be portable to such
old systems may need to include these header files.
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
The following limits on shared memory segment resources affect the
- System wide maximum of shared memory pages (on Linux, this
limit can be read and modified via /proc/sys/kernel/shmall).
- Maximum size in bytes for a shared memory segment: policy
dependent (on Linux, this limit can be read and modified via
- Minimum size in bytes for a shared memory segment:
implementation dependent (currently 1 byte, though PAGE_SIZE is the
effective minimum size).
- 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
The implementation has no specific limits for the per-process maximum number of
shared memory segments (SHMSEG
Until version 2.3.30 Linux would return EIDRM
for a shmget
() on a
shared memory segment scheduled for deletion.
The name choice IPC_PRIVATE
was perhaps unfortunate, IPC_NEW
more clearly show its function.
This page is part of release 3.44 of the Linux man-pages
description of the project, and information about reporting bugs, can be found