SHMGET(2) Linux Programmer's Manual SHMGET(2)
shmget - allocates a shared memory segment
int shmget(key_t key, size_t size, int shmflg);
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 seg-
IPC_EXCL used with IPC_CREAT to ensure failure if the segment
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
SHM_NORESERVE (since Linux 2.6.15)
This flag serves the same purpose as the mmap(2) MAP_NORE-
SERVE 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 initial-
ized 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 veri-
fied, 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:
EACCES The user does not have permission to access the shared mem-
ory segment, and does not have the CAP_IPC_OWNER capabil-
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
ENFILE The system limit on the total number of open files has been
ENOENT No segment exists for the given key, and IPC_CREAT was not
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
EPERM The SHM_HUGETLB flag was specified, but the caller was not
privileged (did not have the CAP_IPC_LOCK capability).
SHM_HUGETLB is a non-portable Linux extension.
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 sig-
nificant 9 bits of shmflg and creates a new shared memory segment (on
The following limits on shared memory segment resources affect the
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
SHMMIN Minimum size in bytes for a shared memory segment: implemen-
tation dependent (currently 1 byte, though PAGE_SIZE is the
effective minimum size).
SHMMNI System wide maximum number of shared memory segments: imple-
mentation 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 would more
clearly show its function.
shmat(2), shmctl(2), shmdt(2), ftok(3), capabilities(7), shm_over-
This page is part of release 3.05 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/.
Linux 2006-05-02 SHMGET(2)