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int shmget(key_t key, size_t size, int shmflg);
shmget() returns the identifier of the System V 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 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_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_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.
On success, a valid shared memory identifier is returned. On error, -1
is returned, and errno is set to indicate the error.
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 > SHM-
MAX, 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
ENOENT No segment exists for the given key, and IPC_CREAT was not spec-
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 sys-
tem to exceed the system-wide limit on shared memory (SHMALL).
EPERM The SHM_HUGETLB flag was specified, but the caller was not priv-
ileged (did not have the CAP_IPC_LOCK capability).
SHM_HUGETLB is a nonportable Linux extension.
The inclusion of <sys/types.h> and <sys/ipc.h> isn't 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.
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
SHMMNI System wide maximum number of shared memory segments: implemen-
tation dependent (currently 4096, was 128 before Linux 2.3.99;
on Linux, this limit can be read and modified via /proc/sys/ker-
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),
Linux 2013-04-19 SHMGET(2)