shmget - allocates a System V shared memory segment
Standard C library (
libc,
-lc)
#include <sys/shm.h>
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. It may be used
either to obtain the identifier of a previously created shared memory segment
(when
shmflg is zero and
key does not have the value
IPC_PRIVATE), or to create a new set.
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
- 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
- This flag is used with IPC_CREAT to ensure that this
call creates the segment. If the segment already exists, the call
fails.
-
SHM_HUGETLB (since Linux 2.6)
- Allocate the segment using "huge" pages. See the
Linux kernel source file
Documentation/admin-guide/mm/hugetlbpage.rst for further
information.
-
SHM_HUGE_2MB, SHM_HUGE_1GB (since Linux
3.8)
- Used in conjunction with SHM_HUGETLB to select
alternative hugetlb page sizes (respectively, 2 MB and 1 GB)
on systems that support multiple hugetlb page sizes.
- More generally, the desired huge page size can be
configured by encoding the base-2 logarithm of the desired page size in
the six bits at the offset SHM_HUGE_SHIFT. Thus, the above two
constants are defined as:
-
#define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT)
#define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT)
- For some additional details, see the discussion of the
similarly named constants in mmap(2).
-
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).
In addition to the above flags, the least significant 9 bits of
shmflg
specify the permissions granted to the owner, group, and others. These bits
have the same format, and the same meaning, as the
mode argument of
open(2). Presently, execute permissions are not used by the system.
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.
On success, a valid shared memory identifier is returned. On error, -1 is
returned, and
errno is set to indicate the error.
- EACCES
- The user does not have permission to access the shared
memory segment, and does not have the CAP_IPC_OWNER capability in
the user namespace that governs its IPC namespace.
- EEXIST
-
IPC_CREAT and IPC_EXCL were specified in
shmflg, but a shared memory segment already exists for
key.
- EINVAL
- A new segment was to be created and size is less
than SHMMIN or greater than SHMMAX.
- EINVAL
- A segment for the given key exists, but size
is greater than the size of that segment.
- ENFILE
- The system-wide 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) and
is not a member of the sysctl_hugetlb_shm_group group; see the
description of /proc/sys/vm/sysctl_hugetlb_shm_group in
proc(5).
POSIX.1-2001, POSIX.1-2008, SVr4.
SHM_HUGETLB and
SHM_NORESERVE are Linux extensions.
IPC_PRIVATE isn't a flag field but a
key_t type. If this special
value is used for
key, the system call ignores all 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
shmget() call:
- SHMALL
- System-wide limit on the total amount of shared memory,
measured in units of the system page size.
- On Linux, this limit can be read and modified via
/proc/sys/kernel/shmall. Since Linux 3.16, the default value for
this limit is:
-
ULONG_MAX - 2^24
- The effect of this value (which is suitable for both 32-bit
and 64-bit systems) is to impose no limitation on allocations. This value,
rather than ULONG_MAX, was chosen as the default to prevent some
cases where historical applications simply raised the existing limit
without first checking its current value. Such applications would cause
the value to overflow if the limit was set at ULONG_MAX.
- From Linux 2.4 up to Linux 3.15, the default value for this
limit was:
-
SHMMAX / PAGE_SIZE * (SHMMNI / 16)
- If SHMMAX and SHMMNI were not modified, then
multiplying the result of this formula by the page size (to get a value in
bytes) yielded a value of 8 GB as the limit on the total memory
used by all shared memory segments.
- SHMMAX
- Maximum size in bytes for a shared memory segment.
- On Linux, this limit can be read and modified via
/proc/sys/kernel/shmmax. Since Linux 3.16, the default value for
this limit is:
-
ULONG_MAX - 2^24
- The effect of this value (which is suitable for both 32-bit
and 64-bit systems) is to impose no limitation on allocations. See the
description of SHMALL for a discussion of why this default value
(rather than ULONG_MAX) is used.
- From Linux 2.2 up to Linux 3.15, the default value of this
limit was 0x2000000 (32 MiB).
- Because it is not possible to map just part of a shared
memory segment, the amount of virtual memory places another limit on the
maximum size of a usable segment: for example, on i386 the largest
segments that can be mapped have a size of around 2.8 GB, and on
x86-64 the limit is around 127 TB.
- 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 limit on the number of shared memory segments.
In Linux 2.2, the default value for this limit was 128; since Linux 2.4,
the default value is 4096.
- 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).
Until Linux 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.
See
shmop(2).
memfd_create(2),
shmat(2),
shmctl(2),
shmdt(2),
ftok(3),
capabilities(7),
shm_overview(7),
sysvipc(7)