SHMGET(2) manual page
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shmget - allocates a System V shared
memory segment
#include <sys/ipc.h>
#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. 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/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)
.
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.
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 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 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).
SVr4,
POSIX.1-2001.
SHM_HUGETLB and SHM_NORESERVE are Linux extensions.
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 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 (32MB).
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 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.
memfd_create(2)
, shmat(2)
, shmctl(2)
, shmdt(2)
,
ftok(3)
, capabilities(7)
, shm_overview(7)
, svipc(7)
This page is
part of release 3.78 of the Linux man-pages project. A description of the
project, information about reporting bugs, and the latest version of this
page, can be found at http://www.kernel.org/doc/man-pages/.
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