SHMGET
Section: Linux Programmer's Manual (2)
Updated: 2017-09-15
Index
Return to Main Contents
NAME
shmget - allocates a System V shared memory segment
SYNOPSIS
#include <sys/ipc.h>
#include <sys/shm.h>
int shmget(key_t key, size_t size, int shmflg);
DESCRIPTION
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_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.
RETURN VALUE
On success, a valid shared memory identifier is returned.
On error, -1 is returned, and
errno
is set to indicate the error.
ERRORS
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 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).
CONFORMING TO
POSIX.1-2001, POSIX.1-2008, SVr4.
SHM_HUGETLB
and
SHM_NORESERVE
are Linux extensions.
NOTES
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.
Shared memory limits
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 MB).
-
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).
Linux notes
Until version 2.3.30, Linux would return
EIDRM
for a
shmget()
on a shared memory segment scheduled for deletion.
BUGS
The name choice
IPC_PRIVATE
was perhaps unfortunate,
IPC_NEW
would more clearly show its function.
SEE ALSO
memfd_create(2),
shmat(2),
shmctl(2),
shmdt(2),
ftok(3),
capabilities(7),
shm_overview(7),
svipc(7)
COLOPHON
This page is part of release 4.13 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
https://www.kernel.org/doc/man-pages/.
Index
- NAME
-
- SYNOPSIS
-
- DESCRIPTION
-
- RETURN VALUE
-
- ERRORS
-
- CONFORMING TO
-
- NOTES
-
- Shared memory limits
-
- Linux notes
-
- BUGS
-
- SEE ALSO
-
- COLOPHON
-
|
|
- Running on 
-
:
: 