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_CREATCreate 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_EXCLThis 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.
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:
The user does not have permission to access the
shared memory segment, and does not have the
CAP_IPC_OWNER
capability.
IPC_CREAT and
IPC_EXCL were specified
in shmflg, but
a shared memory segment already exists for key.
A new segment was to be created and size is less than
SHMMIN or greater than
SHMMAX.
A segment for the given key exists, but
size is greater
than the size of that segment.
The system-wide limit on the total number of open files has been reached.
No segment exists for the given key, and IPC_CREAT was not specified.
No memory could be allocated for segment overhead.
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).
The SHM_HUGETLB flag
was specified, but the caller was not privileged (did
not have the CAP_IPC_LOCK
capability).
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:
SHMALLSystem-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.
SHMMAXMaximum 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.
SHMMINMinimum size in bytes for a shared memory segment:
implementation dependent (currently 1 byte, though
PAGE_SIZE is the
effective minimum size).
SHMMNISystem-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).
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 4.07 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/.
|
Copyright (c) 1993 Luigi P. Bai (lpbsoftint.com) July 28, 1993 %%%LICENSE_START(VERBATIM) Permission is granted to make and distribute verbatim copies of this manual provided the copyright notice and this permission notice are preserved on all copies. Permission is granted to copy and distribute modified versions of this manual under the conditions for verbatim copying, provided that the entire resulting derived work is distributed under the terms of a permission notice identical to this one. Since the Linux kernel and libraries are constantly changing, this manual page may be incorrect or out-of-date. The author(s) assume no responsibility for errors or omissions, or for damages resulting from the use of the information contained herein. The author(s) may not have taken the same level of care in the production of this manual, which is licensed free of charge, as they might when working professionally. Formatted or processed versions of this manual, if unaccompanied by the source, must acknowledge the copyright and authors of this work. %%%LICENSE_END Modified Wed Jul 28 10:57:35 1993, Rik Faith <faithcs.unc.edu> Modified Sun Nov 28 16:43:30 1993, Rik Faith <faithcs.unc.edu> with material from Giorgio Ciucci <giorgiocrcc.it> Portions Copyright 1993 Giorgio Ciucci <giorgiocrcc.it> Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond <esrthyrsus.com> Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpagesgmail.com> Removed EIDRM from errors - that can't happen... Modified, 27 May 2004, Michael Kerrisk <mtk.manpagesgmail.com> Added notes on capability requirements Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpagesgmail.com> Language and formatting clean-ups Added notes on /proc files |