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NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUE | ERRORS | STANDARDS | HISTORY | NOTES | BUGS | EXAMPLES | SEE ALSO | COLOPHON |
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shmget(2) System Calls Manual shmget(2)
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 hugetlb_shm_group group; see the
description of /proc/sys/vm/hugetlb_shm_group in proc(5).
POSIX.1-2008.
SHM_HUGETLB and SHM_NORESERVE are Linux extensions.
POSIX.1-2001, SVr4.
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 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).
Linux notes
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)
This page is part of the man-pages (Linux kernel and C library
user-space interface documentation) project. Information about
the project can be found at
⟨https://www.kernel.org/doc/man-pages/⟩. If you have a bug report
for this manual page, see
⟨https://git.kernel.org/pub/scm/docs/man-pages/man-pages.git/tree/CONTRIBUTING⟩.
This page was obtained from the tarball man-pages-6.15.tar.gz
fetched from
⟨https://mirrors.edge.kernel.org/pub/linux/docs/man-pages/⟩ on
2025-08-11. If you discover any rendering problems in this HTML
version of the page, or you believe there is a better or more up-
to-date source for the page, or you have corrections or
improvements to the information in this COLOPHON (which is not
part of the original manual page), send a mail to
man-pages@man7.org
Linux man-pages 6.15 2025-05-31 shmget(2)
Pages that refer to this page: ipcrm(1), ipcs(1), lsipc(1), pcp-ipcs(1), getrlimit(2), ipc(2), mbind(2), memfd_create(2), shmctl(2), shmop(2), syscalls(2), umask(2), ftok(3), sem_init(3), proc_sys_kernel(5), proc_sys_vm(5), tmpfs(5), sem_overview(7), shm_overview(7), sysvipc(7)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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