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-.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993
-.\"
-.\" SPDX-License-Identifier: Linux-man-pages-copyleft
-.\"
-.\" Modified Wed Jul 28 10:57:35 1993, Rik Faith <faith@cs.unc.edu>
-.\" Modified Sun Nov 28 16:43:30 1993, Rik Faith <faith@cs.unc.edu>
-.\" with material from Giorgio Ciucci <giorgio@crcc.it>
-.\" Portions Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
-.\" Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond <esr@thyrsus.com>
-.\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com>
-.\" Removed EIDRM from errors - that can't happen...
-.\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
-.\" Added notes on capability requirements
-.\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
-.\" Language and formatting clean-ups
-.\" Added notes on /proc files
-.\"
-.TH shmget 2 2023-10-31 "Linux man-pages 6.7"
-.SH NAME
-shmget \- allocates a System V shared memory segment
-.SH LIBRARY
-Standard C library
-.RI ( libc ", " \-lc )
-.SH SYNOPSIS
-.nf
-.B #include <sys/shm.h>
-.P
-.BI "int shmget(key_t " key ", size_t " size ", int " shmflg );
-.fi
-.SH DESCRIPTION
-.BR shmget ()
-returns the identifier of the System\ V shared memory segment
-associated with the value of the argument
-.IR key .
-It may be used either to obtain the identifier of a previously created
-shared memory segment (when
-.I shmflg
-is zero and
-.I key
-does not have the value
-.BR IPC_PRIVATE ),
-or to create a new set.
-.P
-A new shared memory segment, with size equal to the value of
-.I size
-rounded up to a multiple of
-.BR PAGE_SIZE ,
-is created if
-.I key
-has the value
-.B IPC_PRIVATE
-or
-.I key
-isn't
-.BR IPC_PRIVATE ,
-no shared memory segment corresponding to
-.I key
-exists, and
-.B IPC_CREAT
-is specified in
-.IR shmflg .
-.P
-If
-.I shmflg
-specifies both
-.B IPC_CREAT
-and
-.B IPC_EXCL
-and a shared memory segment already exists for
-.IR key ,
-then
-.BR shmget ()
-fails with
-.I errno
-set to
-.BR EEXIST .
-(This is analogous to the effect of the combination
-.B O_CREAT | O_EXCL
-for
-.BR open (2).)
-.P
-The value
-.I shmflg
-is composed of:
-.TP
-.B IPC_CREAT
-Create a new segment.
-If this flag is not used, then
-.BR shmget ()
-will find the segment associated with \fIkey\fP and check to see if
-the user has permission to access the segment.
-.TP
-.B IPC_EXCL
-This flag is used with
-.B IPC_CREAT
-to ensure that this call creates the segment.
-If the segment already exists, the call fails.
-.TP
-.BR SHM_HUGETLB " (since Linux 2.6)"
-Allocate the segment using "huge" pages.
-See the Linux kernel source file
-.I Documentation/admin\-guide/mm/hugetlbpage.rst
-for further information.
-.TP
-.B SHM_HUGE_2MB
-.TQ
-.BR SHM_HUGE_1GB " (since Linux 3.8)"
-.\" See https://lwn.net/Articles/533499/
-Used in conjunction with
-.B SHM_HUGETLB
-to select alternative hugetlb page sizes (respectively, 2\ MB and 1\ GB)
-on systems that support multiple hugetlb page sizes.
-.IP
-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
-.BR SHM_HUGE_SHIFT .
-Thus, the above two constants are defined as:
-.IP
-.in +4n
-.EX
-#define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT)
-#define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT)
-.EE
-.in
-.IP
-For some additional details,
-see the discussion of the similarly named constants in
-.BR mmap (2).
-.TP
-.BR SHM_NORESERVE " (since Linux 2.6.15)"
-This flag serves the same purpose as the
-.BR mmap (2)
-.B 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
-.B SIGSEGV
-upon a write
-if no physical memory is available.
-See also the discussion of the file
-.I /proc/sys/vm/overcommit_memory
-in
-.BR proc (5).
-.\" As at 2.6.17-rc2, this flag has no effect if SHM_HUGETLB was also
-.\" specified.
-.P
-In addition to the above flags, the least significant 9 bits of
-.I shmflg
-specify the permissions granted to the owner, group, and others.
-These bits have the same format, and the same
-meaning, as the
-.I mode
-argument of
-.BR open (2).
-Presently, execute permissions are not used by the system.
-.P
-When a new shared memory segment is created,
-its contents are initialized to zero values, and
-its associated data structure,
-.I shmid_ds
-(see
-.BR shmctl (2)),
-is initialized as follows:
-.IP \[bu] 3
-.I shm_perm.cuid
-and
-.I shm_perm.uid
-are set to the effective user ID of the calling process.
-.IP \[bu]
-.I shm_perm.cgid
-and
-.I shm_perm.gid
-are set to the effective group ID of the calling process.
-.IP \[bu]
-The least significant 9 bits of
-.I shm_perm.mode
-are set to the least significant 9 bit of
-.IR shmflg .
-.IP \[bu]
-.I shm_segsz
-is set to the value of
-.IR size .
-.IP \[bu]
-.IR shm_lpid ,
-.IR shm_nattch ,
-.IR shm_atime ,
-and
-.I shm_dtime
-are set to 0.
-.IP \[bu]
-.I shm_ctime
-is set to the current time.
-.P
-If the shared memory segment already exists, the permissions are
-verified, and a check is made to see if it is marked for destruction.
-.SH RETURN VALUE
-On success, a valid shared memory identifier is returned.
-On error, \-1 is returned, and
-.I errno
-is set to indicate the error.
-.SH ERRORS
-.TP
-.B EACCES
-The user does not have permission to access the
-shared memory segment, and does not have the
-.B CAP_IPC_OWNER
-capability in the user namespace that governs its IPC namespace.
-.TP
-.B EEXIST
-.B IPC_CREAT
-and
-.B IPC_EXCL
-were specified in
-.IR shmflg ,
-but a shared memory segment already exists for
-.IR key .
-.TP
-.B EINVAL
-A new segment was to be created and
-.I size
-is less than
-.B SHMMIN
-or greater than
-.BR SHMMAX .
-.TP
-.B EINVAL
-A segment for the given
-.I key
-exists, but \fIsize\fP is greater than the size
-of that segment.
-.TP
-.B ENFILE
-.\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
-The system-wide limit on the total number of open files has been reached.
-.TP
-.B ENOENT
-No segment exists for the given \fIkey\fP, and
-.B IPC_CREAT
-was not specified.
-.TP
-.B ENOMEM
-No memory could be allocated for segment overhead.
-.TP
-.B ENOSPC
-All possible shared memory IDs have been taken
-.RB ( SHMMNI ),
-or allocating a segment of the requested
-.I size
-would cause the system to exceed the system-wide limit on shared memory
-.RB ( SHMALL ).
-.TP
-.B EPERM
-The
-.B SHM_HUGETLB
-flag was specified, but the caller was not privileged (did not have the
-.B CAP_IPC_LOCK
-capability)
-and is not a member of the
-.I sysctl_hugetlb_shm_group
-group; see the description of
-.I /proc/sys/vm/sysctl_hugetlb_shm_group
-in
-.BR proc (5).
-.SH STANDARDS
-POSIX.1-2008.
-.P
-.B SHM_HUGETLB
-and
-.B SHM_NORESERVE
-are Linux extensions.
-.SH HISTORY
-POSIX.1-2001, SVr4.
-.\" SVr4 documents an additional error condition EEXIST.
-.SH NOTES
-.B IPC_PRIVATE
-isn't a flag field but a
-.I key_t
-type.
-If this special value is used for
-.IR key ,
-the system call ignores all but the least significant 9 bits of
-.I shmflg
-and creates a new shared memory segment.
-.\"
-.SS Shared memory limits
-The following limits on shared memory segment resources affect the
-.BR shmget ()
-call:
-.TP
-.B SHMALL
-System-wide limit on the total amount of shared memory,
-measured in units of the system page size.
-.IP
-On Linux, this limit can be read and modified via
-.IR /proc/sys/kernel/shmall .
-Since Linux 3.16,
-.\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31
-the default value for this limit is:
-.IP
-.in +4n
-.EX
-ULONG_MAX - 2\[ha]24
-.EE
-.in
-.IP
-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
-.BR 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
-.BR ULONG_MAX .
-.IP
-From Linux 2.4 up to Linux 3.15,
-the default value for this limit was:
-.IP
-.in +4n
-.EX
-SHMMAX / PAGE_SIZE * (SHMMNI / 16)
-.EE
-.in
-.IP
-If
-.B SHMMAX
-and
-.B 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.
-.TP
-.B SHMMAX
-Maximum size in bytes for a shared memory segment.
-.IP
-On Linux, this limit can be read and modified via
-.IR /proc/sys/kernel/shmmax .
-Since Linux 3.16,
-.\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31
-the default value for this limit is:
-.IP
-.in +4n
-.EX
-ULONG_MAX - 2\[ha]24
-.EE
-.in
-.IP
-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
-.B SHMALL
-for a discussion of why this default value (rather than
-.BR ULONG_MAX )
-is used.
-.IP
-From Linux 2.2 up to Linux 3.15, the default value of
-this limit was 0x2000000 (32\ MiB).
-.IP
-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.
-.TP
-.B SHMMIN
-Minimum size in bytes for a shared memory segment: implementation
-dependent (currently 1 byte, though
-.B PAGE_SIZE
-is the effective minimum size).
-.TP
-.B 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.
-.IP
-On Linux, this limit can be read and modified via
-.IR /proc/sys/kernel/shmmni .
-.\" Kernels between Linux 2.4.x and Linux 2.6.8 had an off-by-one error
-.\" that meant that we could create one more segment than SHMMNI -- MTK
-.\" This /proc file is not available in Linux 2.2 and earlier -- MTK
-.P
-The implementation has no specific limits for the per-process maximum
-number of shared memory segments
-.RB ( SHMSEG ).
-.SS Linux notes
-Until Linux 2.3.30, Linux would return
-.B EIDRM
-for a
-.BR shmget ()
-on a shared memory segment scheduled for deletion.
-.SH BUGS
-The name choice
-.B IPC_PRIVATE
-was perhaps unfortunate,
-.B IPC_NEW
-would more clearly show its function.
-.SH EXAMPLES
-See
-.BR shmop (2).
-.SH SEE ALSO
-.BR memfd_create (2),
-.BR shmat (2),
-.BR shmctl (2),
-.BR shmdt (2),
-.BR ftok (3),
-.BR capabilities (7),
-.BR shm_overview (7),
-.BR sysvipc (7)