+++ /dev/null
-.\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.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-.\" manual page may be incorrect or out-of-date.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 <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 2015-01-10 "Linux" "Linux Programmer's Manual"
-.SH NAME
-shmget \- allocates a System V shared memory segment
-.SH SYNOPSIS
-.ad l
-.B #include <sys/ipc.h>
-.br
-.B #include <sys/shm.h>
-.sp
-.BI "int shmget(key_t " key ", size_t " size ", int " shmflg );
-.ad b
-.SH DESCRIPTION
-.BR shmget ()
-returns the identifier of the System\ V shared memory segment
-associated with the value of the argument
-.IR key .
-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 .
-.PP
-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).)
-.PP
-The value
-.I shmflg
-is composed of:
-.TP 12
-.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/vm/hugetlbpage.txt
-for further information.
-.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.
-.PP
-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.
-.PP
-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
-.I shm_perm.cuid
-and
-.I shm_perm.uid
-are set to the effective user ID of the calling process.
-.IP
-.I shm_perm.cgid
-and
-.I shm_perm.gid
-are set to the effective group ID of the calling process.
-.IP
-The least significant 9 bits of
-.I shm_perm.mode
-are set to the least significant 9 bit of
-.IR shmflg .
-.IP
-.I shm_segsz
-is set to the value of
-.IR size .
-.IP
-.IR shm_lpid ,
-.IR shm_nattch ,
-.IR shm_atime ,
-and
-.I shm_dtime
-are set to 0.
-.IP
-.I shm_ctime
-is set to the current time.
-.PP
-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
-On failure,
-.I errno
-is set to one of the following:
-.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.
-.TP
-.B EEXIST
-.BR IPC_CREAT
-and
-.BR 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 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).
-.SH CONFORMING TO
-SVr4, POSIX.1-2001.
-.\" SVr4 documents an additional error condition EEXIST.
-
-.B SHM_HUGETLB
-and
-.B SHM_NORESERVE
-are Linux extensions.
-.SH NOTES
-The inclusion of
-.I <sys/types.h>
-and
-.I <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.
-.\" Like Linux, the FreeBSD man pages still document
-.\" the inclusion of these header files.
-
-.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.
-
-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:
-
- 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
-.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 .
-
-From Linux 2.4 up to Linux 3.15,
-the default value for this limit was:
-
- SHMMAX / PAGE_SIZE * (SHMMNI / 16)
-
-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.
-
-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:
-
- 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
-.BR SHMALL
-for a discussion of why this default value (rather than
-.BR 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.
-.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.
-
-On Linux, this limit can be read and modified via
-.IR /proc/sys/kernel/shmmni .
-.\" Kernels between 2.4.x and 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
-.PP
-The implementation has no specific limits for the per-process maximum
-number of shared memory segments
-.RB ( SHMSEG ).
-.SS Linux notes
-Until version 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 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 svipc (7)
-.SH COLOPHON
-This page is part of release 3.79 of the Linux
-.I 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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