1 .\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993
3 .\" Permission is granted to make and distribute verbatim copies of this
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7 .\" Permission is granted to copy and distribute modified versions of this
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9 .\" entire resulting derived work is distributed under the terms of a
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12 .\" Since the Linux kernel and libraries are constantly changing, this
13 .\" manual page may be incorrect or out-of-date. The author(s) assume no
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17 .\" which is licensed free of charge, as they might when working
20 .\" Formatted or processed versions of this manual, if unaccompanied by
21 .\" the source, must acknowledge the copyright and authors of this work.
23 .\" Modified Wed Jul 28 10:57:35 1993, Rik Faith <faith@cs.unc.edu>
24 .\" Modified Sun Nov 28 16:43:30 1993, Rik Faith <faith@cs.unc.edu>
25 .\" with material from Giorgio Ciucci <giorgio@crcc.it>
26 .\" Portions Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
27 .\" Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond <esr@thyrsus.com>
28 .\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com>
29 .\" Removed EIDRM from errors - that can't happen...
30 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
31 .\" Added notes on capability requirements
32 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
33 .\" Language and formatting clean-ups
34 .\" Added notes on /proc files
36 .TH SHMGET 2 2006-05-02 "Linux" "Linux Programmer's Manual"
38 shmget \- allocates a shared memory segment
41 .B #include <sys/ipc.h>
43 .B #include <sys/shm.h>
45 .BI "int shmget(key_t " key ", size_t " size ", int " shmflg );
49 returns the identifier of the shared memory segment
50 associated with the value of the argument
52 A new shared memory segment, with size equal to the value of
54 rounded up to a multiple of
64 no shared memory segment corresponding to
77 and a shared memory segment already exists for
85 (This is analogous to the effect of the combination
95 to create a new segment.
96 If this flag is not used, then
98 will find the segment associated with \fIkey\fP and check to see if
99 the user has permission to access the segment.
102 used with \fBIPC_CREAT\fP to ensure failure if the segment already exists.
105 (least significant 9 bits)
106 specifying the permissions granted to the owner, group, and world.
107 These bits have the same format, and the same
112 Presently, the execute permissions are not used by the system.
114 .BR SHM_HUGETLB " (since Linux 2.6)"
115 Allocate the segment using "huge pages."
116 See the kernel source file
117 .I Documentation/vm/hugetlbpage.txt
118 for further information.
120 .BR SHM_NORESERVE " (since Linux 2.6.15)"
121 This flag serves the same purpose as the
125 Do not reserve swap space for this segment.
126 When swap space is reserved, one has the guarantee
127 that it is possible to modify the segment.
128 When swap space is not reserved one might get
131 if no physical memory is available.
132 See also the discussion of the file
133 .I /proc/sys/vm/overcommit_memory
136 .\" As at 2.6.17-rc2, this flag has no effect if SHM_HUGETLB was also
139 When a new shared memory segment is created,
140 its contents are initialized to zero values, and
141 its associated data structure,
145 is initialized as follows:
150 are set to the effective user ID of the calling process.
155 are set to the effective group ID of the calling process.
157 The least significant 9 bits of
159 are set to the least significant 9 bit of
163 is set to the value of
174 is set to the current time.
176 If the shared memory segment already exists, the permissions are
177 verified, and a check is made to see if it is marked for destruction.
179 A valid segment identifier,
181 is returned on success, \-1 on error.
185 is set to one of the following:
188 The user does not have permission to access the
189 shared memory segment, and does not have the
194 .B IPC_CREAT | IPC_EXCL
195 was specified and the segment exists.
198 A new segment was to be created and \fIsize\fP < \fBSHMMIN\fP
199 or \fIsize\fP > \fBSHMMAX\fP, or no new segment was to be created,
200 a segment with given key existed, but \fIsize\fP is greater than the size
204 .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
205 The system limit on the total number of open files has been reached.
208 No segment exists for the given \fIkey\fP, and
213 No memory could be allocated for segment overhead.
216 All possible shared memory IDs have been taken
218 or allocating a segment of the requested
220 would cause the system to exceed the system-wide limit on shared memory
226 flag was specified, but the caller was not privileged (did not have the
231 .\" SVr4 documents an additional error condition EEXIST.
234 is a nonportable Linux extension.
237 isn't a flag field but a
240 If this special value is used for
242 the system call ignores everything but the least significant 9 bits of
244 and creates a new shared memory segment (on success).
246 The following limits on shared memory segment resources affect the
251 System wide maximum of shared memory pages
252 (on Linux, this limit can be read and modified via
253 .IR /proc/sys/kernel/shmall ).
256 Maximum size in bytes for a shared memory segment: policy dependent
257 (on Linux, this limit can be read and modified via
258 .IR /proc/sys/kernel/shmmax ).
261 Minimum size in bytes for a shared memory segment: implementation
262 dependent (currently 1 byte, though
264 is the effective minimum size).
267 System wide maximum number of shared memory segments: implementation
268 dependent (currently 4096, was 128 before Linux 2.3.99;
269 on Linux, this limit can be read and modified via
270 .IR /proc/sys/kernel/shmmni ).
271 .\" Kernels between 2.4.x and 2.6.8 had an off-by-one error that meant
272 .\" that we could create one more segment than SHMMNI -- MTK
273 .\" This /proc file is not available in Linux 2.2 and earlier -- MTK
275 The implementation has no specific limits for the per-process maximum
276 number of shared memory segments
279 Until version 2.3.30 Linux would return
283 on a shared memory segment scheduled for deletion.
287 was perhaps unfortunate,
289 would more clearly show its function.
295 .BR capabilities (7),
296 .BR shm_overview (7),