1 .\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993
3 .\" %%%LICENSE_START(VERBATIM)
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13 .\" Since the Linux kernel and libraries are constantly changing, this
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25 .\" Modified Wed Jul 28 10:57:35 1993, Rik Faith <faith@cs.unc.edu>
26 .\" Modified Sun Nov 28 16:43:30 1993, Rik Faith <faith@cs.unc.edu>
27 .\" with material from Giorgio Ciucci <giorgio@crcc.it>
28 .\" Portions Copyright 1993 Giorgio Ciucci <giorgio@crcc.it>
29 .\" Modified Tue Oct 22 22:03:17 1996 by Eric S. Raymond <esr@thyrsus.com>
30 .\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com>
31 .\" Removed EIDRM from errors - that can't happen...
32 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
33 .\" Added notes on capability requirements
34 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
35 .\" Language and formatting clean-ups
36 .\" Added notes on /proc files
38 .TH SHMGET 2 2013-04-19 "Linux" "Linux Programmer's Manual"
40 shmget \- allocates a System V shared memory segment
43 .B #include <sys/ipc.h>
45 .B #include <sys/shm.h>
47 .BI "int shmget(key_t " key ", size_t " size ", int " shmflg );
51 returns the identifier of the System V shared memory segment
52 associated with the value of the argument
54 A new shared memory segment, with size equal to the value of
56 rounded up to a multiple of
66 no shared memory segment corresponding to
79 and a shared memory segment already exists for
87 (This is analogous to the effect of the combination
97 to create a new segment.
98 If this flag is not used, then
100 will find the segment associated with \fIkey\fP and check to see if
101 the user has permission to access the segment.
104 used with \fBIPC_CREAT\fP to ensure failure if the segment already exists.
107 (least significant 9 bits)
108 specifying the permissions granted to the owner, group, and world.
109 These bits have the same format, and the same
114 Presently, the execute permissions are not used by the system.
116 .BR SHM_HUGETLB " (since Linux 2.6)"
117 Allocate the segment using "huge pages."
118 See the Linux kernel source file
119 .I Documentation/vm/hugetlbpage.txt
120 for further information.
122 .BR SHM_NORESERVE " (since Linux 2.6.15)"
123 This flag serves the same purpose as the
127 Do not reserve swap space for this segment.
128 When swap space is reserved, one has the guarantee
129 that it is possible to modify the segment.
130 When swap space is not reserved one might get
133 if no physical memory is available.
134 See also the discussion of the file
135 .I /proc/sys/vm/overcommit_memory
138 .\" As at 2.6.17-rc2, this flag has no effect if SHM_HUGETLB was also
141 When a new shared memory segment is created,
142 its contents are initialized to zero values, and
143 its associated data structure,
147 is initialized as follows:
152 are set to the effective user ID of the calling process.
157 are set to the effective group ID of the calling process.
159 The least significant 9 bits of
161 are set to the least significant 9 bit of
165 is set to the value of
176 is set to the current time.
178 If the shared memory segment already exists, the permissions are
179 verified, and a check is made to see if it is marked for destruction.
181 On success, a valid shared memory identifier is returned.
182 On error, \-1 is returned, and
184 is set to indicate the error.
188 is set to one of the following:
191 The user does not have permission to access the
192 shared memory segment, and does not have the
197 .B IPC_CREAT | IPC_EXCL
198 was specified and the segment exists.
201 A new segment was to be created and \fIsize\fP < \fBSHMMIN\fP
202 or \fIsize\fP > \fBSHMMAX\fP, or no new segment was to be created,
203 a segment with given key existed, but \fIsize\fP is greater than the size
207 .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
208 The system limit on the total number of open files has been reached.
211 No segment exists for the given \fIkey\fP, and
216 No memory could be allocated for segment overhead.
219 All possible shared memory IDs have been taken
221 or allocating a segment of the requested
223 would cause the system to exceed the system-wide limit on shared memory
229 flag was specified, but the caller was not privileged (did not have the
234 .\" SVr4 documents an additional error condition EEXIST.
237 is a nonportable Linux extension.
243 isn't required on Linux or by any version of POSIX.
245 some old implementations required the inclusion of these header files,
246 and the SVID also documented their inclusion.
247 Applications intended to be portable to such old systems may need
248 to include these header files.
249 .\" Like Linux, the FreeBSD man pages still document
250 .\" the inclusion of these header files.
253 isn't a flag field but a
256 If this special value is used for
258 the system call ignores everything but the least significant 9 bits of
260 and creates a new shared memory segment (on success).
262 The following limits on shared memory segment resources affect the
267 System wide maximum of shared memory pages
268 (on Linux, this limit can be read and modified via
269 .IR /proc/sys/kernel/shmall ).
272 Maximum size in bytes for a shared memory segment: policy dependent
273 (on Linux, this limit can be read and modified via
274 .IR /proc/sys/kernel/shmmax ).
277 Minimum size in bytes for a shared memory segment: implementation
278 dependent (currently 1 byte, though
280 is the effective minimum size).
283 System wide maximum number of shared memory segments: implementation
284 dependent (currently 4096, was 128 before Linux 2.3.99;
285 on Linux, this limit can be read and modified via
286 .IR /proc/sys/kernel/shmmni ).
287 .\" Kernels between 2.4.x and 2.6.8 had an off-by-one error that meant
288 .\" that we could create one more segment than SHMMNI -- MTK
289 .\" This /proc file is not available in Linux 2.2 and earlier -- MTK
291 The implementation has no specific limits for the per-process maximum
292 number of shared memory segments
295 Until version 2.3.30 Linux would return
299 on a shared memory segment scheduled for deletion.
303 was perhaps unfortunate,
305 would more clearly show its function.
311 .BR capabilities (7),
312 .BR shm_overview (7),