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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18 .\" which is licensed free of charge, as they might when working
21 .\" Formatted or processed versions of this manual, if unaccompanied by
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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 2021-03-22 "Linux" "Linux Programmer's Manual"
40 shmget \- allocates a System V shared memory segment
44 .B #include <sys/ipc.h>
45 .B #include <sys/shm.h>
47 .BI "int shmget(key_t " key ", size_t " size ", int " shmflg );
52 returns the identifier of the System\ V shared memory segment
53 associated with the value of the argument
55 It may be used either to obtain the identifier of a previously created
56 shared memory segment (when
60 does not have the value
62 or to create a new set.
64 A new shared memory segment, with size equal to the value of
66 rounded up to a multiple of
76 no shared memory segment corresponding to
89 and a shared memory segment already exists for
97 (This is analogous to the effect of the combination
107 Create a new segment.
108 If this flag is not used, then
110 will find the segment associated with \fIkey\fP and check to see if
111 the user has permission to access the segment.
114 This flag is used with
116 to ensure that this call creates the segment.
117 If the segment already exists, the call fails.
119 .BR SHM_HUGETLB " (since Linux 2.6)"
120 Allocate the segment using "huge" pages.
121 See the Linux kernel source file
122 .I Documentation/admin\-guide/mm/hugetlbpage.rst
123 for further information.
125 .BR SHM_HUGE_2MB ", " SHM_HUGE_1GB " (since Linux 3.8)"
126 .\" See https://lwn.net/Articles/533499/
127 Used in conjunction with
129 to select alternative hugetlb page sizes (respectively, 2\ MB and 1\ GB)
130 on systems that support multiple hugetlb page sizes.
132 More generally, the desired huge page size can be configured by encoding
133 the base-2 logarithm of the desired page size in the six bits at the offset
135 Thus, the above two constants are defined as:
139 #define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT)
140 #define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT)
144 For some additional details,
145 see the discussion of the similarly named constants in
148 .BR SHM_NORESERVE " (since Linux 2.6.15)"
149 This flag serves the same purpose as the
153 Do not reserve swap space for this segment.
154 When swap space is reserved, one has the guarantee
155 that it is possible to modify the segment.
156 When swap space is not reserved one might get
159 if no physical memory is available.
160 See also the discussion of the file
161 .I /proc/sys/vm/overcommit_memory
164 .\" As at 2.6.17-rc2, this flag has no effect if SHM_HUGETLB was also
167 In addition to the above flags, the least significant 9 bits of
169 specify the permissions granted to the owner, group, and others.
170 These bits have the same format, and the same
175 Presently, execute permissions are not used by the system.
177 When a new shared memory segment is created,
178 its contents are initialized to zero values, and
179 its associated data structure,
183 is initialized as follows:
188 are set to the effective user ID of the calling process.
193 are set to the effective group ID of the calling process.
195 The least significant 9 bits of
197 are set to the least significant 9 bit of
201 is set to the value of
212 is set to the current time.
214 If the shared memory segment already exists, the permissions are
215 verified, and a check is made to see if it is marked for destruction.
217 On success, a valid shared memory identifier is returned.
218 On error, \-1 is returned, and
220 is set to indicate the error.
224 The user does not have permission to access the
225 shared memory segment, and does not have the
227 capability in the user namespace that governs its IPC namespace.
235 but a shared memory segment already exists for
239 A new segment was to be created and
247 A segment for the given
249 exists, but \fIsize\fP is greater than the size
253 .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp()
254 The system-wide limit on the total number of open files has been reached.
257 No segment exists for the given \fIkey\fP, and
262 No memory could be allocated for segment overhead.
265 All possible shared memory IDs have been taken
267 or allocating a segment of the requested
269 would cause the system to exceed the system-wide limit on shared memory
275 flag was specified, but the caller was not privileged (did not have the
279 POSIX.1-2001, POSIX.1-2008, SVr4.
280 .\" SVr4 documents an additional error condition EEXIST.
285 are Linux extensions.
291 isn't required on Linux or by any version of POSIX.
293 some old implementations required the inclusion of these header files,
294 and the SVID also documented their inclusion.
295 Applications intended to be portable to such old systems may need
296 to include these header files.
297 .\" Like Linux, the FreeBSD man pages still document
298 .\" the inclusion of these header files.
301 isn't a flag field but a
304 If this special value is used for
306 the system call ignores all but the least significant 9 bits of
308 and creates a new shared memory segment.
310 .SS Shared memory limits
311 The following limits on shared memory segment resources affect the
316 System-wide limit on the total amount of shared memory,
317 measured in units of the system page size.
319 On Linux, this limit can be read and modified via
320 .IR /proc/sys/kernel/shmall .
322 .\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31
323 the default value for this limit is:
327 The effect of this value
328 (which is suitable for both 32-bit and 64-bit systems)
329 is to impose no limitation on allocations.
330 This value, rather than
332 was chosen as the default to prevent some cases where historical
333 applications simply raised the existing limit without first checking
335 Such applications would cause the value to overflow if the limit was set at
338 From Linux 2.4 up to Linux 3.15,
339 the default value for this limit was:
341 SHMMAX / PAGE_SIZE * (SHMMNI / 16)
347 were not modified, then multiplying the result of this formula
348 by the page size (to get a value in bytes) yielded a value of 8\ GB
349 as the limit on the total memory used by all shared memory segments.
352 Maximum size in bytes for a shared memory segment.
354 On Linux, this limit can be read and modified via
355 .IR /proc/sys/kernel/shmmax .
357 .\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31
358 the default value for this limit is:
362 The effect of this value
363 (which is suitable for both 32-bit and 64-bit systems)
364 is to impose no limitation on allocations.
365 See the description of
367 for a discussion of why this default value (rather than
371 From Linux 2.2 up to Linux 3.15, the default value of
372 this limit was 0x2000000 (32\ MB).
374 Because it is not possible to map just part of a shared memory segment,
375 the amount of virtual memory places another limit on the maximum size of a
377 for example, on i386 the largest segments that can be mapped have a
378 size of around 2.8\ GB, and on x86-64 the limit is around 127 TB.
381 Minimum size in bytes for a shared memory segment: implementation
382 dependent (currently 1 byte, though
384 is the effective minimum size).
387 System-wide limit on the number of shared memory segments.
388 In Linux 2.2, the default value for this limit was 128;
389 since Linux 2.4, the default value is 4096.
391 On Linux, this limit can be read and modified via
392 .IR /proc/sys/kernel/shmmni .
393 .\" Kernels between 2.4.x and 2.6.8 had an off-by-one error that meant
394 .\" that we could create one more segment than SHMMNI -- MTK
395 .\" This /proc file is not available in Linux 2.2 and earlier -- MTK
397 The implementation has no specific limits for the per-process maximum
398 number of shared memory segments
401 Until version 2.3.30, Linux would return
405 on a shared memory segment scheduled for deletion.
409 was perhaps unfortunate,
411 would more clearly show its function.
416 .BR memfd_create (2),
421 .BR capabilities (7),
422 .BR shm_overview (7),