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1 | .\" Copyright (c) 1993 Luigi P. Bai (lpb@softint.com) July 28, 1993 |
2 | .\" | |
93015253 | 3 | .\" %%%LICENSE_START(VERBATIM) |
fea681da MK |
4 | .\" Permission is granted to make and distribute verbatim copies of this |
5 | .\" manual provided the copyright notice and this permission notice are | |
6 | .\" preserved on all copies. | |
7 | .\" | |
8 | .\" Permission is granted to copy and distribute modified versions of this | |
9 | .\" manual under the conditions for verbatim copying, provided that the | |
10 | .\" entire resulting derived work is distributed under the terms of a | |
11 | .\" permission notice identical to this one. | |
c13182ef | 12 | .\" |
fea681da MK |
13 | .\" Since the Linux kernel and libraries are constantly changing, this |
14 | .\" manual page may be incorrect or out-of-date. The author(s) assume no | |
15 | .\" responsibility for errors or omissions, or for damages resulting from | |
16 | .\" the use of the information contained herein. The author(s) may not | |
17 | .\" have taken the same level of care in the production of this manual, | |
18 | .\" which is licensed free of charge, as they might when working | |
19 | .\" professionally. | |
c13182ef | 20 | .\" |
fea681da MK |
21 | .\" Formatted or processed versions of this manual, if unaccompanied by |
22 | .\" the source, must acknowledge the copyright and authors of this work. | |
4b72fb64 | 23 | .\" %%%LICENSE_END |
fea681da MK |
24 | .\" |
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> | |
c11b1abf | 30 | .\" Modified, 8 Jan 2003, Michael Kerrisk, <mtk.manpages@gmail.com> |
fea681da | 31 | .\" Removed EIDRM from errors - that can't happen... |
c11b1abf | 32 | .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com> |
fea681da | 33 | .\" Added notes on capability requirements |
c11b1abf | 34 | .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com> |
c952e226 MK |
35 | .\" Language and formatting clean-ups |
36 | .\" Added notes on /proc files | |
fea681da | 37 | .\" |
b8efb414 | 38 | .TH SHMGET 2 2016-10-08 "Linux" "Linux Programmer's Manual" |
fea681da | 39 | .SH NAME |
47f065d6 | 40 | shmget \- allocates a System V shared memory segment |
fea681da MK |
41 | .SH SYNOPSIS |
42 | .ad l | |
43 | .B #include <sys/ipc.h> | |
d314dbfd | 44 | .br |
fea681da | 45 | .B #include <sys/shm.h> |
68e4db0a | 46 | .PP |
f7174672 | 47 | .BI "int shmget(key_t " key ", size_t " size ", int " shmflg ); |
fea681da MK |
48 | .ad b |
49 | .SH DESCRIPTION | |
c952e226 | 50 | .BR shmget () |
efbfd7ec | 51 | returns the identifier of the System\ V shared memory segment |
fea681da MK |
52 | associated with the value of the argument |
53 | .IR key . | |
54 | A new shared memory segment, with size equal to the value of | |
55 | .I size | |
56 | rounded up to a multiple of | |
57 | .BR PAGE_SIZE , | |
58 | is created if | |
59 | .I key | |
60 | has the value | |
61 | .B IPC_PRIVATE | |
62 | or | |
63 | .I key | |
64 | isn't | |
65 | .BR IPC_PRIVATE , | |
66 | no shared memory segment corresponding to | |
0daa9e92 | 67 | .I key |
fea681da MK |
68 | exists, and |
69 | .B IPC_CREAT | |
c952e226 MK |
70 | is specified in |
71 | .IR shmflg . | |
72 | .PP | |
73 | If | |
fea681da | 74 | .I shmflg |
c952e226 MK |
75 | specifies both |
76 | .B IPC_CREAT | |
77 | and | |
78 | .B IPC_EXCL | |
79 | and a shared memory segment already exists for | |
80 | .IR key , | |
81 | then | |
82 | .BR shmget () | |
c13182ef | 83 | fails with |
c952e226 MK |
84 | .I errno |
85 | set to | |
86 | .BR EEXIST . | |
c13182ef | 87 | (This is analogous to the effect of the combination |
c952e226 MK |
88 | .B O_CREAT | O_EXCL |
89 | for | |
90 | .BR open (2).) | |
fea681da MK |
91 | .PP |
92 | The value | |
93 | .I shmflg | |
94 | is composed of: | |
95 | .TP 12 | |
96 | .B IPC_CREAT | |
130a878d | 97 | Create a new segment. |
c13182ef | 98 | If this flag is not used, then |
c952e226 | 99 | .BR shmget () |
c13182ef | 100 | will find the segment associated with \fIkey\fP and check to see if |
fea681da MK |
101 | the user has permission to access the segment. |
102 | .TP | |
103 | .B IPC_EXCL | |
fbaf6d6e MK |
104 | This flag is used with |
105 | .B IPC_CREAT | |
106 | to ensure that this call creates the segment. | |
107 | If the segment already exists, the call fails. | |
fea681da | 108 | .TP |
32047971 | 109 | .BR SHM_HUGETLB " (since Linux 2.6)" |
c13182ef | 110 | Allocate the segment using "huge pages." |
66a9882e | 111 | See the Linux kernel source file |
eb57338f MK |
112 | .I Documentation/vm/hugetlbpage.txt |
113 | for further information. | |
32047971 | 114 | .TP |
6ca52c17 MK |
115 | .BR SHM_HUGE_2MB ", " SHM_HUGE_1GB " (since Linux 3.8)" |
116 | .\" See https://lwn.net/Articles/533499/ | |
117 | Used in conjunction with | |
118 | .B SHM_HUGETLB | |
c4b7e5ac | 119 | to select alternative hugetlb page sizes (respectively, 2\ MB and 1\ GB) |
6ca52c17 | 120 | on systems that support multiple hugetlb page sizes. |
efeece04 | 121 | .IP |
6ca52c17 MK |
122 | More generally, the desired huge page size can be configured by encoding |
123 | the base-2 logarithm of the desired page size in the six bits at the offset | |
124 | .BR SHM_HUGE_SHIFT . | |
125 | Thus, the above two constants are defined as: | |
efeece04 | 126 | .IP |
6ca52c17 MK |
127 | .nf |
128 | .in +4 | |
129 | #define SHM_HUGE_2MB (21 << SHM_HUGE_SHIFT) | |
130 | #define SHM_HUGE_1GB (30 << SHM_HUGE_SHIFT) | |
6ca52c17 | 131 | .fi |
e646a1ba | 132 | .in |
efeece04 | 133 | .IP |
6ca52c17 MK |
134 | For some additional details, |
135 | see the discussion of the similarly named constants in | |
136 | .BR mmap (2). | |
137 | .TP | |
32047971 MK |
138 | .BR SHM_NORESERVE " (since Linux 2.6.15)" |
139 | This flag serves the same purpose as the | |
140 | .BR mmap (2) | |
141 | .B MAP_NORESERVE | |
142 | flag. | |
c13182ef | 143 | Do not reserve swap space for this segment. |
32047971 MK |
144 | When swap space is reserved, one has the guarantee |
145 | that it is possible to modify the segment. | |
8bd58774 MK |
146 | When swap space is not reserved one might get |
147 | .B SIGSEGV | |
148 | upon a write | |
32047971 MK |
149 | if no physical memory is available. |
150 | See also the discussion of the file | |
151 | .I /proc/sys/vm/overcommit_memory | |
c13182ef | 152 | in |
defcceb3 | 153 | .BR proc (5). |
32047971 MK |
154 | .\" As at 2.6.17-rc2, this flag has no effect if SHM_HUGETLB was also |
155 | .\" specified. | |
fea681da | 156 | .PP |
e1d802d5 MK |
157 | In addition to the above flags, the least significant 9 bits of |
158 | .I shmflg | |
159 | specify the permissions granted to the owner, group, and others. | |
160 | These bits have the same format, and the same | |
161 | meaning, as the | |
162 | .I mode | |
163 | argument of | |
164 | .BR open (2). | |
165 | Presently, execute permissions are not used by the system. | |
166 | .PP | |
6319e935 | 167 | When a new shared memory segment is created, |
d9bfdb9c | 168 | its contents are initialized to zero values, and |
6319e935 | 169 | its associated data structure, |
fea681da | 170 | .I shmid_ds |
c952e226 | 171 | (see |
6319e935 | 172 | .BR shmctl (2)), |
d9bfdb9c | 173 | is initialized as follows: |
fea681da | 174 | .IP |
c952e226 | 175 | .I shm_perm.cuid |
fea681da | 176 | and |
c952e226 | 177 | .I shm_perm.uid |
499d62f2 | 178 | are set to the effective user ID of the calling process. |
fea681da | 179 | .IP |
c952e226 | 180 | .I shm_perm.cgid |
fea681da | 181 | and |
c952e226 | 182 | .I shm_perm.gid |
499d62f2 | 183 | are set to the effective group ID of the calling process. |
fea681da | 184 | .IP |
c952e226 MK |
185 | The least significant 9 bits of |
186 | .I shm_perm.mode | |
187 | are set to the least significant 9 bit of | |
fea681da MK |
188 | .IR shmflg . |
189 | .IP | |
c952e226 | 190 | .I shm_segsz |
fea681da MK |
191 | is set to the value of |
192 | .IR size . | |
193 | .IP | |
c952e226 MK |
194 | .IR shm_lpid , |
195 | .IR shm_nattch , | |
a797afac | 196 | .IR shm_atime , |
fea681da | 197 | and |
c952e226 MK |
198 | .I shm_dtime |
199 | are set to 0. | |
fea681da | 200 | .IP |
c952e226 | 201 | .I shm_ctime |
fea681da MK |
202 | is set to the current time. |
203 | .PP | |
c952e226 | 204 | If the shared memory segment already exists, the permissions are |
fea681da | 205 | verified, and a check is made to see if it is marked for destruction. |
47297adb | 206 | .SH RETURN VALUE |
716c8c3f | 207 | On success, a valid shared memory identifier is returned. |
0bca57f3 | 208 | On error, \-1 is returned, and |
716c8c3f MK |
209 | .I errno |
210 | is set to indicate the error. | |
fea681da MK |
211 | .SH ERRORS |
212 | On failure, | |
c952e226 | 213 | .I errno |
fea681da | 214 | is set to one of the following: |
89b3c6b8 | 215 | .TP |
fea681da | 216 | .B EACCES |
c13182ef | 217 | The user does not have permission to access the |
fea681da MK |
218 | shared memory segment, and does not have the |
219 | .B CAP_IPC_OWNER | |
3294109d | 220 | capability in the user namespace that governs its IPC namespace. |
fea681da MK |
221 | .TP |
222 | .B EEXIST | |
247e5615 MK |
223 | .BR IPC_CREAT |
224 | and | |
225 | .BR IPC_EXCL | |
226 | were specified in | |
227 | .IR shmflg , | |
228 | but a shared memory segment already exists for | |
229 | .IR key . | |
fea681da | 230 | .TP |
fea681da | 231 | .B EINVAL |
a350124d MK |
232 | A new segment was to be created and |
233 | .I size | |
234 | is less than | |
8804c8da | 235 | .B SHMMIN |
a350124d | 236 | or greater than |
41651a35 MK |
237 | .BR SHMMAX . |
238 | .TP | |
239 | .B EINVAL | |
11bae399 | 240 | A segment for the given |
41651a35 MK |
241 | .I key |
242 | exists, but \fIsize\fP is greater than the size | |
fea681da MK |
243 | of that segment. |
244 | .TP | |
245 | .B ENFILE | |
246 | .\" [2.6.7] shmem_zero_setup()-->shmem_file_setup()-->get_empty_filp() | |
e258766b | 247 | The system-wide limit on the total number of open files has been reached. |
fea681da MK |
248 | .TP |
249 | .B ENOENT | |
250 | No segment exists for the given \fIkey\fP, and | |
251 | .B IPC_CREAT | |
252 | was not specified. | |
253 | .TP | |
254 | .B ENOMEM | |
255 | No memory could be allocated for segment overhead. | |
256 | .TP | |
257 | .B ENOSPC | |
357cf3fe | 258 | All possible shared memory IDs have been taken |
fea681da MK |
259 | .RB ( SHMMNI ), |
260 | or allocating a segment of the requested | |
261 | .I size | |
262 | would cause the system to exceed the system-wide limit on shared memory | |
263 | .RB ( SHMALL ). | |
eb57338f MK |
264 | .TP |
265 | .B EPERM | |
266 | The | |
267 | .B SHM_HUGETLB | |
268 | flag was specified, but the caller was not privileged (did not have the | |
269 | .B CAP_IPC_LOCK | |
270 | capability). | |
47297adb | 271 | .SH CONFORMING TO |
6131817b | 272 | POSIX.1-2001, POSIX.1-2008, SVr4. |
a1d5f77c | 273 | .\" SVr4 documents an additional error condition EEXIST. |
efeece04 | 274 | .PP |
a1d5f77c | 275 | .B SHM_HUGETLB |
53b81588 MK |
276 | and |
277 | .B SHM_NORESERVE | |
278 | are Linux extensions. | |
fea681da | 279 | .SH NOTES |
f6652463 MK |
280 | The inclusion of |
281 | .I <sys/types.h> | |
282 | and | |
283 | .I <sys/ipc.h> | |
284 | isn't required on Linux or by any version of POSIX. | |
285 | However, | |
286 | some old implementations required the inclusion of these header files, | |
287 | and the SVID also documented their inclusion. | |
288 | Applications intended to be portable to such old systems may need | |
289 | to include these header files. | |
290 | .\" Like Linux, the FreeBSD man pages still document | |
291 | .\" the inclusion of these header files. | |
efeece04 | 292 | .PP |
fea681da MK |
293 | .B IPC_PRIVATE |
294 | isn't a flag field but a | |
c952e226 | 295 | .I key_t |
fea681da MK |
296 | type. |
297 | If this special value is used for | |
298 | .IR key , | |
ec64ab7f | 299 | the system call ignores all but the least significant 9 bits of |
fea681da | 300 | .I shmflg |
ec64ab7f | 301 | and creates a new shared memory segment. |
72c68b6a MK |
302 | .\" |
303 | .SS Shared memory limits | |
540036b2 | 304 | The following limits on shared memory segment resources affect the |
c952e226 | 305 | .BR shmget () |
fea681da | 306 | call: |
89b3c6b8 | 307 | .TP |
fea681da | 308 | .B SHMALL |
24ade49d MK |
309 | System-wide limit on the total amount of shared memory, |
310 | measured in units of the system page size. | |
efeece04 | 311 | .IP |
9a413675 DB |
312 | On Linux, this limit can be read and modified via |
313 | .IR /proc/sys/kernel/shmall . | |
562ac2f2 MK |
314 | Since Linux 3.16, |
315 | .\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31 | |
9a413675 | 316 | the default value for this limit is: |
efeece04 | 317 | .IP |
9a413675 | 318 | ULONG_MAX - 2^24 |
efeece04 | 319 | .IP |
9a413675 DB |
320 | The effect of this value |
321 | (which is suitable for both 32-bit and 64-bit systems) | |
322 | is to impose no limitation on allocations. | |
323 | This value, rather than | |
324 | .BR ULONG_MAX , | |
325 | was chosen as the default to prevent some cases where historical | |
326 | applications simply raised the existing limit without first checking | |
327 | its current value. | |
328 | Such applications would cause the value to overflow if the limit was set at | |
329 | .BR ULONG_MAX . | |
efeece04 | 330 | .IP |
f037b098 | 331 | From Linux 2.4 up to Linux 3.15, |
d4bf702f | 332 | the default value for this limit was: |
efeece04 | 333 | .IP |
e6a38ea6 | 334 | SHMMAX / PAGE_SIZE * (SHMMNI / 16) |
efeece04 | 335 | .IP |
23c95909 MS |
336 | If |
337 | .B SHMMAX | |
338 | and | |
339 | .B SHMMNI | |
d4bf702f | 340 | were not modified, then multiplying the result of this formula |
c4b7e5ac | 341 | by the page size (to get a value in bytes) yielded a value of 8\ GB |
d4bf702f | 342 | as the limit on the total memory used by all shared memory segments. |
fea681da MK |
343 | .TP |
344 | .B SHMMAX | |
c00811f3 | 345 | Maximum size in bytes for a shared memory segment. |
efeece04 | 346 | .IP |
9a413675 DB |
347 | On Linux, this limit can be read and modified via |
348 | .IR /proc/sys/kernel/shmmax . | |
562ac2f2 MK |
349 | Since Linux 3.16, |
350 | .\" commit 060028bac94bf60a65415d1d55a359c3a17d5c31 | |
9a413675 | 351 | the default value for this limit is: |
efeece04 | 352 | .IP |
9a413675 | 353 | ULONG_MAX - 2^24 |
efeece04 | 354 | .IP |
9a413675 DB |
355 | The effect of this value |
356 | (which is suitable for both 32-bit and 64-bit systems) | |
357 | is to impose no limitation on allocations. | |
358 | See the description of | |
359 | .BR SHMALL | |
360 | for a discussion of why this default value (rather than | |
361 | .BR ULONG_MAX ) | |
362 | is used. | |
efeece04 | 363 | .IP |
f037b098 | 364 | From Linux 2.2 up to Linux 3.15, the default value of |
ee8655b5 | 365 | this limit was 0x2000000 (32\ MB). |
efeece04 | 366 | .IP |
1fabbf12 | 367 | Because it is not possible to map just part of a shared memory segment, |
e5cceccf | 368 | the amount of virtual memory places another limit on the maximum size of a |
1fabbf12 MK |
369 | usable segment: |
370 | for example, on i386 the largest segments that can be mapped have a | |
c4b7e5ac | 371 | size of around 2.8\ GB, and on x86_64 the limit is around 127 TB. |
fea681da MK |
372 | .TP |
373 | .B SHMMIN | |
374 | Minimum size in bytes for a shared memory segment: implementation | |
375 | dependent (currently 1 byte, though | |
376 | .B PAGE_SIZE | |
377 | is the effective minimum size). | |
378 | .TP | |
379 | .B SHMMNI | |
ab99b254 | 380 | System-wide limit on the number of shared memory segments. |
098c6998 MK |
381 | In Linux 2.2, the default value for this limit was 128; |
382 | since Linux 2.4, the default value is 4096. | |
efeece04 | 383 | .IP |
8ab18f6c MK |
384 | On Linux, this limit can be read and modified via |
385 | .IR /proc/sys/kernel/shmmni . | |
ed978af2 | 386 | .\" Kernels between 2.4.x and 2.6.8 had an off-by-one error that meant |
cd31a0d6 | 387 | .\" that we could create one more segment than SHMMNI -- MTK |
c952e226 | 388 | .\" This /proc file is not available in Linux 2.2 and earlier -- MTK |
fea681da | 389 | .PP |
5b55dde2 | 390 | The implementation has no specific limits for the per-process maximum |
fea681da MK |
391 | number of shared memory segments |
392 | .RB ( SHMSEG ). | |
c634028a | 393 | .SS Linux notes |
2ddef207 | 394 | Until version 2.3.30, Linux would return |
0daa9e92 | 395 | .B EIDRM |
2f0af33b | 396 | for a |
4fb31341 MK |
397 | .BR shmget () |
398 | on a shared memory segment scheduled for deletion. | |
fea681da | 399 | .SH BUGS |
2f0af33b MK |
400 | The name choice |
401 | .B IPC_PRIVATE | |
9091595d MK |
402 | was perhaps unfortunate, |
403 | .B IPC_NEW | |
fea681da | 404 | would more clearly show its function. |
47297adb | 405 | .SH SEE ALSO |
c4d76cd9 | 406 | .BR memfd_create (2), |
fea681da MK |
407 | .BR shmat (2), |
408 | .BR shmctl (2), | |
409 | .BR shmdt (2), | |
410 | .BR ftok (3), | |
92243659 | 411 | .BR capabilities (7), |
f93af9c6 | 412 | .BR shm_overview (7), |
2c5e151c | 413 | .BR svipc (7) |