summaryrefslogtreecommitdiffstats
path: root/man2/semget.2
blob: bd2b69335290d24fa51108d3e8474f627bd920da (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
.\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
.\" and Copyright (C) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
.\"
.\" SPDX-License-Identifier: Linux-man-pages-copyleft
.\"
.\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com>
.\" Modified 1 Jan 2002, Martin Schulze <joey@infodrom.org>
.\" Modified 4 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
.\" 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
.\"	Rewrote BUGS note about semget()'s failure to initialize
.\"		semaphore values
.\"
.TH semget 2 2023-05-03 "Linux man-pages 6.05.01"
.SH NAME
semget \- get a System V semaphore set identifier
.SH LIBRARY
Standard C library
.RI ( libc ", " \-lc )
.SH SYNOPSIS
.nf
.B #include <sys/sem.h>
.fi
.PP
.BI "int semget(key_t " key ,
.BI "int " nsems ,
.BI "int " semflg );
.SH DESCRIPTION
The
.BR semget ()
system call returns the System\ V semaphore set identifier
associated with the argument
.IR key .
It may be used either to obtain the identifier of a previously created
semaphore set (when
.I semflg
is zero and
.I key
does not have the value
.BR IPC_PRIVATE ),
or to create a new set.
.PP
A new set of
.I nsems
semaphores is created if
.I key
has the value
.B IPC_PRIVATE
or if no existing semaphore set is associated with
.I key
and
.B IPC_CREAT
is specified in
.IR semflg .
.PP
If
.I semflg
specifies both
.B IPC_CREAT
and
.B IPC_EXCL
and a semaphore set already exists for
.IR key ,
then
.BR semget ()
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
Upon creation, the least significant 9 bits of the argument
.I semflg
define the permissions (for owner, group, and others)
for the semaphore set.
These bits have the same format, and the same
meaning, as the
.I mode
argument of
.BR open (2)
(though the execute permissions are
not meaningful for semaphores, and write permissions mean permission
to alter semaphore values).
.PP
When creating a new semaphore set,
.BR semget ()
initializes the set's associated data structure,
.I semid_ds
(see
.BR semctl (2)),
as follows:
.IP \[bu] 3
.I sem_perm.cuid
and
.I sem_perm.uid
are set to the effective user ID of the calling process.
.IP \[bu]
.I sem_perm.cgid
and
.I sem_perm.gid
are set to the effective group ID of the calling process.
.IP \[bu]
The least significant 9 bits of
.I sem_perm.mode
are set to the least significant 9 bits of
.IR semflg .
.IP \[bu]
.I sem_nsems
is set to the value of
.IR nsems .
.IP \[bu]
.I sem_otime
is set to 0.
.IP \[bu]
.I sem_ctime
is set to the current time.
.PP
The argument
.I nsems
can be 0
(a don't care)
when a semaphore set is not being created.
Otherwise,
.I nsems
must be greater than 0
and less than or equal to the maximum number of semaphores per semaphore set
.RB ( SEMMSL ).
.PP
If the semaphore set already exists, the permissions are
verified.
.\" and a check is made to see if it is marked for destruction.
.SH RETURN VALUE
On success,
.BR semget ()
returns the semaphore set identifier (a nonnegative integer).
On failure, \-1 is returned, and
.I errno
is set to indicate the error.
.SH ERRORS
.TP
.B EACCES
A semaphore set exists for
.IR key ,
but the calling process does not have permission to access the set,
and does not have the
.B CAP_IPC_OWNER
capability in the user namespace that governs its IPC namespace.
.TP
.B EEXIST
.B IPC_CREAT
and
.B IPC_EXCL
were specified in
.IR semflg ,
but a semaphore set already exists for
.IR key .
.\" .TP
.\" .B EIDRM
.\" The semaphore set is marked to be deleted.
.TP
.B EINVAL
.I nsems
is less than 0 or greater than the limit on the number
of semaphores per semaphore set
.RB ( SEMMSL ).
.TP
.B EINVAL
A semaphore set corresponding to
.I key
already exists, but
.I nsems
is larger than the number of semaphores in that set.
.TP
.B ENOENT
No semaphore set exists for
.I key
and
.I semflg
did not specify
.BR IPC_CREAT .
.TP
.B ENOMEM
A semaphore set has to be created but the system does not have
enough memory for the new data structure.
.TP
.B ENOSPC
A semaphore set has to be created but the system limit for the maximum
number of semaphore sets
.RB ( SEMMNI ),
or the system wide maximum number of semaphores
.RB ( SEMMNS ),
would be exceeded.
.SH STANDARDS
POSIX.1-2008.
.SH HISTORY
SVr4, POSIX.1-2001.
.\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
.\" ERANGE, EFAULT.
.SH NOTES
.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 semflg
and creates a new semaphore set (on success).
.\"
.SS Semaphore initialization
The values of the semaphores in a newly created set are indeterminate.
(POSIX.1-2001 and POSIX.1-2008 are explicit on this point,
although POSIX.1-2008 notes that a future version of the standard
may require an implementation to initialize the semaphores to 0.)
Although Linux, like many other implementations,
initializes the semaphore values to 0,
a portable application cannot rely on this:
it should explicitly initialize the semaphores to the desired values.
.\" In truth, every one of the many implementations that I've tested sets
.\" the values to zero, but I suppose there is/was some obscure
.\" implementation out there that does not.
.PP
Initialization can be done using
.BR semctl (2)
.B SETVAL
or
.B SETALL
operation.
Where multiple peers do not know who will be the first to
initialize the set, checking for a nonzero
.I sem_otime
in the associated data structure retrieved by a
.BR semctl (2)
.B IPC_STAT
operation can be used to avoid races.
.\"
.SS Semaphore limits
The following limits on semaphore set resources affect the
.BR semget ()
call:
.TP
.B SEMMNI
System-wide limit on the number of semaphore sets.
Before Linux 3.19,
the default value for this limit was 128.
Since Linux 3.19,
.\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
the default value is 32,000.
On Linux, this limit can be read and modified via the fourth field of
.IR /proc/sys/kernel/sem .
.\" This /proc file is not available in Linux 2.2 and earlier -- MTK
.TP
.B SEMMSL
Maximum number of semaphores per semaphore ID.
Before Linux 3.19,
the default value for this limit was 250.
Since Linux 3.19,
.\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
the default value is 32,000.
On Linux, this limit can be read and modified via the first field of
.IR /proc/sys/kernel/sem .
.TP
.B SEMMNS
System-wide limit on the number of semaphores: policy dependent
(on Linux, this limit can be read and modified via the second field of
.IR /proc/sys/kernel/sem ).
Note that the number of semaphores system-wide
is also limited by the product of
.B SEMMSL
and
.BR SEMMNI .
.SH BUGS
The name choice
.B IPC_PRIVATE
was perhaps unfortunate,
.B IPC_NEW
would more clearly show its function.
.SH EXAMPLES
The program shown below uses
.BR semget ()
to create a new semaphore set or retrieve the ID of an existing set.
It generates the
.I key
for
.BR semget ()
using
.BR ftok (3).
The first two command-line arguments are used as the
.I pathname
and
.I proj_id
arguments for
.BR ftok (3).
The third command-line argument is an integer that specifies the
.I nsems
argument for
.BR semget ().
Command-line options can be used to specify the
.B IPC_CREAT
.RI ( \-c )
and
.B IPC_EXCL
.RI ( \-x )
flags for the call to
.BR semget ().
The usage of this program is demonstrated below.
.PP
We first create two files that will be used to generate keys using
.BR ftok (3),
create two semaphore sets using those files, and then list the sets using
.BR ipcs (1):
.PP
.in +4n
.EX
$ \fBtouch mykey mykey2\fP
$ \fB./t_semget \-c mykey p 1\fP
ID = 9
$ \fB./t_semget \-c mykey2 p 2\fP
ID = 10
$ \fBipcs \-s\fP
\&
\-\-\-\-\-\- Semaphore Arrays \-\-\-\-\-\-\-\-
key        semid      owner      perms      nsems
0x7004136d 9          mtk        600        1
0x70041368 10         mtk        600        2
.EE
.in
.PP
Next, we demonstrate that when
.BR semctl (2)
is given the same
.I key
(as generated by the same arguments to
.BR ftok (3)),
it returns the ID of the already existing semaphore set:
.PP
.in +4n
.EX
$ \fB./t_semget \-c mykey p 1\fP
ID = 9
.EE
.in
.PP
Finally, we demonstrate the kind of collision that can occur when
.BR ftok (3)
is given different
.I pathname
arguments that have the same inode number:
.PP
.in +4n
.EX
$ \fBln mykey link\fP
$ \fBls \-i1 link mykey\fP
2233197 link
2233197 mykey
$ \fB./t_semget link p 1\fP       # Generates same key as \[aq]mykey\[aq]
ID = 9
.EE
.in
.SS Program source
\&
.\" SRC BEGIN (t_semget.c)
.EX
/* t_semget.c
\&
   Licensed under GNU General Public License v2 or later.
*/
#include <stdio.h>
#include <stdlib.h>
#include <sys/ipc.h>
#include <sys/sem.h>
#include <unistd.h>
\&
static void
usage(const char *pname)
{
    fprintf(stderr, "Usage: %s [\-cx] pathname proj\-id num\-sems\en",
            pname);
    fprintf(stderr, "    \-c           Use IPC_CREAT flag\en");
    fprintf(stderr, "    \-x           Use IPC_EXCL flag\en");
    exit(EXIT_FAILURE);
}
\&
int
main(int argc, char *argv[])
{
    int    semid, nsems, flags, opt;
    key_t  key;
\&
    flags = 0;
    while ((opt = getopt(argc, argv, "cx")) != \-1) {
        switch (opt) {
        case \[aq]c\[aq]: flags |= IPC_CREAT;   break;
        case \[aq]x\[aq]: flags |= IPC_EXCL;    break;
        default:  usage(argv[0]);
        }
    }
\&
    if (argc != optind + 3)
        usage(argv[0]);
\&
    key = ftok(argv[optind], argv[optind + 1][0]);
    if (key == \-1) {
        perror("ftok");
        exit(EXIT_FAILURE);
    }
\&
    nsems = atoi(argv[optind + 2]);
\&
    semid = semget(key, nsems, flags | 0600);
    if (semid == \-1) {
        perror("semget");
        exit(EXIT_FAILURE);
    }
\&
    printf("ID = %d\en", semid);
\&
    exit(EXIT_SUCCESS);
}
.EE
.\" SRC END
.SH SEE ALSO
.BR semctl (2),
.BR semop (2),
.BR ftok (3),
.BR capabilities (7),
.BR sem_overview (7),
.BR sysvipc (7)