man2/semget.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\" and Copyright (C) 2020 Michael Kerrisk <mtk.manpages@gmail.com>
   3 .\"
   4 .\" SPDX-License-Identifier: Linux-man-pages-copyleft
   5 .\"
   6 .\" Modified Tue Oct 22 17:54:56 1996 by Eric S. Raymond <esr@thyrsus.com>
   7 .\" Modified 1 Jan 2002, Martin Schulze <joey@infodrom.org>
   8 .\" Modified 4 Jan 2002, Michael Kerrisk <mtk.manpages@gmail.com>
   9 .\" Modified, 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  10 .\"     Added notes on capability requirements
  11 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  12 .\"     Language and formatting clean-ups
  13 .\"     Added notes on /proc files
  14 .\"     Rewrote BUGS note about semget()'s failure to initialize
  15 .\"             semaphore values
  16 .\"
  17 .TH SEMGET 2 2021-03-22 "Linux man-pages (unreleased)"
  18 .SH NAME
  19 semget \- get a System V semaphore set identifier
  20 .SH LIBRARY
  21 Standard C library
  22 .RI ( libc ", " \-lc )
  23 .SH SYNOPSIS
  24 .nf
  25 .B #include <sys/sem.h>
  26 .fi
  27 .PP
  28 .BI "int semget(key_t " key ,
  29 .BI "int " nsems ,
  30 .BI "int " semflg );
  31 .SH DESCRIPTION
  32 The
  33 .BR semget ()
  34 system call returns the System\ V semaphore set identifier
  35 associated with the argument
  36 .IR key .
  37 It may be used either to obtain the identifier of a previously created
  38 semaphore set (when
  39 .I semflg
  40 is zero and
  41 .I key
  42 does not have the value
  43 .BR IPC_PRIVATE ),
  44 or to create a new set.
  45 .PP
  46 A new set of
  47 .I nsems
  48 semaphores is created if
  49 .I key
  50 has the value
  51 .B IPC_PRIVATE
  52 or if no existing semaphore set is associated with
  53 .I key
  54 and
  55 .B IPC_CREAT
  56 is specified in
  57 .IR semflg .
  58 .PP
  59 If
  60 .I semflg
  61 specifies both
  62 .B IPC_CREAT
  63 and
  64 .B IPC_EXCL
  65 and a semaphore set already exists for
  66 .IR key ,
  67 then
  68 .BR semget ()
  69 fails with
  70 .I errno
  71 set to
  72 .BR EEXIST .
  73 (This is analogous to the effect of the combination
  74 .B O_CREAT | O_EXCL
  75 for
  76 .BR open (2).)
  77 .PP
  78 Upon creation, the least significant 9 bits of the argument
  79 .I semflg
  80 define the permissions (for owner, group, and others)
  81 for the semaphore set.
  82 These bits have the same format, and the same
  83 meaning, as the
  84 .I mode
  85 argument of
  86 .BR open (2)
  87 (though the execute permissions are
  88 not meaningful for semaphores, and write permissions mean permission
  89 to alter semaphore values).
  90 .PP
  91 When creating a new semaphore set,
  92 .BR semget ()
  93 initializes the set's associated data structure,
  94 .I semid_ds
  95 (see
  96 .BR semctl (2)),
  97 as follows:
  98 .IP \(bu 2
  99 .I sem_perm.cuid
 100 and
 101 .I sem_perm.uid
 102 are set to the effective user ID of the calling process.
 103 .IP \(bu
 104 .I sem_perm.cgid
 105 and
 106 .I sem_perm.gid
 107 are set to the effective group ID of the calling process.
 108 .IP \(bu
 109 The least significant 9 bits of
 110 .I sem_perm.mode
 111 are set to the least significant 9 bits of
 112 .IR semflg .
 113 .IP \(bu
 114 .I sem_nsems
 115 is set to the value of
 116 .IR nsems .
 117 .IP \(bu
 118 .I sem_otime
 119 is set to 0.
 120 .IP \(bu
 121 .I sem_ctime
 122 is set to the current time.
 123 .PP
 124 The argument
 125 .I nsems
 126 can be 0
 127 (a don't care)
 128 when a semaphore set is not being created.
 129 Otherwise,
 130 .I nsems
 131 must be greater than 0
 132 and less than or equal to the maximum number of semaphores per semaphore set
 133 .RB ( SEMMSL ).
 134 .PP
 135 If the semaphore set already exists, the permissions are
 136 verified.
 137 .\" and a check is made to see if it is marked for destruction.
 138 .SH RETURN VALUE
 139 On success,
 140 .BR semget ()
 141 returns the semaphore set identifier (a nonnegative integer).
 142 On failure, \-1 is returned, and
 143 .I errno
 144 is set to indicate the error.
 145 .SH ERRORS
 146 .TP
 147 .B EACCES
 148 A semaphore set exists for
 149 .IR key ,
 150 but the calling process does not have permission to access the set,
 151 and does not have the
 152 .B CAP_IPC_OWNER
 153 capability in the user namespace that governs its IPC namespace.
 154 .TP
 155 .B EEXIST
 156 .B IPC_CREAT
 157 and
 158 .B IPC_EXCL
 159 were specified in
 160 .IR semflg ,
 161 but a semaphore set already exists for
 162 .IR key .
 163 .\" .TP
 164 .\" .B EIDRM
 165 .\" The semaphore set is marked to be deleted.
 166 .TP
 167 .B EINVAL
 168 .I nsems
 169 is less than 0 or greater than the limit on the number
 170 of semaphores per semaphore set
 171 .RB ( SEMMSL ).
 172 .TP
 173 .B EINVAL
 174 A semaphore set corresponding to
 175 .I key
 176 already exists, but
 177 .I nsems
 178 is larger than the number of semaphores in that set.
 179 .TP
 180 .B ENOENT
 181 No semaphore set exists for
 182 .I key
 183 and
 184 .I semflg
 185 did not specify
 186 .BR IPC_CREAT .
 187 .TP
 188 .B ENOMEM
 189 A semaphore set has to be created but the system does not have
 190 enough memory for the new data structure.
 191 .TP
 192 .B ENOSPC
 193 A semaphore set has to be created but the system limit for the maximum
 194 number of semaphore sets
 195 .RB ( SEMMNI ),
 196 or the system wide maximum number of semaphores
 197 .RB ( SEMMNS ),
 198 would be exceeded.
 199 .SH STANDARDS
 200 SVr4, POSIX.1-2001.
 201 .\" SVr4 documents additional error conditions EFBIG, E2BIG, EAGAIN,
 202 .\" ERANGE, EFAULT.
 203 .SH NOTES
 204 .B IPC_PRIVATE
 205 isn't a flag field but a
 206 .I key_t
 207 type.
 208 If this special value is used for
 209 .IR key ,
 210 the system call ignores all but the least significant 9 bits of
 211 .I semflg
 212 and creates a new semaphore set (on success).
 213 .\"
 214 .SS Semaphore initialization
 215 The values of the semaphores in a newly created set are indeterminate.
 216 (POSIX.1-2001 and POSIX.1-2008 are explicit on this point,
 217 although POSIX.1-2008 notes that a future version of the standard
 218 may require an implementation to initialize the semaphores to 0.)
 219 Although Linux, like many other implementations,
 220 initializes the semaphore values to 0,
 221 a portable application cannot rely on this:
 222 it should explicitly initialize the semaphores to the desired values.
 223 .\" In truth, every one of the many implementations that I've tested sets
 224 .\" the values to zero, but I suppose there is/was some obscure
 225 .\" implementation out there that does not.
 226 .PP
 227 Initialization can be done using
 228 .BR semctl (2)
 229 .B SETVAL
 230 or
 231 .B SETALL
 232 operation.
 233 Where multiple peers do not know who will be the first to
 234 initialize the set, checking for a nonzero
 235 .I sem_otime
 236 in the associated data structure retrieved by a
 237 .BR semctl (2)
 238 .B IPC_STAT
 239 operation can be used to avoid races.
 240 .\"
 241 .SS Semaphore limits
 242 The following limits on semaphore set resources affect the
 243 .BR semget ()
 244 call:
 245 .TP
 246 .B SEMMNI
 247 System-wide limit on the number of semaphore sets.
 248 On Linux systems before version 3.19,
 249 the default value for this limit was 128.
 250 Since Linux 3.19,
 251 .\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
 252 the default value is 32,000.
 253 On Linux, this limit can be read and modified via the fourth field of
 254 .IR /proc/sys/kernel/sem .
 255 .\" This /proc file is not available in Linux 2.2 and earlier -- MTK
 256 .TP
 257 .B SEMMSL
 258 Maximum number of semaphores per semaphore ID.
 259 On Linux systems before version 3.19,
 260 the default value for this limit was 250.
 261 Since Linux 3.19,
 262 .\" commit e843e7d2c88b7db107a86bd2c7145dc715c058f4
 263 the default value is 32,000.
 264 On Linux, this limit can be read and modified via the first field of
 265 .IR /proc/sys/kernel/sem .
 266 .TP
 267 .B SEMMNS
 268 System-wide limit on the number of semaphores: policy dependent
 269 (on Linux, this limit can be read and modified via the second field of
 270 .IR /proc/sys/kernel/sem ).
 271 Note that the number of semaphores system-wide
 272 is also limited by the product of
 273 .B SEMMSL
 274 and
 275 .BR SEMMNI .
 276 .SH BUGS
 277 The name choice
 278 .B IPC_PRIVATE
 279 was perhaps unfortunate,
 280 .B IPC_NEW
 281 would more clearly show its function.
 282 .SH EXAMPLES
 283 The program shown below uses
 284 .BR semget ()
 285 to create a new semaphore set or retrieve the ID of an existing set.
 286 It generates the
 287 .I key
 288 for
 289 .BR semget ()
 290 using
 291 .BR ftok (3).
 292 The first two command-line arguments are used as the
 293 .I pathname
 294 and
 295 .I proj_id
 296 arguments for
 297 .BR ftok (3).
 298 The third command-line argument is an integer that specifies the
 299 .I nsems
 300 argument for
 301 .BR semget ().
 302 Command-line options can be used to specify the
 303 .B IPC_CREAT
 304 .RI ( \-c )
 305 and
 306 .B IPC_EXCL
 307 .RI ( \-x )
 308 flags for the call to
 309 .BR semget ().
 310 The usage of this program is demonstrated below.
 311 .PP
 312 We first create two files that will be used to generate keys using
 313 .BR ftok (3),
 314 create two semaphore sets using those files, and then list the sets using
 315 .BR ipcs (1):
 316 .PP
 317 .in +4n
 318 .EX
 319 $ \fBtouch mykey mykey2\fP
 320 $ \fB./t_semget \-c mykey p 1\fP
 321 ID = 9
 322 $ \fB./t_semget \-c mykey2 p 2\fP
 323 ID = 10
 324 $ \fBipcs \-s\fP
 325
 326 \-\-\-\-\-\- Semaphore Arrays \-\-\-\-\-\-\-\-
 327 key        semid      owner      perms      nsems
 328 0x7004136d 9          mtk        600        1
 329 0x70041368 10         mtk        600        2
 330 .EE
 331 .in
 332 .PP
 333 Next, we demonstrate that when
 334 .BR semctl (2)
 335 is given the same
 336 .I key
 337 (as generated by the same arguments to
 338 .BR ftok (3)),
 339 it returns the ID of the already existing semaphore set:
 340 .PP
 341 .in +4n
 342 .EX
 343 $ \fB./t_semget \-c mykey p 1\fP
 344 ID = 9
 345 .EE
 346 .in
 347 .PP
 348 Finally, we demonstrate the kind of collision that can occur when
 349 .BR ftok (3)
 350 is given different
 351 .I pathname
 352 arguments that have the same inode number:
 353 .PP
 354 .in +4n
 355 .EX
 356 $ \fBln mykey link\fP
 357 $ \fBls \-i1 link mykey\fP
 358 2233197 link
 359 2233197 mykey
 360 $ \fB./t_semget link p 1\fP       # Generates same key as \(aqmykey\(aq
 361 ID = 9
 362 .EE
 363 .in
 364 .SS Program source
 365 \&
 366 .\" SRC BEGIN (t_semget.c)
 367 .EX
 368 /* t_semget.c
 369
 370    Licensed under GNU General Public License v2 or later.
 371 */
 372 #include <stdio.h>
 373 #include <stdlib.h>
 374 #include <sys/ipc.h>
 375 #include <sys/sem.h>
 376 #include <unistd.h>
 377
 378 static void
 379 usage(const char *pname)
 380 {
 381     fprintf(stderr, "Usage: %s [\-cx] pathname proj\-id num\-sems\en",
 382             pname);
 383     fprintf(stderr, "    \-c           Use IPC_CREAT flag\en");
 384     fprintf(stderr, "    \-x           Use IPC_EXCL flag\en");
 385     exit(EXIT_FAILURE);
 386 }
 387
 388 int
 389 main(int argc, char *argv[])
 390 {
 391     int semid, nsems, flags, opt;
 392     key_t key;
 393
 394     flags = 0;
 395     while ((opt = getopt(argc, argv, "cx")) != \-1) {
 396         switch (opt) {
 397         case \(aqc\(aq: flags |= IPC_CREAT;   break;
 398         case \(aqx\(aq: flags |= IPC_EXCL;    break;
 399         default:  usage(argv[0]);
 400         }
 401     }
 402
 403     if (argc != optind + 3)
 404         usage(argv[0]);
 405
 406     key = ftok(argv[optind], argv[optind + 1][0]);
 407     if (key == \-1) {
 408         perror("ftok");
 409         exit(EXIT_FAILURE);
 410     }
 411
 412     nsems = atoi(argv[optind + 2]);
 413
 414     semid = semget(key, nsems, flags | 0600);
 415     if (semid == \-1) {
 416         perror("semget");
 417         exit(EXIT_FAILURE);
 418     }
 419
 420     printf("ID = %d\en", semid);
 421
 422     exit(EXIT_SUCCESS);
 423 }
 424 .EE
 425 .\" SRC END
 426 .SH SEE ALSO
 427 .BR semctl (2),
 428 .BR semop (2),
 429 .BR ftok (3),
 430 .BR capabilities (7),
 431 .BR sem_overview (7),
 432 .BR sysvipc (7)