man2/semctl.2

   1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
   2 .\" and Copyright 2004, 2005 Michael Kerrisk <mtk.manpages@gmail.com>
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  22 .\" the source, must acknowledge the copyright and authors of this work.
  23 .\"
  24 .\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esr@thyrsus.com>
  25 .\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aeb@cwi.nl>
  26 .\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aeb@cwi.nl>
  27 .\" Modified 20 Dec 2001, Michael Kerrisk <mtk.manpages@gmail.com>
  28 .\" Modified 21 Dec 2001, aeb
  29 .\" Modified 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  30 .\"     Added notes on CAP_IPC_OWNER requirement
  31 .\" Modified 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  32 .\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
  33 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  34 .\"     Language and formatting clean-ups
  35 .\"     Rewrote semun text
  36 .\"     Added semid_ds and ipc_perm structure definitions
  37 .\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions.
  38 .\"
  39 .TH SEMCTL 2 2009-09-27 "Linux" "Linux Programmer's Manual"
  40 .SH NAME
  41 semctl \- semaphore control operations
  42 .SH SYNOPSIS
  43 .nf
  44 .B #include <sys/types.h>
  45 .B #include <sys/ipc.h>
  46 .B #include <sys/sem.h>
  47 .sp
  48 .BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"
  49 .fi
  50 .SH DESCRIPTION
  51 .BR semctl ()
  52 performs the control operation specified by
  53 .I cmd
  54 on the semaphore set identified by
  55 .IR semid ,
  56 or on the
  57 .IR semnum -th
  58 semaphore of that set.
  59 (The semaphores in a set are numbered starting at 0.)
  60 .PP
  61 This function has three or four arguments, depending on
  62 .IR cmd .
  63 When there are four, the fourth has the type
  64 .IR "union semun" .
  65 The \fIcalling program\fP must define this union as follows:
  66
  67 .nf
  68 .in +4n
  69 union semun {
  70     int              val;    /* Value for SETVAL */
  71     struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */
  72     unsigned short  *array;  /* Array for GETALL, SETALL */
  73     struct seminfo  *__buf;  /* Buffer for IPC_INFO
  74                                 (Linux-specific) */
  75 };
  76 .in
  77 .fi
  78 .PP
  79 The
  80 .I semid_ds
  81 data structure is defined in \fI<sys/sem.h>\fP as follows:
  82 .nf
  83 .in +4n
  84
  85 struct semid_ds {
  86     struct ipc_perm sem_perm;  /* Ownership and permissions */
  87     time_t          sem_otime; /* Last semop time */
  88     time_t          sem_ctime; /* Last change time */
  89     unsigned short  sem_nsems; /* No. of semaphores in set */
  90 };
  91 .in
  92 .fi
  93 .PP
  94 The
  95 .I ipc_perm
  96 structure is defined in \fI<sys/ipc.h>\fP as follows
  97 (the highlighted fields are settable using
  98 .BR IPC_SET ):
  99 .PP
 100 .nf
 101 .in +4n
 102 struct ipc_perm {
 103     key_t          __key; /* Key supplied to semget(2) */
 104     uid_t          \fBuid\fP;   /* Effective UID of owner */
 105     gid_t          \fBgid\fP;   /* Effective GID of owner */
 106     uid_t          cuid;  /* Effective UID of creator */
 107     gid_t          cgid;  /* Effective GID of creator */
 108     unsigned short \fBmode\fP;  /* Permissions */
 109     unsigned short __seq; /* Sequence number */
 110 };
 111 .in
 112 .fi
 113 .PP
 114 Valid values for
 115 .I cmd
 116 are:
 117 .TP 10
 118 .B IPC_STAT
 119 Copy information from the kernel data structure associated with
 120 .I semid
 121 into the
 122 .I semid_ds
 123 structure pointed to by
 124 .IR arg.buf .
 125 The argument
 126 .I semnum
 127 is ignored.
 128 The calling process must have read permission on the semaphore set.
 129 .TP
 130 .B IPC_SET
 131 Write the values of some members of the
 132 .I semid_ds
 133 structure pointed to by
 134 .I arg.buf
 135 to the kernel data structure associated with this semaphore set,
 136 updating also its
 137 .I sem_ctime
 138 member.
 139 The following members of the structure are updated:
 140 .IR sem_perm.uid ,
 141 .IR sem_perm.gid ,
 142 and (the least significant 9 bits of)
 143 .IR sem_perm.mode .
 144 The effective UID of the calling process must match the owner
 145 .RI ( sem_perm.uid )
 146 or creator
 147 .RI ( sem_perm.cuid )
 148 of the semaphore set, or the caller must be privileged.
 149 The argument
 150 .I semnum
 151 is ignored.
 152 .TP
 153 .B IPC_RMID
 154 Immediately remove the semaphore set,
 155 awakening all processes blocked in
 156 .BR semop (2)
 157 calls on the set (with an error return and
 158 .I errno
 159 set to
 160 .BR EIDRM ).
 161 The effective user ID of the calling process must
 162 match the creator or owner of the semaphore set,
 163 or the caller must be privileged.
 164 The argument
 165 .I semnum
 166 is ignored.
 167 .TP
 168 .BR IPC_INFO " (Linux-specific)"
 169 Returns information about system-wide semaphore limits and
 170 parameters in the structure pointed to by
 171 .IR arg.__buf .
 172 This structure is of type
 173 .IR seminfo ,
 174 defined in
 175 .I <sys/sem.h>
 176 if the
 177 .B _GNU_SOURCE
 178 feature test macro is defined:
 179 .nf
 180 .in +4n
 181
 182 struct  seminfo {
 183     int semmap;  /* Number of entries in semaphore
 184                     map; unused within kernel */
 185     int semmni;  /* Maximum number of semaphore sets */
 186     int semmns;  /* Maximum number of semaphores in all
 187                     semaphore sets */
 188     int semmnu;  /* System-wide maximum number of undo
 189                     structures; unused within kernel */
 190     int semmsl;  /* Maximum number of semaphores in a
 191                     set */
 192     int semopm;  /* Maximum number of operations for
 193                     semop(2) */
 194     int semume;  /* Maximum number of undo entries per
 195                     process; unused within kernel */
 196     int semusz;  /* Size of struct sem_undo */
 197     int semvmx;  /* Maximum semaphore value */
 198     int semaem;  /* Max. value that can be recorded for
 199                     semaphore adjustment (SEM_UNDO) */
 200 };
 201
 202 .in
 203 .fi
 204 The
 205 .IR semmsl ,
 206 .IR semmns ,
 207 .IR semopm ,
 208 and
 209 .I semmni
 210 settings can be changed via
 211 .IR /proc/sys/kernel/sem ;
 212 see
 213 .BR proc (5)
 214 for details.
 215 .TP
 216 .BR SEM_INFO " (Linux-specific)"
 217 Returns a
 218 .I seminfo
 219 structure containing the same information as for
 220 .BR IPC_INFO ,
 221 except that the following fields are returned with information
 222 about system resources consumed by semaphores: the
 223 .I semusz
 224 field returns the number of semaphore sets that currently exist
 225 on the system; and the
 226 .I semaem
 227 field returns the total number of semaphores in all semaphore sets
 228 on the system.
 229 .TP
 230 .BR SEM_STAT " (Linux-specific)"
 231 Returns a
 232 .I semid_ds
 233 structure as for
 234 .BR IPC_STAT .
 235 However, the
 236 .I semid
 237 argument is not a semaphore identifier, but instead an index into
 238 the kernel's internal array that maintains information about
 239 all semaphore sets on the system.
 240 .TP
 241 .B GETALL
 242 Return
 243 .B semval
 244 (i.e., the current value)
 245 for all semaphores of the set into
 246 .IR arg.array .
 247 The argument
 248 .I semnum
 249 is ignored.
 250 The calling process must have read permission on the semaphore set.
 251 .TP
 252 .B GETNCNT
 253 The system call returns the value of
 254 .B semncnt
 255 for the
 256 .IR semnum \-th
 257 semaphore of the set
 258 (i.e., the number of processes waiting for an increase of
 259 .B semval
 260 for the
 261 .IR semnum \-th
 262 semaphore of the set).
 263 The calling process must have read permission on the semaphore set.
 264 .TP
 265 .B GETPID
 266 The system call returns the value of
 267 .B sempid
 268 for the
 269 .IR semnum \-th
 270 semaphore of the set
 271 (i.e., the PID of the process that executed the last
 272 .BR semop (2)
 273 call for the
 274 .IR semnum \-th
 275 semaphore of the set).
 276 The calling process must have read permission on the semaphore set.
 277 .TP
 278 .B GETVAL
 279 The system call returns the value of
 280 .B semval
 281 for the
 282 .IR semnum \-th
 283 semaphore of the set.
 284 The calling process must have read permission on the semaphore set.
 285 .TP
 286 .B GETZCNT
 287 The system call returns the value of
 288 .B semzcnt
 289 for the
 290 .IR semnum \-th
 291 semaphore of the set
 292 (i.e., the number of processes waiting for
 293 .B semval
 294 of the
 295 .IR semnum \-th
 296 semaphore of the set to become 0).
 297 The calling process must have read permission on the semaphore set.
 298 .TP
 299 .B SETALL
 300 Set
 301 .B semval
 302 for all semaphores of the set using
 303 .IR arg.array ,
 304 updating also the
 305 .I sem_ctime
 306 member of the
 307 .I semid_ds
 308 structure associated with the set.
 309 Undo entries (see
 310 .BR semop (2))
 311 are cleared for altered semaphores in all processes.
 312 If the changes to semaphore values would permit blocked
 313 .BR semop (2)
 314 calls in other processes to proceed, then those processes are woken up.
 315 The argument
 316 .I semnum
 317 is ignored.
 318 The calling process must have alter (write) permission on
 319 the semaphore set.
 320 .TP
 321 .B SETVAL
 322 Set the value of
 323 .B semval
 324 to
 325 .I arg.val
 326 for the
 327 .IR semnum \-th
 328 semaphore of the set, updating also the
 329 .I sem_ctime
 330 member of the
 331 .I semid_ds
 332 structure associated with the set.
 333 Undo entries are cleared for altered semaphores in all processes.
 334 If the changes to semaphore values would permit blocked
 335 .BR semop (2)
 336 calls in other processes to proceed, then those processes are woken up.
 337 The calling process must have alter permission on the semaphore set.
 338 .SH "RETURN VALUE"
 339 On failure
 340 .BR semctl ()
 341 returns \-1
 342 with
 343 .I errno
 344 indicating the error.
 345
 346 Otherwise the system call returns a nonnegative value depending on
 347 .I cmd
 348 as follows:
 349 .TP 12
 350 .B GETNCNT
 351 the value of
 352 .BR semncnt .
 353 .TP
 354 .B GETPID
 355 the value of
 356 .BR sempid .
 357 .TP
 358 .B GETVAL
 359 the value of
 360 .BR semval .
 361 .TP
 362 .B GETZCNT
 363 the value of
 364 .BR semzcnt .
 365 .TP
 366 .B IPC_INFO
 367 the index of the highest used entry in the
 368 kernel's internal array recording information about all
 369 semaphore sets.
 370 (This information can be used with repeated
 371 .B SEM_STAT
 372 operations to obtain information about all semaphore sets on the system.)
 373 .TP
 374 .B SEM_INFO
 375 As for
 376 .BR IPC_INFO .
 377 .TP
 378 .B SEM_STAT
 379 the identifier of the semaphore set whose index was given in
 380 .IR semid .
 381 .LP
 382 All other
 383 .I cmd
 384 values return 0 on success.
 385 .SH ERRORS
 386 On failure,
 387 .I errno
 388 will be set to one of the following:
 389 .TP
 390 .B EACCES
 391 The argument
 392 .I cmd
 393 has one of the values
 394 .BR GETALL ,
 395 .BR GETPID ,
 396 .BR GETVAL ,
 397 .BR GETNCNT ,
 398 .BR GETZCNT ,
 399 .BR IPC_STAT ,
 400 .BR SEM_STAT ,
 401 .BR SETALL ,
 402 or
 403 .B SETVAL
 404 and the calling process does not have the required
 405 permissions on the semaphore set and does not have the
 406 .B CAP_IPC_OWNER
 407 capability.
 408 .TP
 409 .B EFAULT
 410 The address pointed to by
 411 .I arg.buf
 412 or
 413 .I arg.array
 414 isn't accessible.
 415 .TP
 416 .B EIDRM
 417 The semaphore set was removed.
 418 .TP
 419 .B EINVAL
 420 Invalid value for
 421 .I cmd
 422 or
 423 .IR semid .
 424 Or: for a
 425 .B SEM_STAT
 426 operation, the index value specified in
 427 .I semid
 428 referred to an array slot that is currently unused.
 429 .TP
 430 .B EPERM
 431 The argument
 432 .I cmd
 433 has the value
 434 .B IPC_SET
 435 or
 436 .B IPC_RMID
 437 but the effective user ID of the calling process is not the creator
 438 (as found in
 439 .IR sem_perm.cuid )
 440 or the owner
 441 (as found in
 442 .IR sem_perm.uid )
 443 of the semaphore set,
 444 and the process does not have the
 445 .B CAP_SYS_ADMIN
 446 capability.
 447 .TP
 448 .B ERANGE
 449 The argument
 450 .I cmd
 451 has the value
 452 .B SETALL
 453 or
 454 .B SETVAL
 455 and the value to which
 456 .B semval
 457 is to be set (for some semaphore of the set) is less than 0
 458 or greater than the implementation limit
 459 .BR SEMVMX .
 460 .SH "CONFORMING TO"
 461 SVr4, POSIX.1-2001.
 462 .\" SVr4 documents more error conditions EINVAL and EOVERFLOW.
 463 .SH NOTES
 464 The
 465 .BR IPC_INFO ,
 466 .B SEM_STAT
 467 and
 468 .B SEM_INFO
 469 operations are used by the
 470 .BR ipcs (8)
 471 program to provide information on allocated resources.
 472 In the future these may modified or moved to a /proc file system
 473 interface.
 474 .LP
 475 Various fields in a \fIstruct semid_ds\fP were typed as
 476 .I short
 477 under Linux 2.2
 478 and have become
 479 .I long
 480 under Linux 2.4.
 481 To take advantage of this,
 482 a recompilation under glibc-2.1.91 or later should suffice.
 483 (The kernel distinguishes old and new calls by an
 484 .B IPC_64
 485 flag in
 486 .IR cmd .)
 487 .PP
 488 In some earlier versions of glibc, the
 489 .I semun
 490 union was defined in \fI<sys/sem.h>\fP, but POSIX.1-2001 requires
 491 that the caller define this union.
 492 On versions of glibc where this union is \fInot\fP defined,
 493 the macro
 494 .B _SEM_SEMUN_UNDEFINED
 495 is defined in \fI<sys/sem.h>\fP.
 496 .PP
 497 The following system limit on semaphore sets affects a
 498 .BR semctl ()
 499 call:
 500 .TP
 501 .B SEMVMX
 502 Maximum value for
 503 .BR semval :
 504 implementation dependent (32767).
 505 .LP
 506 For greater portability it is best to always call
 507 .BR semctl ()
 508 with four arguments.
 509 .SH "SEE ALSO"
 510 .BR ipc (2),
 511 .BR semget (2),
 512 .BR semop (2),
 513 .BR capabilities (7),
 514 .BR sem_overview (7),
 515 .BR svipc (7)