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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  25 .\"
  26 .\" Modified Tue Oct 22 17:53:56 1996 by Eric S. Raymond <esr@thyrsus.com>
  27 .\" Modified Fri Jun 19 10:59:15 1998 by Andries Brouwer <aeb@cwi.nl>
  28 .\" Modified Sun Feb 18 01:59:29 2001 by Andries Brouwer <aeb@cwi.nl>
  29 .\" Modified 20 Dec 2001, Michael Kerrisk <mtk.manpages@gmail.com>
  30 .\" Modified 21 Dec 2001, aeb
  31 .\" Modified 27 May 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  32 .\"     Added notes on CAP_IPC_OWNER requirement
  33 .\" Modified 17 Jun 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  34 .\"     Added notes on CAP_SYS_ADMIN requirement for IPC_SET and IPC_RMID
  35 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
  36 .\"     Language and formatting clean-ups
  37 .\"     Rewrote semun text
  38 .\"     Added semid_ds and ipc_perm structure definitions
  39 .\" 2005-08-02, mtk: Added IPC_INFO, SEM_INFO, SEM_STAT descriptions.
  40 .\" 2018-03-20, dbueso: Added SEM_STAT_ANY description.
  41 .\"
  42 .TH SEMCTL 2 2020-04-11 "Linux" "Linux Programmer's Manual"
  43 .SH NAME
  44 semctl \- System V semaphore control operations
  45 .SH SYNOPSIS
  46 .nf
  47 .B #include <sys/types.h>
  48 .B #include <sys/ipc.h>
  49 .B #include <sys/sem.h>
  50 .PP
  51 .BI "int semctl(int " semid ", int " semnum ", int " cmd ", ...);"
  52 .fi
  53 .SH DESCRIPTION
  54 .BR semctl ()
  55 performs the control operation specified by
  56 .I cmd
  57 on the System\ V semaphore set identified by
  58 .IR semid ,
  59 or on the
  60 .IR semnum -th
  61 semaphore of that set.
  62 (The semaphores in a set are numbered starting at 0.)
  63 .PP
  64 This function has three or four arguments, depending on
  65 .IR cmd .
  66 When there are four, the fourth has the type
  67 .IR "union semun" .
  68 The \fIcalling program\fP must define this union as follows:
  69 .PP
  70 .in +4n
  71 .EX
  72 union semun {
  73     int              val;    /* Value for SETVAL */
  74     struct semid_ds *buf;    /* Buffer for IPC_STAT, IPC_SET */
  75     unsigned short  *array;  /* Array for GETALL, SETALL */
  76     struct seminfo  *__buf;  /* Buffer for IPC_INFO
  77                                 (Linux-specific) */
  78 };
  79 .EE
  80 .in
  81 .PP
  82 The
  83 .I semid_ds
  84 data structure is defined in \fI<sys/sem.h>\fP as follows:
  85 .PP
  86 .in +4n
  87 .EX
  88 struct semid_ds {
  89     struct ipc_perm sem_perm;  /* Ownership and permissions */
  90     time_t          sem_otime; /* Last semop time */
  91     time_t          sem_ctime; /* Creation time/time of last
  92                                   modification via semctl() */
  93     unsigned long   sem_nsems; /* No. of semaphores in set */
  94 };
  95 .EE
  96 .in
  97 The fields of the
  98 .I semid_ds
  99 structure are as follows:
 100 .TP 11
 101 .I sem_perm
 102 This is an
 103 .I ipc_perm
 104 structure (see below) that specifies the access permissions on the semaphore
 105 set.
 106 .TP
 107 .I sem_otime
 108 Time of last
 109 .BR semop (2)
 110 system call.
 111 .TP
 112 .I sem_ctime
 113 Time of creation of semaphore set or time of last
 114 .BR semctl ()
 115 .BR IPCSET ,
 116 .BR SETVAL ,
 117 or
 118 .BR SETVALL
 119 operation.
 120 .TP
 121 .I sem_nsems
 122 Number of semaphores in the set.
 123 Each semaphore of the set is referenced by a nonnegative integer
 124 ranging from
 125 .B 0
 126 to
 127 .IR sem_nsems\-1 .
 128 .PP
 129 The
 130 .I ipc_perm
 131 structure is defined as follows
 132 (the highlighted fields are settable using
 133 .BR IPC_SET ):
 134 .PP
 135 .in +4n
 136 .EX
 137 struct ipc_perm {
 138     key_t          __key; /* Key supplied to semget(2) */
 139     uid_t          \fBuid\fP;   /* Effective UID of owner */
 140     gid_t          \fBgid\fP;   /* Effective GID of owner */
 141     uid_t          cuid;  /* Effective UID of creator */
 142     gid_t          cgid;  /* Effective GID of creator */
 143     unsigned short \fBmode\fP;  /* Permissions */
 144     unsigned short __seq; /* Sequence number */
 145 };
 146 .EE
 147 .in
 148 .PP
 149 The least significant 9 bits of the
 150 .I mode
 151 field of the
 152 .I ipc_perm
 153 structure define the access permissions for the shared memory segment.
 154 The permission bits are as follows:
 155 .TS
 156 l l.
 157 0400    Read by user
 158 0200    Write by user
 159 0040    Read by group
 160 0020    Write by group
 161 0004    Read by others
 162 0002    Write by others
 163 .TE
 164 .PP
 165 In effect, "write" means "alter" for a semaphore set.
 166 Bits 0100, 0010, and 0001 (the execute bits) are unused by the system.
 167 .PP
 168 Valid values for
 169 .I cmd
 170 are:
 171 .TP
 172 .B IPC_STAT
 173 Copy information from the kernel data structure associated with
 174 .I semid
 175 into the
 176 .I semid_ds
 177 structure pointed to by
 178 .IR arg.buf .
 179 The argument
 180 .I semnum
 181 is ignored.
 182 The calling process must have read permission on the semaphore set.
 183 .TP
 184 .B IPC_SET
 185 Write the values of some members of the
 186 .I semid_ds
 187 structure pointed to by
 188 .I arg.buf
 189 to the kernel data structure associated with this semaphore set,
 190 updating also its
 191 .I sem_ctime
 192 member.
 193 The following members of the structure are updated:
 194 .IR sem_perm.uid ,
 195 .IR sem_perm.gid ,
 196 and (the least significant 9 bits of)
 197 .IR sem_perm.mode .
 198 The effective UID of the calling process must match the owner
 199 .RI ( sem_perm.uid )
 200 or creator
 201 .RI ( sem_perm.cuid )
 202 of the semaphore set, or the caller must be privileged.
 203 The argument
 204 .I semnum
 205 is ignored.
 206 .TP
 207 .B IPC_RMID
 208 Immediately remove the semaphore set,
 209 awakening all processes blocked in
 210 .BR semop (2)
 211 calls on the set (with an error return and
 212 .I errno
 213 set to
 214 .BR EIDRM ).
 215 The effective user ID of the calling process must
 216 match the creator or owner of the semaphore set,
 217 or the caller must be privileged.
 218 The argument
 219 .I semnum
 220 is ignored.
 221 .TP
 222 .BR IPC_INFO " (Linux-specific)"
 223 Return information about system-wide semaphore limits and
 224 parameters in the structure pointed to by
 225 .IR arg.__buf .
 226 This structure is of type
 227 .IR seminfo ,
 228 defined in
 229 .I <sys/sem.h>
 230 if the
 231 .B _GNU_SOURCE
 232 feature test macro is defined:
 233 .IP
 234 .in +4n
 235 .EX
 236 struct  seminfo {
 237     int semmap;  /* Number of entries in semaphore
 238                     map; unused within kernel */
 239     int semmni;  /* Maximum number of semaphore sets */
 240     int semmns;  /* Maximum number of semaphores in all
 241                     semaphore sets */
 242     int semmnu;  /* System-wide maximum number of undo
 243                     structures; unused within kernel */
 244     int semmsl;  /* Maximum number of semaphores in a
 245                     set */
 246     int semopm;  /* Maximum number of operations for
 247                     semop(2) */
 248     int semume;  /* Maximum number of undo entries per
 249                     process; unused within kernel */
 250     int semusz;  /* Size of struct sem_undo */
 251     int semvmx;  /* Maximum semaphore value */
 252     int semaem;  /* Max. value that can be recorded for
 253                     semaphore adjustment (SEM_UNDO) */
 254 };
 255 .EE
 256 .in
 257 .IP
 258 The
 259 .IR semmsl ,
 260 .IR semmns ,
 261 .IR semopm ,
 262 and
 263 .I semmni
 264 settings can be changed via
 265 .IR /proc/sys/kernel/sem ;
 266 see
 267 .BR proc (5)
 268 for details.
 269 .TP
 270 .BR SEM_INFO " (Linux-specific)"
 271 Return a
 272 .I seminfo
 273 structure containing the same information as for
 274 .BR IPC_INFO ,
 275 except that the following fields are returned with information
 276 about system resources consumed by semaphores: the
 277 .I semusz
 278 field returns the number of semaphore sets that currently exist
 279 on the system; and the
 280 .I semaem
 281 field returns the total number of semaphores in all semaphore sets
 282 on the system.
 283 .TP
 284 .BR SEM_STAT " (Linux-specific)"
 285 Return a
 286 .I semid_ds
 287 structure as for
 288 .BR IPC_STAT .
 289 However, the
 290 .I semid
 291 argument is not a semaphore identifier, but instead an index into
 292 the kernel's internal array that maintains information about
 293 all semaphore sets on the system.
 294 .TP
 295 .BR SEM_STAT_ANY " (Linux-specific, since Linux 4.17)"
 296 Return a
 297 .I seminfo
 298 structure containing the same information as for
 299 .BR SEM_STAT .
 300 However,
 301 .I sem_perm.mode
 302 is not checked for read access for
 303 .IR semid
 304 meaning that any user can employ this operation (just as any user may read
 305 .IR /proc/sysvipc/sem
 306 to obtain the same information).
 307 .TP
 308 .B GETALL
 309 Return
 310 .B semval
 311 (i.e., the current value)
 312 for all semaphores of the set into
 313 .IR arg.array .
 314 The argument
 315 .I semnum
 316 is ignored.
 317 The calling process must have read permission on the semaphore set.
 318 .TP
 319 .B GETNCNT
 320 Return the
 321 .B semncnt
 322 value for the
 323 .IR semnum \-th
 324 semaphore of the set
 325 (i.e., the number of processes waiting for the semaphore's value to increase).
 326 The calling process must have read permission on the semaphore set.
 327 .TP
 328 .B GETPID
 329 Return the
 330 .B sempid
 331 value for the
 332 .IR semnum \-th
 333 semaphore of the set.
 334 This is the PID of the process that last performed an operation on
 335 that semaphore (but see NOTES).
 336 The calling process must have read permission on the semaphore set.
 337 .TP
 338 .B GETVAL
 339 Return
 340 .B semval
 341 (i.e., the semaphore value) for the
 342 .IR semnum \-th
 343 semaphore of the set.
 344 The calling process must have read permission on the semaphore set.
 345 .TP
 346 .B GETZCNT
 347 Return the
 348 .B semzcnt
 349 value for the
 350 .IR semnum \-th
 351 semaphore of the set
 352 (i.e., the number of processes waiting for the semaphore value to become 0).
 353 The calling process must have read permission on the semaphore set.
 354 .TP
 355 .B SETALL
 356 Set the
 357 .B semval
 358 values for all semaphores of the set using
 359 .IR arg.array ,
 360 updating also the
 361 .I sem_ctime
 362 member of the
 363 .I semid_ds
 364 structure associated with the set.
 365 Undo entries (see
 366 .BR semop (2))
 367 are cleared for altered semaphores in all processes.
 368 If the changes to semaphore values would permit blocked
 369 .BR semop (2)
 370 calls in other processes to proceed, then those processes are woken up.
 371 The argument
 372 .I semnum
 373 is ignored.
 374 The calling process must have alter (write) permission on
 375 the semaphore set.
 376 .TP
 377 .B SETVAL
 378 Set the semaphore value
 379 .BR ( semval )
 380 to
 381 .I arg.val
 382 for the
 383 .IR semnum \-th
 384 semaphore of the set, updating also the
 385 .I sem_ctime
 386 member of the
 387 .I semid_ds
 388 structure associated with the set.
 389 Undo entries are cleared for altered semaphores in all processes.
 390 If the changes to semaphore values would permit blocked
 391 .BR semop (2)
 392 calls in other processes to proceed, then those processes are woken up.
 393 The calling process must have alter permission on the semaphore set.
 394 .SH RETURN VALUE
 395 On failure,
 396 .BR semctl ()
 397 returns \-1
 398 with
 399 .I errno
 400 indicating the error.
 401 .PP
 402 Otherwise, the system call returns a nonnegative value depending on
 403 .I cmd
 404 as follows:
 405 .TP
 406 .B GETNCNT
 407 the value of
 408 .BR semncnt .
 409 .TP
 410 .B GETPID
 411 the value of
 412 .BR sempid .
 413 .TP
 414 .B GETVAL
 415 the value of
 416 .BR semval .
 417 .TP
 418 .B GETZCNT
 419 the value of
 420 .BR semzcnt .
 421 .TP
 422 .B IPC_INFO
 423 the index of the highest used entry in the
 424 kernel's internal array recording information about all
 425 semaphore sets.
 426 (This information can be used with repeated
 427 .B SEM_STAT
 428 or
 429 .B SEM_STAT_ANY
 430 operations to obtain information about all semaphore sets on the system.)
 431 .TP
 432 .B SEM_INFO
 433 as for
 434 .BR IPC_INFO .
 435 .TP
 436 .B SEM_STAT
 437 the identifier of the semaphore set whose index was given in
 438 .IR semid .
 439 .TP
 440 .B SEM_STAT_ANY
 441 as for
 442 .BR SEM_STAT .
 443 .PP
 444 All other
 445 .I cmd
 446 values return 0 on success.
 447 .SH ERRORS
 448 On failure,
 449 .I errno
 450 will be set to one of the following:
 451 .TP
 452 .B EACCES
 453 The argument
 454 .I cmd
 455 has one of the values
 456 .BR GETALL ,
 457 .BR GETPID ,
 458 .BR GETVAL ,
 459 .BR GETNCNT ,
 460 .BR GETZCNT ,
 461 .BR IPC_STAT ,
 462 .BR SEM_STAT ,
 463 .BR SEM_STAT_ANY ,
 464 .BR SETALL ,
 465 or
 466 .B SETVAL
 467 and the calling process does not have the required
 468 permissions on the semaphore set and does not have the
 469 .B CAP_IPC_OWNER
 470 capability in the user namespace that governs its IPC namespace.
 471 .TP
 472 .B EFAULT
 473 The address pointed to by
 474 .I arg.buf
 475 or
 476 .I arg.array
 477 isn't accessible.
 478 .TP
 479 .B EIDRM
 480 The semaphore set was removed.
 481 .TP
 482 .B EINVAL
 483 Invalid value for
 484 .I cmd
 485 or
 486 .IR semid .
 487 Or: for a
 488 .B SEM_STAT
 489 operation, the index value specified in
 490 .I semid
 491 referred to an array slot that is currently unused.
 492 .TP
 493 .B EPERM
 494 The argument
 495 .I cmd
 496 has the value
 497 .B IPC_SET
 498 or
 499 .B IPC_RMID
 500 but the effective user ID of the calling process is not the creator
 501 (as found in
 502 .IR sem_perm.cuid )
 503 or the owner
 504 (as found in
 505 .IR sem_perm.uid )
 506 of the semaphore set,
 507 and the process does not have the
 508 .B CAP_SYS_ADMIN
 509 capability.
 510 .TP
 511 .B ERANGE
 512 The argument
 513 .I cmd
 514 has the value
 515 .B SETALL
 516 or
 517 .B SETVAL
 518 and the value to which
 519 .B semval
 520 is to be set (for some semaphore of the set) is less than 0
 521 or greater than the implementation limit
 522 .BR SEMVMX .
 523 .SH CONFORMING TO
 524 POSIX.1-2001, POSIX.1-2008, SVr4.
 525 .\" SVr4 documents more error conditions EINVAL and EOVERFLOW.
 526 .PP
 527 POSIX.1 specifies the
 528 .\" POSIX.1-2001, POSIX.1-2008
 529 .I sem_nsems
 530 field of the
 531 .I semid_ds
 532 structure as having the type
 533 .IR "unsigned\ short" ,
 534 and the field is so defined on most other systems.
 535 It was also so defined on Linux 2.2 and earlier,
 536 but, since Linux 2.4, the field has the type
 537 .IR "unsigned\ long" .
 538 .SH NOTES
 539 The inclusion of
 540 .I <sys/types.h>
 541 and
 542 .I <sys/ipc.h>
 543 isn't required on Linux or by any version of POSIX.
 544 However,
 545 some old implementations required the inclusion of these header files,
 546 and the SVID also documented their inclusion.
 547 Applications intended to be portable to such old systems may need
 548 to include these header files.
 549 .\" Like Linux, the FreeBSD man pages still document
 550 .\" the inclusion of these header files.
 551 .PP
 552 The
 553 .BR IPC_INFO ,
 554 .B SEM_STAT
 555 and
 556 .B SEM_INFO
 557 operations are used by the
 558 .BR ipcs (1)
 559 program to provide information on allocated resources.
 560 In the future these may modified or moved to a
 561 .I /proc
 562 filesystem interface.
 563 .PP
 564 Various fields in a \fIstruct semid_ds\fP were typed as
 565 .I short
 566 under Linux 2.2
 567 and have become
 568 .I long
 569 under Linux 2.4.
 570 To take advantage of this,
 571 a recompilation under glibc-2.1.91 or later should suffice.
 572 (The kernel distinguishes old and new calls by an
 573 .B IPC_64
 574 flag in
 575 .IR cmd .)
 576 .PP
 577 In some earlier versions of glibc, the
 578 .I semun
 579 union was defined in \fI<sys/sem.h>\fP, but POSIX.1 requires
 580 .\" POSIX.1-2001, POSIX.1-2008
 581 that the caller define this union.
 582 On versions of glibc where this union is \fInot\fP defined,
 583 the macro
 584 .B _SEM_SEMUN_UNDEFINED
 585 is defined in \fI<sys/sem.h>\fP.
 586 .PP
 587 The following system limit on semaphore sets affects a
 588 .BR semctl ()
 589 call:
 590 .TP
 591 .B SEMVMX
 592 Maximum value for
 593 .BR semval :
 594 implementation dependent (32767).
 595 .PP
 596 For greater portability, it is best to always call
 597 .BR semctl ()
 598 with four arguments.
 599 .\"
 600 .SS The sempid value
 601 POSIX.1 defines
 602 .I sempid
 603 as the "process ID of [the] last operation" on a semaphore,
 604 and explicitly notes that this value is set by a successful
 605 .BR semop (2)
 606 call, with the implication that no other interface affects the
 607 .I sempid
 608 value.
 609 .PP
 610 While some implementations conform to the behavior specified in POSIX.1,
 611 others do not.
 612 (The fault here probably lies with POSIX.1 inasmuch as it likely failed
 613 to capture the full range of existing implementation behaviors.)
 614 Various other implementations
 615 .\" At least OpenSolaris (and, one supposes, older Solaris) and Darwin
 616 also update
 617 .I sempid
 618 for the other operations that update the value of a semaphore: the
 619 .B SETVAL
 620 and
 621 .B SETALL
 622 operations, as well as the semaphore adjustments performed
 623 on process termination as a consequence of the use of the
 624 .B SEM_UNDO
 625 flag (see
 626 .BR semop (2)).
 627 .PP
 628 Linux also updates
 629 .I sempid
 630 for
 631 .BR SETVAL
 632 operations and semaphore adjustments.
 633 However, somewhat inconsistently, up to and including Linux 4.5,
 634 the kernel did not update
 635 .I sempid
 636 for
 637 .BR SETALL
 638 operations.
 639 This was rectified
 640 .\" commit a5f4db877177d2a3d7ae62a7bac3a5a27e083d7f
 641 in Linux 4.6.
 642 .SH EXAMPLE
 643 See
 644 .BR shmop (2).
 645 .SH SEE ALSO
 646 .BR ipc (2),
 647 .BR semget (2),
 648 .BR semop (2),
 649 .BR capabilities (7),
 650 .BR sem_overview (7),
 651 .BR sysvipc (7)