1 .\" Copyright 1993 Giorgio Ciucci (giorgio@crcc.it)
3 .\" %%%LICENSE_START(VERBATIM)
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25 .\" Modified 1996-10-22, Eric S. Raymond <esr@thyrsus.com>
26 .\" Modified 2002-01-08, Michael Kerrisk <mtk.manpages@gmail.com>
27 .\" Modified 2003-04-28, Ernie Petrides <petrides@redhat.com>
28 .\" Modified 2004-05-27, Michael Kerrisk <mtk.manpages@gmail.com>
29 .\" Modified, 11 Nov 2004, Michael Kerrisk <mtk.manpages@gmail.com>
30 .\" Language and formatting clean-ups
31 .\" Added notes on /proc files
32 .\" 2005-04-08, mtk, Noted kernel version numbers for semtimedop()
33 .\" 2007-07-09, mtk, Added an EXAMPLE code segment.
35 .TH SEMOP 2 2014-05-10 "Linux" "Linux Programmer's Manual"
37 semop, semtimedop \- System V semaphore operations
40 .B #include <sys/types.h>
41 .B #include <sys/ipc.h>
42 .B #include <sys/sem.h>
44 .BI "int semop(int " semid ", struct sembuf *" sops ", size_t " nsops );
46 .BI "int semtimedop(int " semid ", struct sembuf *" sops ", size_t " nsops ,
47 .BI " const struct timespec *" timeout );
51 Feature Test Macro Requirements for glibc (see
52 .BR feature_test_macros (7)):
58 Each semaphore in a System\ V semaphore set
59 has the following associated values:
63 unsigned short semval; /* semaphore value */
64 unsigned short semzcnt; /* # waiting for zero */
65 unsigned short semncnt; /* # waiting for increase */
66 pid_t sempid; /* ID of process that did last op */
71 performs operations on selected semaphores in the set indicated by
75 elements in the array pointed to by
77 specifies an operation to be performed on a single semaphore.
78 The elements of this structure are of type
80 containing the following members:
84 unsigned short sem_num; /* semaphore number */
85 short sem_op; /* semaphore operation */
86 short sem_flg; /* operation flags */
96 If an operation specifies
98 it will be automatically undone when the process terminates.
100 The set of operations contained in
106 that is, the operations are performed either as a complete unit,
108 The behavior of the system call if not all operations can be
109 performed immediately depends on the presence of the
111 flag in the individual
113 fields, as noted below.
115 Each operation is performed on the
117 semaphore of the semaphore set, where the first semaphore of the set
119 There are three types of operation, distinguished by the value of
124 is a positive integer, the operation adds this value to
129 is specified for this operation, the system subtracts the value
131 from the semaphore adjustment
133 value for this semaphore.
134 This operation can always proceed\(emit never forces a thread to wait.
135 The calling process must have alter permission on the semaphore set.
139 is zero, the process must have read permission on the semaphore
141 This is a "wait-for-zero" operation: if
143 is zero, the operation can immediately proceed.
153 (and none of the operations in
158 (the count of threads waiting until this semaphore's value becomes zero)
159 is incremented by one and the thread sleeps until
160 one of the following occurs:
163 becomes 0, at which time the value of
175 The calling thread catches a signal:
185 The time limit specified by
198 is less than zero, the process must have alter permission on the
202 is greater than or equal to the absolute value of
204 the operation can proceed immediately:
205 the absolute value of
211 is specified for this operation, the system adds the absolute value of
213 to the semaphore adjustment
215 value for this semaphore.
216 If the absolute value of
229 (and none of the operations in
234 (the counter of threads waiting for this semaphore's value to increase)
235 is incremented by one and the thread sleeps until
236 one of the following occurs:
239 becomes greater than or equal to the absolute value of
241 the operation now proceeds, as described above.
243 The semaphore set is removed from the system:
250 The calling thread catches a signal:
260 The time limit specified by
264 call expires: the system call fails, with
269 On successful completion, the
271 value for each semaphore specified in the array pointed to by
273 is set to the caller's process ID.
278 is set to the current time.
281 behaves identically to
283 except that in those cases where the calling thread would sleep,
284 the duration of that sleep is limited by the amount of elapsed
285 time specified by the
287 structure whose address is passed in the
290 (This sleep interval will be rounded up to the system clock granularity,
291 and kernel scheduling delays mean that the interval
292 may overrun by a small amount.)
293 If the specified time limit has been reached,
299 (and none of the operations in
315 otherwise they return \-1
318 indicating the error.
322 is set to one of the following:
329 the maximum number of operations allowed per system
333 The calling process does not have the permissions required
334 to perform the specified semaphore operations,
335 and does not have the
340 An operation could not proceed immediately and either
344 or the time limit specified in
349 An address specified in either the
353 argument isn't accessible.
356 For some operation the value of
358 is less than 0 or greater than or equal to the number
359 of semaphores in the set.
362 The semaphore set was removed.
365 While blocked in this system call, the thread caught a signal; see
369 The semaphore set doesn't exist, or
371 is less than zero, or
373 has a nonpositive value.
378 of some operation specified
380 and the system does not have enough memory to allocate the undo
388 the implementation dependent maximum value for
392 first appeared in Linux 2.5.52,
393 and was subsequently backported into kernel 2.4.22.
396 first appeared in version 2.3.3.
399 .\" SVr4 documents additional error conditions EINVAL, EFBIG, ENOSPC.
405 isn't required on Linux or by any version of POSIX.
407 some old implementations required the inclusion of these header files,
408 and the SVID also documented their inclusion.
409 Applications intended to be portable to such old systems may need
410 to include these header files.
411 .\" Like Linux, the FreeBSD man pages still document
412 .\" the inclusion of these header files.
416 structures of a process aren't inherited by the child produced by
418 but they are inherited across an
423 is never automatically restarted after being interrupted by a signal handler,
424 regardless of the setting of the
426 flag when establishing a signal handler.
428 A semaphore adjustment
430 value is a per-process, per-semaphore integer that is the negated sum
431 of all operations performed on a semaphore specifying the
434 Each process has a list of
436 values\(emone value for each semaphore on which it has operated using
438 When a process terminates, each of its per-semaphore
440 values is added to the corresponding semaphore,
441 thus undoing the effect of that process's operations on the semaphore
442 (but see BUGS below).
443 When a semaphore's value is directly set using the
451 values in all processes are cleared.
453 The \fIsemval\fP, \fIsempid\fP, \fIsemzcnt\fP, and \fIsemnct\fP values
454 for a semaphore can all be retrieved using appropriate
458 The following limits on semaphore set resources affect the
463 Maximum number of operations allowed for one
466 (on Linux, this limit can be read and modified via the third field of
467 .IR /proc/sys/kernel/sem ).
468 .\" This /proc file is not available in Linux 2.2 and earlier -- MTK
471 Maximum allowable value for
473 implementation dependent (32767).
475 The implementation has no intrinsic limits for
476 the adjust on exit maximum value
478 the system wide maximum number of undo structures
480 and the per-process maximum number of undo entries system parameters.
482 When a process terminates, its set of associated
484 structures is used to undo the effect of all of the
485 semaphore operations it performed with the
488 This raises a difficulty: if one (or more) of these semaphore adjustments
489 would result in an attempt to decrease a semaphore's value below zero,
490 what should an implementation do?
491 One possible approach would be to block until all the semaphore
492 adjustments could be performed.
493 This is however undesirable since it could force process termination to
494 block for arbitrarily long periods.
495 Another possibility is that such semaphore adjustments could be ignored
496 altogether (somewhat analogously to failing when
498 is specified for a semaphore operation).
499 Linux adopts a third approach: decreasing the semaphore value
500 as far as possible (i.e., to zero) and allowing process
501 termination to proceed immediately.
503 In kernels 2.6.x, x <= 10, there is a bug that in some circumstances
504 prevents a thread that is waiting for a semaphore value to become
505 zero from being woken up when the value does actually become zero.
506 This bug is fixed in kernel 2.6.11.
508 .\" http://marc.theaimsgroup.com/?l=linux-kernel&m=110260821123863&w=2
510 .\" http://marc.theaimsgroup.com/?l=linux-kernel&m=110261701025794&w=2
512 The following code segment uses
514 to atomically wait for the value of semaphore 0 to become zero,
515 and then increment the semaphore value by one.
518 struct sembuf sops[2];
521 /* Code to set \fIsemid\fP omitted */
523 sops[0].sem_num = 0; /* Operate on semaphore 0 */
524 sops[0].sem_op = 0; /* Wait for value to equal 0 */
527 sops[1].sem_num = 0; /* Operate on semaphore 0 */
528 sops[1].sem_op = 1; /* Increment value by one */
531 if (semop(semid, sops, 2) == \-1) {
541 .BR capabilities (7),
542 .BR sem_overview (7),
546 This page is part of release 3.68 of the Linux
549 A description of the project,
550 information about reporting bugs,
551 and the latest version of this page,
553 \%http://www.kernel.org/doc/man\-pages/.