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33 .Nd kernel event notification mechanism
41 .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
42 .Fn EV_SET "kev" ident filter flags fflags data udata
47 provides a generic method of notifying the user when an event
48 happens or a condition holds, based on the results of small
49 pieces of kernel code termed filters.
50 A kevent is identified by the (ident, filter) pair; there may only
51 be one unique kevent per kqueue.
53 The filter is executed upon the initial registration of a kevent
54 in order to detect whether a preexisting condition is present, and is also
55 executed whenever an event is passed to the filter for evaluation.
56 If the filter determines that the condition should be reported,
57 then the kevent is placed on the kqueue for the user to retrieve.
59 The filter is also run when the user attempts to retrieve the kevent
61 If the filter indicates that the condition that triggered
62 the event no longer holds, the kevent is removed from the kqueue and
65 Multiple events which trigger the filter do not result in multiple
66 kevents being placed on the kqueue; instead, the filter will aggregate
67 the events into a single struct kevent.
70 on a file descriptor will remove any kevents that reference the descriptor.
75 creates a new kernel event queue and returns a descriptor.
76 The queue is not inherited by a child created with
82 flag, then the descriptor table is shared,
83 which will allow sharing of the kqueue between two processes.
88 is used to register events with the queue, and return any pending
93 is a pointer to an array of
95 structures, as defined in
97 All changes contained in the
99 are applied before any pending events are read from the queue.
108 is a pointer to an array of kevent structures.
112 determines the size of
118 will return immediately even if there is a
124 is a non-NULL pointer, it specifies a maximum interval to wait
125 for an event, which will be interpreted as a struct timespec.
131 To effect a poll, the
133 argument should be non-NULL, pointing to a zero-valued
136 The same array may be used for the
143 macro is provided for ease of initializing a
148 structure is defined as:
151 uintptr_t ident; /* identifier for this event */
152 short filter; /* filter for event */
153 u_short flags; /* action flags for kqueue */
154 u_int fflags; /* filter flag value */
155 int64_t data; /* filter data value */
156 void *udata; /* opaque user data identifier */
157 uint64_t ext[4]; /* extensions */
164 .Bl -tag -width "Fa filter"
166 Value used to identify this event.
167 The exact interpretation is determined by the attached filter,
168 but often is a file descriptor.
170 Identifies the kernel filter used to process this event.
172 system filters are described below.
174 Actions to perform on the event.
176 Filter-specific flags.
178 Filter-specific data value.
180 Opaque user-defined value passed through the kernel unchanged.
182 Extended data passed to and from kernel.
187 members use is defined by the filter.
188 If the filter does not use them, the members are copied unchanged.
193 members are always passed through the kernel as-is,
194 making additional context available to application.
199 field can contain the following values:
200 .Bl -tag -width EV_DISPATCH
202 Adds the event to the kqueue.
203 Re-adding an existing event
204 will modify the parameters of the original event, and not result
205 in a duplicate entry.
206 Adding an event automatically enables it,
207 unless overridden by the EV_DISABLE flag.
211 to return the event if it is triggered.
216 The filter itself is not disabled.
218 Disable the event source immediately after delivery of an event.
223 Removes the event from the kqueue.
224 Events which are attached to
225 file descriptors are automatically deleted on the last close of
228 This flag is useful for making bulk changes to a kqueue without draining
230 When passed as input, it forces
232 to always be returned.
233 When a filter is successfully added the
237 Causes the event to return only the first occurrence of the filter
239 After the user retrieves the event from the kqueue,
242 After the event is retrieved by the user, its state is reset.
243 This is useful for filters which report state transitions
244 instead of the current state.
245 Note that some filters may automatically
246 set this flag internally.
248 Filters may set this flag to indicate filter-specific EOF condition.
255 The predefined system filters are listed below.
256 Arguments may be passed to and from the filter via the
260 fields in the kevent structure.
261 .Bl -tag -width "Dv EVFILT_PROCDESC"
263 Takes a descriptor as the identifier, and returns whenever
264 there is data available to read.
265 The behavior of the filter is slightly different depending
266 on the descriptor type.
269 Sockets which have previously been passed to
271 return when there is an incoming connection pending.
273 contains the size of the listen backlog.
275 Other socket descriptors return when there is data to be read,
278 value of the socket buffer.
279 This may be overridden with a per-filter low water mark at the
280 time the filter is added by setting the
284 and specifying the new low water mark in
288 contains the number of bytes of protocol data available to read.
290 If the read direction of the socket has shutdown, then the filter
295 and returns the socket error (if any) in
297 It is possible for EOF to be returned (indicating the connection is gone)
298 while there is still data pending in the socket buffer.
300 Returns when the file pointer is not at the end of file.
302 contains the offset from current position to end of file,
305 This behavior is different from
307 where read events are triggered for regular files unconditionally.
308 This event can be triggered unconditionally by setting the
313 Returns when the there is data to read;
315 contains the number of bytes available.
317 When the last writer disconnects, the filter will set
321 This may be cleared by passing in
324 filter will resume waiting for data to become available before
327 Returns when the BPF buffer is full, the BPF timeout has expired, or
330 enabled and there is any data to read;
332 contains the number of bytes available.
335 Takes a descriptor as the identifier, and returns whenever
336 it is possible to write to the descriptor.
340 will contain the amount of space remaining in the write buffer.
341 The filter will set EV_EOF when the reader disconnects, and for
342 the fifo case, this may be cleared by use of
344 Note that this filter is not supported for vnodes or BPF devices.
346 For sockets, the low water mark and socket error handling is
351 Takes a descriptor as the identifier, and returns whenever
352 there is no remaining data in the write buffer.
354 Events for this filter are not registered with
356 directly but are registered via the
358 member of an asynchronous I/O request when it is scheduled via an
359 asynchronous I/O system call such as
361 The filter returns under the same conditions as
363 For more details on this filter see
367 Takes a file descriptor as the identifier and the events to watch for in
369 and returns when one or more of the requested events occurs on the descriptor.
370 The events to monitor are:
371 .Bl -tag -width "Dv NOTE_CLOSE_WRITE"
373 The file referenced by the descriptor had its attributes changed.
375 A file descriptor referencing the monitored file, was closed.
376 The closed file descriptor did not have write access.
377 .It Dv NOTE_CLOSE_WRITE
378 A file descriptor referencing the monitored file, was closed.
379 The closed file descriptor had write access.
381 This note, as well as
383 are not activated when files are closed forcibly by
388 is sent for such events.
392 system call was called on the file referenced by the descriptor.
394 For regular file, the file referenced by the descriptor was extended.
396 For directory, reports that a directory entry was added or removed,
397 as the result of rename operation.
400 event is not reported when a name is changed inside the directory.
402 The link count on the file changed.
405 event is reported if a subdirectory was created or deleted inside
406 the directory referenced by the descriptor.
408 The file referenced by the descriptor was opened.
410 A read occurred on the file referenced by the descriptor.
412 The file referenced by the descriptor was renamed.
414 Access to the file was revoked via
416 or the underlying file system was unmounted.
418 A write occurred on the file referenced by the descriptor.
423 contains the events which triggered the filter.
425 Takes the process ID to monitor as the identifier and the events to watch for
428 and returns when the process performs one or more of the requested events.
429 If a process can normally see another process, it can attach an event to it.
430 The events to monitor are:
431 .Bl -tag -width "Dv NOTE_TRACKERR"
433 The process has exited.
434 The exit status will be stored in
437 The process has called
440 The process has executed a new process via
444 Follow a process across
447 The parent process registers a new kevent to monitor the child process
450 as the original event.
451 The child process will signal an event with
455 and the parent PID in
458 If the parent process fails to register a new kevent
459 .Pq usually due to resource limitations ,
460 it will signal an event with
464 and the child process will not signal a
471 contains the events which triggered the filter.
472 .It Dv EVFILT_PROCDESC
473 Takes the process descriptor created by
475 to monitor as the identifier and the events to watch for in
477 and returns when the associated process performs one or more of the
479 The events to monitor are:
480 .Bl -tag -width "Dv NOTE_EXIT"
482 The process has exited.
483 The exit status will be stored in
489 contains the events which triggered the filter.
491 Takes the signal number to monitor as the identifier and returns
492 when the given signal is delivered to the process.
493 This coexists with the
497 facilities, and has a lower precedence.
498 The filter will record
499 all attempts to deliver a signal to a process, even if the signal has
504 signal, which, if ignored, will not be recorded by the filter.
505 Event notification happens after normal
506 signal delivery processing.
508 returns the number of times the signal has occurred since the last call to
510 This filter automatically sets the
514 Establishes an arbitrary timer identified by
518 specifies the moment to fire the timer (for
520 or the timeout period.
521 The timer will be periodic unless
528 contains the number of times the timeout has expired since the last call to
530 For non-monotonic timers, this filter automatically sets the
534 The filter accepts the following flags in the
537 .Bl -tag -width "Dv NOTE_MSECONDS"
551 The specified expiration time is absolute.
556 is not set, the default is milliseconds.
559 contains the events which triggered the filter.
561 If an existing timer is re-added, the existing timer will be
562 effectively canceled (throwing away any undelivered record of previous
563 timer expiration) and re-started using the new parameters contained in
568 There is a system wide limit on the number of timers
569 which is controlled by the
570 .Va kern.kq_calloutmax
573 Establishes a user event identified by
575 which is not associated with any kernel mechanism but is triggered by
577 The lower 24 bits of the
579 may be used for user defined flags and manipulated using the following:
580 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
593 .It Dv NOTE_FFCTRLMASK
596 .It Dv NOTE_FFLAGSMASK
597 User defined flag mask for
601 A user event is triggered for output with the following:
602 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
604 Cause the event to be triggered.
609 contains the users defined flags in the lower 24 bits.
611 .Sh CANCELLATION BEHAVIOUR
614 is non-zero, i.e., the function is potentially blocking, the call
615 is a cancellation point.
618 is zero, the call is not cancellable.
619 Cancellation can only occur before any changes are made to the kqueue,
620 or when the call was blocked and no changes to the queue were requested.
625 creates a new kernel event queue and returns a file descriptor.
626 If there was an error creating the kernel event queue, a value of -1 is
627 returned and errno set.
632 returns the number of events placed in the
634 up to the value given by
636 If an error occurs while processing an element of the
638 and there is enough room in the
640 then the event will be placed in the
646 and the system error in
650 will be returned, and
652 will be set to indicate the error condition.
653 If the time limit expires, then
657 .Bd -literal -compact
658 #include <sys/event.h>
666 main(int argc, char **argv)
668 struct kevent event; /* Event we want to monitor */
669 struct kevent tevent; /* Event triggered */
673 err(EXIT_FAILURE, "Usage: %s path\en", argv[0]);
674 fd = open(argv[1], O_RDONLY);
676 err(EXIT_FAILURE, "Failed to open '%s'", argv[1]);
681 err(EXIT_FAILURE, "kqueue() failed");
683 /* Initialize kevent structure. */
684 EV_SET(&event, fd, EVFILT_VNODE, EV_ADD | EV_CLEAR, NOTE_WRITE,
686 /* Attach event to the kqueue. */
687 ret = kevent(kq, &event, 1, NULL, 0, NULL);
689 err(EXIT_FAILURE, "kevent register");
690 if (event.flags & EV_ERROR)
691 errx(EXIT_FAILURE, "Event error: %s", strerror(event.data));
694 /* Sleep until something happens. */
695 ret = kevent(kq, NULL, 0, &tevent, 1, NULL);
697 err(EXIT_FAILURE, "kevent wait");
698 } else if (ret > 0) {
699 printf("Something was written in '%s'\en", argv[1]);
707 system call fails if:
710 The kernel failed to allocate enough memory for the kernel queue.
717 for the current user would be exceeded.
719 The per-process descriptor table is full.
721 The system file table is full.
726 system call fails if:
729 The process does not have permission to register a filter.
731 There was an error reading or writing the
735 The specified descriptor is invalid.
737 A signal was delivered before the timeout expired and before any
738 events were placed on the kqueue for return.
740 A cancellation request was delivered to the thread, but not yet handled.
742 The specified time limit or filter is invalid.
744 The event could not be found to be modified or deleted.
746 No memory was available to register the event
747 or, in the special case of a timer, the maximum number of
748 timers has been exceeded.
749 This maximum is configurable via the
750 .Va kern.kq_calloutmax
753 The specified process to attach to does not exist.
760 error, all changes in the
772 .Xr pthread_setcancelstate 3 ,
779 system calls first appeared in
784 system and this manual page were written by
785 .An Jonathan Lemon Aq Mt jlemon@FreeBSD.org .
789 value is limited to 24 hours; longer timeouts will be silently
790 reinterpreted as 24 hours.
792 In versions older than
795 failed to parse without including