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33 .Nd kernel event notification mechanism
43 .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
44 .Fn EV_SET "kev" ident filter flags fflags data udata
49 provides a generic method of notifying the user when an event
50 happens or a condition holds, based on the results of small
51 pieces of kernel code termed filters.
52 A kevent is identified by the (ident, filter) pair; there may only
53 be one unique kevent per kqueue.
55 The filter is executed upon the initial registration of a kevent
56 in order to detect whether a preexisting condition is present, and is also
57 executed whenever an event is passed to the filter for evaluation.
58 If the filter determines that the condition should be reported,
59 then the kevent is placed on the kqueue for the user to retrieve.
61 The filter is also run when the user attempts to retrieve the kevent
63 If the filter indicates that the condition that triggered
64 the event no longer holds, the kevent is removed from the kqueue and
67 Multiple events which trigger the filter do not result in multiple
68 kevents being placed on the kqueue; instead, the filter will aggregate
69 the events into a single struct kevent.
72 on a file descriptor will remove any kevents that reference the descriptor.
77 creates a new kernel event queue and returns a descriptor.
78 The queue is not inherited by a child created with
84 flag, then the descriptor table is shared,
85 which will allow sharing of the kqueue between two processes.
90 is used to register events with the queue, and return any pending
95 is a pointer to an array of
97 structures, as defined in
99 All changes contained in the
101 are applied before any pending events are read from the queue.
110 is a pointer to an array of kevent structures.
114 determines the size of
120 will return immediately even if there is a
126 is a non-NULL pointer, it specifies a maximum interval to wait
127 for an event, which will be interpreted as a struct timespec.
133 To effect a poll, the
135 argument should be non-NULL, pointing to a zero-valued
138 The same array may be used for the
145 macro is provided for ease of initializing a
150 structure is defined as:
153 uintptr_t ident; /* identifier for this event */
154 short filter; /* filter for event */
155 u_short flags; /* action flags for kqueue */
156 u_int fflags; /* filter flag value */
157 intptr_t data; /* filter data value */
158 void *udata; /* opaque user data identifier */
165 .Bl -tag -width "Fa filter"
167 Value used to identify this event.
168 The exact interpretation is determined by the attached filter,
169 but often is a file descriptor.
171 Identifies the kernel filter used to process this event.
173 system filters are described below.
175 Actions to perform on the event.
177 Filter-specific flags.
179 Filter-specific data value.
181 Opaque user-defined value passed through the kernel unchanged.
186 field can contain the following values:
187 .Bl -tag -width EV_DISPATCH
189 Adds the event to the kqueue.
190 Re-adding an existing event
191 will modify the parameters of the original event, and not result
192 in a duplicate entry.
193 Adding an event automatically enables it,
194 unless overridden by the EV_DISABLE flag.
198 to return the event if it is triggered.
203 The filter itself is not disabled.
205 Disable the event source immediately after delivery of an event.
210 Removes the event from the kqueue.
211 Events which are attached to
212 file descriptors are automatically deleted on the last close of
215 This flag is useful for making bulk changes to a kqueue without draining
217 When passed as input, it forces
219 to always be returned.
220 When a filter is successfully added the
224 Causes the event to return only the first occurrence of the filter
226 After the user retrieves the event from the kqueue,
229 After the event is retrieved by the user, its state is reset.
230 This is useful for filters which report state transitions
231 instead of the current state.
232 Note that some filters may automatically
233 set this flag internally.
235 Filters may set this flag to indicate filter-specific EOF condition.
242 The predefined system filters are listed below.
243 Arguments may be passed to and from the filter via the
247 fields in the kevent structure.
248 .Bl -tag -width "Dv EVFILT_PROCDESC"
250 Takes a descriptor as the identifier, and returns whenever
251 there is data available to read.
252 The behavior of the filter is slightly different depending
253 on the descriptor type.
256 Sockets which have previously been passed to
258 return when there is an incoming connection pending.
260 contains the size of the listen backlog.
262 Other socket descriptors return when there is data to be read,
265 value of the socket buffer.
266 This may be overridden with a per-filter low water mark at the
267 time the filter is added by setting the
271 and specifying the new low water mark in
275 contains the number of bytes of protocol data available to read.
277 If the read direction of the socket has shutdown, then the filter
282 and returns the socket error (if any) in
284 It is possible for EOF to be returned (indicating the connection is gone)
285 while there is still data pending in the socket buffer.
287 Returns when the file pointer is not at the end of file.
289 contains the offset from current position to end of file,
292 Returns when the there is data to read;
294 contains the number of bytes available.
296 When the last writer disconnects, the filter will set
300 This may be cleared by passing in
303 filter will resume waiting for data to become available before
306 Returns when the BPF buffer is full, the BPF timeout has expired, or
309 enabled and there is any data to read;
311 contains the number of bytes available.
314 Takes a descriptor as the identifier, and returns whenever
315 it is possible to write to the descriptor.
319 will contain the amount of space remaining in the write buffer.
320 The filter will set EV_EOF when the reader disconnects, and for
321 the fifo case, this may be cleared by use of
323 Note that this filter is not supported for vnodes or BPF devices.
325 For sockets, the low water mark and socket error handling is
330 Events for this filter are not registered with
332 directly but are registered via the
334 member of an asychronous I/O request when it is scheduled via an asychronous I/O
337 The filter returns under the same conditions as
339 For more details on this filter see
343 Takes a file descriptor as the identifier and the events to watch for in
345 and returns when one or more of the requested events occurs on the descriptor.
346 The events to monitor are:
347 .Bl -tag -width "Dv NOTE_CLOSE_WRITE"
349 The file referenced by the descriptor had its attributes changed.
351 A file descriptor referencing the monitored file, was closed.
352 The closed file descriptor did not have write access.
353 .It Dv NOTE_CLOSE_WRITE
354 A file descriptor referencing the monitored file, was closed.
355 The closed file descriptor had write access.
357 This note, as well as
359 are not activated when files are closed forcibly by
364 is sent for such events.
368 system call was called on the file referenced by the descriptor.
370 For regular file, the file referenced by the descriptor was extended.
372 For directory, reports that a directory entry was added or removed,
373 as the result of rename operation.
376 event is not reported when a name is changed inside the directory.
378 The link count on the file changed.
381 event is reported if a subdirectory was created or deleted inside
382 the directory referenced by the descriptor.
384 The file referenced by the descriptor was opened.
386 A read occurred on the file referenced by the descriptor.
388 The file referenced by the descriptor was renamed.
390 Access to the file was revoked via
392 or the underlying file system was unmounted.
394 A write occurred on the file referenced by the descriptor.
399 contains the events which triggered the filter.
401 Takes the process ID to monitor as the identifier and the events to watch for
404 and returns when the process performs one or more of the requested events.
405 If a process can normally see another process, it can attach an event to it.
406 The events to monitor are:
407 .Bl -tag -width "Dv NOTE_TRACKERR"
409 The process has exited.
410 The exit status will be stored in
413 The process has called
416 The process has executed a new process via
420 Follow a process across
423 The parent process registers a new kevent to monitor the child process
426 as the original event.
427 The child process will signal an event with
431 and the parent PID in
434 If the parent process fails to register a new kevent
435 .Pq usually due to resource limitations ,
436 it will signal an event with
440 and the child process will not signal a
447 contains the events which triggered the filter.
449 Takes the signal number to monitor as the identifier and returns
450 when the given signal is delivered to the process.
451 This coexists with the
455 facilities, and has a lower precedence.
456 The filter will record
457 all attempts to deliver a signal to a process, even if the signal has
462 signal, which, if ignored, won't be recorded by the filter.
463 Event notification happens after normal
464 signal delivery processing.
466 returns the number of times the signal has occurred since the last call to
468 This filter automatically sets the
472 Establishes an arbitrary timer identified by
476 specifies the timeout period.
477 The timer will be periodic unless
482 contains the number of times the timeout has expired since the last call to
484 This filter automatically sets the EV_CLEAR flag internally.
485 There is a system wide limit on the number of timers
486 which is controlled by the
487 .Va kern.kq_calloutmax
489 .Bl -tag -width "Dv NOTE_USECONDS"
506 is not set, the default is milliseconds. On return,
508 contains the events which triggered the filter.
510 Establishes a user event identified by
512 which is not associated with any kernel mechanism but is triggered by
514 The lower 24 bits of the
516 may be used for user defined flags and manipulated using the following:
517 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
530 .It Dv NOTE_FFCTRLMASK
533 .It Dv NOTE_FFLAGSMASK
534 User defined flag mask for
538 A user event is triggered for output with the following:
539 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
541 Cause the event to be triggered.
546 contains the users defined flags in the lower 24 bits.
548 .Sh CANCELLATION BEHAVIOUR
551 is non-zero, i.e. the function is potentially blocking, the call
552 is a cancellation point.
555 is zero, the call is not cancellable.
556 Cancellation can only occur before any changes are made to the kqueue,
557 or when the call was blocked and no changes to the queue were requested.
562 creates a new kernel event queue and returns a file descriptor.
563 If there was an error creating the kernel event queue, a value of -1 is
564 returned and errno set.
569 returns the number of events placed in the
571 up to the value given by
573 If an error occurs while processing an element of the
575 and there is enough room in the
577 then the event will be placed in the
583 and the system error in
587 will be returned, and
589 will be set to indicate the error condition.
590 If the time limit expires, then
594 .Bd -literal -compact
595 #include <sys/types.h>
596 #include <sys/event.h>
597 #include <sys/time.h>
606 main(int argc, char **argv)
608 struct kevent event; /* Event we want to monitor */
609 struct kevent tevent; /* Event triggered */
613 err(EXIT_FAILURE, "Usage: %s path\en", argv[0]);
614 fd = open(argv[1], O_RDONLY);
616 err(EXIT_FAILURE, "Failed to open '%s'", argv[1]);
621 err(EXIT_FAILURE, "kqueue() failed");
623 /* Initialize kevent structure. */
624 EV_SET(&event, fd, EVFILT_VNODE, EV_ADD | EV_CLEAR, NOTE_WRITE,
626 /* Attach event to the kqueue. */
627 ret = kevent(kq, &event, 1, NULL, 0, NULL);
629 err(EXIT_FAILURE, "kevent register");
630 if (event.flags & EV_ERROR)
631 errx(EXIT_FAILURE, "Event error: %s", strerror(event.data));
634 /* Sleep until something happens. */
635 ret = kevent(kq, NULL, 0, &tevent, 1, NULL);
637 err(EXIT_FAILURE, "kevent wait");
638 } else if (ret > 0) {
639 printf("Something was written in '%s'\en", argv[1]);
647 system call fails if:
650 The kernel failed to allocate enough memory for the kernel queue.
652 The per-process descriptor table is full.
654 The system file table is full.
659 system call fails if:
662 The process does not have permission to register a filter.
664 There was an error reading or writing the
668 The specified descriptor is invalid.
670 A signal was delivered before the timeout expired and before any
671 events were placed on the kqueue for return.
673 A cancellation request was delivered to the thread, but not yet handled.
675 The specified time limit or filter is invalid.
677 The event could not be found to be modified or deleted.
679 No memory was available to register the event
680 or, in the special case of a timer, the maximum number of
681 timers has been exceeded.
682 This maximum is configurable via the
683 .Va kern.kq_calloutmax
686 The specified process to attach to does not exist.
693 error, all changes in the
705 .Xr pthread_setcancelstate 3 ,
712 system calls first appeared in
717 system and this manual page were written by
718 .An Jonathan Lemon Aq jlemon@FreeBSD.org .
722 value is limited to 24 hours; longer timeouts will be silently
723 reinterpreted as 24 hours.