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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 The sigevent portion of the AIO request is filled in, with
331 .Va sigev_notify_kqueue
332 containing the descriptor of the kqueue that the event should
334 .Va sigev_notify_kevent_flags
335 containing the kevent flags which should be
341 containing the udata value, and
347 system call is made, the event will be registered
348 with the specified kqueue, and the
355 The filter returns under the same conditions as
358 Takes a file descriptor as the identifier and the events to watch for in
360 and returns when one or more of the requested events occurs on the descriptor.
361 The events to monitor are:
362 .Bl -tag -width "Dv NOTE_CLOSE_WRITE"
364 The file referenced by the descriptor had its attributes changed.
366 A file descriptor referencing the monitored file, was closed.
367 The closed file descriptor did not have write access.
368 .It Dv NOTE_CLOSE_WRITE
369 A file descriptor referencing the monitored file, was closed.
370 The closed file descriptor had write access.
372 This note, as well as
374 are not activated when files are closed forcibly by
379 is sent for such events.
383 system call was called on the file referenced by the descriptor.
385 For regular file, the file referenced by the descriptor was extended.
387 For directory, reports that a directory entry was added or removed,
388 as the result of rename operation.
391 event is not reported when a name is changed inside the directory.
393 The link count on the file changed.
396 event is reported if a subdirectory was created or deleted inside
397 the directory referenced by the descriptor.
399 The file referenced by the descriptor was opened.
401 A read occurred on the file referenced by the descriptor.
403 The file referenced by the descriptor was renamed.
405 Access to the file was revoked via
407 or the underlying file system was unmounted.
409 A write occurred on the file referenced by the descriptor.
414 contains the events which triggered the filter.
416 Takes the process ID to monitor as the identifier and the events to watch for
419 and returns when the process performs one or more of the requested events.
420 If a process can normally see another process, it can attach an event to it.
421 The events to monitor are:
422 .Bl -tag -width "Dv NOTE_TRACKERR"
424 The process has exited.
425 The exit status will be stored in
428 The process has called
431 The process has executed a new process via
435 Follow a process across
438 The parent process registers a new kevent to monitor the child process
441 as the original event.
442 The child process will signal an event with
446 and the parent PID in
449 If the parent process fails to register a new kevent
450 .Pq usually due to resource limitations ,
451 it will signal an event with
455 and the child process will not signal a
462 contains the events which triggered the filter.
464 Takes the signal number to monitor as the identifier and returns
465 when the given signal is delivered to the process.
466 This coexists with the
470 facilities, and has a lower precedence.
471 The filter will record
472 all attempts to deliver a signal to a process, even if the signal has
477 signal, which, if ignored, won't be recorded by the filter.
478 Event notification happens after normal
479 signal delivery processing.
481 returns the number of times the signal has occurred since the last call to
483 This filter automatically sets the
487 Establishes an arbitrary timer identified by
491 specifies the timeout period.
492 The timer will be periodic unless
497 contains the number of times the timeout has expired since the last call to
499 This filter automatically sets the EV_CLEAR flag internally.
500 There is a system wide limit on the number of timers
501 which is controlled by the
502 .Va kern.kq_calloutmax
504 .Bl -tag -width "Dv NOTE_USECONDS"
521 is not set, the default is milliseconds. On return,
523 contains the events which triggered the filter.
525 Establishes a user event identified by
527 which is not associated with any kernel mechanism but is triggered by
529 The lower 24 bits of the
531 may be used for user defined flags and manipulated using the following:
532 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
545 .It Dv NOTE_FFCTRLMASK
548 .It Dv NOTE_FFLAGSMASK
549 User defined flag mask for
553 A user event is triggered for output with the following:
554 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
556 Cause the event to be triggered.
561 contains the users defined flags in the lower 24 bits.
563 .Sh CANCELLATION BEHAVIOUR
566 is non-zero, i.e. the function is potentially blocking, the call
567 is a cancellation point.
570 is zero, the call is not cancellable.
571 Cancellation can only occur before any changes are made to the kqueue,
572 or when the call was blocked and no changes to the queue were requested.
577 creates a new kernel event queue and returns a file descriptor.
578 If there was an error creating the kernel event queue, a value of -1 is
579 returned and errno set.
584 returns the number of events placed in the
586 up to the value given by
588 If an error occurs while processing an element of the
590 and there is enough room in the
592 then the event will be placed in the
598 and the system error in
602 will be returned, and
604 will be set to indicate the error condition.
605 If the time limit expires, then
609 .Bd -literal -compact
610 #include <sys/types.h>
611 #include <sys/event.h>
612 #include <sys/time.h>
621 main(int argc, char **argv)
623 struct kevent event; /* Event we want to monitor */
624 struct kevent tevent; /* Event triggered */
628 err(EXIT_FAILURE, "Usage: %s path\en", argv[0]);
629 fd = open(argv[1], O_RDONLY);
631 err(EXIT_FAILURE, "Failed to open '%s'", argv[1]);
636 err(EXIT_FAILURE, "kqueue() failed");
638 /* Initialize kevent structure. */
639 EV_SET(&event, fd, EVFILT_VNODE, EV_ADD | EV_CLEAR, NOTE_WRITE,
641 /* Attach event to the kqueue. */
642 ret = kevent(kq, &event, 1, NULL, 0, NULL);
644 err(EXIT_FAILURE, "kevent register");
645 if (event.flags & EV_ERROR)
646 errx(EXIT_FAILURE, "Event error: %s", strerror(event.data));
649 /* Sleep until something happens. */
650 ret = kevent(kq, NULL, 0, &tevent, 1, NULL);
652 err(EXIT_FAILURE, "kevent wait");
653 } else if (ret > 0) {
654 printf("Something was written in '%s'\en", argv[1]);
662 system call fails if:
665 The kernel failed to allocate enough memory for the kernel queue.
667 The per-process descriptor table is full.
669 The system file table is full.
674 system call fails if:
677 The process does not have permission to register a filter.
679 There was an error reading or writing the
683 The specified descriptor is invalid.
685 A signal was delivered before the timeout expired and before any
686 events were placed on the kqueue for return.
688 A cancellation request was delivered to the thread, but not yet handled.
690 The specified time limit or filter is invalid.
692 The event could not be found to be modified or deleted.
694 No memory was available to register the event
695 or, in the special case of a timer, the maximum number of
696 timers has been exceeded.
697 This maximum is configurable via the
698 .Va kern.kq_calloutmax
701 The specified process to attach to does not exist.
708 error, all changes in the
720 .Xr pthread_setcancelstate 3 ,
727 system calls first appeared in
732 system and this manual page were written by
733 .An Jonathan Lemon Aq jlemon@FreeBSD.org .
737 value is limited to 24 hours; longer timeouts will be silently
738 reinterpreted as 24 hours.