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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 XXXfilter
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 XXXEV_ONESHOT
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 Removes the event from the kqueue.
206 Events which are attached to
207 file descriptors are automatically deleted on the last close of
210 Causes the event to return only the first occurrence of the filter
212 After the user retrieves the event from the kqueue,
215 After the event is retrieved by the user, its state is reset.
216 This is useful for filters which report state transitions
217 instead of the current state.
218 Note that some filters may automatically
219 set this flag internally.
221 Filters may set this flag to indicate filter-specific EOF condition.
228 The predefined system filters are listed below.
229 Arguments may be passed to and from the filter via the
233 fields in the kevent structure.
234 .Bl -tag -width EVFILT_SIGNAL
236 Takes a descriptor as the identifier, and returns whenever
237 there is data available to read.
238 The behavior of the filter is slightly different depending
239 on the descriptor type.
243 Sockets which have previously been passed to
245 return when there is an incoming connection pending.
247 contains the size of the listen backlog.
249 Other socket descriptors return when there is data to be read,
252 value of the socket buffer.
253 This may be overridden with a per-filter low water mark at the
254 time the filter is added by setting the
258 and specifying the new low water mark in
262 contains the number of bytes of protocol data available to read.
264 If the read direction of the socket has shutdown, then the filter
267 and returns the socket error (if any) in
269 It is possible for EOF to be returned (indicating the connection is gone)
270 while there is still data pending in the socket buffer.
272 Returns when the file pointer is not at the end of file.
274 contains the offset from current position to end of file,
277 Returns when the there is data to read;
279 contains the number of bytes available.
281 When the last writer disconnects, the filter will set EV_EOF in
283 This may be cleared by passing in EV_CLEAR, at which point the
284 filter will resume waiting for data to become available before
287 Returns when the BPF buffer is full, the BPF timeout has expired, or
290 enabled and there is any data to read;
292 contains the number of bytes available.
295 Takes a descriptor as the identifier, and returns whenever
296 it is possible to write to the descriptor.
300 will contain the amount of space remaining in the write buffer.
301 The filter will set EV_EOF when the reader disconnects, and for
302 the fifo case, this may be cleared by use of EV_CLEAR.
303 Note that this filter is not supported for vnodes or BPF devices.
305 For sockets, the low water mark and socket error handling is
306 identical to the EVFILT_READ case.
308 The sigevent portion of the AIO request is filled in, with
309 .Va sigev_notify_kqueue
310 containing the descriptor of the kqueue that the event should
313 containing the udata value, and
318 system call is made, the event will be registered
319 with the specified kqueue, and the
326 The filter returns under the same conditions as aio_error.
328 Takes a file descriptor as the identifier and the events to watch for in
330 and returns when one or more of the requested events occurs on the descriptor.
331 The events to monitor are:
332 .Bl -tag -width XXNOTE_RENAME
337 was called on the file referenced by the descriptor.
339 A write occurred on the file referenced by the descriptor.
341 The file referenced by the descriptor was extended.
343 The file referenced by the descriptor had its attributes changed.
345 The link count on the file changed.
347 The file referenced by the descriptor was renamed.
349 Access to the file was revoked via
351 or the underlying file system was unmounted.
356 contains the events which triggered the filter.
358 Takes the process ID to monitor as the identifier and the events to watch for
361 and returns when the process performs one or more of the requested events.
362 If a process can normally see another process, it can attach an event to it.
363 The events to monitor are:
364 .Bl -tag -width XXNOTE_TRACKERR
366 The process has exited.
367 The exit status will be stored in
370 The process has called
373 The process has executed a new process via
377 Follow a process across
380 The parent process will return with NOTE_TRACK set in the
382 field, while the child process will return with NOTE_CHILD set in
384 and the parent PID in
387 This flag is returned if the system was unable to attach an event to
388 the child process, usually due to resource limitations.
393 contains the events which triggered the filter.
395 Takes the signal number to monitor as the identifier and returns
396 when the given signal is delivered to the process.
397 This coexists with the
401 facilities, and has a lower precedence.
402 The filter will record
403 all attempts to deliver a signal to a process, even if the signal has
404 been marked as SIG_IGN.
405 Event notification happens after normal
406 signal delivery processing.
408 returns the number of times the signal has occurred since the last call to
410 This filter automatically sets the EV_CLEAR flag internally.
412 Establishes an arbitrary timer identified by
416 specifies the timeout period in milliseconds.
417 The timer will be periodic unless EV_ONESHOT is specified.
420 contains the number of times the timeout has expired since the last call to
422 This filter automatically sets the EV_CLEAR flag internally.
423 There is a system wide limit on the number of timers
424 which is controlled by the
425 .Va kern.kq_calloutmax
428 Takes a descriptor to a network interface as the identifier, and the events to watch for in
430 It returns, when one or more of the requested events occur on the descriptor.
431 The events to monitor are:
432 .Bl -tag -width XXNOTE_LINKDOWN
438 The link state is invalid.
443 contains the events which triggered the filter.
449 creates a new kernel event queue and returns a file descriptor.
450 If there was an error creating the kernel event queue, a value of -1 is
451 returned and errno set.
456 returns the number of events placed in the
458 up to the value given by
460 If an error occurs while processing an element of the
462 and there is enough room in the
464 then the event will be placed in the
470 and the system error in
474 will be returned, and
476 will be set to indicate the error condition.
477 If the time limit expires, then
483 system call fails if:
486 The kernel failed to allocate enough memory for the kernel queue.
488 The per-process descriptor table is full.
490 The system file table is full.
495 system call fails if:
498 The process does not have permission to register a filter.
500 There was an error reading or writing the
504 The specified descriptor is invalid.
506 A signal was delivered before the timeout expired and before any
507 events were placed on the kqueue for return.
509 The specified time limit or filter is invalid.
511 The event could not be found to be modified or deleted.
513 No memory was available to register the event
514 or, in the special case of a timer, the maximum number of
515 timers has been exceeded.
516 This maximum is configurable via the
517 .Va kern.kq_calloutmax
520 The specified process to attach to does not exist.
537 system calls first appeared in
542 system and this manual page were written by
543 .An Jonathan Lemon Aq jlemon@FreeBSD.org .
547 filter is currently only implemented for devices that use the
549 driver for LINKUP and LINKDOWN operations.
550 Therefore, it will not work with many non-ethernet devices.
554 value is limited to 24 hours; longer timeouts will be silently
555 reinterpreted as 24 hours.