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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_RENAME"
367 was called on the file referenced by the descriptor.
369 A write occurred on the file referenced by the descriptor.
371 The file referenced by the descriptor was extended.
373 The file referenced by the descriptor had its attributes changed.
375 The link count on the file changed.
377 The file referenced by the descriptor was renamed.
379 Access to the file was revoked via
381 or the underlying file system was unmounted.
386 contains the events which triggered the filter.
388 Takes the process ID to monitor as the identifier and the events to watch for
391 and returns when the process performs one or more of the requested events.
392 If a process can normally see another process, it can attach an event to it.
393 The events to monitor are:
394 .Bl -tag -width "Dv NOTE_TRACKERR"
396 The process has exited.
397 The exit status will be stored in
400 The process has called
403 The process has executed a new process via
407 Follow a process across
410 The parent process registers a new kevent to monitor the child process
413 as the original event.
414 The child process will signal an event with
418 and the parent PID in
421 If the parent process fails to register a new kevent
422 .Pq usually due to resource limitations ,
423 it will signal an event with
427 and the child process will not signal a
434 contains the events which triggered the filter.
435 .It Dv EVFILT_PROCDESC
436 Takes the process descriptor created by
438 to monitor as the identifier and the events to watch for in
440 and returns when the associated process performs one or more of the
442 The events to monitor are:
443 .Bl -tag -width "Dv NOTE_EXIT"
445 The process has exited.
446 The exit status will be stored in
452 contains the events which triggered the filter.
454 Takes the signal number to monitor as the identifier and returns
455 when the given signal is delivered to the process.
456 This coexists with the
460 facilities, and has a lower precedence.
461 The filter will record
462 all attempts to deliver a signal to a process, even if the signal has
467 signal, which, if ignored, won't be recorded by the filter.
468 Event notification happens after normal
469 signal delivery processing.
471 returns the number of times the signal has occurred since the last call to
473 This filter automatically sets the
477 Establishes an arbitrary timer identified by
481 specifies the timeout period.
482 The timer will be periodic unless
487 contains the number of times the timeout has expired since the last call to
489 This filter automatically sets the EV_CLEAR flag internally.
490 There is a system wide limit on the number of timers
491 which is controlled by the
492 .Va kern.kq_calloutmax
494 .Bl -tag -width "Dv NOTE_USECONDS"
511 is not set, the default is milliseconds. On return,
513 contains the events which triggered the filter.
515 Establishes a user event identified by
517 which is not associated with any kernel mechanism but is triggered by
519 The lower 24 bits of the
521 may be used for user defined flags and manipulated using the following:
522 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
535 .It Dv NOTE_FFCTRLMASK
538 .It Dv NOTE_FFLAGSMASK
539 User defined flag mask for
543 A user event is triggered for output with the following:
544 .Bl -tag -width "Dv NOTE_FFLAGSMASK"
546 Cause the event to be triggered.
551 contains the users defined flags in the lower 24 bits.
553 .Sh CANCELLATION BEHAVIOUR
556 is non-zero, i.e. the function is potentially blocking, the call
557 is a cancellation point.
560 is zero, the call is not cancellable.
561 Cancellation can only occur before any changes are made to the kqueue,
562 or when the call was blocked and no changes to the queue were requested.
567 creates a new kernel event queue and returns a file descriptor.
568 If there was an error creating the kernel event queue, a value of -1 is
569 returned and errno set.
574 returns the number of events placed in the
576 up to the value given by
578 If an error occurs while processing an element of the
580 and there is enough room in the
582 then the event will be placed in the
588 and the system error in
592 will be returned, and
594 will be set to indicate the error condition.
595 If the time limit expires, then
601 system call fails if:
604 The kernel failed to allocate enough memory for the kernel queue.
611 for the current user would be exceeded.
613 The per-process descriptor table is full.
615 The system file table is full.
620 system call fails if:
623 The process does not have permission to register a filter.
625 There was an error reading or writing the
629 The specified descriptor is invalid.
631 A signal was delivered before the timeout expired and before any
632 events were placed on the kqueue for return.
634 A cancellation request was delivered to the thread, but not yet handled.
636 The specified time limit or filter is invalid.
638 The event could not be found to be modified or deleted.
640 No memory was available to register the event
641 or, in the special case of a timer, the maximum number of
642 timers has been exceeded.
643 This maximum is configurable via the
644 .Va kern.kq_calloutmax
647 The specified process to attach to does not exist.
654 error, all changes in the
666 .Xr pthread_setcancelstate 3 ,
673 system calls first appeared in
678 system and this manual page were written by
679 .An Jonathan Lemon Aq Mt jlemon@FreeBSD.org .
683 value is limited to 24 hours; longer timeouts will be silently
684 reinterpreted as 24 hours.