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25 .\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.1.2.16 2002/07/02 21:05:08 mp Exp $
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
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.
73 creates a new kernel event queue and returns a descriptor.
74 The queue is not inherited by a child created with
80 flag, then the descriptor table is shared,
81 which will allow sharing of the kqueue between two processes.
84 is used to register events with the queue, and return any pending
87 is a pointer to an array of
89 structures, as defined in
91 All changes contained in the
93 are applied before any pending events are read from the queue.
98 is a pointer to an array of kevent structures.
100 determines the size of
104 is a non-NULL pointer, it specifies a maximum interval to wait
105 for an event, which will be interpreted as a struct timespec.
111 To effect a poll, the
113 argument should be non-NULL, pointing to a zero-valued
116 The same array may be used for the
122 is a macro which is provided for ease of initializing a
127 structure is defined as:
130 uintptr_t ident; /* identifier for this event */
131 short filter; /* filter for event */
132 u_short flags; /* action flags for kqueue */
133 u_int fflags; /* filter flag value */
134 intptr_t data; /* filter data value */
135 void *udata; /* opaque user data identifier */
142 .Bl -tag -width XXXfilter
144 Value used to identify this event.
145 The exact interpretation is determined by the attached filter,
146 but often is a file descriptor.
148 Identifies the kernel filter used to process this event.
149 The pre-defined system filters are described below.
151 Actions to perform on the event.
153 Filter-specific flags.
155 Filter-specific data value.
157 Opaque user-defined value passed through the kernel unchanged.
162 field can contain the following values:
163 .Bl -tag -width ".Dv EV_DISPATCH"
165 Adds the event to the kqueue.
166 Re-adding an existing event will modify the parameters of the original
167 event, and not result in a duplicate entry.
168 Adding an event automatically enables it, unless overridden by the
174 to return the event if it is triggered.
179 The filter itself is not disabled.
181 Disable the event source immediately after delivery of an event.
186 Removes the event from the kqueue.
187 Events which are attached to file descriptors are automatically
188 deleted on the last close of the descriptor.
190 This flag is useful for making bulk changes to a kqueue without draining
192 When passed as input, it forces
194 to always be returned.
195 When a filter is successfully added the
199 Causes the event to return only the first occurrence of the filter
201 After the user retrieves the event from the kqueue, it is deleted.
203 After the event is retrieved by the user, its state is reset.
204 This is useful for filters which report state transitions
205 instead of the current state.
206 Note that some filters may automatically set this flag internally.
208 Filters may set this flag to indicate filter-specific EOF condition.
215 The predefined system filters are listed below.
216 Arguments may be passed to and from the filter via the
220 fields in the kevent structure.
221 .Bl -tag -width ".Dv EVFILT_SIGNAL"
223 Takes a descriptor as the identifier, and returns whenever
224 there is data available to read.
225 The behavior of the filter is slightly different depending
226 on the descriptor type.
229 Sockets which have previously been passed to
231 return when there is an incoming connection pending.
233 contains the size of the listen backlog.
235 Other socket descriptors return when there is data to be read,
238 value of the socket buffer.
239 This may be overridden with a per-filter low water mark at the
240 time the filter is added by setting the
244 and specifying the new low water mark in
248 contains the number of bytes in the socket buffer.
250 If the read direction of the socket has shutdown, then the filter also sets
254 and returns the socket error (if any) in
256 It is possible for EOF to be returned (indicating the connection is gone)
257 while there is still data pending in the socket buffer.
259 Returns when the file pointer is not at the end of file.
261 contains the offset from current position to end of file,
264 Returns when the there is data to read;
266 contains the number of bytes available.
268 When the last writer disconnects, the filter will set
272 This may be cleared by passing in
274 at which point the filter will resume waiting for data to become
275 available before returning.
278 Takes a descriptor as the identifier, and returns whenever
279 it is possible to write to the descriptor.
280 For sockets, pipes and fifos,
282 will contain the amount of space remaining in the write buffer.
285 when the reader disconnects, and for the fifo case, this may be cleared
288 Note that this filter is not supported for vnodes.
290 For sockets, the low water mark and socket error handling is
295 Takes a descriptor as the identifier, and returns whenever one of the
296 specified exceptional conditions has occurred on the descriptor. Conditions
299 Currently, a filter can monitor the reception of out-of-band data with
302 The sigevent portion of the AIO request is filled in, with
303 .Va sigev_notify_kqueue
304 containing the descriptor of the kqueue that the event should
307 containing the udata value, and
311 When the aio_* function is called, the event will be registered
312 with the specified kqueue, and the
316 returned by the aio_* function.
317 The filter returns under the same conditions as aio_error.
319 Alternatively, a kevent structure may be initialized, with
321 containing the descriptor of the kqueue, and the
322 address of the kevent structure placed in the
324 field of the AIO request.
325 However, this approach will not work on architectures with 64-bit
326 pointers, and should be considered deprecated.
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 ".Dv NOTE_RENAME"
335 was called on the file referenced by the descriptor.
337 A write occurred on the file referenced by the descriptor.
339 The file referenced by the descriptor was extended.
341 The file referenced by the descriptor had its attributes changed.
343 The link count on the file changed.
345 The file referenced by the descriptor was renamed.
347 Access to the file was revoked via
349 or the underlying fileystem was unmounted.
354 contains the events which triggered the filter.
356 Takes the process ID to monitor as the identifier and the events to watch for
359 and returns when the process performs one or more of the requested events.
360 If a process can normally see another process, it can attach an event to it.
361 The events to monitor are:
362 .Bl -tag -width ".Dv NOTE_TRACKERR"
364 The process has exited.
366 The process has called
369 The process has executed a new process via
373 Follow a process across
376 The parent process will return with
380 field, while the child process will return with
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 all attempts to deliver a signal to a process,
403 even if the signal has been marked as
405 Event notification happens after normal signal delivery processing.
407 returns the number of times the signal has occurred since the last call to
409 This filter automatically sets the
413 Establishes an arbitrary timer identified by
417 specifies the timeout period in milliseconds.
418 The timer will be periodic unless
423 contains the number of times the timeout has expired since the last call to
425 This filter automatically sets the
431 creates a new kernel event queue and returns a file descriptor.
432 If there was an error creating the kernel event queue, a value of -1 is
438 returns the number of events placed in the
440 up to the value given by
442 If an error occurs while processing an element of the
444 and there is enough room in the
446 then the event will be placed in the
452 and the system error in
456 will be returned, and
458 will be set to indicate the error condition.
459 If the time limit expires, then
468 The kernel failed to allocate enough memory for the kernel queue.
470 The per-process descriptor table is full.
472 The system file table is full.
480 The process does not have permission to register a filter.
482 There was an error reading or writing the
486 The specified descriptor is invalid.
488 A signal was delivered before the timeout expired and before any
489 events were placed on the kqueue for return.
491 The specified time limit or filter is invalid.
493 The event could not be found to be modified or deleted.
495 No memory was available to register the event.
497 The specified process to attach to does not exist.
514 functions first appeared in
519 system and this manual page were written by
520 .An Jonathan Lemon Aq Mt jlemon@FreeBSD.org .
522 Currently it is only possible to watch a