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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 $
26 .\" $DragonFly: src/lib/libc/sys/kqueue.2,v 1.6 2007/09/07 08:14:57 swildner Exp $
34 .Nd kernel event notification mechanism
44 .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
45 .Fn EV_SET "&kev" ident filter flags fflags data udata
48 provides a generic method of notifying the user when an event
49 happens or a condition holds, based on the results of small
50 pieces of kernel code termed filters.
51 A kevent is identified by the (ident, filter) pair; there may only
52 be one unique kevent per kqueue.
54 The filter is executed upon the initial registration of a kevent
55 in order to detect whether a preexisting condition is present, and is also
56 executed whenever an event is passed to the filter for evaluation.
57 If the filter determines that the condition should be reported,
58 then the kevent is placed on the kqueue for the user to retrieve.
60 The filter is also run when the user attempts to retrieve the kevent
62 If the filter indicates that the condition that triggered
63 the event no longer holds, the kevent is removed from the kqueue and
66 Multiple events which trigger the filter do not result in multiple
67 kevents being placed on the kqueue; instead, the filter will aggregate
68 the events into a single struct kevent.
71 on a file descriptor will remove any kevents that reference the descriptor.
74 creates a new kernel event queue and returns a descriptor.
75 The queue is not inherited by a child created with
81 flag, then the descriptor table is shared,
82 which will allow sharing of the kqueue between two processes.
85 is used to register events with the queue, and return any pending
88 is a pointer to an array of
90 structures, as defined in
92 All changes contained in the
94 are applied before any pending events are read from the queue.
99 is a pointer to an array of kevent structures.
101 determines the size of
105 is a non-NULL pointer, it specifies a maximum interval to wait
106 for an event, which will be interpreted as a struct timespec.
112 To effect a poll, the
114 argument should be non-NULL, pointing to a zero-valued
117 The same array may be used for the
123 is a macro which is provided for ease of initializing a
128 structure is defined as:
131 uintptr_t ident; /* identifier for this event */
132 short filter; /* filter for event */
133 u_short flags; /* action flags for kqueue */
134 u_int fflags; /* filter flag value */
135 intptr_t data; /* filter data value */
136 void *udata; /* opaque user data identifier */
143 .Bl -tag -width XXXfilter
145 Value used to identify this event.
146 The exact interpretation is determined by the attached filter,
147 but often is a file descriptor.
149 Identifies the kernel filter used to process this event.
150 The pre-defined system filters are described below.
152 Actions to perform on the event.
154 Filter-specific flags.
156 Filter-specific data value.
158 Opaque user-defined value passed through the kernel unchanged.
163 field can contain the following values:
164 .Bl -tag -width ".Dv EV_ONESHOT"
166 Adds the event to the kqueue.
167 Re-adding an existing event will modify the parameters of the original
168 event, and not result in a duplicate entry.
169 Adding an event automatically enables it, unless overridden by the
175 to return the event if it is triggered.
180 The filter itself is not disabled.
182 Removes the event from the kqueue.
183 Events which are attached to file descriptors are automatically
184 deleted on the last close of the descriptor.
186 Causes the event to return only the first occurrence of the filter
188 After the user retrieves the event from the kqueue, it is deleted.
190 After the event is retrieved by the user, its state is reset.
191 This is useful for filters which report state transitions
192 instead of the current state.
193 Note that some filters may automatically set this flag internally.
195 Filters may set this flag to indicate filter-specific EOF condition.
202 The predefined system filters are listed below.
203 Arguments may be passed to and from the filter via the
207 fields in the kevent structure.
208 .Bl -tag -width ".Dv EVFILT_SIGNAL"
210 Takes a descriptor as the identifier, and returns whenever
211 there is data available to read.
212 The behavior of the filter is slightly different depending
213 on the descriptor type.
217 Sockets which have previously been passed to
219 return when there is an incoming connection pending.
221 contains the size of the listen backlog.
223 Other socket descriptors return when there is data to be read,
226 value of the socket buffer.
227 This may be overridden with a per-filter low water mark at the
228 time the filter is added by setting the
232 and specifying the new low water mark in
236 contains the number of bytes in the socket buffer.
238 If the read direction of the socket has shutdown, then the filter also sets
242 and returns the socket error (if any) in
244 It is possible for EOF to be returned (indicating the connection is gone)
245 while there is still data pending in the socket buffer.
247 Returns when the file pointer is not at the end of file.
249 contains the offset from current position to end of file,
252 Returns when the there is data to read;
254 contains the number of bytes available.
256 When the last writer disconnects, the filter will set
260 This may be cleared by passing in
262 at which point the filter will resume waiting for data to become
263 available before returning.
266 Takes a descriptor as the identifier, and returns whenever
267 it is possible to write to the descriptor.
268 For sockets, pipes and fifos,
270 will contain the amount of space remaining in the write buffer.
273 when the reader disconnects, and for the fifo case, this may be cleared
276 Note that this filter is not supported for vnodes.
278 For sockets, the low water mark and socket error handling is
283 The sigevent portion of the AIO request is filled in, with
284 .Va sigev_notify_kqueue
285 containing the descriptor of the kqueue that the event should
288 containing the udata value, and
292 When the aio_* function is called, the event will be registered
293 with the specified kqueue, and the
297 returned by the aio_* function.
298 The filter returns under the same conditions as aio_error.
300 Alternatively, a kevent structure may be initialized, with
302 containing the descriptor of the kqueue, and the
303 address of the kevent structure placed in the
305 field of the AIO request.
306 However, this approach will not work on architectures with 64-bit
307 pointers, and should be considered deprecated.
309 Takes a file descriptor as the identifier and the events to watch for in
311 and returns when one or more of the requested events occurs on the descriptor.
312 The events to monitor are:
313 .Bl -tag -width ".Dv NOTE_RENAME"
316 was called on the file referenced by the descriptor.
318 A write occurred on the file referenced by the descriptor.
320 The file referenced by the descriptor was extended.
322 The file referenced by the descriptor had its attributes changed.
324 The link count on the file changed.
326 The file referenced by the descriptor was renamed.
328 Access to the file was revoked via
330 or the underlying fileystem was unmounted.
335 contains the events which triggered the filter.
337 Takes the process ID to monitor as the identifier and the events to watch for
340 and returns when the process performs one or more of the requested events.
341 If a process can normally see another process, it can attach an event to it.
342 The events to monitor are:
343 .Bl -tag -width ".Dv NOTE_TRACKERR"
345 The process has exited.
347 The process has called
350 The process has executed a new process via
354 Follow a process across
357 The parent process will return with
361 field, while the child process will return with
365 and the parent PID in
368 This flag is returned if the system was unable to attach an event to
369 the child process, usually due to resource limitations.
374 contains the events which triggered the filter.
376 Takes the signal number to monitor as the identifier and returns
377 when the given signal is delivered to the process.
378 This coexists with the
382 facilities, and has a lower precedence.
383 The filter will record all attempts to deliver a signal to a process,
384 even if the signal has been marked as
386 Event notification happens after normal signal delivery processing.
388 returns the number of times the signal has occurred since the last call to
390 This filter automatically sets the
394 Establishes an arbitrary timer identified by
398 specifies the timeout period in milliseconds.
399 The timer will be periodic unless
404 contains the number of times the timeout has expired since the last call to
406 This filter automatically sets the
412 creates a new kernel event queue and returns a file descriptor.
413 If there was an error creating the kernel event queue, a value of -1 is
419 returns the number of events placed in the
421 up to the value given by
423 If an error occurs while processing an element of the
425 and there is enough room in the
427 then the event will be placed in the
433 and the system error in
437 will be returned, and
439 will be set to indicate the error condition.
440 If the time limit expires, then
449 The kernel failed to allocate enough memory for the kernel queue.
451 The per-process descriptor table is full.
453 The system file table is full.
461 The process does not have permission to register a filter.
463 There was an error reading or writing the
467 The specified descriptor is invalid.
469 A signal was delivered before the timeout expired and before any
470 events were placed on the kqueue for return.
472 The specified time limit or filter is invalid.
474 The event could not be found to be modified or deleted.
476 No memory was available to register the event.
478 The specified process to attach to does not exist.
495 functions first appeared in
500 system and this manual page were written by
501 .An Jonathan Lemon Aq jlemon@FreeBSD.org .
503 It is currently not possible to watch a
505 that resides on anything but