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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_ONESHOT"
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 Removes the event from the kqueue.
182 Events which are attached to file descriptors are automatically
183 deleted on the last close of the descriptor.
185 Causes the event to return only the first occurrence of the filter
187 After the user retrieves the event from the kqueue, it is deleted.
189 After the event is retrieved by the user, its state is reset.
190 This is useful for filters which report state transitions
191 instead of the current state.
192 Note that some filters may automatically set this flag internally.
194 Filters may set this flag to indicate filter-specific EOF condition.
201 The predefined system filters are listed below.
202 Arguments may be passed to and from the filter via the
206 fields in the kevent structure.
207 .Bl -tag -width ".Dv EVFILT_SIGNAL"
209 Takes a descriptor as the identifier, and returns whenever
210 there is data available to read.
211 The behavior of the filter is slightly different depending
212 on the descriptor type.
215 Sockets which have previously been passed to
217 return when there is an incoming connection pending.
219 contains the size of the listen backlog.
221 Other socket descriptors return when there is data to be read,
224 value of the socket buffer.
225 This may be overridden with a per-filter low water mark at the
226 time the filter is added by setting the
230 and specifying the new low water mark in
234 contains the number of bytes in the socket buffer.
236 If the read direction of the socket has shutdown, then the filter also sets
240 and returns the socket error (if any) in
242 It is possible for EOF to be returned (indicating the connection is gone)
243 while there is still data pending in the socket buffer.
245 Returns when the file pointer is not at the end of file.
247 contains the offset from current position to end of file,
250 Returns when the there is data to read;
252 contains the number of bytes available.
254 When the last writer disconnects, the filter will set
258 This may be cleared by passing in
260 at which point the filter will resume waiting for data to become
261 available before returning.
264 Takes a descriptor as the identifier, and returns whenever
265 it is possible to write to the descriptor.
266 For sockets, pipes and fifos,
268 will contain the amount of space remaining in the write buffer.
271 when the reader disconnects, and for the fifo case, this may be cleared
274 Note that this filter is not supported for vnodes.
276 For sockets, the low water mark and socket error handling is
281 Takes a descriptor as the identifier, and returns whenever one of the
282 specified exceptional conditions has occurred on the descriptor. Conditions
285 Currently, a filter can monitor the reception of out-of-band data with
288 The sigevent portion of the AIO request is filled in, with
289 .Va sigev_notify_kqueue
290 containing the descriptor of the kqueue that the event should
293 containing the udata value, and
297 When the aio_* function is called, the event will be registered
298 with the specified kqueue, and the
302 returned by the aio_* function.
303 The filter returns under the same conditions as aio_error.
305 Alternatively, a kevent structure may be initialized, with
307 containing the descriptor of the kqueue, and the
308 address of the kevent structure placed in the
310 field of the AIO request.
311 However, this approach will not work on architectures with 64-bit
312 pointers, and should be considered deprecated.
314 Takes a file descriptor as the identifier and the events to watch for in
316 and returns when one or more of the requested events occurs on the descriptor.
317 The events to monitor are:
318 .Bl -tag -width ".Dv NOTE_RENAME"
321 was called on the file referenced by the descriptor.
323 A write occurred on the file referenced by the descriptor.
325 The file referenced by the descriptor was extended.
327 The file referenced by the descriptor had its attributes changed.
329 The link count on the file changed.
331 The file referenced by the descriptor was renamed.
333 Access to the file was revoked via
335 or the underlying fileystem was unmounted.
340 contains the events which triggered the filter.
342 Takes the process ID to monitor as the identifier and the events to watch for
345 and returns when the process performs one or more of the requested events.
346 If a process can normally see another process, it can attach an event to it.
347 The events to monitor are:
348 .Bl -tag -width ".Dv NOTE_TRACKERR"
350 The process has exited.
352 The process has called
355 The process has executed a new process via
359 Follow a process across
362 The parent process will return with
366 field, while the child process will return with
370 and the parent PID in
373 This flag is returned if the system was unable to attach an event to
374 the child process, usually due to resource limitations.
379 contains the events which triggered the filter.
381 Takes the signal number to monitor as the identifier and returns
382 when the given signal is delivered to the process.
383 This coexists with the
387 facilities, and has a lower precedence.
388 The filter will record all attempts to deliver a signal to a process,
389 even if the signal has been marked as
391 Event notification happens after normal signal delivery processing.
393 returns the number of times the signal has occurred since the last call to
395 This filter automatically sets the
399 Establishes an arbitrary timer identified by
403 specifies the timeout period in milliseconds.
404 The timer will be periodic unless
409 contains the number of times the timeout has expired since the last call to
411 This filter automatically sets the
417 creates a new kernel event queue and returns a file descriptor.
418 If there was an error creating the kernel event queue, a value of -1 is
424 returns the number of events placed in the
426 up to the value given by
428 If an error occurs while processing an element of the
430 and there is enough room in the
432 then the event will be placed in the
438 and the system error in
442 will be returned, and
444 will be set to indicate the error condition.
445 If the time limit expires, then
454 The kernel failed to allocate enough memory for the kernel queue.
456 The per-process descriptor table is full.
458 The system file table is full.
466 The process does not have permission to register a filter.
468 There was an error reading or writing the
472 The specified descriptor is invalid.
474 A signal was delivered before the timeout expired and before any
475 events were placed on the kqueue for return.
477 The specified time limit or filter is invalid.
479 The event could not be found to be modified or deleted.
481 No memory was available to register the event.
483 The specified process to attach to does not exist.
500 functions first appeared in
505 system and this manual page were written by
506 .An Jonathan Lemon Aq Mt jlemon@FreeBSD.org .
508 Currently it is only possible to watch a