bsd/man/man2/kqueue.2

   1 .\" Copyright (c) 2000 Jonathan Lemon
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  25 .\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.32 2002/12/19 09:40:25 ru Exp $
  26 .\"
  27 .Dd April 14, 2000
  28 .Dt KQUEUE 2
  29 .Os
  30 .Sh NAME
  31 .Nm kqueue ,
  32 .Nm kevent
  33 .Nd kernel event notification mechanism
  34 .Sh LIBRARY
  35 .Lb libc
  36 .Sh SYNOPSIS
  37 .In sys/types.h
  38 .In sys/event.h
  39 .In sys/time.h
  40 .Ft int
  41 .Fn kqueue "void"
  42 .Ft int
  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
  45 .Sh DESCRIPTION
  46 The
  47 .Fn kqueue
  48 system call
  49 provides a generic method of notifying the user when an kernel
  50 event (kevent) happens or a condition holds, based on the results
  51 of small pieces of kernel code termed filters.
  52 A kevent is identified by an (ident, filter) pair and specifies
  53 the interesting conditions to be notified about for that pair.
  54 An (ident, filter) pair can only appear once is a given kqueue.
  55 Subsequent attempts to register the same pair for a given kqueue
  56 will result in the replacement of the conditions being watched,
  57 not an addition.
  58 .Pp
  59 The filter identified in a kevent is executed upon the initial
  60 registration of that event in order to detect whether a preexisting
  61 condition is present, and is also executed whenever an event is
  62 passed to the filter for evaluation.
  63 If the filter determines that the condition should be reported,
  64 then the kevent is placed on the kqueue for the user to retrieve.
  65 .Pp
  66 The filter is also run when the user attempts to retrieve the kevent
  67 from the kqueue.
  68 If the filter indicates that the condition that triggered
  69 the event no longer holds, the kevent is removed from the kqueue and
  70 is not returned.
  71 .Pp
  72 Multiple events which trigger the filter do not result in multiple
  73 kevents being placed on the kqueue; instead, the filter will aggregate
  74 the events into a single struct kevent.
  75 Calling
  76 .Fn close
  77 on a file descriptor will remove any kevents that reference the descriptor.
  78 .Pp
  79 The
  80 .Fn kqueue
  81 system call
  82 creates a new kernel event queue and returns a descriptor.
  83 The queue is not inherited by a child created with
  84 .Xr fork 2 .
  85 .Pp
  86 The
  87 .Fn kevent
  88 system call
  89 is used to register events with the queue, and return any pending
  90 events to the user.
  91 The
  92 .Fa changelist
  93 argument
  94 is a pointer to an array of
  95 .Va kevent
  96 structures, as defined in
  97 .Aq Pa sys/event.h .
  98 All changes contained in the
  99 .Fa changelist
 100 are applied before any pending events are read from the queue.
 101 The
 102 .Fa nchanges
 103 argument
 104 gives the size of
 105 .Fa changelist .
 106 The
 107 .Fa eventlist
 108 argument
 109 is a pointer to an array of kevent structures.
 110 The
 111 .Fa nevents
 112 argument
 113 determines the size of
 114 .Fa eventlist .
 115 If
 116 .Fa timeout
 117 is a non-NULL pointer, it specifies a maximum interval to wait
 118 for an event, which will be interpreted as a struct timespec.  If
 119 .Fa timeout
 120 is a NULL pointer,
 121 .Fn kevent
 122 waits indefinitely.  To effect a poll, the
 123 .Fa timeout
 124 argument should be non-NULL, pointing to a zero-valued
 125 .Va timespec
 126 structure.  The same array may be used for the
 127 .Fa changelist
 128 and
 129 .Fa eventlist .
 130 .Pp
 131 The
 132 .Fn EV_SET
 133 macro is provided for ease of initializing a
 134 kevent structure.
 135 .Pp
 136 The
 137 .Va kevent
 138 structure is defined as:
 139 .Bd -literal
 140 struct kevent {
 141         uintptr_t ident;        /* identifier for this event */
 142         short     filter;       /* filter for event */
 143         u_short   flags;        /* action flags for kqueue */
 144         u_int     fflags;       /* filter flag value */
 145         intptr_t  data;         /* filter data value */
 146         void      *udata;       /* opaque user data identifier */
 147 };
 148 .Ed
 149 .Pp
 150 The fields of
 151 .Fa struct kevent
 152 are:
 153 .Bl -tag -width XXXfilter
 154 .It ident
 155 Value used to identify this event.
 156 The exact interpretation is determined by the attached filter,
 157 but often is a file descriptor.
 158 .It filter
 159 Identifies the kernel filter used to process this event.  The pre-defined
 160 system filters are described below.
 161 .It flags
 162 Actions to perform on the event.
 163 .It fflags
 164 Filter-specific flags.
 165 .It data
 166 Filter-specific data value.
 167 .It udata
 168 Opaque user-defined value passed through the kernel unchanged.
 169 .El
 170 .Pp
 171 The
 172 .Va flags
 173 field can contain the following values:
 174 .Bl -tag -width XXXEV_ONESHOT
 175 .It EV_ADD
 176 Adds the event to the kqueue.  Re-adding an existing event
 177 will modify the parameters of the original event, and not result
 178 in a duplicate entry.  Adding an event automatically enables it,
 179 unless overridden by the EV_DISABLE flag.
 180 .It EV_ENABLE
 181 Permit
 182 .Fn kevent
 183 to return the event if it is triggered.
 184 .It EV_DISABLE
 185 Disable the event so
 186 .Fn kevent
 187 will not return it.  The filter itself is not disabled.
 188 .It EV_DELETE
 189 Removes the event from the kqueue.  Events which are attached to
 190 file descriptors are automatically deleted on the last close of
 191 the descriptor.
 192 .It EV_ONESHOT
 193 Causes the event to return only the first occurrence of the filter
 194 being triggered.  After the user retrieves the event from the kqueue,
 195 it is deleted.
 196 .It EV_CLEAR
 197 After the event is retrieved by the user, its state is reset.
 198 This is useful for filters which report state transitions
 199 instead of the current state.  Note that some filters may automatically
 200 set this flag internally.
 201 .It EV_EOF
 202 Filters may set this flag to indicate filter-specific EOF condition.
 203 .It EV_ERROR
 204 See
 205 .Sx RETURN VALUES
 206 below.
 207 .El
 208 .Pp
 209 The predefined system filters are listed below.
 210 Arguments may be passed to and from the filter via the
 211 .Va fflags
 212 and
 213 .Va data
 214 fields in the kevent structure.
 215 .Bl -tag -width EVFILT_SIGNAL
 216 .It EVFILT_READ
 217 Takes a file descriptor as the identifier, and returns whenever
 218 there is data available to read.
 219 The behavior of the filter is slightly different depending
 220 on the descriptor type.
 221 .Pp
 222 .Bl -tag -width 2n
 223 .It Sockets
 224 Sockets which have previously been passed to
 225 .Fn listen
 226 return when there is an incoming connection pending.
 227 .Va data
 228 contains the size of the listen backlog.
 229 .Pp
 230 Other socket descriptors return when there is data to be read,
 231 subject to the
 232 .Dv SO_RCVLOWAT
 233 value of the socket buffer.
 234 This may be overridden with a per-filter low water mark at the
 235 time the filter is added by setting the
 236 NOTE_LOWAT
 237 flag in
 238 .Va fflags ,
 239 and specifying the new low water mark in
 240 .Va data .
 241 On return,
 242 .Va data
 243 contains the number of bytes of protocol data available to read.
 244 .Pp
 245 If the read direction of the socket has shutdown, then the filter
 246 also sets EV_EOF in
 247 .Va flags ,
 248 and returns the socket error (if any) in
 249 .Va fflags .
 250 It is possible for EOF to be returned (indicating the connection is gone)
 251 while there is still data pending in the socket buffer.
 252 .It Vnodes
 253 Returns when the file pointer is not at the end of file.
 254 .Va data
 255 contains the offset from current position to end of file,
 256 and may be negative.
 257 .It "Fifos, Pipes"
 258 Returns when the there is data to read;
 259 .Va data
 260 contains the number of bytes available.
 261 .Pp
 262 When the last writer disconnects, the filter will set EV_EOF in
 263 .Va flags .
 264 This may be cleared by passing in EV_CLEAR, at which point the
 265 filter will resume waiting for data to become available before
 266 returning.
 267 .El
 268 .It EVFILT_WRITE
 269 Takes a file descriptor as the identifier, and returns whenever
 270 it is possible to write to the descriptor.  For sockets, pipes
 271 and fifos,
 272 .Va data
 273 will contain the amount of space remaining in the write buffer.
 274 The filter will set EV_EOF when the reader disconnects, and for
 275 the fifo case, this may be cleared by use of EV_CLEAR.
 276 Note that this filter is not supported for vnodes.
 277 .Pp
 278 For sockets, the low water mark and socket error handling is
 279 identical to the EVFILT_READ case.
 280 .It EVFILT_AIO
 281 This filter is currently unsupported.
 282 .\"The sigevent portion of the AIO request is filled in, with
 283 .\".Va sigev_notify_kqueue
 284 .\"containing the descriptor of the kqueue that the event should
 285 .\"be attached to,
 286 .\".Va sigev_value
 287 .\"containing the udata value, and
 288 .\".Va sigev_notify
 289 .\"set to SIGEV_KEVENT.
 290 .\"When the
 291 .\".Fn aio_*
 292 .\"system call is made, the event will be registered
 293 .\"with the specified kqueue, and the
 294 .\".Va ident
 295 .\"argument set to the
 296 .\".Fa struct aiocb
 297 .\"returned by the
 298 .\".Fn aio_*
 299 .\"system call.
 300 .\"The filter returns under the same conditions as aio_error.
 301 .\".Pp
 302 .\"Alternatively, a kevent structure may be initialized, with
 303 .\".Va ident
 304 .\"containing the descriptor of the kqueue, and the
 305 .\"address of the kevent structure placed in the
 306 .\".Va aio_lio_opcode
 307 .\"field of the AIO request.  However, this approach will not work on
 308 .\"architectures with 64-bit pointers, and should be considered deprecated.
 309 .It EVFILT_VNODE
 310 Takes a file descriptor as the identifier and the events to watch for in
 311 .Va fflags ,
 312 and returns when one or more of the requested events occurs on the descriptor.
 313 The events to monitor are:
 314 .Bl -tag -width XXNOTE_RENAME
 315 .It NOTE_DELETE
 316 The
 317 .Fn unlink
 318 system call
 319 was called on the file referenced by the descriptor.
 320 .It NOTE_WRITE
 321 A write occurred on the file referenced by the descriptor.
 322 .It NOTE_EXTEND
 323 The file referenced by the descriptor was extended.
 324 .It NOTE_ATTRIB
 325 The file referenced by the descriptor had its attributes changed.
 326 .It NOTE_LINK
 327 The link count on the file changed.
 328 .It NOTE_RENAME
 329 The file referenced by the descriptor was renamed.
 330 .It NOTE_REVOKE
 331 Access to the file was revoked via
 332 .Xr revoke 2
 333 or the underlying fileystem was unmounted.
 334 .El
 335 .Pp
 336 On return,
 337 .Va fflags
 338 contains the events which triggered the filter.
 339 .It EVFILT_PROC
 340 Takes the process ID to monitor as the identifier and the events to watch for
 341 in
 342 .Va fflags ,
 343 and returns when the process performs one or more of the requested events.
 344 If a process can normally see another process, it can attach an event to it.
 345 The events to monitor are:
 346 .Bl -tag -width XXNOTE_TRACKERR
 347 .It NOTE_EXIT
 348 The process has exited.
 349 .It NOTE_FORK
 350 The process has called
 351 .Fn fork .
 352 .It NOTE_EXEC
 353 The process has executed a new process via
 354 .Xr execve 2
 355 or similar call.
 356 .It NOTE_TRACK
 357 Follow a process across
 358 .Fn fork
 359 calls.  The parent process will return with NOTE_TRACK set in the
 360 .Va fflags
 361 field, while the child process will return with NOTE_CHILD set in
 362 .Va fflags
 363 and the parent PID in
 364 .Va data .
 365 .It NOTE_TRACKERR
 366 This flag is returned if the system was unable to attach an event to
 367 the child process, usually due to resource limitations.
 368 .El
 369 .Pp
 370 On return,
 371 .Va fflags
 372 contains the events which triggered the filter.
 373 .It EVFILT_SIGNAL
 374 Takes the signal number to monitor as the identifier and returns
 375 when the given signal is delivered to the process.
 376 This coexists with the
 377 .Fn signal
 378 and
 379 .Fn sigaction
 380 facilities, and has a lower precedence.  The filter will record
 381 all attempts to deliver a signal to a process, even if the signal has
 382 been marked as SIG_IGN.  Event notification happens after normal
 383 signal delivery processing.
 384 .Va data
 385 returns the number of times the signal has occurred since the last call to
 386 .Fn kevent .
 387 This filter automatically sets the EV_CLEAR flag internally.
 388 .It EVFILT_TIMER
 389 This filter is currently unsupported.
 390 .\"Establishes an arbitrary timer identified by
 391 .\".Va ident .
 392 .\"When adding a timer,
 393 .\".Va data
 394 .\"specifies the timeout period in milliseconds.
 395 .\"The timer will be periodic unless EV_ONESHOT is specified.
 396 .\"On return,
 397 .\".Va data
 398 .\"contains the number of times the timeout has expired since the last call to
 399 .\".Fn kevent .
 400 .\"This filter automatically sets the EV_CLEAR flag internally.
 401 .El
 402 .Sh RETURN VALUES
 403 The
 404 .Fn kqueue
 405 system call
 406 creates a new kernel event queue and returns a file descriptor.
 407 If there was an error creating the kernel event queue, a value of -1 is
 408 returned and errno set.
 409 .Pp
 410 The
 411 .Fn kevent
 412 system call
 413 returns the number of events placed in the
 414 .Fa eventlist ,
 415 up to the value given by
 416 .Fa nevents .
 417 If an error occurs while processing an element of the
 418 .Fa changelist
 419 and there is enough room in the
 420 .Fa eventlist ,
 421 then the event will be placed in the
 422 .Fa eventlist
 423 with
 424 .Dv EV_ERROR
 425 set in
 426 .Va flags
 427 and the system error in
 428 .Va data .
 429 Otherwise,
 430 .Dv -1
 431 will be returned, and
 432 .Dv errno
 433 will be set to indicate the error condition.
 434 If the time limit expires, then
 435 .Fn kevent
 436 returns 0.
 437 .Sh ERRORS
 438 The
 439 .Fn kqueue
 440 system call fails if:
 441 .Bl -tag -width Er
 442 .It Bq Er ENOMEM
 443 The kernel failed to allocate enough memory for the kernel queue.
 444 .It Bq Er EMFILE
 445 The per-process descriptor table is full.
 446 .It Bq Er ENFILE
 447 The system file table is full.
 448 .El
 449 .Pp
 450 The
 451 .Fn kevent
 452 system call fails if:
 453 .Bl -tag -width Er
 454 .It Bq Er EACCES
 455 The process does not have permission to register a filter.
 456 .It Bq Er EFAULT
 457 There was an error reading or writing the
 458 .Va kevent
 459 structure.
 460 .It Bq Er EBADF
 461 The specified descriptor is invalid.
 462 .It Bq Er EINTR
 463 A signal was delivered before the timeout expired and before any
 464 events were placed on the kqueue for return.
 465 .It Bq Er EINVAL
 466 The specified time limit or filter is invalid.
 467 .It Bq Er ENOENT
 468 The event could not be found to be modified or deleted.
 469 .It Bq Er ENOMEM
 470 No memory was available to register the event.
 471 .It Bq Er ESRCH
 472 The specified process to attach to does not exist.
 473 .El
 474 .Sh SEE ALSO
 475 .Xr aio_error 2 ,
 476 .Xr aio_read 2 ,
 477 .Xr aio_return 2 ,
 478 .Xr read 2 ,
 479 .Xr select 2 ,
 480 .Xr sigaction 2 ,
 481 .Xr write 2 ,
 482 .Xr signal 3
 483 .Sh HISTORY
 484 The
 485 .Fn kqueue
 486 and
 487 .Fn kevent
 488 system calls first appeared in
 489 .Fx 4.1 .
 490 .Sh AUTHORS
 491 The
 492 .Fn kqueue
 493 system and this manual page were written by
 494 .An Jonathan Lemon Aq jlemon@FreeBSD.org .
 495 .Sh BUGS
 496 Not all filesystem types support kqueue-style notifications.
 497 And even some that do, like some remote filesystems, may only
 498 support a subset of the notification semantics described
 499 here.