bsd/man/man2/kqueue.2

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  48 .\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.32 2002/12/19 09:40:25 ru Exp $
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  50 .Dd October 21, 2008
  51 .Dt KQUEUE 2
  52 .Os
  53 .Sh NAME
  54 .Nm kqueue ,
  55 .Nm kevent ,
  56 and
  57 .Nm kevent64
  58 .Nd kernel event notification mechanism
  59 .Sh LIBRARY
  60 .Lb libc
  61 .Sh SYNOPSIS
  62 .In sys/types.h
  63 .In sys/event.h
  64 .In sys/time.h
  65 .Ft int
  66 .Fn kqueue "void"
  67 .Ft int
  68 .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout"
  69 .Ft int
  70 .Fn kevent64 "int kq" "const struct kevent64_s *changelist" "int nchanges" "struct kevent64_s *eventlist" "int nevents" "unsigned int flags" "const struct timespec *timeout"
  71 .Fn EV_SET "&kev" ident filter flags fflags data udata
  72 .Fn EV_SET64 "&kev" ident filter flags fflags data udata "ext[0]" "ext[1]"
  73 .Sh DESCRIPTION
  74 The
  75 .Fn kqueue
  76 system call
  77 provides a generic method of notifying the user when a kernel
  78 event (kevent) happens or a condition holds, based on the results
  79 of small pieces of kernel code termed filters.
  80 A kevent is identified by an (ident, filter) pair and specifies
  81 the interesting conditions to be notified about for that pair.
  82 An (ident, filter) pair can only appear once in a given kqueue.
  83 Subsequent attempts to register the same pair for a given kqueue
  84 will result in the replacement of the conditions being watched,
  85 not an addition.
  86 .Pp
  87 The filter identified in a kevent is executed upon the initial
  88 registration of that event in order to detect whether a preexisting
  89 condition is present, and is also executed whenever an event is
  90 passed to the filter for evaluation.
  91 If the filter determines that the condition should be reported,
  92 then the kevent is placed on the kqueue for the user to retrieve.
  93 .Pp
  94 The filter is also run when the user attempts to retrieve the kevent
  95 from the kqueue.
  96 If the filter indicates that the condition that triggered
  97 the event no longer holds, the kevent is removed from the kqueue and
  98 is not returned.
  99 .Pp
 100 Multiple events which trigger the filter do not result in multiple
 101 kevents being placed on the kqueue; instead, the filter will aggregate
 102 the events into a single struct kevent.
 103 Calling
 104 .Fn close
 105 on a file descriptor will remove any kevents that reference the descriptor.
 106 .Pp
 107 The
 108 .Fn kqueue
 109 system call
 110 creates a new kernel event queue and returns a descriptor.
 111 The queue is not inherited by a child created with
 112 .Xr fork 2 .
 113 .Pp
 114 The
 115 .Fn kevent
 116 and
 117 .Fn kevent64
 118 system calls
 119 are used to register events with the queue, and return any pending
 120 events to the user.
 121 The
 122 .Fa changelist
 123 argument
 124 is a pointer to an array of
 125 .Va kevent
 126 or
 127 .Va kevent64_s
 128 structures, as defined in
 129 .Aq Pa sys/event.h .
 130 All changes contained in the
 131 .Fa changelist
 132 are applied before any pending events are read from the queue.
 133 The
 134 .Fa nchanges
 135 argument
 136 gives the size of
 137 .Fa changelist .
 138 The
 139 .Fa eventlist
 140 argument
 141 is a pointer to an array of
 142 .Va kevent
 143 or
 144 .Va kevent64_s
 145 structures.
 146 The
 147 .Fa nevents
 148 argument
 149 determines the size of
 150 .Fa eventlist .
 151 If
 152 .Fa timeout
 153 is a non-NULL pointer, it specifies a maximum interval to wait
 154 for an event, which will be interpreted as a struct timespec.  If
 155 .Fa timeout
 156 is a NULL pointer, both
 157 .Fn kevent
 158 and
 159 .Fn kevent64
 160 wait indefinitely.  To effect a poll, the
 161 .Fa timeout
 162 argument should be non-NULL, pointing to a zero-valued
 163 .Va timespec
 164 structure.  The same array may be used for the
 165 .Fa changelist
 166 and
 167 .Fa eventlist .
 168 .Pp
 169 The
 170 .Fn EV_SET
 171 macro is provided for ease of initializing a
 172 .Va kevent
 173 structure. Similarly,
 174 .Fn EV_SET64
 175 initializes a
 176 .Va kevent64_s
 177 structure.
 178 .Pp
 179 The
 180 .Va kevent
 181 and
 182 .Va kevent64_s
 183 structures are defined as:
 184 .Bd -literal
 185 struct kevent {
 186         uintptr_t       ident;          /* identifier for this event */
 187         int16_t         filter;         /* filter for event */
 188         uint16_t        flags;          /* general flags */
 189         uint32_t        fflags;         /* filter-specific flags */
 190         intptr_t        data;           /* filter-specific data */
 191         void            *udata;         /* opaque user data identifier */
 192 };
 193
 194
 195 struct kevent64_s {
 196         uint64_t        ident;          /* identifier for this event */
 197         int16_t         filter;         /* filter for event */
 198         uint16_t        flags;          /* general flags */
 199         uint32_t        fflags;         /* filter-specific flags */
 200         int64_t         data;           /* filter-specific data */
 201         uint64_t        udata;          /* opaque user data identifier */
 202         uint64_t        ext[2];         /* filter-specific extensions */
 203 };
 204 .Ed
 205 .Pp
 206 ----
 207 .Pp
 208 The fields of
 209 .Fa struct kevent
 210 and
 211 .Fa struct kevent64_s
 212 are:
 213 .Bl -tag -width XXXfilter
 214 .It ident
 215 Value used to identify this event.
 216 The exact interpretation is determined by the attached filter,
 217 but often is a file descriptor.
 218 .It filter
 219 Identifies the kernel filter used to process this event.  The pre-defined
 220 system filters are described below.
 221 .It flags
 222 Actions to perform on the event.
 223 .It fflags
 224 Filter-specific flags.
 225 .It data
 226 Filter-specific data value.
 227 .It udata
 228 Opaque user-defined value passed through the kernel unchanged.
 229 .El
 230 .Pp
 231 In addition,
 232 .Fa struct kevent64_s
 233 contains:
 234 .Bl -tag -width XXXfilter
 235 .It ext[2]
 236 This field stores extensions for the event's filter. What type of extension depends on
 237 what type of filter is being used.
 238 .El
 239 .Pp
 240 ----
 241 .Pp
 242 The
 243 .Va flags
 244 field can contain the following values:
 245 .Bl -tag -width XXXEV_ONESHOT
 246 .It EV_ADD
 247 Adds the event to the kqueue.  Re-adding an existing event
 248 will modify the parameters of the original event, and not result
 249 in a duplicate entry.  Adding an event automatically enables it,
 250 unless overridden by the EV_DISABLE flag.
 251 .It EV_ENABLE
 252 Permit
 253 .Fn kevent
 254 and
 255 .Fn kevent64
 256 to return the event if it is triggered.
 257 .It EV_DISABLE
 258 Disable the event so
 259 .Fn kevent
 260 and
 261 .Fn kevent64
 262 will not return it.  The filter itself is not disabled.
 263 .It EV_DELETE
 264 Removes the event from the kqueue.  Events which are attached to
 265 file descriptors are automatically deleted on the last close of
 266 the descriptor.
 267 .It EV_RECEIPT
 268 This flag is useful for making bulk changes to a kqueue without draining any
 269 pending events. When passed as input, it forces EV_ERROR to always be returned.
 270 When a filter is successfully added, the
 271 .Va data
 272 field will be zero.
 273 .It EV_ONESHOT
 274 Causes the event to return only the first occurrence of the filter
 275 being triggered.  After the user retrieves the event from the kqueue,
 276 it is deleted.
 277 .It EV_CLEAR
 278 After the event is retrieved by the user, its state is reset.
 279 This is useful for filters which report state transitions
 280 instead of the current state.  Note that some filters may automatically
 281 set this flag internally.
 282 .It EV_EOF
 283 Filters may set this flag to indicate filter-specific EOF condition.
 284 .It EV_ERROR
 285 See
 286 .Sx RETURN VALUES
 287 below.
 288 .El
 289 .Pp
 290 ----
 291 .Pp
 292 The predefined system filters are listed below.
 293 Arguments may be passed to and from the filter via the
 294 .Va fflags
 295 and
 296 .Va data
 297 fields in the
 298 .Va kevent
 299 or
 300 .Va kevent64_s
 301 structure.
 302 .Bl -tag -width EVFILT_MACHPORT
 303 .It EVFILT_READ
 304 Takes a file descriptor as the identifier, and returns whenever
 305 there is data available to read.
 306 The behavior of the filter is slightly different depending
 307 on the descriptor type.
 308 .Pp
 309 .Bl -tag -width 2n
 310 .It Sockets
 311 Sockets which have previously been passed to
 312 .Fn listen
 313 return when there is an incoming connection pending.
 314 .Va data
 315 contains the size of the listen backlog.
 316 .Pp
 317 Other socket descriptors return when there is data to be read,
 318 subject to the
 319 .Dv SO_RCVLOWAT
 320 value of the socket buffer.
 321 This may be overridden with a per-filter low water mark at the
 322 time the filter is added by setting the
 323 NOTE_LOWAT
 324 flag in
 325 .Va fflags ,
 326 and specifying the new low water mark in
 327 .Va data .
 328 On return,
 329 .Va data
 330 contains the number of bytes of protocol data available to read.
 331 .Pp
 332 If the read direction of the socket has shutdown, then the filter
 333 also sets EV_EOF in
 334 .Va flags ,
 335 and returns the socket error (if any) in
 336 .Va fflags .
 337 It is possible for EOF to be returned (indicating the connection is gone)
 338 while there is still data pending in the socket buffer.
 339 .It Vnodes
 340 Returns when the file pointer is not at the end of file.
 341 .Va data
 342 contains the offset from current position to end of file,
 343 and may be negative.
 344 .It "Fifos, Pipes"
 345 Returns when the there is data to read;
 346 .Va data
 347 contains the number of bytes available.
 348 .Pp
 349 When the last writer disconnects, the filter will set EV_EOF in
 350 .Va flags .
 351 This may be cleared by passing in EV_CLEAR, at which point the
 352 filter will resume waiting for data to become available before
 353 returning.
 354 .El
 355 .It EVFILT_WRITE
 356 Takes a file descriptor as the identifier, and returns whenever
 357 it is possible to write to the descriptor.  For sockets, pipes
 358 and fifos,
 359 .Va data
 360 will contain the amount of space remaining in the write buffer.
 361 The filter will set EV_EOF when the reader disconnects, and for
 362 the fifo case, this may be cleared by use of EV_CLEAR.
 363 Note that this filter is not supported for vnodes.
 364 .Pp
 365 For sockets, the low water mark and socket error handling is
 366 identical to the EVFILT_READ case.
 367 .It EVFILT_AIO
 368 This filter is currently unsupported.
 369 .\"The sigevent portion of the AIO request is filled in, with
 370 .\".Va sigev_notify_kqueue
 371 .\"containing the descriptor of the kqueue that the event should
 372 .\"be attached to,
 373 .\".Va sigev_value
 374 .\"containing the udata value, and
 375 .\".Va sigev_notify
 376 .\"set to SIGEV_KEVENT.
 377 .\"When the
 378 .\".Fn aio_*
 379 .\"system call is made, the event will be registered
 380 .\"with the specified kqueue, and the
 381 .\".Va ident
 382 .\"argument set to the
 383 .\".Fa struct aiocb
 384 .\"returned by the
 385 .\".Fn aio_*
 386 .\"system call.
 387 .\"The filter returns under the same conditions as aio_error.
 388 .\".Pp
 389 .\"Alternatively, a kevent structure may be initialized, with
 390 .\".Va ident
 391 .\"containing the descriptor of the kqueue, and the
 392 .\"address of the kevent structure placed in the
 393 .\".Va aio_lio_opcode
 394 .\"field of the AIO request.  However, this approach will not work on
 395 .\"architectures with 64-bit pointers, and should be considered deprecated.
 396 .It EVFILT_VNODE
 397 Takes a file descriptor as the identifier and the events to watch for in
 398 .Va fflags ,
 399 and returns when one or more of the requested events occurs on the descriptor.
 400 The events to monitor are:
 401 .Bl -tag -width XXNOTE_RENAME
 402 .It NOTE_DELETE
 403 The
 404 .Fn unlink
 405 system call
 406 was called on the file referenced by the descriptor.
 407 .It NOTE_WRITE
 408 A write occurred on the file referenced by the descriptor.
 409 .It NOTE_EXTEND
 410 The file referenced by the descriptor was extended.
 411 .It NOTE_ATTRIB
 412 The file referenced by the descriptor had its attributes changed.
 413 .It NOTE_LINK
 414 The link count on the file changed.
 415 .It NOTE_RENAME
 416 The file referenced by the descriptor was renamed.
 417 .It NOTE_REVOKE
 418 Access to the file was revoked via
 419 .Xr revoke 2
 420 or the underlying fileystem was unmounted.
 421 .El
 422 .Pp
 423 On return,
 424 .Va fflags
 425 contains the events which triggered the filter.
 426 .It EVFILT_PROC
 427 Takes the process ID to monitor as the identifier and the events to watch for
 428 in
 429 .Va fflags ,
 430 and returns when the process performs one or more of the requested events.
 431 If a process can normally see another process, it can attach an event to it.
 432 The events to monitor are:
 433 .Bl -tag -width NOTE_SIGNAL
 434 .It NOTE_EXIT
 435 The process has exited.
 436 .It NOTE_EXITSTATUS
 437 The process has exited and its exit status is in filter specific data. Valid only on child processes and to be used along with NOTE_EXIT.
 438 .It NOTE_FORK
 439 The process created a child process via
 440 .Xr fork 2
 441 or similar call.
 442 .It NOTE_EXEC
 443 The process executed a new process via
 444 .Xr execve 2
 445 or similar call.
 446 .It NOTE_SIGNAL
 447 The process was sent a signal. Status can be checked via
 448 .Xr waitpid 2
 449 or similar call.
 450 .It NOTE_REAP
 451 The process was reaped by the parent via
 452 .Xr wait 2
 453 or similar call. Deprecated, use NOTE_EXIT.
 454 .El
 455 .Pp
 456 On return,
 457 .Va fflags
 458 contains the events which triggered the filter.
 459 .It EVFILT_SIGNAL
 460 Takes the signal number to monitor as the identifier and returns
 461 when the given signal is generated for the process.
 462 This coexists with the
 463 .Fn signal
 464 and
 465 .Fn sigaction
 466 facilities, and has a lower precedence.  Only signals sent to the process,
 467 not to a particular thread, will trigger the filter. The filter will record
 468 all attempts to deliver a signal to a process, even if the signal has
 469 been marked as SIG_IGN.  Event notification happens before normal
 470 signal delivery processing.
 471 .Va data
 472 returns the number of times the signal has been generated since the last call to
 473 .Fn kevent .
 474 This filter automatically sets the EV_CLEAR flag internally.
 475 .It EVFILT_MACHPORT
 476 Takes the name of a mach port, or port set, in
 477 .Va ident
 478 and waits until a message is received on the port or port set. When a message
 479 is recieved, the size of the message is returned in
 480 .Va data
 481 and if
 482 .Va fflags
 483 is set to MACH_RCV_MSG, a pointer to the message is returned in ext[0].
 484 .It EVFILT_TIMER
 485 Establishes an interval timer with the data
 486 timer identified by
 487 .Va ident .
 488 When adding a timer,
 489 .Va data
 490 specifies the timeout period and
 491 .Va fflags
 492 can be set to one of the following:
 493 .Bl -tag -width NOTE_ABSOLUTE
 494 .It NOTE_SECONDS
 495 data is in seconds
 496 .It NOTE_USECONDS
 497 data is in microseconds
 498 .It NOTE_NSECONDS
 499 data is in nanoseconds
 500 .It NOTE_ABSOLUTE
 501 data is an absolute timeout
 502 .It NOTE_CRITICAL
 503 system makes a best effort to fire this timer as scheduled.
 504 .It NOTE_BACKGROUND
 505 system has extra leeway to coalesce this timer.
 506 .It NOTE_LEEWAY
 507 ext[1] holds user-supplied slop in deadline for timer coalescing.
 508 .El
 509 .Pp
 510 If fflags is not set, the default is milliseconds. The timer will be periodic unless EV_ONESHOT is specified.
 511 On return,
 512 .Va data
 513 contains the number of times the timeout has expired since the last call to
 514 .Fn kevent
 515 or
 516 .Fn kevent64 .
 517 This filter automatically sets the EV_CLEAR flag internally.
 518 .El
 519 .Pp
 520 ----
 521 .Pp
 522 In the
 523 .Va ext[2]
 524 field of the
 525 .Va kevent64_s
 526 struture,
 527 .Va ext[0]
 528 is only used with the EVFILT_MACHPORT filter.
 529 With other filters,
 530 .Va ext[0]
 531 is passed through
 532 .Fn kevent64
 533 much like
 534 .Va udata .
 535 .Va ext[1]
 536 can always be used like
 537 .Va udata .
 538 For the use of ext[0], see the EVFILT_MACHPORT filter above.
 539 .Sh RETURN VALUES
 540 The
 541 .Fn kqueue
 542 system call
 543 creates a new kernel event queue and returns a file descriptor.
 544 If there was an error creating the kernel event queue, a value of -1 is
 545 returned and errno set.
 546 .Pp
 547 The
 548 .Fn kevent
 549 and
 550 .Fn kevent64
 551 system calls
 552 return the number of events placed in the
 553 .Fa eventlist ,
 554 up to the value given by
 555 .Fa nevents .
 556 If an error occurs while processing an element of the
 557 .Fa changelist
 558 and there is enough room in the
 559 .Fa eventlist ,
 560 then the event will be placed in the
 561 .Fa eventlist
 562 with
 563 .Dv EV_ERROR
 564 set in
 565 .Va flags
 566 and the system error in
 567 .Va data .
 568 Otherwise,
 569 .Dv -1
 570 will be returned, and
 571 .Dv errno
 572 will be set to indicate the error condition.
 573 If the time limit expires, then
 574 .Fn kevent
 575 and
 576 .Fn kevent64
 577 return 0.
 578 .Sh ERRORS
 579 The
 580 .Fn kqueue
 581 system call fails if:
 582 .Bl -tag -width Er
 583 .It Bq Er ENOMEM
 584 The kernel failed to allocate enough memory for the kernel queue.
 585 .It Bq Er EMFILE
 586 The per-process descriptor table is full.
 587 .It Bq Er ENFILE
 588 The system file table is full.
 589 .El
 590 .Pp
 591 The
 592 .Fn kevent
 593 and
 594 .Fn kevent64
 595 system calls fail if:
 596 .Bl -tag -width Er
 597 .It Bq Er EACCES
 598 The process does not have permission to register a filter.
 599 .It Bq Er EFAULT
 600 There was an error reading or writing the
 601 .Va kevent
 602 or
 603 .Va kevent64_s
 604 structure.
 605 .It Bq Er EBADF
 606 The specified descriptor is invalid.
 607 .It Bq Er EINTR
 608 A signal was delivered before the timeout expired and before any
 609 events were placed on the kqueue for return.
 610 .It Bq Er EINVAL
 611 The specified time limit or filter is invalid.
 612 .It Bq Er ENOENT
 613 The event could not be found to be modified or deleted.
 614 .It Bq Er ENOMEM
 615 No memory was available to register the event.
 616 .It Bq Er ESRCH
 617 The specified process to attach to does not exist.
 618 .El
 619 .Sh SEE ALSO
 620 .Xr aio_error 2 ,
 621 .Xr aio_read 2 ,
 622 .Xr aio_return 2 ,
 623 .Xr read 2 ,
 624 .Xr select 2 ,
 625 .Xr sigaction 2 ,
 626 .Xr write 2 ,
 627 .Xr signal 3
 628 .Sh HISTORY
 629 The
 630 .Fn kqueue
 631 and
 632 .Fn kevent
 633 system calls first appeared in
 634 .Fx 4.1 .
 635 .Sh AUTHORS
 636 The
 637 .Fn kqueue
 638 system and this manual page were written by
 639 .An Jonathan Lemon Aq jlemon@FreeBSD.org .
 640 .Sh BUGS
 641 Not all filesystem types support kqueue-style notifications.
 642 And even some that do, like some remote filesystems, may only
 643 support a subset of the notification semantics described
 644 here.