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 an 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_FORK
 437 The process created a child process via
 438 .Xr fork 2
 439 or similar call.
 440 .It NOTE_EXEC
 441 The process executed a new process via
 442 .Xr execve 2
 443 or similar call.
 444 .It NOTE_SIGNAL
 445 The process was sent a signal. Status can be checked via
 446 .Xr waitpid 2
 447 or similar call.
 448 .It NOTE_REAP
 449 The process was reaped by the parent via
 450 .Xr wait 2
 451 or similar call.
 452 .El
 453 .Pp
 454 On return,
 455 .Va fflags
 456 contains the events which triggered the filter.
 457 .It EVFILT_SIGNAL
 458 Takes the signal number to monitor as the identifier and returns
 459 when the given signal is generated for the process.
 460 This coexists with the
 461 .Fn signal
 462 and
 463 .Fn sigaction
 464 facilities, and has a lower precedence.  Only signals sent to the process,
 465 not to a particular thread, will trigger the filter. The filter will record
 466 all attempts to deliver a signal to a process, even if the signal has
 467 been marked as SIG_IGN.  Event notification happens before normal
 468 signal delivery processing.
 469 .Va data
 470 returns the number of times the signal has been generated since the last call to
 471 .Fn kevent .
 472 This filter automatically sets the EV_CLEAR flag internally.
 473 .It EVFILT_MACHPORT
 474 Takes the name of a mach port, or port set, in
 475 .Va ident
 476 and waits until a message is received on the port or port set. When a message
 477 is recieved, the size of the message is returned in
 478 .Va data
 479 and if
 480 .Va fflags
 481 is set to MACH_RCV_MSG, a pointer to the message is returned in ext[0].
 482 .It EVFILT_TIMER
 483 Establishes an interval timer with the data
 484 timer identified by
 485 .Va ident .
 486 When adding a timer,
 487 .Va data
 488 specifies the timeout period and
 489 .Va fflags
 490 can be set to one of the following:
 491 .Bl -tag -width NOTE_ABSOLUTE
 492 .It NOTE_SECONDS
 493 data is in seconds
 494 .It NOTE_USECONDS
 495 data is in microseconds
 496 .It NOTE_NSECONDS
 497 data is in nanoseconds
 498 .It NOTE_ABSOLUTE
 499 data is an absolute timeout
 500 .El
 501 .Pp
 502 If fflags is not set, the default is milliseconds. The timer will be periodic unless EV_ONESHOT is specified.
 503 On return,
 504 .Va data
 505 contains the number of times the timeout has expired since the last call to
 506 .Fn kevent
 507 or
 508 .Fn kevent64 .
 509 This filter automatically sets the EV_CLEAR flag internally.
 510 .It EVFILT_SESSION
 511 Takes the audit session ID to monitor as the identifier and the events to watch for in
 512 .Va fflags ,
 513 and returns when one or more of the requested session events occurs.
 514 To monitor for events for any audit session the value AS_ANY_ASID
 515 should be used as the identifier.  With AS_ANY_ASID, as new audit
 516 sessions are created they are included as if the were added
 517 individually.  The events to monitor are:
 518 .Bl -tag -width NOTE_AS_UPDATE
 519 .It NOTE_AS_START
 520 A new audit session has started.
 521 .It NOTE_AS_END
 522 All the processes in the audit session have exited.
 523 .It NOTE_AS_CLOSE
 524 This audit session is no longer valid in the kernel.  In other words, it
 525 is now safe to dispose of any cached information about this session or
 526 reuse its session ID for a new audit session.
 527 .It NOTE_AS_UPDATE
 528 The audit session information was updated.  The audit session information is
 529 considered immutable once initially set.  If this becomes enforced in
 530 the kernel then this event may no longer be needed and may become
 531 obsolete.
 532 .It NOTE_AS_ERR
 533 This flag is returned if the system was unable to attach an event to a
 534 new session when the audit session ID of AS_ANY_ASID
 535 is used.  This is usually due to resource limitations.
 536 .El
 537 .Pp
 538 On return,
 539 .Va fflags
 540 contains the events which triggered the filter,
 541 .Va ident
 542 contains the audit session ID, and
 543 .Va data
 544 contains the audit user ID.
 545 This filter automatically sets the EV_CLEAR flag internally.
 546 .El
 547 .Pp
 548 ----
 549 .Pp
 550 In the
 551 .Va ext[2]
 552 field of the
 553 .Va kevent64_s
 554 struture,
 555 .Va ext[0]
 556 is only used with the EVFILT_MACHPORT filter.
 557 With other filters,
 558 .Va ext[0]
 559 is passed through
 560 .Fn kevent64
 561 much like
 562 .Va udata .
 563 .Va ext[1]
 564 can always be used like
 565 .Va udata .
 566 For the use of ext[0], see the EVFILT_MACHPORT filter above.
 567 .Sh RETURN VALUES
 568 The
 569 .Fn kqueue
 570 system call
 571 creates a new kernel event queue and returns a file descriptor.
 572 If there was an error creating the kernel event queue, a value of -1 is
 573 returned and errno set.
 574 .Pp
 575 The
 576 .Fn kevent
 577 and
 578 .Fn kevent64
 579 system calls
 580 return the number of events placed in the
 581 .Fa eventlist ,
 582 up to the value given by
 583 .Fa nevents .
 584 If an error occurs while processing an element of the
 585 .Fa changelist
 586 and there is enough room in the
 587 .Fa eventlist ,
 588 then the event will be placed in the
 589 .Fa eventlist
 590 with
 591 .Dv EV_ERROR
 592 set in
 593 .Va flags
 594 and the system error in
 595 .Va data .
 596 Otherwise,
 597 .Dv -1
 598 will be returned, and
 599 .Dv errno
 600 will be set to indicate the error condition.
 601 If the time limit expires, then
 602 .Fn kevent
 603 and
 604 .Fn kevent64
 605 return 0.
 606 .Sh ERRORS
 607 The
 608 .Fn kqueue
 609 system call fails if:
 610 .Bl -tag -width Er
 611 .It Bq Er ENOMEM
 612 The kernel failed to allocate enough memory for the kernel queue.
 613 .It Bq Er EMFILE
 614 The per-process descriptor table is full.
 615 .It Bq Er ENFILE
 616 The system file table is full.
 617 .El
 618 .Pp
 619 The
 620 .Fn kevent
 621 and
 622 .Fn kevent64
 623 system calls fail if:
 624 .Bl -tag -width Er
 625 .It Bq Er EACCES
 626 The process does not have permission to register a filter.
 627 .It Bq Er EFAULT
 628 There was an error reading or writing the
 629 .Va kevent
 630 or
 631 .Va kevent64_s
 632 structure.
 633 .It Bq Er EBADF
 634 The specified descriptor is invalid.
 635 .It Bq Er EINTR
 636 A signal was delivered before the timeout expired and before any
 637 events were placed on the kqueue for return.
 638 .It Bq Er EINVAL
 639 The specified time limit or filter is invalid.
 640 .It Bq Er ENOENT
 641 The event could not be found to be modified or deleted.
 642 .It Bq Er ENOMEM
 643 No memory was available to register the event.
 644 .It Bq Er ESRCH
 645 The specified process to attach to does not exist.
 646 .El
 647 .Sh SEE ALSO
 648 .Xr aio_error 2 ,
 649 .Xr aio_read 2 ,
 650 .Xr aio_return 2 ,
 651 .Xr read 2 ,
 652 .Xr select 2 ,
 653 .Xr sigaction 2 ,
 654 .Xr write 2 ,
 655 .Xr signal 3
 656 .Sh HISTORY
 657 The
 658 .Fn kqueue
 659 and
 660 .Fn kevent
 661 system calls first appeared in
 662 .Fx 4.1 .
 663 .Sh AUTHORS
 664 The
 665 .Fn kqueue
 666 system and this manual page were written by
 667 .An Jonathan Lemon Aq jlemon@FreeBSD.org .
 668 .Sh BUGS
 669 Not all filesystem types support kqueue-style notifications.
 670 And even some that do, like some remote filesystems, may only
 671 support a subset of the notification semantics described
 672 here.