1.\" Copyright (c) 2000 Jonathan Lemon 2.\" All rights reserved. 3.\" 4.\" Redistribution and use in source and binary forms, with or without 5.\" modification, are permitted provided that the following conditions 6.\" are met: 7.\" 1. Redistributions of source code must retain the above copyright 8.\" notice, this list of conditions and the following disclaimer. 9.\" 2. Redistributions in binary form must reproduce the above copyright 10.\" notice, this list of conditions and the following disclaimer in the 11.\" documentation and/or other materials provided with the distribution. 12.\" 13.\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND 14.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 15.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 16.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE 17.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL 18.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS 19.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) 20.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT 21.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY 22.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF 23.\" SUCH DAMAGE. 24.\" 25.\" $FreeBSD$ 26.\" 27.Dd February 15, 2012 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 46The 47.Fn kqueue 48system call 49provides a generic method of notifying the user when an event 50happens or a condition holds, based on the results of small 51pieces of kernel code termed filters. 52A kevent is identified by the (ident, filter) pair; there may only 53be one unique kevent per kqueue. 54.Pp 55The filter is executed upon the initial registration of a kevent 56in order to detect whether a preexisting condition is present, and is also 57executed whenever an event is passed to the filter for evaluation. 58If the filter determines that the condition should be reported, 59then the kevent is placed on the kqueue for the user to retrieve. 60.Pp 61The filter is also run when the user attempts to retrieve the kevent 62from the kqueue. 63If the filter indicates that the condition that triggered 64the event no longer holds, the kevent is removed from the kqueue and 65is not returned. 66.Pp 67Multiple events which trigger the filter do not result in multiple 68kevents being placed on the kqueue; instead, the filter will aggregate 69the events into a single struct kevent. 70Calling 71.Fn close 72on a file descriptor will remove any kevents that reference the descriptor. 73.Pp 74The 75.Fn kqueue 76system call 77creates a new kernel event queue and returns a descriptor. 78The queue is not inherited by a child created with 79.Xr fork 2 . 80However, if 81.Xr rfork 2 82is called without the 83.Dv RFFDG 84flag, then the descriptor table is shared, 85which will allow sharing of the kqueue between two processes. 86.Pp 87The 88.Fn kevent 89system call 90is used to register events with the queue, and return any pending 91events to the user. 92The 93.Fa changelist 94argument 95is a pointer to an array of 96.Va kevent 97structures, as defined in 98.In sys/event.h . 99All changes contained in the 100.Fa changelist 101are applied before any pending events are read from the queue. 102The 103.Fa nchanges 104argument 105gives the size of 106.Fa changelist . 107The 108.Fa eventlist 109argument 110is a pointer to an array of kevent structures. 111The 112.Fa nevents 113argument 114determines the size of 115.Fa eventlist . 116When 117.Fa nevents 118is zero, 119.Fn kevent 120will return immediately even if there is a 121.Fa timeout 122specified unlike 123.Xr select 2 . 124If 125.Fa timeout 126is a non-NULL pointer, it specifies a maximum interval to wait 127for an event, which will be interpreted as a struct timespec. 128If 129.Fa timeout 130is a NULL pointer, 131.Fn kevent 132waits indefinitely. 133To effect a poll, the 134.Fa timeout 135argument should be non-NULL, pointing to a zero-valued 136.Va timespec 137structure. 138The same array may be used for the 139.Fa changelist 140and 141.Fa eventlist . 142.Pp 143The 144.Fn EV_SET 145macro is provided for ease of initializing a 146kevent structure. 147.Pp 148The 149.Va kevent 150structure is defined as: 151.Bd -literal 152struct kevent { 153 uintptr_t ident; /* identifier for this event */ 154 short filter; /* filter for event */ 155 u_short flags; /* action flags for kqueue */ 156 u_int fflags; /* filter flag value */ 157 intptr_t data; /* filter data value */ 158 void *udata; /* opaque user data identifier */ 159}; 160.Ed 161.Pp 162The fields of 163.Fa struct kevent 164are: 165.Bl -tag -width XXXfilter 166.It ident 167Value used to identify this event. 168The exact interpretation is determined by the attached filter, 169but often is a file descriptor. 170.It filter 171Identifies the kernel filter used to process this event. 172The pre-defined 173system filters are described below. 174.It flags 175Actions to perform on the event. 176.It fflags 177Filter-specific flags. 178.It data 179Filter-specific data value. 180.It udata 181Opaque user-defined value passed through the kernel unchanged. 182.El 183.Pp 184The 185.Va flags 186field can contain the following values: 187.Bl -tag -width XXXEV_ONESHOT 188.It EV_ADD 189Adds the event to the kqueue. 190Re-adding an existing event 191will modify the parameters of the original event, and not result 192in a duplicate entry. 193Adding an event automatically enables it, 194unless overridden by the EV_DISABLE flag. 195.It EV_ENABLE 196Permit 197.Fn kevent 198to return the event if it is triggered. 199.It EV_DISABLE 200Disable the event so 201.Fn kevent 202will not return it. 203The filter itself is not disabled. 204.It EV_DISPATCH 205Disable the event source immediately after delivery of an event. 206See 207.Dv EV_DISABLE 208above. 209.It EV_DELETE 210Removes the event from the kqueue. 211Events which are attached to 212file descriptors are automatically deleted on the last close of 213the descriptor. 214.It EV_RECEIPT 215This flag is useful for making bulk changes to a kqueue without draining 216any pending events. 217When passed as input, it forces 218.Dv EV_ERROR 219to always be returned. 220When a filter is successfully added the 221.Va data 222field will be zero. 223.It EV_ONESHOT 224Causes the event to return only the first occurrence of the filter 225being triggered. 226After the user retrieves the event from the kqueue, 227it is deleted. 228.It EV_CLEAR 229After the event is retrieved by the user, its state is reset. 230This is useful for filters which report state transitions 231instead of the current state. 232Note that some filters may automatically 233set this flag internally. 234.It EV_EOF 235Filters may set this flag to indicate filter-specific EOF condition. 236.It EV_ERROR 237See 238.Sx RETURN VALUES 239below. 240.El 241.Pp 242The predefined system filters are listed below. 243Arguments may be passed to and from the filter via the 244.Va fflags 245and 246.Va data 247fields in the kevent structure. 248.Bl -tag -width EVFILT_SIGNAL 249.It EVFILT_READ 250Takes a descriptor as the identifier, and returns whenever 251there is data available to read. 252The behavior of the filter is slightly different depending 253on the descriptor type. 254.Bl -tag -width 2n 255.It Sockets 256Sockets which have previously been passed to 257.Fn listen 258return when there is an incoming connection pending. 259.Va data 260contains the size of the listen backlog. 261.Pp 262Other socket descriptors return when there is data to be read, 263subject to the 264.Dv SO_RCVLOWAT 265value of the socket buffer. 266This may be overridden with a per-filter low water mark at the 267time the filter is added by setting the 268NOTE_LOWAT 269flag in 270.Va fflags , 271and specifying the new low water mark in 272.Va data . 273On return, 274.Va data 275contains the number of bytes of protocol data available to read. 276.Pp 277If the read direction of the socket has shutdown, then the filter 278also sets EV_EOF in 279.Va flags , 280and returns the socket error (if any) in 281.Va fflags . 282It is possible for EOF to be returned (indicating the connection is gone) 283while there is still data pending in the socket buffer. 284.It Vnodes 285Returns when the file pointer is not at the end of file. 286.Va data 287contains the offset from current position to end of file, 288and may be negative. 289.It "Fifos, Pipes" 290Returns when the there is data to read; 291.Va data 292contains the number of bytes available. 293.Pp 294When the last writer disconnects, the filter will set EV_EOF in 295.Va flags . 296This may be cleared by passing in EV_CLEAR, at which point the 297filter will resume waiting for data to become available before 298returning. 299.It "BPF devices" 300Returns when the BPF buffer is full, the BPF timeout has expired, or 301when the BPF has 302.Dq immediate mode 303enabled and there is any data to read; 304.Va data 305contains the number of bytes available. 306.El 307.It EVFILT_WRITE 308Takes a descriptor as the identifier, and returns whenever 309it is possible to write to the descriptor. 310For sockets, pipes 311and fifos, 312.Va data 313will contain the amount of space remaining in the write buffer. 314The filter will set EV_EOF when the reader disconnects, and for 315the fifo case, this may be cleared by use of EV_CLEAR. 316Note that this filter is not supported for vnodes or BPF devices. 317.Pp 318For sockets, the low water mark and socket error handling is 319identical to the EVFILT_READ case. 320.It EVFILT_AIO 321The sigevent portion of the AIO request is filled in, with 322.Va sigev_notify_kqueue 323containing the descriptor of the kqueue that the event should 324be attached to, 325.Va sigev_notify_kevent_flags 326containing the kevent flags which should be EV_ONESHOT, EV_CLEAR or 327EV_DISPATCH, 328.Va sigev_value 329containing the udata value, and 330.Va sigev_notify 331set to SIGEV_KEVENT. 332When the 333.Fn aio_* 334system call is made, the event will be registered 335with the specified kqueue, and the 336.Va ident 337argument set to the 338.Fa struct aiocb 339returned by the 340.Fn aio_* 341system call. 342The filter returns under the same conditions as aio_error. 343.It EVFILT_VNODE 344Takes a file descriptor as the identifier and the events to watch for in 345.Va fflags , 346and returns when one or more of the requested events occurs on the descriptor. 347The events to monitor are: 348.Bl -tag -width XXNOTE_RENAME 349.It NOTE_DELETE 350The 351.Fn unlink 352system call 353was called on the file referenced by the descriptor. 354.It NOTE_WRITE 355A write occurred on the file referenced by the descriptor. 356.It NOTE_EXTEND 357The file referenced by the descriptor was extended. 358.It NOTE_ATTRIB 359The file referenced by the descriptor had its attributes changed. 360.It NOTE_LINK 361The link count on the file changed. 362.It NOTE_RENAME 363The file referenced by the descriptor was renamed. 364.It NOTE_REVOKE 365Access to the file was revoked via 366.Xr revoke 2 367or the underlying file system was unmounted. 368.El 369.Pp 370On return, 371.Va fflags 372contains the events which triggered the filter. 373.It EVFILT_PROC 374Takes the process ID to monitor as the identifier and the events to watch for 375in 376.Va fflags , 377and returns when the process performs one or more of the requested events. 378If a process can normally see another process, it can attach an event to it. 379The events to monitor are: 380.Bl -tag -width XXNOTE_TRACKERR 381.It NOTE_EXIT 382The process has exited. 383The exit status will be stored in 384.Va data . 385.It NOTE_FORK 386The process has called 387.Fn fork . 388.It NOTE_EXEC 389The process has executed a new process via 390.Xr execve 2 391or similar call. 392.It NOTE_TRACK 393Follow a process across 394.Fn fork 395calls. 396The parent process will return with NOTE_TRACK set in the 397.Va fflags 398field, while the child process will return with NOTE_CHILD set in 399.Va fflags 400and the parent PID in 401.Va data . 402.It NOTE_TRACKERR 403This flag is returned if the system was unable to attach an event to 404the child process, usually due to resource limitations. 405.El 406.Pp 407On return, 408.Va fflags 409contains the events which triggered the filter. 410.It EVFILT_SIGNAL 411Takes the signal number to monitor as the identifier and returns 412when the given signal is delivered to the process. 413This coexists with the 414.Fn signal 415and 416.Fn sigaction 417facilities, and has a lower precedence. 418The filter will record 419all attempts to deliver a signal to a process, even if the signal has 420been marked as SIG_IGN. 421Event notification happens after normal 422signal delivery processing. 423.Va data 424returns the number of times the signal has occurred since the last call to 425.Fn kevent . 426This filter automatically sets the EV_CLEAR flag internally. 427.It EVFILT_TIMER 428Establishes an arbitrary timer identified by 429.Va ident . 430When adding a timer, 431.Va data 432specifies the timeout period in milliseconds. 433The timer will be periodic unless EV_ONESHOT is specified. 434On return, 435.Va data 436contains the number of times the timeout has expired since the last call to 437.Fn kevent . 438This filter automatically sets the EV_CLEAR flag internally. 439There is a system wide limit on the number of timers 440which is controlled by the 441.Va kern.kq_calloutmax 442sysctl. 443.Pp 444On return, 445.Va fflags 446contains the events which triggered the filter. 447.It Dv EVFILT_USER 448Establishes a user event identified by 449.Va ident 450which is not associated with any kernel mechanism but is triggered by 451user level code. 452The lower 24 bits of the 453.Va fflags 454may be used for user defined flags and manipulated using the following: 455.Bl -tag -width XXNOTE_FFLAGSMASK 456.It Dv NOTE_FFNOP 457Ignore the input 458.Va fflags . 459.It Dv NOTE_FFAND 460Bitwise AND 461.Va fflags . 462.It Dv NOTE_FFOR 463Bitwise OR 464.Va fflags . 465.It Dv NOTE_FFCOPY 466Copy 467.Va fflags . 468.It Dv NOTE_FFCTRLMASK 469Control mask for 470.Va fflags . 471.It Dv NOTE_FFLAGSMASK 472User defined flag mask for 473.Va fflags . 474.El 475.Pp 476A user event is triggered for output with the following: 477.Bl -tag -width XXNOTE_FFLAGSMASK 478.It Dv NOTE_TRIGGER 479Cause the event to be triggered. 480.El 481.Pp 482On return, 483.Va fflags 484contains the users defined flags in the lower 24 bits. 485.El 486.Sh RETURN VALUES 487The 488.Fn kqueue 489system call 490creates a new kernel event queue and returns a file descriptor. 491If there was an error creating the kernel event queue, a value of -1 is 492returned and errno set. 493.Pp 494The 495.Fn kevent 496system call 497returns the number of events placed in the 498.Fa eventlist , 499up to the value given by 500.Fa nevents . 501If an error occurs while processing an element of the 502.Fa changelist 503and there is enough room in the 504.Fa eventlist , 505then the event will be placed in the 506.Fa eventlist 507with 508.Dv EV_ERROR 509set in 510.Va flags 511and the system error in 512.Va data . 513Otherwise, 514.Dv -1 515will be returned, and 516.Dv errno 517will be set to indicate the error condition. 518If the time limit expires, then 519.Fn kevent 520returns 0. 521.Sh ERRORS 522The 523.Fn kqueue 524system call fails if: 525.Bl -tag -width Er 526.It Bq Er ENOMEM 527The kernel failed to allocate enough memory for the kernel queue. 528.It Bq Er EMFILE 529The per-process descriptor table is full. 530.It Bq Er ENFILE 531The system file table is full. 532.El 533.Pp 534The 535.Fn kevent 536system call fails if: 537.Bl -tag -width Er 538.It Bq Er EACCES 539The process does not have permission to register a filter. 540.It Bq Er EFAULT 541There was an error reading or writing the 542.Va kevent 543structure. 544.It Bq Er EBADF 545The specified descriptor is invalid. 546.It Bq Er EINTR 547A signal was delivered before the timeout expired and before any 548events were placed on the kqueue for return. 549.It Bq Er EINVAL 550The specified time limit or filter is invalid. 551.It Bq Er ENOENT 552The event could not be found to be modified or deleted. 553.It Bq Er ENOMEM 554No memory was available to register the event 555or, in the special case of a timer, the maximum number of 556timers has been exceeded. 557This maximum is configurable via the 558.Va kern.kq_calloutmax 559sysctl. 560.It Bq Er ESRCH 561The specified process to attach to does not exist. 562.El 563.Sh SEE ALSO 564.Xr aio_error 2 , 565.Xr aio_read 2 , 566.Xr aio_return 2 , 567.Xr poll 2 , 568.Xr read 2 , 569.Xr select 2 , 570.Xr sigaction 2 , 571.Xr write 2 , 572.Xr signal 3 573.Sh HISTORY 574The 575.Fn kqueue 576and 577.Fn kevent 578system calls first appeared in 579.Fx 4.1 . 580.Sh AUTHORS 581The 582.Fn kqueue 583system and this manual page were written by 584.An Jonathan Lemon Aq [email protected] . 585.Sh BUGS 586The 587.Fa timeout 588value is limited to 24 hours; longer timeouts will be silently 589reinterpreted as 24 hours. 590