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 September 15, 2009 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.Pp 255.Bl -tag -width 2n 256.It Sockets 257Sockets which have previously been passed to 258.Fn listen 259return when there is an incoming connection pending. 260.Va data 261contains the size of the listen backlog. 262.Pp 263Other socket descriptors return when there is data to be read, 264subject to the 265.Dv SO_RCVLOWAT 266value of the socket buffer. 267This may be overridden with a per-filter low water mark at the 268time the filter is added by setting the 269NOTE_LOWAT 270flag in 271.Va fflags , 272and specifying the new low water mark in 273.Va data . 274On return, 275.Va data 276contains the number of bytes of protocol data available to read. 277.Pp 278If the read direction of the socket has shutdown, then the filter 279also sets EV_EOF in 280.Va flags , 281and returns the socket error (if any) in 282.Va fflags . 283It is possible for EOF to be returned (indicating the connection is gone) 284while there is still data pending in the socket buffer. 285.It Vnodes 286Returns when the file pointer is not at the end of file. 287.Va data 288contains the offset from current position to end of file, 289and may be negative. 290.It "Fifos, Pipes" 291Returns when the there is data to read; 292.Va data 293contains the number of bytes available. 294.Pp 295When the last writer disconnects, the filter will set EV_EOF in 296.Va flags . 297This may be cleared by passing in EV_CLEAR, at which point the 298filter will resume waiting for data to become available before 299returning. 300.It "BPF devices" 301Returns when the BPF buffer is full, the BPF timeout has expired, or 302when the BPF has 303.Dq immediate mode 304enabled and there is any data to read; 305.Va data 306contains the number of bytes available. 307.El 308.It EVFILT_WRITE 309Takes a descriptor as the identifier, and returns whenever 310it is possible to write to the descriptor. 311For sockets, pipes 312and fifos, 313.Va data 314will contain the amount of space remaining in the write buffer. 315The filter will set EV_EOF when the reader disconnects, and for 316the fifo case, this may be cleared by use of EV_CLEAR. 317Note that this filter is not supported for vnodes or BPF devices. 318.Pp 319For sockets, the low water mark and socket error handling is 320identical to the EVFILT_READ case. 321.It EVFILT_AIO 322The sigevent portion of the AIO request is filled in, with 323.Va sigev_notify_kqueue 324containing the descriptor of the kqueue that the event should 325be attached to, 326.Va sigev_value 327containing the udata value, and 328.Va sigev_notify 329set to SIGEV_KEVENT. 330When the 331.Fn aio_* 332system call is made, the event will be registered 333with the specified kqueue, and the 334.Va ident 335argument set to the 336.Fa struct aiocb 337returned by the 338.Fn aio_* 339system call. 340The filter returns under the same conditions as aio_error. 341.It EVFILT_VNODE 342Takes a file descriptor as the identifier and the events to watch for in 343.Va fflags , 344and returns when one or more of the requested events occurs on the descriptor. 345The events to monitor are: 346.Bl -tag -width XXNOTE_RENAME 347.It NOTE_DELETE 348The 349.Fn unlink 350system call 351was called on the file referenced by the descriptor. 352.It NOTE_WRITE 353A write occurred on the file referenced by the descriptor. 354.It NOTE_EXTEND 355The file referenced by the descriptor was extended. 356.It NOTE_ATTRIB 357The file referenced by the descriptor had its attributes changed. 358.It NOTE_LINK 359The link count on the file changed. 360.It NOTE_RENAME 361The file referenced by the descriptor was renamed. 362.It NOTE_REVOKE 363Access to the file was revoked via 364.Xr revoke 2 365or the underlying file system was unmounted. 366.El 367.Pp 368On return, 369.Va fflags 370contains the events which triggered the filter. 371.It EVFILT_PROC 372Takes the process ID to monitor as the identifier and the events to watch for 373in 374.Va fflags , 375and returns when the process performs one or more of the requested events. 376If a process can normally see another process, it can attach an event to it. 377The events to monitor are: 378.Bl -tag -width XXNOTE_TRACKERR 379.It NOTE_EXIT 380The process has exited. 381The exit status will be stored in 382.Va data . 383.It NOTE_FORK 384The process has called 385.Fn fork . 386.It NOTE_EXEC 387The process has executed a new process via 388.Xr execve 2 389or similar call. 390.It NOTE_TRACK 391Follow a process across 392.Fn fork 393calls. 394The parent process will return with NOTE_TRACK set in the 395.Va fflags 396field, while the child process will return with NOTE_CHILD set in 397.Va fflags 398and the parent PID in 399.Va data . 400.It NOTE_TRACKERR 401This flag is returned if the system was unable to attach an event to 402the child process, usually due to resource limitations. 403.El 404.Pp 405On return, 406.Va fflags 407contains the events which triggered the filter. 408.It EVFILT_SIGNAL 409Takes the signal number to monitor as the identifier and returns 410when the given signal is delivered to the process. 411This coexists with the 412.Fn signal 413and 414.Fn sigaction 415facilities, and has a lower precedence. 416The filter will record 417all attempts to deliver a signal to a process, even if the signal has 418been marked as SIG_IGN. 419Event notification happens after normal 420signal delivery processing. 421.Va data 422returns the number of times the signal has occurred since the last call to 423.Fn kevent . 424This filter automatically sets the EV_CLEAR flag internally. 425.It EVFILT_TIMER 426Establishes an arbitrary timer identified by 427.Va ident . 428When adding a timer, 429.Va data 430specifies the timeout period in milliseconds. 431The timer will be periodic unless EV_ONESHOT is specified. 432On return, 433.Va data 434contains the number of times the timeout has expired since the last call to 435.Fn kevent . 436This filter automatically sets the EV_CLEAR flag internally. 437There is a system wide limit on the number of timers 438which is controlled by the 439.Va kern.kq_calloutmax 440sysctl. 441.Pp 442On return, 443.Va fflags 444contains the events which triggered the filter. 445.It Dv EVFILT_USER 446Establishes a user event identified by 447.Va ident 448which is not assosicated with any kernel mechanism but is triggered by 449user level code. 450The lower 24 bits of the 451.Va fflags 452may be used for user defined flags and manipulated using the following: 453.Bl -tag -width XXNOTE_FFLAGSMASK 454.It Dv NOTE_FFNOP 455Ignore the input 456.Va fflags . 457.It Dv NOTE_FFAND 458Bitwise AND 459.Va fflags . 460.It Dv NOTE_FFOR 461Bitwise OR 462.Va fflags . 463.It Dv NOTE_COPY 464Copy 465.Va fflags . 466.It Dv NOTE_FFCTRLMASK 467Control mask for 468.Va fflags . 469.It Dv NOTE_FFLAGSMASK 470User defined flag mask for 471.Va fflags . 472.El 473.Pp 474A user event is triggered for output with the following: 475.Bl -tag -width XXNOTE_FFLAGSMASK 476.It Dv NOTE_TRIGGER 477Cause the event to be triggered. 478.El 479.Pp 480On return, 481.Va fflags 482contains the users defined flags in the lower 24 bits. 483.El 484.Sh RETURN VALUES 485The 486.Fn kqueue 487system call 488creates a new kernel event queue and returns a file descriptor. 489If there was an error creating the kernel event queue, a value of -1 is 490returned and errno set. 491.Pp 492The 493.Fn kevent 494system call 495returns the number of events placed in the 496.Fa eventlist , 497up to the value given by 498.Fa nevents . 499If an error occurs while processing an element of the 500.Fa changelist 501and there is enough room in the 502.Fa eventlist , 503then the event will be placed in the 504.Fa eventlist 505with 506.Dv EV_ERROR 507set in 508.Va flags 509and the system error in 510.Va data . 511Otherwise, 512.Dv -1 513will be returned, and 514.Dv errno 515will be set to indicate the error condition. 516If the time limit expires, then 517.Fn kevent 518returns 0. 519.Sh ERRORS 520The 521.Fn kqueue 522system call fails if: 523.Bl -tag -width Er 524.It Bq Er ENOMEM 525The kernel failed to allocate enough memory for the kernel queue. 526.It Bq Er EMFILE 527The per-process descriptor table is full. 528.It Bq Er ENFILE 529The system file table is full. 530.El 531.Pp 532The 533.Fn kevent 534system call fails if: 535.Bl -tag -width Er 536.It Bq Er EACCES 537The process does not have permission to register a filter. 538.It Bq Er EFAULT 539There was an error reading or writing the 540.Va kevent 541structure. 542.It Bq Er EBADF 543The specified descriptor is invalid. 544.It Bq Er EINTR 545A signal was delivered before the timeout expired and before any 546events were placed on the kqueue for return. 547.It Bq Er EINVAL 548The specified time limit or filter is invalid. 549.It Bq Er ENOENT 550The event could not be found to be modified or deleted. 551.It Bq Er ENOMEM 552No memory was available to register the event 553or, in the special case of a timer, the maximum number of 554timers has been exceeded. 555This maximum is configurable via the 556.Va kern.kq_calloutmax 557sysctl. 558.It Bq Er ESRCH 559The specified process to attach to does not exist. 560.El 561.Sh SEE ALSO 562.Xr aio_error 2 , 563.Xr aio_read 2 , 564.Xr aio_return 2 , 565.Xr poll 2 , 566.Xr read 2 , 567.Xr select 2 , 568.Xr sigaction 2 , 569.Xr write 2 , 570.Xr signal 3 571.Sh HISTORY 572The 573.Fn kqueue 574and 575.Fn kevent 576system calls first appeared in 577.Fx 4.1 . 578.Sh AUTHORS 579The 580.Fn kqueue 581system and this manual page were written by 582.An Jonathan Lemon Aq [email protected] . 583.Sh BUGS 584The 585.Fa timeout 586value is limited to 24 hours; longer timeouts will be silently 587reinterpreted as 24 hours. 588