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 July 18, 2014 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 "Fa filter" 166.It Fa ident 167Value used to identify this event. 168The exact interpretation is determined by the attached filter, 169but often is a file descriptor. 170.It Fa filter 171Identifies the kernel filter used to process this event. 172The pre-defined 173system filters are described below. 174.It Fa flags 175Actions to perform on the event. 176.It Fa fflags 177Filter-specific flags. 178.It Fa data 179Filter-specific data value. 180.It Fa 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 EV_DISPATCH 188.It Dv 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 Dv EV_ENABLE 196Permit 197.Fn kevent 198to return the event if it is triggered. 199.It Dv EV_DISABLE 200Disable the event so 201.Fn kevent 202will not return it. 203The filter itself is not disabled. 204.It Dv EV_DISPATCH 205Disable the event source immediately after delivery of an event. 206See 207.Dv EV_DISABLE 208above. 209.It Dv 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 Dv 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 Dv 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 Dv 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 Dv EV_EOF 235Filters may set this flag to indicate filter-specific EOF condition. 236.It Dv 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 "Dv EVFILT_PROCDESC" 249.It Dv 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 268.Dv NOTE_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 279.Dv EV_EOF 280in 281.Va flags , 282and returns the socket error (if any) in 283.Va fflags . 284It is possible for EOF to be returned (indicating the connection is gone) 285while there is still data pending in the socket buffer. 286.It Vnodes 287Returns when the file pointer is not at the end of file. 288.Va data 289contains the offset from current position to end of file, 290and may be negative. 291.It "Fifos, Pipes" 292Returns when the there is data to read; 293.Va data 294contains the number of bytes available. 295.Pp 296When the last writer disconnects, the filter will set 297.Dv EV_EOF 298in 299.Va flags . 300This may be cleared by passing in 301.Dv EV_CLEAR , 302at which point the 303filter will resume waiting for data to become available before 304returning. 305.It "BPF devices" 306Returns when the BPF buffer is full, the BPF timeout has expired, or 307when the BPF has 308.Dq immediate mode 309enabled and there is any data to read; 310.Va data 311contains the number of bytes available. 312.El 313.It Dv EVFILT_WRITE 314Takes a descriptor as the identifier, and returns whenever 315it is possible to write to the descriptor. 316For sockets, pipes 317and fifos, 318.Va data 319will contain the amount of space remaining in the write buffer. 320The filter will set EV_EOF when the reader disconnects, and for 321the fifo case, this may be cleared by use of 322.Dv EV_CLEAR . 323Note that this filter is not supported for vnodes or BPF devices. 324.Pp 325For sockets, the low water mark and socket error handling is 326identical to the 327.Dv EVFILT_READ 328case. 329.It Dv EVFILT_AIO 330The sigevent portion of the AIO request is filled in, with 331.Va sigev_notify_kqueue 332containing the descriptor of the kqueue that the event should 333be attached to, 334.Va sigev_notify_kevent_flags 335containing the kevent flags which should be 336.Dv EV_ONESHOT , 337.Dv EV_CLEAR 338or 339.Dv EV_DISPATCH , 340.Va sigev_value 341containing the udata value, and 342.Va sigev_notify 343set to 344.Dv SIGEV_KEVENT . 345When the 346.Fn aio_* 347system call is made, the event will be registered 348with the specified kqueue, and the 349.Va ident 350argument set to the 351.Fa struct aiocb 352returned by the 353.Fn aio_* 354system call. 355The filter returns under the same conditions as 356.Fn aio_error . 357.It Dv EVFILT_VNODE 358Takes a file descriptor as the identifier and the events to watch for in 359.Va fflags , 360and returns when one or more of the requested events occurs on the descriptor. 361The events to monitor are: 362.Bl -tag -width "Dv NOTE_RENAME" 363.It Dv NOTE_DELETE 364The 365.Fn unlink 366system call 367was called on the file referenced by the descriptor. 368.It Dv NOTE_WRITE 369A write occurred on the file referenced by the descriptor. 370.It Dv NOTE_EXTEND 371The file referenced by the descriptor was extended. 372.It Dv NOTE_ATTRIB 373The file referenced by the descriptor had its attributes changed. 374.It Dv NOTE_LINK 375The link count on the file changed. 376.It Dv NOTE_RENAME 377The file referenced by the descriptor was renamed. 378.It Dv NOTE_REVOKE 379Access to the file was revoked via 380.Xr revoke 2 381or the underlying file system was unmounted. 382.El 383.Pp 384On return, 385.Va fflags 386contains the events which triggered the filter. 387.It Dv EVFILT_PROC 388Takes the process ID to monitor as the identifier and the events to watch for 389in 390.Va fflags , 391and returns when the process performs one or more of the requested events. 392If a process can normally see another process, it can attach an event to it. 393The events to monitor are: 394.Bl -tag -width "Dv NOTE_TRACKERR" 395.It Dv NOTE_EXIT 396The process has exited. 397The exit status will be stored in 398.Va data . 399.It Dv NOTE_FORK 400The process has called 401.Fn fork . 402.It Dv NOTE_EXEC 403The process has executed a new process via 404.Xr execve 2 405or a similar call. 406.It Dv NOTE_TRACK 407Follow a process across 408.Fn fork 409calls. 410The parent process registers a new kevent to monitor the child process 411using the same 412.Va fflags 413as the original event. 414The child process will signal an event with 415.Dv NOTE_CHILD 416set in 417.Va fflags 418and the parent PID in 419.Va data . 420.Pp 421If the parent process fails to register a new kevent 422.Pq usually due to resource limitations , 423it will signal an event with 424.Dv NOTE_TRACKERR 425set in 426.Va fflags , 427and the child process will not signal a 428.Dv NOTE_CHILD 429event. 430.El 431.Pp 432On return, 433.Va fflags 434contains the events which triggered the filter. 435.It Dv EVFILT_PROCDESC 436Takes the process descriptor created by 437.Xr pdfork 2 438to monitor as the identifier and the events to watch for in 439.Va fflags , 440and returns when the associated process performs one or more of the 441requested events. 442The events to monitor are: 443.Bl -tag -width "Dv NOTE_EXIT" 444.It Dv NOTE_EXIT 445The process has exited. 446The exit status will be stored in 447.Va data . 448.El 449.Pp 450On return, 451.Va fflags 452contains the events which triggered the filter. 453.It Dv EVFILT_SIGNAL 454Takes the signal number to monitor as the identifier and returns 455when the given signal is delivered to the process. 456This coexists with the 457.Fn signal 458and 459.Fn sigaction 460facilities, and has a lower precedence. 461The filter will record 462all attempts to deliver a signal to a process, even if the signal has 463been marked as 464.Dv SIG_IGN , 465except for the 466.Dv SIGCHLD 467signal, which, if ignored, won't be recorded by the filter. 468Event notification happens after normal 469signal delivery processing. 470.Va data 471returns the number of times the signal has occurred since the last call to 472.Fn kevent . 473This filter automatically sets the 474.Dv EV_CLEAR 475flag internally. 476.It Dv EVFILT_TIMER 477Establishes an arbitrary timer identified by 478.Va ident . 479When adding a timer, 480.Va data 481specifies the timeout period. 482The timer will be periodic unless 483.Dv EV_ONESHOT 484is specified. 485On return, 486.Va data 487contains the number of times the timeout has expired since the last call to 488.Fn kevent . 489This filter automatically sets the EV_CLEAR flag internally. 490There is a system wide limit on the number of timers 491which is controlled by the 492.Va kern.kq_calloutmax 493sysctl. 494.Bl -tag -width "Dv NOTE_USECONDS" 495.It Dv NOTE_SECONDS 496.Va data 497is in seconds. 498.It Dv NOTE_MSECONDS 499.Va data 500is in milliseconds. 501.It Dv NOTE_USECONDS 502.Va data 503is in microseconds. 504.It Dv NOTE_NSECONDS 505.Va data 506is in nanoseconds. 507.El 508.Pp 509If 510.Va fflags 511is not set, the default is milliseconds. On return, 512.Va fflags 513contains the events which triggered the filter. 514.It Dv EVFILT_USER 515Establishes a user event identified by 516.Va ident 517which is not associated with any kernel mechanism but is triggered by 518user level code. 519The lower 24 bits of the 520.Va fflags 521may be used for user defined flags and manipulated using the following: 522.Bl -tag -width "Dv NOTE_FFLAGSMASK" 523.It Dv NOTE_FFNOP 524Ignore the input 525.Va fflags . 526.It Dv NOTE_FFAND 527Bitwise AND 528.Va fflags . 529.It Dv NOTE_FFOR 530Bitwise OR 531.Va fflags . 532.It Dv NOTE_FFCOPY 533Copy 534.Va fflags . 535.It Dv NOTE_FFCTRLMASK 536Control mask for 537.Va fflags . 538.It Dv NOTE_FFLAGSMASK 539User defined flag mask for 540.Va fflags . 541.El 542.Pp 543A user event is triggered for output with the following: 544.Bl -tag -width "Dv NOTE_FFLAGSMASK" 545.It Dv NOTE_TRIGGER 546Cause the event to be triggered. 547.El 548.Pp 549On return, 550.Va fflags 551contains the users defined flags in the lower 24 bits. 552.El 553.Sh RETURN VALUES 554The 555.Fn kqueue 556system call 557creates a new kernel event queue and returns a file descriptor. 558If there was an error creating the kernel event queue, a value of -1 is 559returned and errno set. 560.Pp 561The 562.Fn kevent 563system call 564returns the number of events placed in the 565.Fa eventlist , 566up to the value given by 567.Fa nevents . 568If an error occurs while processing an element of the 569.Fa changelist 570and there is enough room in the 571.Fa eventlist , 572then the event will be placed in the 573.Fa eventlist 574with 575.Dv EV_ERROR 576set in 577.Va flags 578and the system error in 579.Va data . 580Otherwise, 581.Dv -1 582will be returned, and 583.Dv errno 584will be set to indicate the error condition. 585If the time limit expires, then 586.Fn kevent 587returns 0. 588.Sh ERRORS 589The 590.Fn kqueue 591system call fails if: 592.Bl -tag -width Er 593.It Bq Er ENOMEM 594The kernel failed to allocate enough memory for the kernel queue. 595.It Bq Er ENOMEM 596The 597.Dv RLIMIT_KQUEUES 598rlimit 599(see 600.Xr getrlimit 2 ) 601for the current user would be exceeded. 602.It Bq Er EMFILE 603The per-process descriptor table is full. 604.It Bq Er ENFILE 605The system file table is full. 606.El 607.Pp 608The 609.Fn kevent 610system call fails if: 611.Bl -tag -width Er 612.It Bq Er EACCES 613The process does not have permission to register a filter. 614.It Bq Er EFAULT 615There was an error reading or writing the 616.Va kevent 617structure. 618.It Bq Er EBADF 619The specified descriptor is invalid. 620.It Bq Er EINTR 621A signal was delivered before the timeout expired and before any 622events were placed on the kqueue for return. 623.It Bq Er EINVAL 624The specified time limit or filter is invalid. 625.It Bq Er ENOENT 626The event could not be found to be modified or deleted. 627.It Bq Er ENOMEM 628No memory was available to register the event 629or, in the special case of a timer, the maximum number of 630timers has been exceeded. 631This maximum is configurable via the 632.Va kern.kq_calloutmax 633sysctl. 634.It Bq Er ESRCH 635The specified process to attach to does not exist. 636.El 637.Sh SEE ALSO 638.Xr aio_error 2 , 639.Xr aio_read 2 , 640.Xr aio_return 2 , 641.Xr poll 2 , 642.Xr read 2 , 643.Xr select 2 , 644.Xr sigaction 2 , 645.Xr write 2 , 646.Xr signal 3 647.Sh HISTORY 648The 649.Fn kqueue 650and 651.Fn kevent 652system calls first appeared in 653.Fx 4.1 . 654.Sh AUTHORS 655The 656.Fn kqueue 657system and this manual page were written by 658.An Jonathan Lemon Aq Mt [email protected] . 659.Sh BUGS 660The 661.Fa timeout 662value is limited to 24 hours; longer timeouts will be silently 663reinterpreted as 24 hours. 664