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 August 4, 2015 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.Pp 292This behavior is different from 293.Xr poll 2 , 294where read events are triggered for regular files unconditionally. 295This event can be triggered unconditionally by setting the 296.Dv NOTE_FILE_POLL 297flag in 298.Va fflags . 299.It "Fifos, Pipes" 300Returns when the there is data to read; 301.Va data 302contains the number of bytes available. 303.Pp 304When the last writer disconnects, the filter will set 305.Dv EV_EOF 306in 307.Va flags . 308This may be cleared by passing in 309.Dv EV_CLEAR , 310at which point the 311filter will resume waiting for data to become available before 312returning. 313.It "BPF devices" 314Returns when the BPF buffer is full, the BPF timeout has expired, or 315when the BPF has 316.Dq immediate mode 317enabled and there is any data to read; 318.Va data 319contains the number of bytes available. 320.El 321.It Dv EVFILT_WRITE 322Takes a descriptor as the identifier, and returns whenever 323it is possible to write to the descriptor. 324For sockets, pipes 325and fifos, 326.Va data 327will contain the amount of space remaining in the write buffer. 328The filter will set EV_EOF when the reader disconnects, and for 329the fifo case, this may be cleared by use of 330.Dv EV_CLEAR . 331Note that this filter is not supported for vnodes or BPF devices. 332.Pp 333For sockets, the low water mark and socket error handling is 334identical to the 335.Dv EVFILT_READ 336case. 337.It Dv EVFILT_AIO 338The sigevent portion of the AIO request is filled in, with 339.Va sigev_notify_kqueue 340containing the descriptor of the kqueue that the event should 341be attached to, 342.Va sigev_notify_kevent_flags 343containing the kevent flags which should be 344.Dv EV_ONESHOT , 345.Dv EV_CLEAR 346or 347.Dv EV_DISPATCH , 348.Va sigev_value 349containing the udata value, and 350.Va sigev_notify 351set to 352.Dv SIGEV_KEVENT . 353When the 354.Fn aio_* 355system call is made, the event will be registered 356with the specified kqueue, and the 357.Va ident 358argument set to the 359.Fa struct aiocb 360returned by the 361.Fn aio_* 362system call. 363The filter returns under the same conditions as 364.Fn aio_error . 365.It Dv EVFILT_VNODE 366Takes a file descriptor as the identifier and the events to watch for in 367.Va fflags , 368and returns when one or more of the requested events occurs on the descriptor. 369The events to monitor are: 370.Bl -tag -width "Dv NOTE_RENAME" 371.It Dv NOTE_DELETE 372The 373.Fn unlink 374system call 375was called on the file referenced by the descriptor. 376.It Dv NOTE_WRITE 377A write occurred on the file referenced by the descriptor. 378.It Dv NOTE_EXTEND 379The file referenced by the descriptor was extended. 380.It Dv NOTE_ATTRIB 381The file referenced by the descriptor had its attributes changed. 382.It Dv NOTE_LINK 383The link count on the file changed. 384.It Dv NOTE_RENAME 385The file referenced by the descriptor was renamed. 386.It Dv NOTE_REVOKE 387Access to the file was revoked via 388.Xr revoke 2 389or the underlying file system was unmounted. 390.El 391.Pp 392On return, 393.Va fflags 394contains the events which triggered the filter. 395.It Dv EVFILT_PROC 396Takes the process ID to monitor as the identifier and the events to watch for 397in 398.Va fflags , 399and returns when the process performs one or more of the requested events. 400If a process can normally see another process, it can attach an event to it. 401The events to monitor are: 402.Bl -tag -width "Dv NOTE_TRACKERR" 403.It Dv NOTE_EXIT 404The process has exited. 405The exit status will be stored in 406.Va data . 407.It Dv NOTE_FORK 408The process has called 409.Fn fork . 410.It Dv NOTE_EXEC 411The process has executed a new process via 412.Xr execve 2 413or a similar call. 414.It Dv NOTE_TRACK 415Follow a process across 416.Fn fork 417calls. 418The parent process registers a new kevent to monitor the child process 419using the same 420.Va fflags 421as the original event. 422The child process will signal an event with 423.Dv NOTE_CHILD 424set in 425.Va fflags 426and the parent PID in 427.Va data . 428.Pp 429If the parent process fails to register a new kevent 430.Pq usually due to resource limitations , 431it will signal an event with 432.Dv NOTE_TRACKERR 433set in 434.Va fflags , 435and the child process will not signal a 436.Dv NOTE_CHILD 437event. 438.El 439.Pp 440On return, 441.Va fflags 442contains the events which triggered the filter. 443.It Dv EVFILT_PROCDESC 444Takes the process descriptor created by 445.Xr pdfork 2 446to monitor as the identifier and the events to watch for in 447.Va fflags , 448and returns when the associated process performs one or more of the 449requested events. 450The events to monitor are: 451.Bl -tag -width "Dv NOTE_EXIT" 452.It Dv NOTE_EXIT 453The process has exited. 454The exit status will be stored in 455.Va data . 456.El 457.Pp 458On return, 459.Va fflags 460contains the events which triggered the filter. 461.It Dv EVFILT_SIGNAL 462Takes the signal number to monitor as the identifier and returns 463when the given signal is delivered to the process. 464This coexists with the 465.Fn signal 466and 467.Fn sigaction 468facilities, and has a lower precedence. 469The filter will record 470all attempts to deliver a signal to a process, even if the signal has 471been marked as 472.Dv SIG_IGN , 473except for the 474.Dv SIGCHLD 475signal, which, if ignored, won't be recorded by the filter. 476Event notification happens after normal 477signal delivery processing. 478.Va data 479returns the number of times the signal has occurred since the last call to 480.Fn kevent . 481This filter automatically sets the 482.Dv EV_CLEAR 483flag internally. 484.It Dv EVFILT_TIMER 485Establishes an arbitrary timer identified by 486.Va ident . 487When adding a timer, 488.Va data 489specifies the timeout period. 490The timer will be periodic unless 491.Dv EV_ONESHOT 492is specified. 493On return, 494.Va data 495contains the number of times the timeout has expired since the last call to 496.Fn kevent . 497This filter automatically sets the EV_CLEAR flag internally. 498There is a system wide limit on the number of timers 499which is controlled by the 500.Va kern.kq_calloutmax 501sysctl. 502.Bl -tag -width "Dv NOTE_USECONDS" 503.It Dv NOTE_SECONDS 504.Va data 505is in seconds. 506.It Dv NOTE_MSECONDS 507.Va data 508is in milliseconds. 509.It Dv NOTE_USECONDS 510.Va data 511is in microseconds. 512.It Dv NOTE_NSECONDS 513.Va data 514is in nanoseconds. 515.El 516.Pp 517If 518.Va fflags 519is not set, the default is milliseconds. On return, 520.Va fflags 521contains the events which triggered the filter. 522.It Dv EVFILT_USER 523Establishes a user event identified by 524.Va ident 525which is not associated with any kernel mechanism but is triggered by 526user level code. 527The lower 24 bits of the 528.Va fflags 529may be used for user defined flags and manipulated using the following: 530.Bl -tag -width "Dv NOTE_FFLAGSMASK" 531.It Dv NOTE_FFNOP 532Ignore the input 533.Va fflags . 534.It Dv NOTE_FFAND 535Bitwise AND 536.Va fflags . 537.It Dv NOTE_FFOR 538Bitwise OR 539.Va fflags . 540.It Dv NOTE_FFCOPY 541Copy 542.Va fflags . 543.It Dv NOTE_FFCTRLMASK 544Control mask for 545.Va fflags . 546.It Dv NOTE_FFLAGSMASK 547User defined flag mask for 548.Va fflags . 549.El 550.Pp 551A user event is triggered for output with the following: 552.Bl -tag -width "Dv NOTE_FFLAGSMASK" 553.It Dv NOTE_TRIGGER 554Cause the event to be triggered. 555.El 556.Pp 557On return, 558.Va fflags 559contains the users defined flags in the lower 24 bits. 560.El 561.Sh CANCELLATION BEHAVIOUR 562If 563.Fa nevents 564is non-zero, i.e. the function is potentially blocking, the call 565is a cancellation point. 566Otherwise, i.e. if 567.Fa nevents 568is zero, the call is not cancellable. 569Cancellation can only occur before any changes are made to the kqueue, 570or when the call was blocked and no changes to the queue were requested. 571.Sh RETURN VALUES 572The 573.Fn kqueue 574system call 575creates a new kernel event queue and returns a file descriptor. 576If there was an error creating the kernel event queue, a value of -1 is 577returned and errno set. 578.Pp 579The 580.Fn kevent 581system call 582returns the number of events placed in the 583.Fa eventlist , 584up to the value given by 585.Fa nevents . 586If an error occurs while processing an element of the 587.Fa changelist 588and there is enough room in the 589.Fa eventlist , 590then the event will be placed in the 591.Fa eventlist 592with 593.Dv EV_ERROR 594set in 595.Va flags 596and the system error in 597.Va data . 598Otherwise, 599.Dv -1 600will be returned, and 601.Dv errno 602will be set to indicate the error condition. 603If the time limit expires, then 604.Fn kevent 605returns 0. 606.Sh ERRORS 607The 608.Fn kqueue 609system call fails if: 610.Bl -tag -width Er 611.It Bq Er ENOMEM 612The kernel failed to allocate enough memory for the kernel queue. 613.It Bq Er ENOMEM 614The 615.Dv RLIMIT_KQUEUES 616rlimit 617(see 618.Xr getrlimit 2 ) 619for the current user would be exceeded. 620.It Bq Er EMFILE 621The per-process descriptor table is full. 622.It Bq Er ENFILE 623The system file table is full. 624.El 625.Pp 626The 627.Fn kevent 628system call fails if: 629.Bl -tag -width Er 630.It Bq Er EACCES 631The process does not have permission to register a filter. 632.It Bq Er EFAULT 633There was an error reading or writing the 634.Va kevent 635structure. 636.It Bq Er EBADF 637The specified descriptor is invalid. 638.It Bq Er EINTR 639A signal was delivered before the timeout expired and before any 640events were placed on the kqueue for return. 641.It Bq Er EINTR 642A cancellation request was delivered to the thread, but not yet handled. 643.It Bq Er EINVAL 644The specified time limit or filter is invalid. 645.It Bq Er ENOENT 646The event could not be found to be modified or deleted. 647.It Bq Er ENOMEM 648No memory was available to register the event 649or, in the special case of a timer, the maximum number of 650timers has been exceeded. 651This maximum is configurable via the 652.Va kern.kq_calloutmax 653sysctl. 654.It Bq Er ESRCH 655The specified process to attach to does not exist. 656.El 657.Pp 658When 659.Fn kevent 660call fails with 661.Er EINTR 662error, all changes in the 663.Fa changelist 664have been applied. 665.Sh SEE ALSO 666.Xr aio_error 2 , 667.Xr aio_read 2 , 668.Xr aio_return 2 , 669.Xr poll 2 , 670.Xr read 2 , 671.Xr select 2 , 672.Xr sigaction 2 , 673.Xr write 2 , 674.Xr pthread_setcancelstate 3 , 675.Xr signal 3 676.Sh HISTORY 677The 678.Fn kqueue 679and 680.Fn kevent 681system calls first appeared in 682.Fx 4.1 . 683.Sh AUTHORS 684The 685.Fn kqueue 686system and this manual page were written by 687.An Jonathan Lemon Aq Mt [email protected] . 688.Sh BUGS 689The 690.Fa timeout 691value is limited to 24 hours; longer timeouts will be silently 692reinterpreted as 24 hours. 693