.\" $OpenBSD: kqueue.2,v 1.25 2012/04/12 14:25:57 deraadt Exp $ .\" .\" Copyright (c) 2000 Jonathan Lemon .\" All rights reserved. .\" .\" Redistribution and use in source and binary forms, with or without .\" modification, are permitted provided that the following conditions .\" are met: .\" 1. Redistributions of source code must retain the above copyright .\" notice, this list of conditions and the following disclaimer. .\" 2. Redistributions in binary form must reproduce the above copyright .\" notice, this list of conditions and the following disclaimer in the .\" documentation and/or other materials provided with the distribution. .\" .\" THIS SOFTWARE IS PROVIDED ``AS IS'' AND .\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE .\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE .\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE .\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL .\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS .\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) .\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT .\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY .\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF .\" SUCH DAMAGE. .\" .\" $FreeBSD: src/lib/libc/sys/kqueue.2,v 1.18 2001/02/14 08:48:35 guido Exp $ .\" .Dd $Mdocdate: April 12 2012 $ .Dt KQUEUE 2 .Os .Sh NAME .Nm kqueue , .Nm kevent .Nd kernel event notification mechanism .Sh SYNOPSIS .Fd #include .Fd #include .Fd #include .Ft int .Fn kqueue "void" .Ft int .Fn kevent "int kq" "const struct kevent *changelist" "int nchanges" "struct kevent *eventlist" "int nevents" "const struct timespec *timeout" .Fn EV_SET "&kev" ident filter flags fflags data udata .Sh DESCRIPTION .Fn kqueue provides a generic method of notifying the user when an event happens or a condition holds, based on the results of small pieces of kernel code termed .Dq filters . A kevent is identified by the (ident, filter) pair; there may only be one unique kevent per kqueue. .Pp The filter is executed upon the initial registration of a kevent in order to detect whether a preexisting condition is present, and is also executed whenever an event is passed to the filter for evaluation. If the filter determines that the condition should be reported, then the kevent is placed on the kqueue for the user to retrieve. .Pp The filter is also run when the user attempts to retrieve the kevent from the kqueue. If the filter indicates that the condition that triggered the event no longer holds, the kevent is removed from the kqueue and is not returned. .Pp Multiple events which trigger the filter do not result in multiple kevents being placed on the kqueue; instead, the filter will aggregate the events into a single .Li struct kevent . Calling .Fn close on a file descriptor will remove any kevents that reference the descriptor. .Pp .Fn kqueue creates a new kernel event queue and returns a descriptor. The queue is not inherited by a child created with .Xr fork 2 . Similarly, kqueues cannot be passed across UNIX-domain sockets. .Pp .Fn kevent is used to register events with the queue, and return any pending events to the user. .Fa changelist is a pointer to an array of .Va kevent structures, as defined in .Aq Pa sys/event.h . All changes contained in the .Fa changelist are applied before any pending events are read from the queue. .Fa nchanges gives the size of .Fa changelist . .Fa eventlist is a pointer to an array of kevent structures. .Fa nevents determines the size of .Fa eventlist . When .Fa nevents is zero, .Fn kevent will return immediately even if there is a .Fa timeout specified unlike .Xr select 2 . If .Fa timeout is a non-null pointer, it specifies a maximum interval to wait for an event, which will be interpreted as a .Li struct timespec . If .Fa timeout is a null pointer, .Fn kevent waits indefinitely. To effect a poll, the .Fa timeout argument should be non-null, pointing to a zero-valued .Va timespec structure. The same array may be used for the .Fa changelist and .Fa eventlist . .Pp .Fn EV_SET is a macro which is provided for ease of initializing a kevent structure. .Pp The .Va kevent structure is defined as: .Bd -literal struct kevent { u_int ident; /* identifier for this event */ short filter; /* filter for event */ u_short flags; /* action flags for kqueue */ u_int fflags; /* filter flag value */ int data; /* filter data value */ void *udata; /* opaque user data identifier */ }; .Ed .Pp The fields of .Li struct kevent are: .Bl -tag -width XXXfilter .It ident Value used to identify this event. The exact interpretation is determined by the attached filter, but often is a file descriptor. .It filter Identifies the kernel filter used to process this event. The pre-defined system filters are described below. .It flags Actions to perform on the event. .It fflags Filter-specific flags. .It data Filter-specific data value. .It udata Opaque user-defined value passed through the kernel unchanged. .El .Pp The .Va flags field can contain the following values: .Bl -tag -width XXXEV_ONESHOT .It Dv EV_ADD Adds the event to the kqueue. Re-adding an existing event will modify the parameters of the original event, and not result in a duplicate entry. Adding an event automatically enables it, unless overridden by the .Dv EV_DISABLE flag. .It Dv EV_ENABLE Permit .Fn kevent to return the event if it is triggered. .It Dv EV_DISABLE Disable the event so .Fn kevent will not return it. The filter itself is not disabled. .It Dv EV_DELETE Removes the event from the kqueue. Events which are attached to file descriptors are automatically deleted on the last close of the descriptor. .It Dv EV_ONESHOT Causes the event to return only the first occurrence of the filter being triggered. After the user retrieves the event from the kqueue, it is deleted. .It Dv EV_CLEAR After the event is retrieved by the user, its state is reset. This is useful for filters which report state transitions instead of the current state. Note that some filters may automatically set this flag internally. .It Dv EV_EOF Filters may set this flag to indicate filter-specific EOF condition. .It Dv EV_ERROR See .Sx RETURN VALUES below. .El .Pp The predefined system filters are listed below. Arguments may be passed to and from the filter via the .Va fflags and .Va data fields in the kevent structure. .Bl -tag -width EVFILT_SIGNAL .It Dv EVFILT_READ Takes a descriptor as the identifier, and returns whenever there is data available to read. The behavior of the filter is slightly different depending on the descriptor type. .Bl -tag -width 2n .It Sockets Sockets which have previously been passed to .Fn listen return when there is an incoming connection pending. .Va data contains the size of the listen backlog. .Pp Other socket descriptors return when there is data to be read, subject to the .Dv SO_RCVLOWAT value of the socket buffer. This may be overridden with a per-filter low water mark at the time the filter is added by setting the .Dv NOTE_LOWAT flag in .Va fflags , and specifying the new low water mark in .Va data . On return, .Va data contains the number of bytes in the socket buffer. .Pp If the read direction of the socket has shutdown, then the filter also sets .Dv EV_EOF in .Va flags , and returns the socket error (if any) in .Va fflags . It is possible for EOF to be returned (indicating the connection is gone) while there is still data pending in the socket buffer. .It Vnodes Returns when the file pointer is not at the end of file. .Va data contains the offset from current position to end of file, and may be negative. If .Dv NOTE_EOF is set in .Va fflags , .Fn kevent will also return when the file pointer is at the end of file. The end of file condition is indicated by the presence of .Dv NOTE_EOF in .Va fflags on return. .It "Fifos, Pipes" Returns when there is data to read; .Va data contains the number of bytes available. .Pp When the last writer disconnects, the filter will set .Dv EV_EOF in .Va flags . This may be cleared by passing in .Dv EV_CLEAR , at which point the filter will resume waiting for data to become available before returning. .It "BPF devices" Returns when the BPF buffer is full, the BPF timeout has expired, or when the BPF has .Dq immediate mode enabled and there is any data to read; .Va data contains the number of bytes available. .El .It Dv EVFILT_WRITE Takes a descriptor as the identifier, and returns whenever it is possible to write to the descriptor. For sockets, pipes, and FIFOs, .Va data will contain the amount of space remaining in the write buffer. The filter will set .Dv EV_EOF when the reader disconnects, and for the FIFO case, this may be cleared by use of .Dv EV_CLEAR . Note that this filter is not supported for vnodes or BPF devices. .Pp For sockets, the low water mark and socket error handling is identical to the .Dv EVFILT_READ case. .It Dv EVFILT_AIO The sigevent portion of the AIO request is filled in, with .Va sigev_notify_kqueue containing the descriptor of the kqueue that the event should be attached to, .Va sigev_value containing the udata value, and .Va sigev_notify set to .Dv SIGEV_KEVENT . When the aio_* function is called, the event will be registered with the specified kqueue, and the .Va ident argument set to the .Li struct aiocb returned by the aio_* function. The filter returns under the same conditions as aio_error. .Pp Alternatively, a kevent structure may be initialized, with .Va ident containing the descriptor of the kqueue, and the address of the kevent structure placed in the .Va aio_lio_opcode field of the AIO request. However, this approach will not work on architectures with 64-bit pointers, and should be considered deprecated. .It Dv EVFILT_VNODE Takes a file descriptor as the identifier and the events to watch for in .Va fflags , and returns when one or more of the requested events occurs on the descriptor. The events to monitor are: .Bl -tag -width XXNOTE_RENAME .It Dv NOTE_DELETE .Fn unlink was called on the file referenced by the descriptor. .It Dv NOTE_WRITE A write occurred on the file referenced by the descriptor. .It Dv NOTE_EXTEND The file referenced by the descriptor was extended. .It Dv NOTE_TRUNCATE The file referenced by the descriptor was truncated. .It Dv NOTE_ATTRIB The file referenced by the descriptor had its attributes changed. .It Dv NOTE_LINK The link count on the file changed. .It Dv NOTE_RENAME The file referenced by the descriptor was renamed. .It Dv NOTE_REVOKE Access to the file was revoked via .Xr revoke 2 or the underlying file system was unmounted. .El .Pp On return, .Va fflags contains the events which triggered the filter. .It Dv EVFILT_PROC Takes the process ID to monitor as the identifier and the events to watch for in .Va fflags , and returns when the process performs one or more of the requested events. If a process can normally see another process, it can attach an event to it. The events to monitor are: .Bl -tag -width XXNOTE_TRACKERR .It Dv NOTE_EXIT The process has exited. .It Dv NOTE_FORK The process has called .Fn fork . .It Dv NOTE_EXEC The process has executed a new process via .Xr execve 2 or similar call. .It Dv NOTE_TRACK Follow a process across .Fn fork calls. The parent process will return with .Dv NOTE_FORK set in the .Va fflags field, while the child process will return with .Dv NOTE_CHILD set in .Va fflags and the parent PID in .Va data . .It Dv NOTE_TRACKERR This flag is returned if the system was unable to attach an event to the child process, usually due to resource limitations. .El .Pp On return, .Va fflags contains the events which triggered the filter. .It Dv EVFILT_SIGNAL Takes the signal number to monitor as the identifier and returns when the given signal is delivered to the process. This coexists with the .Fn signal and .Fn sigaction facilities, and has a lower precedence. The filter will record all attempts to deliver a signal to a process, even if the signal has been marked as .Dv SIG_IGN . Event notification happens after normal signal delivery processing. .Va data returns the number of times the signal has occurred since the last call to .Fn kevent . This filter automatically sets the .Dv EV_CLEAR flag internally. .It Dv EVFILT_TIMER Establishes an arbitrary timer identified by .Va ident . When adding a timer, .Va data specifies the timeout period in milliseconds. The timer will be periodic unless .Dv EV_ONESHOT is specified. On return, .Va data contains the number of times the timeout has expired since the last call to .Fn kevent . This filter automatically sets the .Dv EV_CLEAR flag internally. .El .Sh RETURN VALUES .Fn kqueue creates a new kernel event queue and returns a file descriptor. If there was an error creating the kernel event queue, a value of -1 is returned and errno set. .Pp .Fn kevent returns the number of events placed in the .Fa eventlist , up to the value given by .Fa nevents . If an error occurs while processing an element of the .Fa changelist and there is enough room in the .Fa eventlist , then the event will be placed in the .Fa eventlist with .Dv EV_ERROR set in .Va flags and the system error in .Va data . Otherwise, .Dv -1 will be returned, and .Dv errno will be set to indicate the error condition. If the time limit expires, then .Fn kevent returns 0. .Sh ERRORS The .Fn kqueue function fails if: .Bl -tag -width Er .It Bq Er ENOMEM The kernel failed to allocate enough memory for the kernel queue. .It Bq Er EMFILE The per-process descriptor table is full. .It Bq Er ENFILE The system file table is full. .El .Pp The .Fn kevent function fails if: .Bl -tag -width Er .It Bq Er EACCES The process does not have permission to register a filter. .It Bq Er EFAULT There was an error reading or writing the .Va kevent structure. .It Bq Er EBADF The specified descriptor is invalid. .It Bq Er EINTR A signal was delivered before the timeout expired and before any events were placed on the kqueue for return. .It Bq Er EINVAL The specified time limit or filter is invalid. .It Bq Er ENOENT The event could not be found to be modified or deleted. .It Bq Er ENOMEM No memory was available to register the event. .It Bq Er ESRCH The specified process to attach to does not exist. .El .Sh SEE ALSO .Xr poll 2 , .Xr read 2 , .Xr select 2 , .Xr sigaction 2 , .Xr write 2 , .Xr signal 3 .Sh HISTORY The .Fn kqueue and .Fn kevent functions first appeared in .Fx 4.1 . .Sh AUTHORS The .Fn kqueue system and this manual page were written by .An Jonathan Lemon Aq jlemon@FreeBSD.org . .Sh BUGS It is currently not possible to watch FIFOs or AIO that reside on anything but a UFS file system. Watching a vnode is possible on UFS, NFS and MS-DOS file systems. .Pp The .Fa timeout value is limited to 24 hours; longer timeouts will be silently reinterpreted as 24 hours.