.\" $NetBSD: kqueue.2,v 1.60 2023/07/28 23:41:16 wiz Exp $ .\" .\" Copyright (c) 2000 Jonathan Lemon .\" All rights reserved. .\" .\" Copyright (c) 2001, 2002, 2003 The NetBSD Foundation, Inc. .\" All rights reserved. .\" .\" Portions of this documentation is derived from text contributed by .\" Luke Mewburn. .\" .\" 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 1.22 2001/06/27 19:55:57 dd Exp $ .\" .Dd June 19, 2023 .Dt KQUEUE 2 .Os .Sh NAME .Nm kqueue , .Nm kqueue1 , .Nm kevent , .Nm EV_SET .Nd kernel event notification mechanism .Sh LIBRARY .Lb libc .Sh SYNOPSIS .In sys/event.h .In sys/time.h .Ft int .Fn kqueue "void" .Ft int .Fn kqueue1 "int flags" .Ft int .Fn kevent "int kq" "const struct kevent *changelist" "size_t nchanges" "struct kevent *eventlist" "size_t nevents" "const struct timespec *timeout" .Fn EV_SET "&kev" ident filter flags fflags data udata .Sh DESCRIPTION The .Fn kqueue system call 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 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 struct kevent. Calling .Xr close 2 on a file descriptor will remove any kevents that reference the descriptor. .Pp The .Fn kqueue system call creates a new kernel event queue and returns a descriptor. .Pp The .Fn kqueue1 system call also allows to set the following .Fa flags on the returned file descriptor: .Bl -column O_NONBLOCK -offset indent .It Dv O_CLOEXEC Set the close on exec property. .It Dv O_NONBLOCK Set non-blocking I/O. .It Dv O_NOSIGPIPE Return .Er EPIPE instead of raising .Dv SIGPIPE . .El .Pp The queue is not inherited by a child created with .Xr fork 2 . .\" However, if .\" .Xr rfork 2 .\" is called without the .\" .Dv RFFDG .\" flag, then the descriptor table is shared, .\" which will allow sharing of the kqueue between two processes. .Pp The .Fn kevent system call is used to register events with the queue, and return any pending events to the user. The .Fa changelist argument is a pointer to an array of .Va kevent structures, as defined in .In sys/event.h . All changes contained in the .Fa changelist are applied before any pending events are read from the queue. The .Fa nchanges argument gives the size of .Fa changelist . The .Fa eventlist argument is a pointer to an array of kevent structures. The .Fa nevents argument 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 .No non- Ns Dv 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 .Dv NULL pointer, .Fn kevent waits indefinitely. To effect a poll, the .Fa timeout argument should be .No non- Ns Dv NULL , pointing to a zero-valued .Xr timespec 3 structure. The same array may be used for the .Fa changelist and .Fa eventlist . .Pp The .Fn EV_SET macro is provided for ease of initializing a kevent structure. This macro does not evaluate its parameters multiple times. .Pp The .Va kevent structure is defined as: .Bd -literal struct kevent { uintptr_t ident; /* identifier for this event */ uint32_t filter; /* filter for event */ uint32_t flags; /* action flags for kqueue */ uint32_t fflags; /* filter flag value */ int64_t data; /* filter data value */ void *udata; /* opaque user data identifier */ uint64_t ext[4]; /* extensions */ }; .Ed .Pp The fields of .Fa struct kevent are: .Bl -tag -width "Fa filter" -offset indent .It ident Value used to identify this event. The exact interpretation is determined by the attached filter, but often is a file descriptor. .It Fa filter Identifies the kernel filter used to process this event. There are pre-defined system filters (which are described below), and other filters may be added by kernel subsystems as necessary. .It Fa flags Actions to perform on the event. .It Fa fflags Filter-specific flags. .It Fa data Filter-specific data value. .It Fa udata Opaque user-defined value passed through the kernel unchanged. .It Fa ext Extended data passed to and from kernel. The .Fa ext[0] and .Fa ext[1] members use is defined by the filter. If the filter does not use them, the members are copied unchanged. The .Fa ext[2] and .Fa ext[3] members are always passed through the kernel as-is, making additional context available to application. .El .Pp The .Va flags field can contain the following values: .Bl -tag -width XXXEV_ONESHOT -offset indent .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 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_DISPATCH Disable the event source immediately after delivery of an event. See .Dv EV_DISABLE above. .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_RECEIPT This flag is useful for making bulk changes to a kqueue without draining any pending events. When passed as input, it forces .Dv EV_ERROR to always be returned. When a filter is successfully added the .Va data field will be zero. Note that if this flag is encountered and there is no remaining space in .Fa eventlist to hold the .Dv EV_ERROR event, then subsequent changes will not get processed. .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 .Ss Filters Filters are identified by a number. There are two types of filters; pre-defined filters which are described below, and third-party filters that may be added with .Xr kfilter_register 9 by kernel sub-systems, third-party device drivers, or loadable kernel modules. .Pp As a third-party filter is referenced by a well-known name instead of a statically assigned number, two .Xr ioctl 2 Ns s are supported on the file descriptor returned by .Fn kqueue to map a filter name to a filter number, and vice-versa (passing arguments in a structure described below): .Bl -tag -width KFILTER_BYFILTER -offset indent .It Dv KFILTER_BYFILTER Map .Va filter to .Va name , which is of size .Va len . .It Dv KFILTER_BYNAME Map .Va name to .Va filter . .Va len is ignored. .El .Pp The following structure is used to pass arguments in and out of the .Xr ioctl 2 : .Bd -literal -offset indent struct kfilter_mapping { char *name; /* name to lookup or return */ size_t len; /* length of name */ uint32_t filter; /* filter to lookup or return */ }; .Ed .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. .Pp The predefined system filters are: .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 .Xr listen 2 return when there is an incoming connection pending. .Va data contains the size of the listen backlog (i.e., the number of connections ready to be accepted with .Xr accept 2 . ) .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 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 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. .\" .Pp .\" This behavior is different from .\" .Xr poll 2 , .\" where read events are triggered for regular files unconditionally. .\" This event can be triggered unconditionally by setting the .\" .Dv NOTE_FILE_POLL .\" flag in .\" .Va fflags . .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 EV_EOF in .Va flags . This may be cleared by passing in 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, fifos, and ttys, .Va data will contain the amount of space remaining in the write buffer. The filter will set EV_EOF when the reader disconnects, and for the fifo case, this may be cleared by use of EV_CLEAR. Note that this filter is not supported for vnodes. .Pp For sockets, the low water mark and socket error handling is identical to the EVFILT_READ case. .It Dv EVFILT_EMPTY Takes a descriptor as the identifier, and returns whenever there is no remaining data in the write buffer. This is currently implemented only for sockets. It's primary purpose is to provide visibility to an application that all previously written data has been acknowledged by the receiver at the TCP layer. .It Dv EVFILT_AIO This is not implemented in .Nx . .ig 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 SIGEV_EVENT. When the aio_* function is called, the event will be registered with the specified kqueue, and the .Va ident argument set to the .Fa 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 NOTE_CLOSE_WRITE .It Dv NOTE_ATTRIB The file referenced by the descriptor had its attributes changed. .It Dv NOTE_CLOSE A file descriptor without write access referencing the file was closed. .It Dv NOTE_CLOSE_WRITE A file descriptor with write access referencing the file was closed. .It Dv NOTE_DELETE .Xr unlink 2 was called on the file referenced by the descriptor. .It Dv NOTE_EXTEND The file referenced by the descriptor was extended. .It Dv NOTE_LINK The link count on the file changed. .It Dv NOTE_OPEN The file referenced by the descriptor was opened. .It Dv NOTE_READ A read occurred on the file referenced by the descriptor. .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. .It Dv NOTE_WRITE A write occurred on the file referenced by the descriptor. .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. The exit code of the process is stored in .Va data . .It Dv NOTE_FORK The process has called .Xr fork 2 . .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 .Xr fork 2 calls. The parent process will return with NOTE_TRACK set in the .Va fflags field, while the child process will return with 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 current process. This coexists with the .Xr signal 3 and .Xr sigaction 2 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 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 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 units described below, or, if .Dv NOTE_ABSTIME is set in .Va fflags , specifies the absolute time at which the timer should fire. The timer will repeat unless .Dv EV_ONESHOT is set in .Va flags or .Dv NOTE_ABSTIME is set in .Va fflags . On return, .Va data contains the number of times the timeout has expired since the last call to .Fn kevent . This filter automatically sets .Dv EV_CLEAR in .Va flags for periodic timers. Timers created with .Dv NOTE_ABSTIME remain activated on the kqueue once the absolute time has passed unless .Dv EV_CLEAR or .Dv EV_ONESHOT are also specified. .Dv CLOCK_REALTIME is the reference clock for timers created with .Dv NOTE_ABSTIME . .Pp The filter accepts the following flags in the .Va fflags argument: .Bl -tag -width XXNOTE_TRACKERR .It Dv NOTE_SECONDS The timer value in .Va data is expressed in seconds. .It Dv NOTE_MSECONDS The timer value in .Va data is expressed in milliseconds. .It Dv NOTE_USECONDS The timer value in .Va data is expressed in microseconds. .It Dv NOTE_NSECONDS The timer value in .Va data is expressed in nanoseconds. .It Dv NOTE_ABSTIME The timer value is an absolute time; see discussion above. .El .Pp Note that .Dv NOTE_SECONDS , .Dv NOTE_MSECONDS , .Dv NOTE_USECONDS , and .Dv NOTE_NSECONDS are mutually exclusive; behavior is undefined if more than one are specified. If a timer value unit is not specified, the default is .Dv NOTE_MSECONDS . .It Dv EVFILT_FS Establishes a file system monitor. Currently it only monitors file system mount and unmount actions. .It Dv EVFILT_USER Establishes a user event identified by .Va ident which is not associated with any kernel mechanism but is triggered by user level code. The lower 24 bits of the .Va fflags may be used for user defined flags and manipulated using the following: .Bl -tag -width "Dv NOTE_FFLAGSMASK" .It Dv NOTE_FFNOP Ignore the input .Va fflags . .It Dv NOTE_FFAND Bitwise AND .Va fflags . .It Dv NOTE_FFOR Bitwise OR .Va fflags . .It Dv NOTE_FFCOPY Copy .Va fflags . .It Dv NOTE_FFCTRLMASK Control mask for .Va fflags . .It Dv NOTE_FFLAGSMASK User defined flag mask for .Va fflags . .El .Pp A user event is triggered for output with the following: .Bl -tag -width "Dv NOTE_FFLAGSMASK" .It Dv NOTE_TRIGGER Cause the event to be triggered. .El .Pp On return, .Va fflags contains the users defined flags in the lower 24 bits. .El .Sh CANCELLATION BEHAVIOUR If .Fa nevents is non-zero, i.e., the function is potentially blocking, the call is a cancellation point. Otherwise, i.e., if .Fa nevents is zero, the call is not cancellable. Cancellation can only occur before any changes are made to the kqueue, or when the call was blocked and no changes to the queue were requested. .Sh RETURN VALUES The .Fn kqueue system call 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 .Dv errno is set. .Pp The .Fn kevent system call 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 EXAMPLES The following example program monitors a file (provided to it as the first argument) and prints information about some common events it receives notifications for: .Bd -literal -offset indent #include #include #include #include #include #include #include #include int main(int argc, char *argv[]) { int fd, kq, nev; struct kevent ev; static const struct timespec tout = { 1, 0 }; if ((fd = open(argv[1], O_RDONLY)) == -1) err(1, "Cannot open `%s'", argv[1]); if ((kq = kqueue()) == -1) err(1, "Cannot create kqueue"); EV_SET(&ev, fd, EVFILT_VNODE, EV_ADD | EV_ENABLE | EV_CLEAR, NOTE_DELETE|NOTE_WRITE|NOTE_EXTEND|NOTE_ATTRIB|NOTE_LINK| NOTE_RENAME|NOTE_REVOKE, 0, 0); if (kevent(kq, &ev, 1, NULL, 0, &tout) == -1) err(1, "kevent"); for (;;) { nev = kevent(kq, NULL, 0, &ev, 1, &tout); if (nev == -1) err(1, "kevent"); if (nev == 0) continue; if (ev.fflags & NOTE_DELETE) { printf("deleted "); ev.fflags &= ~NOTE_DELETE; } if (ev.fflags & NOTE_WRITE) { printf("written "); ev.fflags &= ~NOTE_WRITE; } if (ev.fflags & NOTE_EXTEND) { printf("extended "); ev.fflags &= ~NOTE_EXTEND; } if (ev.fflags & NOTE_ATTRIB) { printf("chmod/chown/utimes "); ev.fflags &= ~NOTE_ATTRIB; } if (ev.fflags & NOTE_LINK) { printf("hardlinked "); ev.fflags &= ~NOTE_LINK; } if (ev.fflags & NOTE_RENAME) { printf("renamed "); ev.fflags &= ~NOTE_RENAME; } if (ev.fflags & NOTE_REVOKE) { printf("revoked "); ev.fflags &= ~NOTE_REVOKE; } printf("\\n"); if (ev.fflags) warnx("unknown event 0x%x\\n", ev.fflags); } } .Ed .Sh ERRORS The .Fn kqueue function fails if: .Bl -tag -width Er .It Bq Er EMFILE The per-process descriptor table is full. .It Bq Er ENFILE The system file table is full. .It Bq Er ENOMEM The kernel failed to allocate enough memory for the kernel queue. .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 EBADF The specified descriptor is invalid. .It Bq Er EFAULT There was an error reading or writing the .Va kevent structure. .It Bq Er EINTR A signal was delivered before the timeout expired and before any events were placed on the kqueue for return. All changes contained in the .Fa changelist are applied before returning this error. .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 EOPNOTSUPP This type of file descriptor is not supported for .Fn kevent operations. .It Bq Er ESRCH The specified process to attach to does not exist. .El .Sh SEE ALSO .\" .Xr aio_error 2 , .\" .Xr aio_read 2 , .\" .Xr aio_return 2 , .Xr fork 2 , .Xr ioctl 2 , .Xr listen 2 , .Xr poll 2 , .Xr read 2 , .Xr select 2 , .Xr sigaction 2 , .Xr unlink 2 , .Xr write 2 , .Xr signal 3 , .Xr timespec 3 , .Xr kfilter_register 9 , .Xr knote 9 .Rs .%A Jonathan Lemon .%T "Kqueue: A Generic and Scalable Event Notification Facility" .%I USENIX Association .%B Proceedings of the FREENIX Track: 2001 USENIX Annual Technical Conference .%D June 25-30, 2001 .%U http://www.usenix.org/event/usenix01/freenix01/full_papers/lemon/lemon.pdf .Re .Sh HISTORY The .Fn kqueue and .Fn kevent functions first appeared in .Fx 4.1 , and then in .Nx 2.0 . The .Fn kqueue1 function first appeared in .Nx 6.0 . .Pp The .Fn EV_SET macro was protected from evaluating multiple times the first argument in .Nx 8.0 . .Pp The .Va udata type was changed from intptr_t to void * in .Nx 10.0 . .Pp Support for .Dv NOTE_SECONDS , .Dv NOTE_MSECONDS , .Dv NOTE_USECONDS , .Dv NOTE_NSECONDS , and .Dv NOTE_ABSTIME filter flags for .Dv EVFILT_TIMER was added in .Nx 10.0 . .Pp Support for .Dv NOTE_OPEN , .Dv NOTE_CLOSE , .Dv NOTE_CLOSE_WRITE , and NOTE_READ filter flags for .Dv EVFILT_VNODE was added in .Nx 10.0 . .Pp Support for .Dv EVFILT_EMPTY was added in .Nx 10.0 .