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KQUEUE(2)                     System Calls Manual                    KQUEUE(2)

     kqueue, kqueue1, kevent, EV_SET - kernel event notification mechanism

     Standard C Library (libc, -lc)

     #include <sys/event.h>
     #include <sys/time.h>


     kqueue1(int flags);

     kevent(int kq, const struct kevent *changelist, size_t nchanges,
         struct kevent *eventlist, size_t nevents,
         const struct timespec *timeout);

     EV_SET(&kev, ident, filter, flags, fflags, data, udata);

     The 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.

     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.

     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.

     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 close(2) on a file
     descriptor will remove any kevents that reference the descriptor.

     The kqueue() system call creates a new kernel event queue and returns a

     The kqueue1() system call also allows to set the following flags on the
     returned file descriptor:

           O_CLOEXEC Set the close on exec property.
           O_NONBLOCK Set non-blocking I/O.
           O_NOSIGPIPE Return EPIPE instead of raising SIGPIPE.

     The queue is not inherited by a child created with fork(2).

     The kevent() system call is used to register events with the queue, and
     return any pending events to the user.  The changelist argument is a
     pointer to an array of kevent structures, as defined in <sys/event.h>.
     All changes contained in the changelist are applied before any pending
     events are read from the queue.  The nchanges argument gives the size of
     changelist.  The eventlist argument is a pointer to an array of kevent
     structures.  The nevents argument determines the size of eventlist.  When
     nevents is zero, kevent() will return immediately even if there is a
     timeout specified unlike select(2).  If timeout is a non-NULL pointer, it
     specifies a maximum interval to wait for an event, which will be
     interpreted as a struct timespec.  If timeout is a NULL pointer, kevent()
     waits indefinitely.  To effect a poll, the timeout argument should be
     non-NULL, pointing to a zero-valued timespec(3) structure.  The same
     array may be used for the changelist and eventlist.

     The EV_SET() macro is provided for ease of initializing a kevent
     structure.  This macro does not evaluate its parameters multiple times.

     The kevent structure is defined as:

     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 */

     The fields of struct kevent are:

           ident      Value used to identify this event.  The exact
                      interpretation is determined by the attached filter, but
                      often is a file descriptor.

           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.

           flags      Actions to perform on the event.

           fflags     Filter-specific flags.

           data       Filter-specific data value.

           udata      Opaque user-defined value passed through the kernel

           ext        Extended data passed to and from kernel.  The ext[0] and
                      ext[1] members use is defined by the filter.  If the
                      filter does not use them, the members are copied
                      unchanged.  The ext[2] and ext[3] members are always
                      passed through the kernel as-is, making additional
                      context available to application.

     The flags field can contain the following values:

           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.

           EV_ENABLE      Permit kevent() to return the event if it is

           EV_DISABLE     Disable the event so kevent() will not return it.
                          The filter itself is not disabled.

           EV_DISPATCH    Disable the event source immediately after delivery
                          of an event.  See EV_DISABLE above.

           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.

           EV_RECEIPT     This flag is useful for making bulk changes to a
                          kqueue without draining any pending events.  When
                          passed as input, it forces EV_ERROR to always be
                          returned.  When a filter is successfully added the
                          data field will be zero.  Note that if this flag is
                          encountered and there is no remaining space in
                          eventlist to hold the EV_ERROR event, then
                          subsequent changes will not get processed.

           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.

           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.

           EV_EOF         Filters may set this flag to indicate filter-
                          specific EOF condition.

           EV_ERROR       See RETURN VALUES below.

     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 kfilter_register(9) by kernel sub-systems, third-party
     device drivers, or loadable kernel modules.

     As a third-party filter is referenced by a well-known name instead of a
     statically assigned number, two ioctl(2)s are supported on the file
     descriptor returned by kqueue() to map a filter name to a filter number,
     and vice-versa (passing arguments in a structure described below):

           KFILTER_BYFILTER  Map filter to name, which is of size len.

           KFILTER_BYNAME    Map name to filter.  len is ignored.

     The following structure is used to pass arguments in and out of the

           struct kfilter_mapping {
                   char     *name;         /* name to lookup or return */
                   size_t   len;           /* length of name */
                   uint32_t filter;        /* filter to lookup or return */

     The predefined system filters are listed below.  Arguments may be passed
     to and from the filter via the fflags and data fields in the kevent

     The predefined system filters are:

     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

                        Sockets which have previously been passed to listen(2)
                        return when there is an incoming connection pending.
                        data contains the size of the listen backlog (i.e.,
                        the number of connections ready to be accepted with

                        Other socket descriptors return when there is data to
                        be read, subject to the 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 fflags, and
                        specifying the new low water mark in data.  On return,
                        data contains the number of bytes in the socket

                        If the read direction of the socket has shutdown, then
                        the filter also sets EV_EOF in flags, and returns the
                        socket error (if any) in fflags.  It is possible for
                        EOF to be returned (indicating the connection is gone)
                        while there is still data pending in the socket

                        Returns when the file pointer is not at the end of
                        file.  data contains the offset from current position
                        to end of file, and may be negative.

                    Fifos, Pipes
                        Returns when there is data to read; data contains the
                        number of bytes available.

                        When the last writer disconnects, the filter will set
                        EV_EOF in 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.

                    BPF devices
                        Returns when the BPF buffer is full, the BPF timeout
                        has expired, or when the BPF has "immediate mode"
                        enabled and there is any data to read; data contains
                        the number of bytes available.

     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, 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.

                    For sockets, the low water mark and socket error handling
                    is identical to the EVFILT_READ case.

     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.

     EVFILT_AIO     This is not implemented in NetBSD.

     EVFILT_VNODE   Takes a file descriptor as the identifier and the events
                    to watch for in fflags, and returns when one or more of
                    the requested events occurs on the descriptor.  The events
                    to monitor are:

                    NOTE_ATTRIB       The file referenced by the descriptor
                                      had its attributes changed.

                    NOTE_CLOSE        A file descriptor without write access
                                      referencing the file was closed.

                    NOTE_CLOSE_WRITE  A file descriptor with write access
                                      referencing the file was closed.

                    NOTE_DELETE       unlink(2) was called on the file
                                      referenced by the descriptor.

                    NOTE_EXTEND       The file referenced by the descriptor
                                      was extended.

                    NOTE_LINK         The link count on the file changed.

                    NOTE_OPEN         The file referenced by the descriptor
                                      was opened.

                    NOTE_READ         A read occurred on the file referenced
                                      by the descriptor.

                    NOTE_RENAME       The file referenced by the descriptor
                                      was renamed.

                    NOTE_REVOKE       Access to the file was revoked via
                                      revoke(2) or the underlying file system
                                      was unmounted.

                    NOTE_WRITE        A write occurred on the file referenced
                                      by the descriptor.

                    On return, fflags contains the events which triggered the

     EVFILT_PROC    Takes the process ID to monitor as the identifier and the
                    events to watch for in 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:

                    NOTE_EXIT        The process has exited.  The exit code of
                                     the process is stored in data.

                    NOTE_FORK        The process has called fork(2).

                    NOTE_EXEC        The process has executed a new process
                                     via execve(2) or similar call.

                    NOTE_TRACK       Follow a process across fork(2) calls.
                                     The parent process will return with
                                     NOTE_TRACK set in the fflags field, while
                                     the child process will return with
                                     NOTE_CHILD set in fflags and the parent
                                     PID in data.

                    NOTE_TRACKERR    This flag is returned if the system was
                                     unable to attach an event to the child
                                     process, usually due to resource

                    On return, fflags contains the events which triggered the

     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 signal(3) and
                    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.  data returns the number of times the signal
                    has occurred since the last call to kevent().  This filter
                    automatically sets the EV_CLEAR flag internally.

     EVFILT_TIMER   Establishes an arbitrary timer identified by ident.  When
                    adding a timer, data specifies the timeout period in units
                    described below, or, if NOTE_ABSTIME is set in fflags,
                    specifies the absolute time at which the timer should
                    fire.  The timer will repeat unless EV_ONESHOT is set in
                    flags or NOTE_ABSTIME is set in fflags.  On return, data
                    contains the number of times the timeout has expired since
                    the last call to kevent().  This filter automatically sets
                    EV_CLEAR in flags for periodic timers.  Timers created
                    with NOTE_ABSTIME remain activated on the kqueue once the
                    absolute time has passed unless EV_CLEAR or EV_ONESHOT are
                    also specified.  CLOCK_REALTIME is the reference clock for
                    timers created with NOTE_ABSTIME.

                    The filter accepts the following flags in the fflags

                    NOTE_SECONDS     The timer value in data is expressed in

                    NOTE_MSECONDS    The timer value in data is expressed in

                    NOTE_USECONDS    The timer value in data is expressed in

                    NOTE_NSECONDS    The timer value in data is expressed in

                    NOTE_ABSTIME     The timer value is an absolute time; see
                                     discussion above.

                    Note that NOTE_SECONDS, NOTE_MSECONDS, NOTE_USECONDS, and
                    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 NOTE_MSECONDS.

     EVFILT_FS      Establishes a file system monitor.  Currently it only
                    monitors file system mount and unmount actions.

     EVFILT_USER    Establishes a user event identified by ident which is not
                    associated with any kernel mechanism but is triggered by
                    user level code.  The lower 24 bits of the fflags may be
                    used for user defined flags and manipulated using the

                    NOTE_FFNOP          Ignore the input fflags.

                    NOTE_FFAND          Bitwise AND fflags.

                    NOTE_FFOR           Bitwise OR fflags.

                    NOTE_FFCOPY         Copy fflags.

                    NOTE_FFCTRLMASK     Control mask for fflags.

                    NOTE_FFLAGSMASK     User defined flag mask for fflags.

                    A user event is triggered for output with the following:

                    NOTE_TRIGGER        Cause the event to be triggered.

                    On return, fflags contains the users defined flags in the
                    lower 24 bits.

     If nevents is non-zero, i.e., the function is potentially blocking, the
     call is a cancellation point.  Otherwise, i.e., if 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.

     The 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 errno is set.

     The kevent() system call returns the number of events placed in the
     eventlist, up to the value given by nevents.  If an error occurs while
     processing an element of the changelist and there is enough room in the
     eventlist, then the event will be placed in the eventlist with EV_ERROR
     set in flags and the system error in data.  Otherwise, -1 will be
     returned, and errno will be set to indicate the error condition.  If the
     time limit expires, then kevent() returns 0.

     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:

           #include <sys/types.h>
           #include <sys/event.h>
           #include <sys/time.h>
           #include <stdio.h>
           #include <unistd.h>
           #include <stdlib.h>
           #include <fcntl.h>
           #include <err.h>

           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_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)
                           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;
                           if (ev.fflags)
                                   warnx("unknown event 0x%x\n", ev.fflags);

     The kqueue() function fails if:

     [EMFILE]           The per-process descriptor table is full.

     [ENFILE]           The system file table is full.

     [ENOMEM]           The kernel failed to allocate enough memory for the
                        kernel queue.

     The kevent() function fails if:

     [EACCES]           The process does not have permission to register a

     [EBADF]            The specified descriptor is invalid.

     [EFAULT]           There was an error reading or writing the kevent

     [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 changelist are
                        applied before returning this error.

     [EINVAL]           The specified time limit or filter is invalid.

     [ENOENT]           The event could not be found to be modified or

     [ENOMEM]           No memory was available to register the event.

     [EOPNOTSUPP]       This type of file descriptor is not supported for
                        kevent() operations.

     [ESRCH]            The specified process to attach to does not exist.

     fork(2), ioctl(2), listen(2), poll(2), read(2), select(2), sigaction(2),
     unlink(2), write(2), signal(3), timespec(3), kfilter_register(9),

     Jonathan Lemon, "Kqueue: A Generic and Scalable Event Notification
     Facility", Proceedings of the FREENIX Track: 2001 USENIX Annual Technical
     Conference, USENIX Association,
     June 25-30, 2001.

     The kqueue() and kevent() functions first appeared in FreeBSD 4.1, and
     then in NetBSD 2.0.  The kqueue1() function first appeared in NetBSD 6.0.

     The EV_SET() macro was protected from evaluating multiple times the first
     argument in NetBSD 8.0.

     The udata type was changed from intptr_t to void * in NetBSD 10.0.

     and NOTE_ABSTIME filter flags for EVFILT_TIMER was added in NetBSD 10.0.

     flags for EVFILT_VNODE was added in NetBSD 10.0.

     Support for EVFILT_EMPTY was added in NetBSD 10.0.

NetBSD 10.99                     June 19, 2023                    NetBSD 10.99