KQUEUE(2)                 NetBSD 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);

     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 filters.  A kevent is identified by the (ident, fil-
     ter) 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 exe-
     cuted 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.

     kqueue() creates a new kernel event queue and returns a descriptor.

     The kqueue1() function 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).

     kevent() is used to register events with the queue, and return any pend-
     ing events to the user.  changelist 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.
     nchanges gives the size of changelist.  eventlist is a pointer to an
     array of kevent structures.  nevents determines the size of eventlist.
     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

     EV_SET() is a macro which 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 */
             intptr_t  udata;        /* opaque user data identifier */

     The fields of struct kevent are:

           ident      Value used to identify this event.  The exact interpre-
                      tation 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 ker-
                      nel 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

     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 trig-

           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.

           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-spe-
                          cific 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 */

     Arguments may be passed to and from the filter via the fflags and data
     fields in the kevent structure.

     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 fil-
                    ter 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-fil-
                        ter 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 con-
                        tains the number of bytes in the socket buffer.

                        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 wait-
                        ing for data to become available before returning.

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

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

                    NOTE_EXTEND    The file referenced by the descriptor was

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

                    NOTE_LINK      The link count on the file changed.

                    NOTE_RENAME    The file referenced by the descriptor was

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

                    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 limita-

                    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 mil-
                    liseconds.  The timer will be periodic unless EV_ONESHOT
                    is specified.  On return, data contains the number of
                    times the timeout has expired since the last call to
                    kevent().  This filter automatically sets the EV_CLEAR
                    flag internally.

     EVFILT_FS      Establishes a file system monitor.  Currently it only mon-
                    itors file system mount and unmount actions.

     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 is set.

     kevent() 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

     [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.

NetBSD 9.0                      January 8, 2018                     NetBSD 9.0

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