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     kqueue, kevent - kernel event notification mechanism


     Standard C Library (libc, -lc)


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


     kevent(int kq, const struct kevent *changelist, int nchanges,
             struct kevent *eventlist, int 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() 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
     descriptor.  The queue is not inherited by a child created with fork(2).
     However, if rfork(2) is called without the RFFDG flag, then the
     descriptor table is shared, which will allow sharing of the kqueue
     between two processes.

     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 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 structure.  The same array
     may be used for the changelist and eventlist.

     The EV_SET() macro is provided for ease of initializing a kevent

     The kevent structure is defined as:

     struct kevent {
             uintptr_t ident;        /* identifier for this event */
             short     filter;       /* filter for event */
             u_short   flags;        /* action flags for kqueue */
             u_int     fflags;       /* filter flag value */
             intptr_t  data;         /* filter data value */
             void      *udata;       /* opaque user data identifier */

     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

     filter     Identifies the kernel filter used to process this event.  The
                pre-defined system filters are described below.

     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 unchanged.

     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 triggered.

     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-specific EOF

     EV_ERROR       See RETURN VALUES below.

     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

     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()
                        return when there is an incoming connection pending.
                        data contains the size of the listen backlog.

                        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 of protocol data
                        available to read.

                        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 the 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 and fifos, 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 or BPF devices.

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

     EVFILT_AIO     The sigevent portion of the AIO request is filled in, with
                    sigev_notify_kqueue containing the descriptor of the
                    kqueue that the event should be attached to, sigev_value
                    containing the udata value, and sigev_notify set to
                    SIGEV_KEVENT.  When the aio_*() system call is made, the
                    event will be registered with the specified kqueue, and
                    the ident argument set to the struct aiocb returned by the
                    aio_*() system call.  The filter returns under the same
                    conditions as aio_error.

     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    The unlink() system call 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 status
                                     will be stored in data.

                    NOTE_FORK        The process has called fork().

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

                    NOTE_TRACK       Follow a process across fork() 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 process.
                    This coexists with the signal() and 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 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
                    milliseconds.  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.  There is a system wide limit on
                    the number of timers which is controlled by the
                    kern.kq_calloutmax sysctl.

     EVFILT_NETDEV  Takes a descriptor to a network interface as the
                    identifier, and the events to watch for in fflags.  It
                    returns, when one or more of the requested events occur on
                    the descriptor.  The events to monitor are:

                    NOTE_LINKUP      The link is up.

                    NOTE_LINKDOWN    The link is down.

                    NOTE_LINKINV     The link state is invalid.

                    On return, fflags contains the events which triggered the

     EVFILT_USER    Establishes a user event identified by ident which is not
                    assosicated 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_COPY          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.


     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 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 kqueue() system call fails if:

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

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

     [ENFILE]           The system file table is full.

     The kevent() system call fails if:

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

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

     [EBADF]            The specified descriptor is invalid.

     [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 or, in
                        the special case of a timer, the maximum number of
                        timers has been exceeded.  This maximum is
                        configurable via the kern.kq_calloutmax sysctl.

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


     aio_error(2), aio_read(2), aio_return(2), poll(2), read(2), select(2),
     sigaction(2), write(2), signal(3)


     The kqueue() and kevent() system calls first appeared in FreeBSD 4.1.


     The kqueue() system and this manual page were written by Jonathan Lemon


     The EVFILT_NETDEV filter is currently only implemented for devices that
     use the miibus(4) driver for LINKUP and LINKDOWN operations.  Therefore,
     it will not work with many non-ethernet devices.

     The timeout value is limited to 24 hours; longer timeouts will be
     silently reinterpreted as 24 hours.