Provided by: libkqueue-dev_2.0.3-1.1_amd64 bug


     kqueue, kevent — kernel event notification mechanism


     #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 <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 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.

     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 descriptor.

     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

     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

     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

     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.

     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 structure.

     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.

                        Sockets which have previously been passed to listen() return when there
                        is an incoming connection pending.  data contains the size of the listen

                        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 buffer.

                        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

                        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

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

     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 extended.

                    NOTE_ATTRIB    The file referenced by the descriptor had its attributes

                    NOTE_LINK      The link count on the file changed.

                    NOTE_RENAME    The file referenced by the descriptor was renamed.

                    On return, fflags contains the events which triggered the filter.

     EVFILT_SIGNAL  Takes the signal number to monitor as the identifier and returns when the
                    given signal is delivered to the process.  This overrides the signal() and
                    sigaction() facilities, and has a higher precedence.  The filter will record
                    all attempts to deliver a signal to a process, even if the signal has been
                    marked as SIG_IGN.  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_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 filter.

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

     [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 return.

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

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

     [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