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KQUEUE(2)                   BSD System Calls Manual                  KQUEUE(2)

NAME
     kqueue, kevent -- kernel event notification mechanism

SYNOPSIS
     #include <&lt;sys/types.h>&gt;
     #include <&lt;sys/event.h>&gt;
     #include <&lt;sys/time.h>&gt;

     int
     kqueue(void);

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

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

DESCRIPTION
     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,
     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 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() on a file
     descriptor will remove any kevents that reference the descriptor.

     kqueue() creates a new kernel event queue and returns a descriptor.  The
     queue is not inherited by a child created with fork(2).  Similarly,
     kqueues cannot be passed across UNIX-domain sockets.

     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.
     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 inter-
     preted 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.

     EV_SET() is a macro which 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 */
             quad_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 automati-
                    cally 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_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_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.

     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 fil-
                    ter is slightly different depending on the descriptor
                    type.

                    Sockets
                        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-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 buf-
                        fer.

                    Vnodes
                        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.  If NOTE_EOF is
                        set in fflags, kevent() will also return when the file
                        pointer is at the end of file.  The end of file condi-
                        tion is indicated by the presence of NOTE_EOF in
                        fflags on return.

                    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.

                    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_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() 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_TRUNCATE  The file referenced by the descriptor was
                                   truncated.

                    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
                                   renamed.

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

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

     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 in the same format
                                     as the status set by wait(2).

                    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_FORK 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-
                                     tions.

                    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 coexists with the signal() and sigaction() facili-
                    ties, 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.

RETURN VALUES
     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 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.

ERRORS
     The kqueue() function 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() function 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.

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

SEE ALSO
     poll(2), read(2), select(2), sigaction(2), wait(2), write(2), signal(3)

HISTORY
     The kqueue() and kevent() functions first appeared in FreeBSD 4.1.

AUTHORS
     The kqueue() system and this manual page were written by Jonathan Lemon
     <jlemon@FreeBSD.org>.

BUGS
     It is currently not possible to watch FIFOs or AIO that reside on any-
     thing but a UFS file system.  Watching a vnode is possible on UFS, NFS
     and MS-DOS file systems.

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

BSD                             March 30, 2017                             BSD