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KQUEUE(2)		  OpenBSD Programmer's Manual		     KQUEUE(2)

     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);

     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
     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.

     kqueue() 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.

     kevent() is used to register events with the queue, and return any
     pending 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
     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.

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

     The kevent structure is defined as:

     struct kevent {
	     u_int   ident;	     /* identifier for this event */
	     short   filter;	     /* filter for event */
	     u_short flags;	     /* action flags for kqueue */
	     u_int   fflags;	     /* filter flag value */
	     int     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_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

     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 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.  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
			condition 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
			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_* function is called, the
		    event will be registered with the specified kqueue, and
		    the ident argument set to the struct aiocb returned by the
		    aio_* function.  The filter returns under the same
		    conditions as aio_error.

		    Alternatively, a kevent structure may be initialized, with
		    ident containing the descriptor of the kqueue, and the
		    address of the kevent structure placed in the
		    aio_lio_opcode field of the AIO request.  However, this
		    approach will not work on architectures with 64-bit
		    pointers, and should be considered deprecated.

     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

		    NOTE_TRUNCATE  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.

		    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

		    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.

     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.

     The kqueue() function fails if:

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

     [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.

     poll(2), read(2), select(2), sigaction(2), write(2), signal(3)

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

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

     It is currently not possible to watch FIFOs, AIO, or a vnode that resides
     on anything but a UFS file system.

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

OpenBSD 4.9			 May 31, 2007			   OpenBSD 4.9

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