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KQUEUE(2)		  FreeBSD System Calls Manual		     KQUEUE(2)

NAME
     kqueue, kevent -- kernel event notification mechanism

LIBRARY
     Standard C	Library	(libc, -lc)

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

     int
     kqueue(void);

     int
     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);

DESCRIPTION
     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 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.

     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 descrip-
     tor 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 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.

     The EV_SET() macro	is provided for	ease of	initializing a kevent struc-
     ture.

     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 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_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
		    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 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.

		    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.

		    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	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_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_notify_kevent_flags containing the kevent flags
		    which should be EV_ONESHOT,	EV_CLEAR or EV_DISPATCH,
		    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
				   extended.

		    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.

		    NOTE_FORK	     The process has called fork().

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

		    NOTE_TRACK	     Follow a process across fork() calls.
				     The parent	process	registers a new	kevent
				     to	monitor	the child process using	the
				     same fflags as the	original event.	 The
				     child process will	signal an event	with
				     NOTE_CHILD	set in fflags and the parent
				     PID in data.

				     If	the parent process fails to register a
				     new kevent	(usually due to	resource
				     limitations), it will signal an event
				     with NOTE_TRACKERR	set in fflags, and the
				     child process will	not signal a
				     NOTE_CHILD	event.

		    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, except for the SIGCHLD
		    signal, which, if ignored, won't be	recorded by the	fil-
		    ter.  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.  There is a system	wide limit on the num-
		    ber	of timers which	is controlled by the
		    kern.kq_calloutmax sysctl.

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

     EVFILT_USER    Establishes	a user event identified	by ident which is not
		    associated 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 fol-
		    lowing:

		    NOTE_FFNOP	       Ignore the input	fflags.

		    NOTE_FFAND	       Bitwise AND fflags.

		    NOTE_FFOR	       Bitwise OR fflags.

		    NOTE_FFCOPY	       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.

RETURN VALUES
     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.

ERRORS
     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 config-
			urable via the kern.kq_calloutmax sysctl.

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

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

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

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

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

FreeBSD	10.1		       January 21, 2013			  FreeBSD 10.1

NAME | LIBRARY | SYNOPSIS | DESCRIPTION | RETURN VALUES | ERRORS | SEE ALSO | HISTORY | AUTHORS | BUGS

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