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

NAME
     kqueue, kevent, EV_SET -- kernel event notification mechanism

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
     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, fil-
     ter) 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(2) on a file de-
     scriptor 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 ar-
     ray 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	not NULL, it specifies
     a maximum interval	to wait	for an event, which will be interpreted	as a
     struct timespec.  If timeout is NULL, kevent() waits indefinitely.	 To
     effect a poll, the	timeout	argument should	not be NULL, pointing to a
     zero-valued struct	timespec.  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 */
	     int64_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 de-
		scriptor.

     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 at-
		    tached to file descriptors are automatically deleted on
		    the	last close of the descriptor.

     EV_RECEIPT	    Causes kevent() to return with EV_ERROR set	without	drain-
		    ing	any pending events after updating events in the
		    kqueue.  When a filter is successfully added the data
		    field will be zero.	 This flag is useful for making	bulk
		    changes to a kqueue.

     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 re-
		    set.  This is useful for filters which report state	tran-
		    sitions instead of the current state.  Note	that some fil-
		    ters 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(2)
			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" en-
			abled and there	is any data to read; data contains the
			number of bytes	available.

     EVFILT_EXCEPT  Takes a descriptor as the identifier, and returns whenever
		    one	of the specified exceptional conditions	has occurred
		    on the descriptor.	Conditions are specified in fflags.
		    Currently, a filter	can monitor the	reception of out-of-
		    band data on a socket or pseudo terminal with NOTE_OOB.

     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(2) 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(2).

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

		    NOTE_TRACK	     Follow a process across fork(2) 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(3) and sigaction(2) 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 re-
		    turns 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.

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

		    NOTE_CHANGE	   A device change event has occurred, e.g. an
				   HDMI	cable has been plugged in to a port.

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

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	re-
			turn.

     [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 and
     have been available since OpenBSD 2.9.

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

FreeBSD	13.0		       November	14, 2020		  FreeBSD 13.0

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

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