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KQUEUE(2)		    BSD 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 kernel event (kevent) happens or a condition holds, based on the
     results of small pieces of kernel code termed filters.  A kevent is iden‐
     tified by an (ident, filter) pair and specifies the interesting condi‐
     tions to be notified about for that pair.	An (ident, filter) pair can
     only appear once is a given kqueue.  Subsequent attempts to register the
     same pair for a given kqueue will result in the replacement of the condi‐
     tions being watched, not an addition.

     The filter identified in a kevent is executed upon the initial registra‐
     tion of that event 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).

     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.  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 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_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 file descriptor as the identifier, and returns
		    whenever there is data available to read.  The behavior of
		    the filter is slightly different depending on the descrip‐
		    tor 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 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.

		    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.

     EVFILT_WRITE   Takes a file 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.

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

     EVFILT_AIO	    This filter is currently unsupported.

     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 fileystem 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.

		    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_TRACK 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   This filter is currently unsupported.

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.

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

SEE ALSO
     aio_error(2), aio_read(2), aio_return(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
     Not all filesystem types support kqueue-style notifications.  And even
     some that do, like some remote filesystems, may only support a subset of
     the notification semantics described here.

BSD				April 14, 2000				   BSD
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