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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
     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() 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 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.
     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
     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 */
	     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 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
		 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.

		    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 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_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 with NOTE_OOB.

     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 condi‐
		    tions 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 point‐
		    ers, 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
				 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   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
     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() 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 a vnode(9) that resides on anything
     but a UFS(5) or a HAMMER(5) file system.

BSD			       December 3, 2008				   BSD
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