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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 identified by an (ident, filter) pair and specifies the
     interesting conditions 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 conditions being watched, not an addition.

     The filter identified in a kevent is executed upon the initial registration 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 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 file descriptor as the identifier, and returns whenever there is data
		    available to read.	The behavior of the filter is slightly different depend-
		    ing 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-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 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 waiting for data to become available before returning.

     EVFILT_WRITE   Takes a file descriptor as the identifier, and returns whenever it is possi-
		    ble to write to the descriptor.  For sockets, pipes and fifos, data will con-
		    tain 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 sim-
				     ilar 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() 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   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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