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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 event hap-
     pens or a condition holds, based on the results of small pieces of kernel code termed fil-
     ters.  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 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).  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.

     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 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_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 descrip-
		  tors 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 trig-
		  gered.  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 filter 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, sub-
			     ject 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 con-
			     nection 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 con-
			     tains 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.

			 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 possi-
			 ble 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 sup-
			 ported 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 refer-
					 enced by the descriptor.

			 NOTE_WRITE	 A write occurred on the file referenced by the descrip-
					 tor.

			 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_PROCDESC	 Takes the process descriptor created by pdfork(2) to monitor as the
			 identifier and the events to watch for in fflags, and returns when the
			 associated process performs one or more of the requested events.  The
			 events to monitor are:

			 NOTE_EXIT     The process has exited.	The exit status will be stored in
				       data.

			 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, except for the SIGCHLD signal,
			 which, if ignored, won't be recorded by the filter.  Event notification
			 happens after normal signal delivery processing.  data returns the num-
			 ber 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.  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 sys-
			 tem wide limit on the number of timers which is controlled by the
			 kern.kq_calloutmax sysctl.

			 NOTE_SECONDS	   data is in seconds.

			 NOTE_MSECONDS	   data is in milliseconds.

			 NOTE_USECONDS	   data is in microseconds.

			 NOTE_NSECONDS	   data is in nanoseconds.

			 If fflags is not set, the default is milliseconds. 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 following:

			 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.

     [ENOMEM]		The RLIMIT_KQUEUES rlimit (see getrlimit(2)) for the current user would
			be exceeded.

     [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
			configurable 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.

BSD					  July 18, 2014 				      BSD
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