Home Man
Today's Posts

Linux & Unix Commands - Search Man Pages
Man Page or Keyword Search:
Select Section of Man Page:
Select Man Page Repository:

NetBSD 6.1.5 - man page for kevent (netbsd section 2)

KQUEUE(2)			     BSD System Calls Manual				KQUEUE(2)

     kqueue, kqueue1, kevent -- kernel event notification mechanism

     Standard C Library (libc, -lc)

     #include <sys/event.h>
     #include <sys/time.h>


     kqueue1(int flags);

     kevent(int kq, const struct kevent *changelist, size_t nchanges, struct kevent *eventlist,
	 size_t nevents, const struct timespec *timeout);

     EV_SET(&kev, ident, filter, flags, fflags, data, udata);

     kqueue() provides a generic method of notifying the user when an event happens or a condi-
     tion holds, based on the results of small pieces of kernel code termed filters.  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.

     kqueue() creates a new kernel event queue and returns a descriptor.

     The kqueue1() also allows to set the following flags on the returned file descriptor:

	   O_CLOEXEC Set the close on exec property.
	   O_NONBLOCK Sets non-blocking I/O.
	   O_NOSIGPIPE Return EPIPE instead of raising SIGPIPE.
     The queue is not inherited by a child created with fork(2).

     kevent() is used to register events with the queue, and return any pending 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

     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 */
	     uint32_t  filter;	     /* filter for event */
	     uint32_t  flags;	     /* action flags for kqueue */
	     uint32_t  fflags;	     /* filter flag value */
	     int64_t   data;	     /* filter data value */
	     intptr_t  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.	There are pre-
		      defined system filters (which are described below), and other filters may
		      be added by kernel subsystems as necessary.

	   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 descrip-

	   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 cur-
			  rent state.  Note that some filters may automatically set this flag

	   EV_EOF	  Filters may set this flag to indicate filter-specific EOF condition.


     Filters are identified by a number.  There are two types of filters; pre-defined filters
     which are described below, and third-party filters that may be added with
     kfilter_register(9) by kernel sub-systems, third-party device drivers, or loadable kernel

     As a third-party filter is referenced by a well-known name instead of a statically assigned
     number, two ioctl(2)s are supported on the file descriptor returned by kqueue() to map a
     filter name to a filter number, and vice-versa (passing arguments in a structure described

	   KFILTER_BYFILTER  Map filter to name, which is of size len.

	   KFILTER_BYNAME    Map name to filter.  len is ignored.

     The following structure is used to pass arguments in and out of the ioctl(2):

	   struct kfilter_mapping {
		   char     *name;	   /* name to lookup or return */
		   size_t   len;	   /* length of name */
		   uint32_t filter;	   /* filter to lookup or return */

     Arguments may be passed to and from the filter via the fflags and data fields in the kevent

     The predefined system filters are:

     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 depend-
		    ing on the descriptor type.

			Sockets which have previously been passed to listen() return when there
			is an incoming connection pending.  data contains the size of the listen
			backlog (i.e., the number of connections ready to be accepted with

			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 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 buffer.

			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 there is data to read; data contains the number of bytes

			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 descriptor as the identifier, and returns whenever it is possible to
		    write to the descriptor.  For sockets, pipes, fifos, and ttys, 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

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

     EVFILT_AIO     This is not implemented in NetBSD.

     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

		    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-

		    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 current 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   Establishes an arbitrary timer identified by ident.  When adding a timer,
		    data specifies the timeout period in milliseconds.	The timer will be peri-
		    odic 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.

     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.

     The following example program monitors a file (provided to it as the first argument) and
     prints information about some common events it receives notifications for:

	   #include <sys/types.h>
	   #include <sys/event.h>
	   #include <sys/time.h>
	   #include <stdio.h>
	   #include <unistd.h>
	   #include <stdlib.h>
	   #include <fcntl.h>
	   #include <err.h>

	   main(int argc, char *argv[])
		   int fd, kq, nev;
		   struct kevent ev;
		   static const struct timespec tout = { 1, 0 };

		   if ((fd = open(argv[1], O_RDONLY)) == -1)
			   err(1, "Cannot open `%s'", argv[1]);

		   if ((kq = kqueue()) == -1)
			   err(1, "Cannot create kqueue");

		       NOTE_RENAME|NOTE_REVOKE, 0, 0);
		   if (kevent(kq, &ev, 1, NULL, 0, &tout) == -1)
			   err(1, "kevent");
		   for (;;) {
			   nev = kevent(kq, NULL, 0, &ev, 1, &tout);
			   if (nev == -1)
				   err(1, "kevent");
			   if (nev == 0)
			   if (ev.fflags & NOTE_DELETE) {
				   printf("deleted ");
				   ev.fflags &= ~NOTE_DELETE;
			   if (ev.fflags & NOTE_WRITE) {
				   printf("written ");
				   ev.fflags &= ~NOTE_WRITE;
			   if (ev.fflags & NOTE_EXTEND) {
				   printf("extended ");
				   ev.fflags &= ~NOTE_EXTEND;
			   if (ev.fflags & NOTE_ATTRIB) {
				   printf("chmod/chown/utimes ");
				   ev.fflags &= ~NOTE_ATTRIB;
			   if (ev.fflags & NOTE_LINK) {
				   printf("hardlinked ");
				   ev.fflags &= ~NOTE_LINK;
			   if (ev.fflags & NOTE_RENAME) {
				   printf("renamed ");
				   ev.fflags &= ~NOTE_RENAME;
			   if (ev.fflags & NOTE_REVOKE) {
				   printf("revoked ");
				   ev.fflags &= ~NOTE_REVOKE;
			   if (ev.fflags)
				   warnx("unknown event 0x%x\n", ev.fflags);

     The kqueue() function fails if:

     [EMFILE]		The per-process descriptor table is full.

     [ENFILE]		The system file table is full.

     [ENOMEM]		The kernel failed to allocate enough memory for the kernel queue.

     The kevent() function fails if:

     [EACCES]		The process does not have permission to register a filter.

     [EBADF]		The specified descriptor is invalid.

     [EFAULT]		There was an error reading or writing the kevent structure.

     [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.

     [EOPNOTSUPP]	This type of file descriptor is not supported for kevent() operations.

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

     ioctl(2), poll(2), read(2), select(2), sigaction(2), write(2), signal(3),
     kfilter_register(9), knote(9)

     Jonathan Lemon, "Kqueue: A Generic and Scalable Event Notification Facility", Proceedings of
     the FREENIX Track: 2001 USENIX Annual Technical Conference, USENIX Association,
     http://www.usenix.org/event/usenix01/freenix01/full_papers/lemon/lemon.pdf, June 25-30,

     The kqueue() and kevent() functions first appeared in FreeBSD 4.1, and then in NetBSD 2.0.
     The kqueue1() function first appeared in NetBSD 6.0.

BSD					November 24, 2012				      BSD

All times are GMT -4. The time now is 04:16 PM.

Unix & Linux Forums Content Copyrightę1993-2018. All Rights Reserved.
Show Password