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INOTIFY(7)		   Linux Programmer's Manual		    INOTIFY(7)

NAME
       inotify - monitoring file system	events

DESCRIPTION
       The inotify API provides	a mechanism for	monitoring file	system events.
       Inotify can be used to monitor individual files,	or to monitor directo-
       ries.   When  a	directory is monitored,	inotify	will return events for
       the directory itself, and for files inside the directory.

       The following system calls are used with	this API: inotify_init(2)  (or
       inotify_init1(2)),  inotify_add_watch(2), inotify_rm_watch(2), read(2),
       and close(2).

       inotify_init(2) creates an inotify instance and returns a file descrip-
       tor   referring	 to  the  inotify  instance.   The  more  recent  ino-
       tify_init1(2) is	like inotify_init(2), but provides  some  extra	 func-
       tionality.

       inotify_add_watch(2)  manipulates  the  "watch list" associated with an
       inotify instance.  Each item ("watch") in the watch list	specifies  the
       pathname	of a file or directory,	along with some	set of events that the
       kernel should monitor for the file referred to by that pathname.	  ino-
       tify_add_watch(2)  either  creates  a  new  watch  item,	or modifies an
       existing	watch.	Each watch has a unique	"watch descriptor", an integer
       returned	by inotify_add_watch(2)	when the watch is created.

       inotify_rm_watch(2) removes an item from	an inotify watch list.

       When  all  file	descriptors referring to an inotify instance have been
       closed, the underlying object and its resources are freed for reuse  by
       the kernel; all associated watches are automatically freed.

       To  determine  what  events have	occurred, an application read(2)s from
       the inotify file	descriptor.  If	no events have so far occurred,	 then,
       assuming	 a blocking file descriptor, read(2) will block	until at least
       one event occurs	(unless	interrupted by a signal,  in  which  case  the
       call fails with the error EINTR;	see signal(7)).

       Each  successful	read(2)	returns	a buffer containing one	or more	of the
       following structures:

	   struct inotify_event	{
	       int	wd;	  /* Watch descriptor */
	       uint32_t	mask;	  /* Mask of events */
	       uint32_t	cookie;	  /* Unique cookie associating related
				     events (for rename(2)) */
	       uint32_t	len;	  /* Size of name field	*/
	       char	name[];	  /* Optional null-terminated name */
	   };

       wd identifies the watch for which this event occurs.  It	is one of  the
       watch descriptors returned by a previous	call to	inotify_add_watch(2).

       mask contains bits that describe	the event that occurred	(see below).

       cookie  is  a  unique  integer that connects related events.  Currently
       this is only used for rename events, and	allows the resulting  pair  of
       IN_MOVE_FROM and	IN_MOVE_TO events to be	connected by the application.

       The  name  field	 is  only present when an event	is returned for	a file
       inside a	watched	directory; it identifies the file pathname relative to
       the  watched  directory.	  This	pathname  is  null-terminated, and may
       include further null bytes to align  subsequent	reads  to  a  suitable
       address boundary.

       The  len	 field	counts	all  of	 the bytes in name, including the null
       bytes; the length of each inotify_event structure is  thus  sizeof(ino-
       tify_event)+len.

       The  behavior  when  the	buffer given to	read(2)	is too small to	return
       information about the next event	depends	on the kernel version: in ker-
       nels  before  2.6.21,  read(2)  returns 0; since	kernel 2.6.21, read(2)
       fails with the error EINVAL.

   inotify events
       The inotify_add_watch(2)	mask argument and the mask field of  the  ino-
       tify_event  structure returned when read(2)ing an inotify file descrip-
       tor are both bit	masks identifying inotify events.  The following  bits
       can  be	specified in mask when calling inotify_add_watch(2) and	may be
       returned	in the mask field returned by read(2):

	   IN_ACCESS	     File was accessed (read) (*).
	   IN_ATTRIB	     Metadata changed, e.g., permissions,  timestamps,
			     extended  attributes,  link  count	 (since	 Linux
			     2.6.25), UID, GID,	etc. (*).
	   IN_CLOSE_WRITE    File opened for writing was closed	(*).
	   IN_CLOSE_NOWRITE  File not opened for writing was closed (*).
	   IN_CREATE	     File/directory created in watched directory (*).
	   IN_DELETE	     File/directory  deleted  from  watched  directory
			     (*).
	   IN_DELETE_SELF    Watched file/directory was	itself deleted.
	   IN_MODIFY	     File was modified (*).
	   IN_MOVE_SELF	     Watched file/directory was	itself moved.
	   IN_MOVED_FROM     File moved	out of watched directory (*).
	   IN_MOVED_TO	     File moved	into watched directory (*).
	   IN_OPEN	     File was opened (*).

       When  monitoring	 a  directory,	the events marked with an asterisk (*)
       above can occur for files in the	directory,  in	which  case  the  name
       field  in  the  returned	inotify_event structure	identifies the name of
       the file	within the directory.

       The IN_ALL_EVENTS macro is defined as a bit mask	of all	of  the	 above
       events.	 This macro can	be used	as the mask argument when calling ino-
       tify_add_watch(2).

       Two  additional	convenience  macros  are  IN_MOVE,  which  equates  to
       IN_MOVED_FROM|IN_MOVED_TO,    and    IN_CLOSE	which	 equates    to
       IN_CLOSE_WRITE|IN_CLOSE_NOWRITE.

       The following further bits can be specified in mask when	 calling  ino-
       tify_add_watch(2):

	   IN_DONT_FOLLOW (since Linux 2.6.15)
			     Don't  dereference	 pathname  if it is a symbolic
			     link.
	   IN_MASK_ADD	     Add (OR) events to	watch mask for	this  pathname
			     if	it already exists (instead of replacing	mask).
	   IN_ONESHOT	     Monitor  pathname for one event, then remove from
			     watch list.
	   IN_ONLYDIR (since Linux 2.6.15)
			     Only watch	pathname if it is a directory.

       The following bits may be set in	the mask field returned	by read(2):

	   IN_IGNORED	     Watch    was     removed	  explicitly	 (ino-
			     tify_rm_watch(2))	 or  automatically  (file  was
			     deleted, or file system was unmounted).
	   IN_ISDIR	     Subject of	this event is a	directory.
	   IN_Q_OVERFLOW     Event queue overflowed (wd	is -1 for this event).
	   IN_UNMOUNT	     File  system  containing	watched	  object   was
			     unmounted.

   /proc interfaces
       The following interfaces	can be used to limit the amount	of kernel mem-
       ory consumed by inotify:

       /proc/sys/fs/inotify/max_queued_events
	      The value	in this	file is	used when an  application  calls  ino-
	      tify_init(2)  to set an upper limit on the number	of events that
	      can be queued to the corresponding inotify instance.  Events  in
	      excess  of this limit are	dropped, but an	IN_Q_OVERFLOW event is
	      always generated.

       /proc/sys/fs/inotify/max_user_instances
	      This specifies an	upper limit on the number of inotify instances
	      that can be created per real user	ID.

       /proc/sys/fs/inotify/max_user_watches
	      This  specifies an upper limit on	the number of watches that can
	      be created per real user ID.

VERSIONS
       Inotify was merged into the 2.6.13 Linux	kernel.	 The required  library
       interfaces  were	 added	to  glibc  in  version	2.4.  (IN_DONT_FOLLOW,
       IN_MASK_ADD, and	IN_ONLYDIR were	only added in version 2.5.)

CONFORMING TO
       The inotify API is Linux-specific.

NOTES
       Inotify file descriptors	can be monitored using select(2), poll(2), and
       epoll(7).  When an event	is available, the file descriptor indicates as
       readable.

       Since Linux 2.6.25, signal-driven I/O  notification  is	available  for
       inotify	file  descriptors;  see	the discussion of F_SETFL (for setting
       the O_ASYNC flag), F_SETOWN, and	F_SETSIG in fcntl(2).	The  siginfo_t
       structure (described in sigaction(2)) that is passed to the signal han-
       dler has	the following fields set: si_fd	is set	to  the	 inotify  file
       descriptor number; si_signo is set to the signal	number;	si_code	is set
       to POLL_IN; and POLLIN is set in	si_band.

       If successive output  inotify  events  produced	on  the	 inotify  file
       descriptor  are	identical  (same wd, mask, cookie, and name) then they
       are coalesced into a single event if the	older event has	not  yet  been
       read (but see BUGS).

       The  events returned by reading from an inotify file descriptor form an
       ordered queue.  Thus, for example, it is	guaranteed that	when  renaming
       from  one  directory to another,	events will be produced	in the correct
       order on	the inotify file descriptor.

       The FIONREAD ioctl(2) returns the number	of  bytes  available  to  read
       from an inotify file descriptor.

       Inotify	monitoring  of directories is not recursive: to	monitor	subdi-
       rectories under a directory, additional watches must be created.

BUGS
       In kernels before 2.6.16, the IN_ONESHOT	mask flag does not work.

       Before kernel 2.6.25, the kernel	code that  was	intended  to  coalesce
       successive  identical  events  (i.e.,  the two most recent events could
       potentially be coalesced	if the older had not yet  been	read)  instead
       checked	if  the	 most  recent event could be coalesced with the	oldest
       unread event.

SEE ALSO
       inotify_add_watch(2),	inotify_init(2),    inotify_init1(2),	  ino-
       tify_rm_watch(2),   read(2),   stat(2),	Documentation/filesystems/ino-
       tify.txt.

COLOPHON
       This page is part of release 3.25 of the	Linux  man-pages  project.   A
       description  of	the project, and information about reporting bugs, can
       be found	at http://www.kernel.org/doc/man-pages/.

Linux				  2008-11-18			    INOTIFY(7)

NAME | DESCRIPTION | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHON

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