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

NAME
       inotify - monitoring filesystem events

DESCRIPTION
       The  inotify API provides a mechanism for monitoring filesystem 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 used only for rename events, and allows the resulting  pair  of
       IN_MOVED_FROM  and  IN_MOVED_TO	events to be connected by the applica‐
       tion.  For all other event types, cookie is set to 0.

       The name field is present only 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 ('\0') to align subsequent reads to a suit‐
       able 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(struct
       inotify_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.  Specifying a buffer of size

	   sizeof(struct inotify_event) + NAME_MAX + 1

       will be sufficient to read at least one event.

   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     Generated for the directory  containing  the  old
			     filename when a file is renamed (*).
	   IN_MOVED_TO	     Generated	for  the  directory containing the new
			     filename when a file is renamed (*).
	   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_EXCL_UNLINK (since Linux 2.6.36)
			     By	 default, when watching events on the children
			     of a directory, events are generated for children
			     even  after  they	have  been  unlinked  from the
			     directory.	 This can result in large  numbers  of
			     uninteresting events for some applications (e.g.,
			     if watching /tmp, in which many applications cre‐
			     ate  temporary  files whose names are immediately
			     unlinked).	 Specifying IN_EXCL_UNLINK changes the
			     default  behavior,	 so that events are not gener‐
			     ated for children after they have	been  unlinked
			     from the watched directory.
	   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 filesystem 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	     Filesystem	   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 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.

   Limitations and caveats
       Inotify monitoring of directories is not recursive: to  monitor	subdi‐
       rectories  under a directory, additional watches must be created.  This
       can take a significant amount time for large directory trees.

       The inotify API provides no information about the user or process  that
       triggered the inotify event.  In particular, there is no easy way for a
       process that is monitoring events via  inotify  to  distinguish	events
       that  it	 triggers  itself  from those that are triggered by other pro‐
       cesses.

       Note that the event queue can overflow.	In this case, events are lost.
       Robust applications should handle the possibility of lost events grace‐
       fully.

       The inotify API identifies affected files by filename.  However, by the
       time  an	 application  processes	 an  inotify  event,  the filename may
       already have been deleted or renamed.

       If monitoring an entire directory subtree, and a	 new  subdirectory  is
       created	in that tree, be aware that by the time you create a watch for
       the new subdirectory, new files may already have been  created  in  the
       subdirectory.  Therefore, you may want to scan the contents of the sub‐
       directory immediately after adding the watch.

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
       inotifywait(1), inotifywatch(1), inotify_add_watch(2), inotify_init(2),
       inotify_init1(2), inotify_rm_watch(2), read(2), stat(2)

       Documentation/filesystems/inotify.txt in the Linux kernel source tree

COLOPHON
       This  page  is  part of release 3.54 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				  2013-09-16			    INOTIFY(7)
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