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

       inotify - monitoring filesystem events

       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) creates an  inotify  instance	 and  returns  a  file
	  descriptor  referring to the inotify instance.  The more recent ino‐
	  tify_init1(2) is like inotify_init(2), but has a flags argument that
	  provides access to some extra functionality.

       *  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.   inotify_add_watch(2) either creates a new watch item, or
	  modifies an existing watch.  Each watch has a unique "watch descrip‐
	  tor",	 an integer returned by inotify_add_watch(2) when the watch is

       *  When events occur for monitored files and directories, those	events
	  are made available to the application as structured data that can be
	  read from the inotify file descriptor using read(2) (see below).

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

       *  When all file descriptors referring to an inotify instance have been
	  closed (using close(2)), the underlying object and its resources are
	  freed for reuse by the kernel; all associated watches are  automati‐
	  cally freed.

	  With	careful	 programming,  an application can use inotify to effi‐
	  ciently monitor and cache the state of a set of filesystem  objects.
	  However,  robust applications should allow for the fact that bugs in
	  the monitoring logic or races of the kind described below may	 leave
	  the  cache  inconsistent  with the filesystem state.	It is probably
	  wise to to do some consistency checking, and rebuild the cache  when
	  inconsistencies are detected.

   Reading events from an inotify file descriptor
       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

       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 (e.g., read(2), execve(2)).

	   IN_ATTRIB (*)
		  Metadata  changed—for example, permissions (e.g., chmod(2)),
		  timestamps (e.g., utimensat(2)), extended attributes	(setx‐
		  attr(2)), link count (since Linux 2.6.25; e.g., for the tar‐
		  get of link(2) and for unlink(2)), and user/group ID	(e.g.,

		  File opened for writing was closed.

		  File not opened for writing was closed.

	   IN_CREATE (*)
		  File/directory  created  in watched directory (e.g., open(2)
		  O_CREAT, mkdir(2), link(2), symlink(2), bind(2)  on  a  UNIX
		  domain socket).

	   IN_DELETE (*)
		  File/directory deleted from watched directory.

		  Watched file/directory was itself deleted.  (This event also
		  occurs if an object is moved to  another  filesystem,	 since
		  mv(1)	 in effect copies the file to the other filesystem and
		  then deletes it from the original filesystem.)  In addition,
		  an  IN_IGNORED  event will subsequently be generated for the
		  watch descriptor.

	   IN_MODIFY (*)
		  File was modified (e.g., write(2), truncate(2)).

		  Watched file/directory was itself moved.

		  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‐

       Two additional convenience macros are defined:

		  Equates to IN_MOVED_FROM | IN_MOVED_TO.


       The  following  further bits can be specified in mask when calling ino‐

	   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 direc‐
		  tory, 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	create	tempo‐
		  rary	files whose names are immediately unlinked).  Specify‐
		  ing IN_EXCL_UNLINK changes the  default  behavior,  so  that
		  events  are  not generated for children after they have been
		  unlinked from the watched directory.

		  Add (OR) events to  watch  mask  for	this  pathname	if  it
		  already exists (instead of replacing mask).

		  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):

		  Watch	 was removed explicitly (inotify_rm_watch(2)) or auto‐
		  matically (file was deleted, or filesystem  was  unmounted).
		  See also BUGS.

		  Subject of this event is a directory.

		  Event queue overflowed (wd is -1 for this event).

		  Filesystem  containing  watched  object  was	unmounted.  In
		  addition, an IN_IGNORED event will subsequently be generated
		  for the watch descriptor.

       Suppose	an  application	 is  watching  the  directory dir and the file
       dir/myfile for all events.  The examples below show  some  events  that
       will be generated for these two objects.

	   fd = open("dir/myfile", O_RDWR);
		  Generates IN_OPEN events for both dir and dir/myfile.

	   read(fd, buf, count);
		  Generates IN_ACCESS events for both dir and dir/myfile.

	   write(fd, buf, count);
		  Generates IN_MODIFY events for both dir and dir/myfile.

	   fchmod(fd, mode);
		  Generates IN_ATTRIB events for both dir and dir/myfile.

		  Generates IN_CLOSE_WRITE events for both dir and dir/myfile.

       Suppose	an  application is watching the directories dir1 and dir2, and
       the file dir1/myfile.  The following examples show some events that may
       be generated.

	   link("dir1/myfile", "dir2/new");
		  Generates  an	 IN_ATTRIB  event  for myfile and an IN_CREATE
		  event for dir2.

	   rename("dir1/myfile", "dir2/myfile");
		  Generates an IN_MOVED_FROM event for	dir1,  an  IN_MOVED_TO
		  event	 for  dir2, and an IN_MOVE_SELF event for myfile.  The
		  IN_MOVED_FROM and IN_MOVED_TO	 events	 will  have  the  same
		  cookie value.

       Suppose that dir1/xx and dir2/yy are (the only) links to the same file,
       and an application is watching dir1, dir2, dir1/xx, and dir2/yy.	  Exe‐
       cuting  the  following calls in the order given below will generate the
       following events:

		  Generates an IN_ATTRIB event for xx (because its link	 count
		  changes) and an IN_DELETE event for dir2.

		  Generates  IN_ATTRIB,	 IN_DELETE_SELF, and IN_IGNORED events
		  for xx, and an IN_DELETE event for dir1.

       Suppose an application is watching the directory dir  and  (the	empty)
       directory dir/subdir.  The following examples show some events that may
       be generated.

	   mkdir("dir/new", mode);
		  Generates an IN_CREATE | IN_ISDIR event for dir.

		  Generates IN_DELETE_SELF and IN_IGNORED events  for  subdir,
		  and an IN_DELETE | IN_ISDIR event for dir.

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

	      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.

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

	      This  specifies an upper limit on the number of watches that can
	      be created per real user ID.

       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 glibc version 2.5.)

       The inotify API is Linux-specific.

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

       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).  This reduces the amount of kernel memory required
       for the event queue, but also means that an application can't use  ino‐
       tify to reliably count file events.

       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
       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‐

       Inotify	reports only events that a user-space program triggers through
       the filesystem API.  As a result, it does not catch remote events  that
       occur  on network filesystems.  (Applications must fall back to polling
       the filesystem to catch such  events.)	Furthermore,  various  pseudo-
       filesystems  such as /proc, /sys, and /dev/pts are not monitorable with

       The inotify API does not report file accesses  and  modifications  that
       may occur because of mmap(2) and msync(2).

       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.

       The  inotify  API  identifies  events via watch descriptors.  It is the
       application's responsibility to cache a	mapping	 (if  one  is  needed)
       between	watch  descriptors  and	 pathnames.   Be  aware that directory
       renamings may affect multiple cached pathnames.

       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.

       If monitoring an entire directory subtree, and a	 new  subdirectory  is
       created	in  that  tree	or  an existing directory is renamed into that
       tree, be aware that by the time you create a watch for the  new	subdi‐
       rectory,	 new  files  (and subdirectories) may already exist inside the
       subdirectory.  Therefore, you may want to scan the contents of the sub‐
       directory  immediately  after adding the watch (and, if desired, recur‐
       sively add watches for any subdirectories that it contains).

       Note that the event queue can overflow.	In this case, events are lost.
       Robust applications should handle the possibility of lost events grace‐
       fully.  For example, it may be necessary to rebuild part or all of  the
       application cache.  (One simple, but possibly expensive, approach is to
       close the inotify file descriptor, empty the cache, create a  new  ino‐
       tify  file descriptor, and then re-create watches and cache entries for
       the objects to be monitored.)

   Dealing with rename() events
       As noted above, the IN_MOVED_FROM and IN_MOVED_TO event	pair  that  is
       generated by rename(2) can be matched up via their shared cookie value.
       However, the task of matching has some challenges.

       These two events are usually consecutive in the event stream  available
       when  reading  from  the inotify file descriptor.  However, this is not
       guaranteed.  If multiple processes are triggering events for  monitored
       objects,	 then  (on rare occasions) an arbitrary number of other events
       may appear between the IN_MOVED_FROM and IN_MOVED_TO events.

       Matching up the IN_MOVED_FROM and IN_MOVED_TO event pair	 generated  by
       rename(2)  is thus inherently racy.  (Don't forget that if an object is
       renamed outside of a monitored directory, there	may  not  even	be  an
       IN_MOVED_TO  event.)  Heuristic approaches (e.g., assume the events are
       always consecutive) can be used to ensure a match in  most  cases,  but
       will  inevitably	 miss  some cases, causing the application to perceive
       the IN_MOVED_FROM and IN_MOVED_TO events as being unrelated.  If	 watch
       descriptors  are destroyed and re-created as a result, then those watch
       descriptors will be inconsistent with  the  watch  descriptors  in  any
       pending	events.	 (Re-creating the inotify file descriptor and rebuild‐
       ing the cache may be useful to deal with this scenario.)

       Applications  should  also  allow  for	the   possibility   that   the
       IN_MOVED_FROM  event  was  the  last event that could fit in the buffer
       returned	 by  the  current  call	 to  read(2),  and  the	  accompanying
       IN_MOVED_TO event might be fetched only on the next read(2).

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

       As  originally  designed	 and  implemented, the IN_ONESHOT flag did not
       cause an IN_IGNORED event to be generated when the  watch  was  dropped
       after  one  event.   However, as an unintended effect of other changes,
       since Linux 2.6.36, an IN_IGNORED event is generated in this case.

       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.

       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

       This  page  is  part of release 3.65 of the Linux man-pages project.  A
       description of the project, and information about reporting  bugs,  can
       be found at

Linux				  2014-04-01			    INOTIFY(7)

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