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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 efficiently
       monitor and cache the state of a set of filesystem  objects.   However,
       robust applications should allow for the fact that bugs in the monitor-
       ing logic or races of the kind described	 below	may  leave  the	 cache
       inconsistent with the filesystem state.	It is probably wise to do some
       consistency checking, and rebuild the cache  when  inconsistencies  are

   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 describing event */
	       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 filename within to the
       watched directory.  This filename is null-terminated, and  may  include
       further	null  bytes  ('\0')  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(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 or directory 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 or directory was opened.

       Inotify monitoring is inode-based: when monitoring a file (but not when
       monitoring  the directory containing a file), an event can be generated
       for activity on any link to the file (in the same or a different direc-

       When monitoring a directory:

       *  the  events marked above with an asterisk (*) can occur both for the
	  directory itself and for objects inside the directory; and

       *  the events marked with a plus sign (+) occur only for objects inside
	  the directory (not for the directory itself).

       Note:  when  monitoring	a  directory, events are not generated for the
       files inside the directory when the events are performed via a pathname
       (i.e., a link) that lies outside the monitored directory.

       When  events  are generated for objects inside a watched directory, 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.

		  If a watch instance already exists for the filesystem object
		  corresponding	 to  pathname,	add (OR) the events in mask to
		  the watch mask (instead of replacing the mask).

		  Monitor the filesystem object corresponding to pathname  for
		  one event, then remove from watch list.

	   IN_ONLYDIR (since Linux 2.6.15)
		  Watch	 pathname  only if it is a directory.  Using this flag
		  provides an application with a  race-free  way  of  ensuring
		  that the monitored object 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 set of watch descriptors that is being  monitored  via  an  inotify
       file  descriptor	 can  be  viewed  via  the  entry for the inotify file
       descriptor in the process's /proc/[pid]/fdinfo directory.  See  proc(5)
       for further details.  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), msync(2), and munmap(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.  Further-
       more, it is not guaranteed that the event pair is  atomically  inserted
       into  the  queue: there may be a brief interval where the IN_MOVED_FROM
       has appeared, but the IN_MOVED_TO has not.

       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), which
       should be done with a (small) timeout to allow for the fact that inser-
       tion  of	 the  IN_MOVED_FROM-IN_MOVED_TO	 event pair is not atomic, and
       also the possibility that there may not be any IN_MOVED_TO event.

       Before Linux 3.19, fallocate(2) did  not	 create	 any  inotify  events.
       Since Linux 3.19, calls to fallocate(2) generate IN_MODIFY events.

       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.

       When a watch descriptor is removed by calling  inotify_rm_watch(2)  (or
       because	a  watch file is deleted or the filesystem that contains it is
       unmounted), any pending unread events for that watch descriptor	remain
       available  to  read.   As  watch descriptors are subsequently allocated
       with inotify_add_watch(2), the kernel cycles through the range of  pos-
       sible  watch descriptors (0 to INT_MAX) incrementally.  When allocating
       a free watch descriptor, no check is made to  see  whether  that	 watch
       descriptor  number  has any pending unread events in the inotify queue.
       Thus, it can happen that a watch descriptor is  reallocated  even  when
       pending	unread	events	exist for a previous incarnation of that watch
       descriptor number, with the result that the application might then read
       those  events  and  interpret  them as belonging to the file associated
       with the newly recycled watch descriptor.  In practice, the  likelihood
       of  hitting  this  bug  may be extremely low, since it requires that an
       application cycle through INT_MAX watch descriptors,  release  a	 watch
       descriptor while leaving unread events for that watch descriptor in the
       queue, and then recycle that watch descriptor.  For  this  reason,  and
       because	there  have been no reports of the bug occurring in real-world
       applications, as of Linux 3.15, no kernel changes have yet been made to
       eliminate this possible bug.

       The  following  program	demonstrates the usage of the inotify API.  It
       marks the directories passed as a command-line arguments and waits  for
       events of type IN_OPEN, IN_CLOSE_NOWRITE and IN_CLOSE_WRITE.

       The   following	 output	  was	recorded   while   editing   the  file
       /home/user/temp/foo and listing directory /tmp.	Before	the  file  and
       the directory were opened, IN_OPEN events occurred.  After the file was
       closed, an IN_CLOSE_WRITE event	occurred.   After  the	directory  was
       closed,	an  IN_CLOSE_NOWRITE event occurred.  Execution of the program
       ended when the user pressed the ENTER key.

   Example output
	   $ ./a.out /tmp /home/user/temp
	   Press enter key to terminate.
	   Listening for events.
	   IN_OPEN: /home/user/temp/foo [file]
	   IN_CLOSE_WRITE: /home/user/temp/foo [file]
	   IN_OPEN: /tmp/ [directory]
	   IN_CLOSE_NOWRITE: /tmp/ [directory]

	   Listening for events stopped.

   Program source
       #include <errno.h>
       #include <poll.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/inotify.h>
       #include <unistd.h>

       /* Read all available inotify events from the file descriptor 'fd'.
	  wd is the table of watch descriptors for the directories in argv.
	  argc is the length of wd and argv.
	  argv is the list of watched directories.
	  Entry 0 of wd and argv is unused. */

       static void
       handle_events(int fd, int *wd, int argc, char* argv[])
	   /* Some systems cannot read integer variables if they are not
	      properly aligned. On other systems, incorrect alignment may
	      decrease performance. Hence, the buffer used for reading from
	      the inotify file descriptor should have the same alignment as
	      struct inotify_event. */

	   char buf[4096]
	       __attribute__ ((aligned(__alignof__(struct inotify_event))));
	   const struct inotify_event *event;
	   int i;
	   ssize_t len;
	   char *ptr;

	   /* Loop while events can be read from inotify file descriptor. */

	   for (;;) {

	       /* Read some events. */

	       len = read(fd, buf, sizeof buf);
	       if (len == -1 && errno != EAGAIN) {

	       /* If the nonblocking read() found no events to read, then
		  it returns -1 with errno set to EAGAIN. In that case,
		  we exit the loop. */

	       if (len <= 0)

	       /* Loop over all events in the buffer */

	       for (ptr = buf; ptr < buf + len;
		       ptr += sizeof(struct inotify_event) + event->len) {

		   event = (const struct inotify_event *) ptr;

		   /* Print event type */

		   if (event->mask & IN_OPEN)
		       printf("IN_OPEN: ");
		   if (event->mask & IN_CLOSE_NOWRITE)
		       printf("IN_CLOSE_NOWRITE: ");
		   if (event->mask & IN_CLOSE_WRITE)
		       printf("IN_CLOSE_WRITE: ");

		   /* Print the name of the watched directory */

		   for (i = 1; i < argc; ++i) {
		       if (wd[i] == event->wd) {
			   printf("%s/", argv[i]);

		   /* Print the name of the file */

		   if (event->len)
		       printf("%s", event->name);

		   /* Print type of filesystem object */

		   if (event->mask & IN_ISDIR)
		       printf(" [directory]\n");
		       printf(" [file]\n");

       main(int argc, char* argv[])
	   char buf;
	   int fd, i, poll_num;
	   int *wd;
	   nfds_t nfds;
	   struct pollfd fds[2];

	   if (argc < 2) {
	       printf("Usage: %s PATH [PATH ...]\n", argv[0]);

	   printf("Press ENTER key to terminate.\n");

	   /* Create the file descriptor for accessing the inotify API */

	   fd = inotify_init1(IN_NONBLOCK);
	   if (fd == -1) {

	   /* Allocate memory for watch descriptors */

	   wd = calloc(argc, sizeof(int));
	   if (wd == NULL) {

	   /* Mark directories for events
	      - file was opened
	      - file was closed */

	   for (i = 1; i < argc; i++) {
	       wd[i] = inotify_add_watch(fd, argv[i],
					 IN_OPEN | IN_CLOSE);
	       if (wd[i] == -1) {
		   fprintf(stderr, "Cannot watch '%s'\n", argv[i]);

	   /* Prepare for polling */

	   nfds = 2;

	   /* Console input */

	   fds[0].fd = STDIN_FILENO;
	   fds[0].events = POLLIN;

	   /* Inotify input */

	   fds[1].fd = fd;
	   fds[1].events = POLLIN;

	   /* Wait for events and/or terminal input */

	   printf("Listening for events.\n");
	   while (1) {
	       poll_num = poll(fds, nfds, -1);
	       if (poll_num == -1) {
		   if (errno == EINTR)

	       if (poll_num > 0) {

		   if (fds[0].revents & POLLIN) {

		       /* Console input is available. Empty stdin and quit */

		       while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\n')

		   if (fds[1].revents & POLLIN) {

		       /* Inotify events are available */

		       handle_events(fd, wd, argc, argv);

	   printf("Listening for events stopped.\n");

	   /* Close inotify file descriptor */



       inotifywait(1), inotifywatch(1), inotify_add_watch(2), inotify_init(2),
       inotify_init1(2), inotify_rm_watch(2), read(2), stat(2), fanotify(7)

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

       This  page  is  part of release 4.10 of the Linux man-pages project.  A
       description of the project, information about reporting bugs,  and  the
       latest	  version     of     this    page,    can    be	   found    at

Linux				  2016-12-12			    INOTIFY(7)