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
       directories.  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  descriptor referring to the inotify instance.  The more recent ino-
       tify_init1(2) is like inotify_init(2), but provides 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 descriptor", an  inte-
       ger 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  ino-
       tify_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
       kernels 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 inotify_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 inotify_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 (inotify_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 memory consumed by inotify:

       /proc/sys/fs/inotify/max_queued_events
	      The value in this file is used when an application calls inotify_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 gen-
	      erated.

       /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 indi-
       cates 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
       handler 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 renam-
       ing 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 subdirectories 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), inotify_rm_watch(2), read(2), stat(2), Documentation/filesystems/inotify.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)