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

NAME         top

       inotify - monitoring file system events

DESCRIPTION         top

       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
       inotify_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 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
       application.  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 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 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
       kernels 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
       inotify_event structure returned when read(2)ing an inotify file
       descriptor 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
       inotify_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_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 create temporary files whose
                             names are immediately unlinked).  Specifying
                             IN_EXCL_UNLINK changes the default behavior, so
                             that events are not generated 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
                             (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 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         top

       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         top

       The inotify API is Linux-specific.

NOTES         top

       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
       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
       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
       subdirectories 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 processes.

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

       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 subdirectory immediately after adding the watch.

BUGS         top

       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         top

       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         top

       This page is part of release 3.51 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                            2013-02-25                       INOTIFY(7)

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