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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 indi-
       vidual 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  ino-
       tify_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 re-use 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 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(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.

   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_MASK_ADD       Add (OR) events to watch mask for this pathname if it already exists (instead of  replac-
                             ing 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
       Inotify was merged into the 2.6.13 Linux kernel.  The required library interfaces were added to glibc  in  ver-
       sion 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 indicates as readable.

       Since Linux 2.6.25, signal-driven I/O notification is available for inotify file descriptors; see  the  discus-
       sion  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.

       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), Documenta-
       tion/filesystems/inotify.txt.

COLOPHON
       This page is part of release 3.22 of the Linux man-pages project.  A description of the project,  and  informa-
       tion about reporting bugs, can be found at http://www.kernel.org/doc/man-pages/.



Linux                             2008-11-18                        INOTIFY(7)