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       inotify - monitoring file system events


       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),
       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.

       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 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.

       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

       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

           IN_ACCESS         File was accessed (read) (*).
           IN_ATTRIB         Metadata    changed   (permissions,   timestamps,
                             extended attributes, 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

       Two  additional  convenience  macros  are  IN_MOVE,  which  equates  to
       IN_MOVED_FROM|IN_MOVED_TO,    and    IN_CLOSE    which    equates    to

       The  following  further  bits  can  be  specified  in mask when calling

           IN_DONT_FOLLOW    Don’t dereference pathname if it  is  a  symbolic
                             link (since Linux 2.6.15).
           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        Only watch pathname if it is a  directory  (since
                             Linux 2.6.15).

       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

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

              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.

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

              This  specifies  a  limit  on  the number of watches that can be
              associated with each inotify instance.


       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.)


       The inotify API is Linux-specific.


       Inotify file descriptors can be monitored using select(2), poll(2), and

       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.

       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.


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


       inotify_add_watch(2),  inotify_init(2),  inotify_rm_watch(2),  read(2),
       stat(2), Documentation/filesystems/inotify.txt.


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