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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)  (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

       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 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, 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

       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 (since Linux 2.6.15)
                             Don't dereference pathname if it  is  a  symbolic
           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

   /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 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 only added in 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
       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.

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

       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.


       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.


       inotify_add_watch(2),         inotify_init(2),        inotify_init1(2),
       inotify_rm_watch(2),                 read(2),                  stat(2),


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