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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_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 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(struct

       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

           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)

       Documentation/filesystems/inotify.txt in the Linux kernel source tree


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