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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 individual files, or to monitor directo-
       ries.   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 descrip-
       tor   referring   to  the  inotify  instance.   The  more  recent  ino-
       tify_init1(2) is like inotify_init(2), but provides  some  extra  func-
       tionality.
       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.   ino-
       tify_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 applica-
       tion.  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 ('\0') to align subsequent reads to a suit-
       able 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 ker-
       nels 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 ino-
       tify_event structure returned when read(2)ing an inotify file  descrip-
       tor  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     Generated for the directory  containing  the  old
                             filename when a file is renamed (*).
           IN_MOVED_TO       Generated  for  the  directory containing the new
                             filename when a file is renamed (*).
           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  ino-
       tify_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 ino-
       tify_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 cre-
                             ate  temporary  files whose names are immediately
                             unlinked).  Specifying IN_EXCL_UNLINK changes the
                             default  behavior,  so that events are not gener-
                             ated 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    (ino-
                             tify_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 mem-
       ory consumed by inotify:
       /proc/sys/fs/inotify/max_queued_events
              The  value  in  this file is used when an application calls ino-
              tify_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  version  2.4.   (IN_DONT_FOLLOW,
       IN_MASK_ADD, and IN_ONLYDIR were 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 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 han-
       dler  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  subdi-
       rectories  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 pro-
       cesses.
       Note that the event queue can overflow.  In this case, events are lost.
       Robust applications should handle the possibility of lost events grace-
       fully.
       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 sub-
       directory immediately after adding the watch.
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),     ino-
       tify_rm_watch(2), read(2), stat(2)
       Documentation/filesystems/inotify.txt in the Linux kernel source tree
COLOPHON
       This  page  is  part of release 3.53 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                             2013-07-21                        INOTIFY(7)