inotify


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

       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.

   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     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 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.,

       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 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 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
       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.

       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/ino-
       tify.txt.

COLOPHON
       This page is part of release 3.35 of the Linux  man-pages  project.   A
       description  of  the project, and information about reporting bugs, can
       be found at http://man7.org/linux/man-pages/.



Linux                             2010-11-12                        INOTIFY(7)
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