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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),
       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.

       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 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  (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 sizeof(ino-
       tify_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.

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

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.

       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 subdi-
       rectories under a directory, additional watches must be created.

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

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

COLOPHON
       This  page  is  part of release 3.05 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                             2008-05-15                        INOTIFY(7)