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           The  inotify API provides a mechanism for monitoring filesystem 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-
           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).
           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--for example, permissions, times-
                                 tamps,  extended  attributes,  link  count (since
                                 Linux 2.6.25), UID, or GID. (*).
               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-
           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 ino-
               IN_DONT_FOLLOW (since Linux 2.6.15)
               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 filesystem 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        Filesystem    containing   watched   object   was
       /proc interfaces
           The following interfaces can be used to limit the amount of kernel mem-
           ory consumed by inotify:
                  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.
                  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 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
           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-
           Note that the event queue can overflow.  In this case, events are lost.
           Robust applications should handle the possibility of lost events grace-
           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.


           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.


           inotifywait(1), inotifywatch(1), 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

    Linux 2013-09-16 INOTIFY(7)


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