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NAME

       inotify - monitoring filesystem events

DESCRIPTION

       The  inotify API provides a mechanism for monitoring filesystem 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) 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 has a  flags  argument
          that provides access to some extra functionality.

       *  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.

       *  When events occur for monitored files and directories, those  events
          are made available to the application as structured data that can be
          read from the inotify file descriptor using read(2) (see below).

       *  inotify_rm_watch(2) removes an item from an inotify watch list.

       *  When all file descriptors referring to an inotify instance have been
          closed (using close(2)), the underlying object and its resources are
          freed  for  reuse  by  the  kernel;  all  associated   watches   are
          automatically freed.

          With   careful  programming,  an  application  can  use  inotify  to
          efficiently monitor and cache the  state  of  a  set  of  filesystem
          objects.   However,  robust  applications  should allow for the fact
          that bugs in the monitoring logic or races  of  the  kind  described
          below  may  leave  the cache inconsistent with the filesystem state.
          It is probably wise to to do some consistency checking, and  rebuild
          the cache when inconsistencies are detected.

   Reading events from an inotify file descriptor
       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
       application.  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  ('')  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
       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
       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
       read(2):

           IN_ACCESS (*)
                  File was accessed (e.g., read(2), execve(2)).

           IN_ATTRIB (*)
                  Metadata changed—for example, permissions (e.g.,  chmod(2)),
                  timestamps   (e.g.,   utimensat(2)),   extended   attributes
                  (setxattr(2)), link count (since Linux 2.6.25; e.g., for the
                  target  of  link(2)  and  for  unlink(2)), and user/group ID
                  (e.g., chown(2)).

           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  (e.g.,  open(2)
                  O_CREAT,  mkdir(2),  link(2),  symlink(2), bind(2) on a UNIX
                  domain socket).

           IN_DELETE (*)
                  File/directory deleted from watched directory.

           IN_DELETE_SELF
                  Watched file/directory was itself deleted.  (This event also
                  occurs  if  an  object is moved to another filesystem, since
                  mv(1) in effect copies the file to the other filesystem  and
                  then deletes it from the original filesystem.)  In addition,
                  an IN_IGNORED event will subsequently be generated  for  the
                  watch descriptor.

           IN_MODIFY (*)
                  File was modified (e.g., write(2), truncate(2)).

           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
       inotify_add_watch(2).

       Two additional convenience macros are defined:

           IN_MOVE
                  Equates to IN_MOVED_FROM | IN_MOVED_TO.

           IN_CLOSE
                  Equates to IN_CLOSE_WRITE | IN_CLOSE_NOWRITE.

       The following further bits  can  be  specified  in  mask  when  calling
       inotify_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 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  filesystem  was
                  unmounted).  See also BUGS.

           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  unmounted.   In
                  addition, an IN_IGNORED event will subsequently be generated
                  for the watch descriptor.

   Examples
       Suppose an application is watching  the  directory  dir  and  the  file
       dir/myfile  for  all  events.  The examples below show some events that
       will be generated for these two objects.

           fd = open("dir/myfile", O_RDWR);
                  Generates IN_OPEN events for both dir and dir/myfile.

           read(fd, buf, count);
                  Generates IN_ACCESS events for both dir and dir/myfile.

           write(fd, buf, count);
                  Generates IN_MODIFY events for both dir and dir/myfile.

           fchmod(fd, mode);
                  Generates IN_ATTRIB events for both dir and dir/myfile.

           close(fd);
                  Generates IN_CLOSE_WRITE events for both dir and dir/myfile.

       Suppose an application is watching the directories dir1 and  dir2,  and
       the file dir1/myfile.  The following examples show some events that may
       be generated.

           link("dir1/myfile", "dir2/new");
                  Generates an IN_ATTRIB event for  myfile  and  an  IN_CREATE
                  event for dir2.

           rename("dir1/myfile", "dir2/myfile");
                  Generates  an  IN_MOVED_FROM  event for dir1, an IN_MOVED_TO
                  event for dir2, and an IN_MOVE_SELF event for  myfile.   The
                  IN_MOVED_FROM  and  IN_MOVED_TO  events  will  have the same
                  cookie value.

       Suppose that dir1/xx and dir2/yy are (the only) links to the same file,
       and  an  application  is  watching  dir1,  dir2,  dir1/xx, and dir2/yy.
       Executing the following calls in the order given  below  will  generate
       the following events:

           unlink("dir2/yy");
                  Generates  an IN_ATTRIB event for xx (because its link count
                  changes) and an IN_DELETE event for dir2.

           unlink("dir1/xx");
                  Generates IN_ATTRIB, IN_DELETE_SELF, and  IN_IGNORED  events
                  for xx, and an IN_DELETE event for dir1.

       Suppose  an  application  is watching the directory dir and (the empty)
       directory dir/subdir.  The following examples show some events that may
       be generated.

           mkdir("dir/new", mode);
                  Generates an IN_CREATE | IN_ISDIR event for dir.

           rmdir("dir/subdir");
                  Generates  IN_DELETE_SELF  and IN_IGNORED events for subdir,
                  and an IN_DELETE | IN_ISDIR event for dir.

   /proc interfaces
       The following interfaces can be used to  limit  the  amount  of  kernel
       memory consumed by inotify:

       /proc/sys/fs/inotify/max_queued_events
              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.

       /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 glibc 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
       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).  This reduces the amount of kernel memory required
       for  the  event  queue,  but  also  means that an application can't use
       inotify to reliably count file events.

       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
       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
       processes.

       Inotify reports only events that a user-space program triggers  through
       the  filesystem API.  As a result, it does not catch remote events that
       occur on network filesystems.  (Applications must fall back to  polling
       the  filesystem  to  catch  such events.)  Furthermore, various pseudo-
       filesystems such as /proc, /sys, and /dev/pts are not monitorable  with
       inotify.

       The  inotify  API  does not report file accesses and modifications that
       may occur because of mmap(2) and msync(2).

       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.

       The inotify API identifies events via watch  descriptors.   It  is  the
       application's  responsibility  to  cache  a  mapping (if one is needed)
       between watch descriptors  and  pathnames.   Be  aware  that  directory
       renamings may affect multiple cached pathnames.

       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.

       If  monitoring  an  entire directory subtree, and a new subdirectory is
       created in that tree or an existing  directory  is  renamed  into  that
       tree,  be  aware  that  by  the  time  you  create  a watch for the new
       subdirectory, new files (and subdirectories) may already  exist  inside
       the  subdirectory.  Therefore, you may want to scan the contents of the
       subdirectory immediately after  adding  the  watch  (and,  if  desired,
       recursively add watches for any subdirectories that it contains).

       Note that the event queue can overflow.  In this case, events are lost.
       Robust applications  should  handle  the  possibility  of  lost  events
       gracefully.  For example, it may be necessary to rebuild part or all of
       the application cache.  (One simple, but possibly  expensive,  approach
       is  to close the inotify file descriptor, empty the cache, create a new
       inotify file descriptor, and then re-create watches and  cache  entries
       for the objects to be monitored.)

   Dealing with rename() events
       As  noted  above,  the IN_MOVED_FROM and IN_MOVED_TO event pair that is
       generated by rename(2) can be matched up via their shared cookie value.
       However, the task of matching has some challenges.

       These  two events are usually consecutive in the event stream available
       when reading from the inotify file descriptor.  However,  this  is  not
       guaranteed.   If multiple processes are triggering events for monitored
       objects, then (on rare occasions) an arbitrary number of  other  events
       may appear between the IN_MOVED_FROM and IN_MOVED_TO events.

       Matching  up  the IN_MOVED_FROM and IN_MOVED_TO event pair generated by
       rename(2) is thus inherently racy.  (Don't forget that if an object  is
       renamed  outside  of  a  monitored  directory, there may not even be an
       IN_MOVED_TO event.)  Heuristic approaches (e.g., assume the events  are
       always  consecutive)  can  be used to ensure a match in most cases, but
       will inevitably miss some cases, causing the  application  to  perceive
       the  IN_MOVED_FROM and IN_MOVED_TO events as being unrelated.  If watch
       descriptors are destroyed and re-created as a result, then those  watch
       descriptors  will  be  inconsistent  with  the watch descriptors in any
       pending  events.   (Re-creating  the  inotify   file   descriptor   and
       rebuilding the cache may be useful to deal with this scenario.)

       Applications   should   also   allow   for  the  possibility  that  the
       IN_MOVED_FROM event was the last event that could  fit  in  the  buffer
       returned   by  the  current  call  to  read(2),  and  the  accompanying
       IN_MOVED_TO event might be fetched only on the next read(2).

BUGS

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

       As originally designed and implemented, the  IN_ONESHOT  flag  did  not
       cause  an  IN_IGNORED  event to be generated when the watch was dropped
       after one event.  However, as an unintended effect  of  other  changes,
       since Linux 2.6.36, an IN_IGNORED event is generated in this case.

       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

       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

COLOPHON

       This page is part of release 3.65 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/.



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