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       inotify - monitoring filesystem events


       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
       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 describing event */
               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 filename within to the
       watched directory.  This filename is null-terminated, and  may  include
       further  null  bytes  ('\0')  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

       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

           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 or directory 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.

                  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)).

                  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 or directory was opened.

       When monitoring a directory:

       *  the  events marked above with an asterisk (*) can occur both for the
          directory itself and for objects inside the directory; and

       *  the events marked with a plus sign (+) occur only for objects inside
          the directory (not for the directory itself).

       When  events  are generated for objects inside a watched directory, 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

       Two additional convenience macros are defined:

                  Equates to IN_MOVED_FROM | IN_MOVED_TO.

                  Equates to IN_CLOSE_WRITE | IN_CLOSE_NOWRITE.

       The  following  further  bits  can  be  specified  in mask when calling

           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.

                  If a watch instance already exists for the filesystem object
                  corresponding  to  pathname,  add (OR) the events in mask to
                  the watch mask (instead of replacing the mask).

                  Monitor the filesystem object corresponding to pathname  for
                  one event, then remove from watch list.

           IN_ONLYDIR (since Linux 2.6.15)
                  Only  watch  pathname if it is a directory.  Using this flag
                  provides an application with a  race-free  way  of  ensuring
                  that the monitored object is a directory.

       The following bits may be set in the mask field returned by read(2):

                  Watch   was   removed  explicitly  (inotify_rm_watch(2))  or
                  automatically  (file  was   deleted,   or   filesystem   was
                  unmounted).  See also BUGS.

                  Subject of this event is a directory.

                  Event queue overflowed (wd is -1 for this event).

                  Filesystem  containing  watched  object  was  unmounted.  In
                  addition, an IN_IGNORED event will subsequently be generated
                  for the watch descriptor.

       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.

                  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:

                  Generates an IN_ATTRIB event for xx (because its link  count
                  changes) and an IN_DELETE event for dir2.

                  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.

                  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:

              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.

              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 glibc 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
       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

       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

       The inotify API does not report file accesses  and  modifications  that
       may occur because of mmap(2), msync(2), and munmap(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.
       Furthermore,  it  is  not  guaranteed that the event pair is atomically
       inserted into the queue: there  may  be  a  brief  interval  where  the
       IN_MOVED_FROM has appeared, but the IN_MOVED_TO has not.

       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),  which
       should  be  done  with  a  (small)  timeout  to allow for the fact that
       insertion of the IN_MOVED_FROM-IN_MOVED_TO event pair  is  not  atomic,
       and also the possibility that there may not be any IN_MOVED_TO event.


       Before  Linux  3.19,  fallocate(2)  did  not create any inotify events.
       Since Linux 3.19, calls to fallocate(2) generate IN_MODIFY events.

       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.

       When  a  watch descriptor is removed by calling inotify_rm_watch(2) (or
       because a watch file is deleted or the filesystem that contains  it  is
       unmounted),  any pending unread events for that watch descriptor remain
       available to read.  As watch  descriptors  are  subsequently  allocated
       with  inotify_add_watch(2),  the  kernel  cycles  through  the range of
       possible  watch  descriptors  (0  to  INT_MAX)   incrementally.    When
       allocating  a  free  watch  descriptor, no check is made to see whether
       that watch descriptor number has  any  pending  unread  events  in  the
       inotify  queue.   Thus,  it  can  happen  that  a  watch  descriptor is
       reallocated even when  pending  unread  events  exist  for  a  previous
       incarnation  of  that watch descriptor number, with the result that the
       application  might  then  read  those  events  and  interpret  them  as
       belonging  to  the  file  associated  with  the  newly  recycled  watch
       descriptor.  In practice, the likelihood of hitting  this  bug  may  be
       extremely  low,  since  it  requires  that an application cycle through
       INT_MAX watch descriptors, release a  watch  descriptor  while  leaving
       unread  events for that watch descriptor in the queue, and then recycle
       that watch descriptor.  For this reason, and because there have been no
       reports  of  the  bug occurring in real-world applications, as of Linux
       3.15, no kernel changes have yet been made to eliminate  this  possible


       The  following  program  demonstrates the usage of the inotify API.  It
       marks the directories passed as a command-line arguments and waits  for
       events of type IN_OPEN, IN_CLOSE_NOWRITE and IN_CLOSE_WRITE.

       The   following   output   was   recorded   while   editing   the  file
       /home/user/temp/foo and listing directory /tmp.  Before  the  file  and
       the directory were opened, IN_OPEN events occurred.  After the file was
       closed, an IN_CLOSE_WRITE event  occurred.   After  the  directory  was
       closed,  an  IN_CLOSE_NOWRITE event occurred.  Execution of the program
       ended when the user pressed the ENTER key.

   Example output
           $ ./a.out /tmp /home/user/temp
           Press enter key to terminate.
           Listening for events.
           IN_OPEN: /home/user/temp/foo [file]
           IN_CLOSE_WRITE: /home/user/temp/foo [file]
           IN_OPEN: /tmp/ [directory]
           IN_CLOSE_NOWRITE: /tmp/ [directory]

           Listening for events stopped.

   Program source
       #include <errno.h>
       #include <poll.h>
       #include <stdio.h>
       #include <stdlib.h>
       #include <sys/inotify.h>
       #include <unistd.h>

       /* Read all available inotify events from the file descriptor 'fd'.
          wd is the table of watch descriptors for the directories in argv.
          argc is the length of wd and argv.
          argv is the list of watched directories.
          Entry 0 of wd and argv is unused. */

       static void
       handle_events(int fd, int *wd, int argc, char* argv[])
           /* Some systems cannot read integer variables if they are not
              properly aligned. On other systems, incorrect alignment may
              decrease performance. Hence, the buffer used for reading from
              the inotify file descriptor should have the same alignment as
              struct inotify_event. */

           char buf[4096]
               __attribute__ ((aligned(__alignof__(struct inotify_event))));
           const struct inotify_event *event;
           int i;
           ssize_t len;
           char *ptr;

           /* Loop while events can be read from inotify file descriptor. */

           for (;;) {

               /* Read some events. */

               len = read(fd, buf, sizeof buf);
               if (len == -1 && errno != EAGAIN) {

               /* If the nonblocking read() found no events to read, then
                  it returns -1 with errno set to EAGAIN. In that case,
                  we exit the loop. */

               if (len <= 0)

               /* Loop over all events in the buffer */

               for (ptr = buf; ptr < buf + len;
                       ptr += sizeof(struct inotify_event) + event->len) {

                   event = (const struct inotify_event *) ptr;

                   /* Print event type */

                   if (event->mask & IN_OPEN)
                       printf("IN_OPEN: ");
                   if (event->mask & IN_CLOSE_NOWRITE)
                       printf("IN_CLOSE_NOWRITE: ");
                   if (event->mask & IN_CLOSE_WRITE)
                       printf("IN_CLOSE_WRITE: ");

                   /* Print the name of the watched directory */

                   for (i = 1; i < argc; ++i) {
                       if (wd[i] == event->wd) {
                           printf("%s/", argv[i]);

                   /* Print the name of the file */

                   if (event->len)
                       printf("%s", event->name);

                   /* Print type of filesystem object */

                   if (event->mask & IN_ISDIR)
                       printf(" [directory]\n");
                       printf(" [file]\n");

       main(int argc, char* argv[])
           char buf;
           int fd, i, poll_num;
           int *wd;
           nfds_t nfds;
           struct pollfd fds[2];

           if (argc < 2) {
               printf("Usage: %s PATH [PATH ...]\n", argv[0]);

           printf("Press ENTER key to terminate.\n");

           /* Create the file descriptor for accessing the inotify API */

           fd = inotify_init1(IN_NONBLOCK);
           if (fd == -1) {

           /* Allocate memory for watch descriptors */

           wd = calloc(argc, sizeof(int));
           if (wd == NULL) {

           /* Mark directories for events
              - file was opened
              - file was closed */

           for (i = 1; i < argc; i++) {
               wd[i] = inotify_add_watch(fd, argv[i],
                                         IN_OPEN | IN_CLOSE);
               if (wd[i] == -1) {
                   fprintf(stderr, "Cannot watch '%s'\n", argv[i]);

           /* Prepare for polling */

           nfds = 2;

           /* Console input */

           fds[0].fd = STDIN_FILENO;
           fds[0].events = POLLIN;

           /* Inotify input */

           fds[1].fd = fd;
           fds[1].events = POLLIN;

           /* Wait for events and/or terminal input */

           printf("Listening for events.\n");
           while (1) {
               poll_num = poll(fds, nfds, -1);
               if (poll_num == -1) {
                   if (errno == EINTR)

               if (poll_num > 0) {

                   if (fds[0].revents & POLLIN) {

                       /* Console input is available. Empty stdin and quit */

                       while (read(STDIN_FILENO, &buf, 1) > 0 && buf != '\n')

                   if (fds[1].revents & POLLIN) {

                       /* Inotify events are available */

                       handle_events(fd, wd, argc, argv);

           printf("Listening for events stopped.\n");

           /* Close inotify file descriptor */




       inotifywait(1), inotifywatch(1), inotify_add_watch(2), inotify_init(2),
       inotify_init1(2), inotify_rm_watch(2), read(2), stat(2), fanotify(7)

       Documentation/filesystems/inotify.txt in the Linux kernel source tree


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       description of the project, information about reporting bugs,  and  the
       latest     version     of     this    page,    can    be    found    at