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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

       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

           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 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 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

           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 inotify_add_watch(2).

       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 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.

                  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

       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

              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 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

   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), 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

       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 bug.


       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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