inotify(7) — Linux manual page


inotify(7)          Miscellaneous Information Manual          inotify(7)

NAME         top

       inotify - monitoring filesystem events

DESCRIPTION         top

       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

       •  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

       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

       mask contains bits that describe the event that occurred (see

       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 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: before Linux 2.6.21, read(2) returns 0; since Linux
       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.

       Inotify monitoring is inode-based: when monitoring a file (but
       not when monitoring the directory containing a file), an event
       can be generated for activity on any link to the file (in the
       same or a different directory).

       When monitoring a directory:

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

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

       Note: when monitoring a directory, events are not generated for
       the files inside the directory when the events are performed via
       a pathname (i.e., a link) that lies outside the monitored

       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

           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); the error EINVAL results if IN_MASK_CREATE is
                  also specified.

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

           IN_ONLYDIR (since Linux 2.6.15)
                  Watch pathname only if it is a directory; the error
                  ENOTDIR results if pathname is not a directory.  Using
                  this flag provides an application with a race-free way
                  of ensuring that the monitored object is a directory.

           IN_MASK_CREATE (since Linux 4.18)
                  Watch pathname only if it does not already have a
                  watch associated with it; the error EEXIST results if
                  pathname is already being watched.

                  Using this flag provides an application with a way of
                  ensuring that new watches do not modify existing ones.
                  This is useful because multiple paths may refer to the
                  same inode, and multiple calls to inotify_add_watch(2)
                  without this flag may clobber existing watch masks.

       The following bits may be set in the mask field returned by

                  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

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

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

                  Generates IN_CLOSE_WRITE events for both dir and

       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.

STANDARDS         top


HISTORY         top

       Inotify was merged into Linux 2.6.13.  The required library
       interfaces were added in glibc 2.4.  (IN_DONT_FOLLOW,
       IN_MASK_ADD, and IN_ONLYDIR were added in glibc 2.5.)

NOTES         top

       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 set of watch descriptors that is being monitored via an
       inotify file descriptor can be viewed via the entry for the
       inotify file descriptor in the process's /proc/pid/fdinfo
       directory.  See proc(5) for further details.  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), 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

       If a filesystem is mounted on top of a monitored directory, no
       event is generated, and no events are generated for objects
       immediately under the new mount point.  If the filesystem is
       subsequently unmounted, events will subsequently be generated for
       the directory and the objects it contains.

   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.

BUGS         top

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

       Before Linux 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 Linux 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 (1 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.

EXAMPLES         top

       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

       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

   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>
       #include <string.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;
           ssize_t len;

           /* 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 (char *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 (size_t 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': %s\n",
                           argv[i], strerror(errno));

           /* Prepare for polling. */

           nfds = 2;

           fds[0].fd = STDIN_FILENO;       /* Console input */
           fds[0].events = POLLIN;

           fds[1].fd = fd;                 /* Inotify input */
           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. */



SEE ALSO         top

       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

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