INOTIFY(7)                  Linux Programmer's Manual                 INOTIFY(7)

       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

   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 the watched
       directory.  This filename is null-terminated, and may include further
       null bytes ('\0') to align subsequent reads to a suitable address

       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 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.,

           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.

       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

       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).

       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 directory.

       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); 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 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

           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

       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 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

       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.)

       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

   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>
       #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 (int 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. */



       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

       This page is part of release 5.12 of the Linux man-pages project.  A
       description of the project, information about reporting bugs, and the
       latest version of this page, can be found at

Linux                              2021-03-22                         INOTIFY(7)