NAME | DESCRIPTION | VERSIONS | CONFORMING TO | NOTES | BUGS | EXAMPLE | SEE ALSO | COLOPHON

INOTIFY(7)                Linux Programmer's 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
       directory.

       The following system calls are used with this API:

       *  inotify_init(2) creates an inotify instance and returns a file
          descriptor referring to the inotify instance.  The more recent
          inotify_init1(2) is like inotify_init(2), but has a flags argument
          that provides access to some extra functionality.

       *  inotify_add_watch(2) manipulates the "watch list" associated with
          an inotify instance.  Each item ("watch") in the watch list
          specifies the pathname of a file or directory, along with some set
          of events that the kernel should monitor for the file referred to
          by that pathname.  inotify_add_watch(2) either creates a new watch
          item, or modifies an existing watch.  Each watch has a unique
          "watch descriptor", an integer returned by inotify_add_watch(2)
          when the watch is created.

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

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

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

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

   Reading events from an inotify file descriptor
       To determine what events have occurred, an application read(2)s from
       the inotify file descriptor.  If no events have so far occurred,
       then, assuming a blocking file descriptor, read(2) will block until
       at least one event occurs (unless interrupted by a signal, in which
       case the call fails with the error EINTR; see signal(7)).

       Each successful read(2) returns a buffer containing one or more of
       the following structures:

           struct inotify_event {
               int      wd;       /* Watch descriptor */
               uint32_t mask;     /* Mask 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 file pathname relative
       to the watched directory.  This pathname 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 not opened for writing was closed.

           IN_CREATE (+)
                  File/directory created in watched directory (e.g., open(2)
                  O_CREAT, mkdir(2), link(2), symlink(2), bind(2) on a UNIX
                  domain socket).

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

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

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

           IN_MOVE_SELF
                  Watched file/directory was itself moved.

           IN_MOVED_FROM (+)
                  Generated for the directory containing the old filename
                  when a file is renamed.

           IN_MOVED_TO (+)
                  Generated for the directory containing the new filename
                  when a file is renamed.

           IN_OPEN (*)
                  File was opened.

       When monitoring a directory:

       *  the events marked 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:

           IN_MOVE
                  Equates to IN_MOVED_FROM | IN_MOVED_TO.

           IN_CLOSE
                  Equates to IN_CLOSE_WRITE | IN_CLOSE_NOWRITE.

       The following further bits can be specified in mask when calling
       inotify_add_watch(2):

           IN_DONT_FOLLOW (since Linux 2.6.15)
                  Don't dereference pathname if it is a symbolic link.

           IN_EXCL_UNLINK (since Linux 2.6.36)
                  By default, when watching events on the children of a
                  directory, events are generated for children even after
                  they have been unlinked from the directory.  This can
                  result in large numbers of uninteresting events for some
                  applications (e.g., if watching /tmp, in which many
                  applications create temporary files whose names are
                  immediately unlinked).  Specifying IN_EXCL_UNLINK changes
                  the default behavior, so that events are not generated for
                  children after they have been unlinked from the watched
                  directory.

           IN_MASK_ADD
                  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).

           IN_ONESHOT
                  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 objeect is a directory.

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

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

           IN_ISDIR
                  Subject of this event is a directory.

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

       /proc/sys/fs/inotify/max_queued_events
              The value in this file is used when an application calls
              inotify_init(2) to set an upper limit on the number of events
              that can be queued to the corresponding inotify instance.
              Events in excess of this limit are dropped, but an
              IN_Q_OVERFLOW event is always generated.

       /proc/sys/fs/inotify/max_user_instances
              This specifies an upper limit on the number of inotify
              instances that can be created per real user ID.

       /proc/sys/fs/inotify/max_user_watches
              This specifies an upper limit on the number of watches that
              can be created per real user ID.

VERSIONS         top

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

CONFORMING TO         top

       The inotify API is Linux-specific.

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 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 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.

BUGS         top

       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 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.

EXAMPLE         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 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) {
                   perror("read");
                   exit(EXIT_FAILURE);
               }

               /* 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)
                   break;

               /* 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]);
                           break;
                       }
                   }

                   /* 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");
                   else
                       printf(" [file]\n");
               }
           }
       }

       int
       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]);
               exit(EXIT_FAILURE);
           }

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

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

           fd = inotify_init1(IN_NONBLOCK);
           if (fd == -1) {
               perror("inotify_init1");
               exit(EXIT_FAILURE);
           }

           /* Allocate memory for watch descriptors */

           wd = calloc(argc, sizeof(int));
           if (wd == NULL) {
               perror("calloc");
               exit(EXIT_FAILURE);
           }

           /* 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]);
                   perror("inotify_add_watch");
                   exit(EXIT_FAILURE);
               }
           }

           /* 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)
                       continue;
                   perror("poll");
                   exit(EXIT_FAILURE);
               }

               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')
                           continue;
                       break;
                   }

                   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 */

           close(fd);

           free(wd);
           exit(EXIT_SUCCESS);
       }

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 tree

COLOPHON         top

       This page is part of release 3.71 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
       http://www.kernel.org/doc/man-pages/.

Linux                            2014-07-08                       INOTIFY(7)