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

NAME
       inotify - monitoring filesystem events

DESCRIPTION
       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  moni‐
       tored,  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  consis‐
       tency 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  struc‐
       tures:

           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.

           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 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:

           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 applica‐
                  tion with a race-free way of ensuring that the monitored object is a directory.

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

           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 ino‐
       tify:

       /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 cre‐
              ated 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
       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
       The inotify API is Linux-specific.

NOTES
       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  descrip‐
       tors; 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 mem‐
       ory 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  ino‐
       tify  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.  (Applica‐
       tions  must fall back to polling the filesystem to catch such events.)  Furthermore, vari‐
       ous 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 applica‐
       tion 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  respon‐
       sibility  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 imme‐
       diately after adding the watch (and, if desired, recursively add watches for any subdirec‐
       tories that it contains).

       Note  that  the event queue can overflow.  In this case, events are lost.  Robust applica‐
       tions 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 direc‐
       tory, 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
       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  unin‐
       tended  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 appli‐
       cation 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 descrip‐
       tor.  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 elimi‐
       nate this possible bug.

EXAMPLE
       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 
       #include 
       #include 
       #include 
       #include 
       #include 

       /* 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
       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
       This  page  is  part of release 4.04 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                                       2015-05-07                                 INOTIFY(7)

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