Manpages - inotify.7

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 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.

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.

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

IN_ONESHOT

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):

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.

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.

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.

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.)

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.

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.

  $ ./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.

  #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) {
              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 (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]);
                      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': %s\n",
                      argv[i], strerror(errno));
              exit(EXIT_FAILURE);
          }
      }

      /* 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)
                  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);
  }