man7.org > Linux > man-pages

NAME | DESCRIPTION | VERSIONS | CONFORMING TO | NOTES | BUGS | SEE ALSO | COLOPHONThe Linux Programming Interface


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

NAME         top

       inotify - monitoring file system events

DESCRIPTION         top

       The inotify API provides a mechanism for monitoring file system 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) (or
       inotify_init1(2)), inotify_add_watch(2), inotify_rm_watch(2), read(2), and
       close(2).

       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 provides 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.

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

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

       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 of events */
               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
       only used for rename events, and allows the resulting pair of IN_MOVE_FROM
       and IN_MOVE_TO events to be connected by the application.  For all other
       event types, cookie is set to 0.

       The name field is only present 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 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 (read) (*).
           IN_ATTRIB         Metadata changed, e.g., permissions, timestamps,
                             extended attributes, link count (since Linux 2.6.25),
                             UID, GID, etc. (*).
           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 (*).
           IN_DELETE         File/directory deleted from watched directory (*).
           IN_DELETE_SELF    Watched file/directory was itself deleted.
           IN_MODIFY         File was modified (*).
           IN_MOVE_SELF      Watched file/directory was itself moved.
           IN_MOVED_FROM     File moved out of watched directory (*).
           IN_MOVED_TO       File moved into watched directory (*).
           IN_OPEN           File was opened (*).

       When monitoring a directory, the events marked with an asterisk (*) above
       can occur for files in the directory, in which case 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 IN_MOVE, which equates to
       IN_MOVED_FROM|IN_MOVED_TO, and IN_CLOSE, which 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       Add (OR) events to watch mask for this pathname if it
                             already exists (instead of replacing mask).
           IN_ONESHOT        Monitor pathname for one event, then remove from
                             watch list.
           IN_ONLYDIR (since Linux 2.6.15)
                             Only watch pathname if it 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 file system was
                             unmounted).
           IN_ISDIR          Subject of this event is a directory.
           IN_Q_OVERFLOW     Event queue overflowed (wd is -1 for this event).
           IN_UNMOUNT        File system containing watched object was unmounted.

/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 only added in 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).

       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

       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.

       The inotify API provides no information about the user or process that
       triggered the inotify event.

       Note that the event queue can overflow.  In this case, events are lost.
       Robust applications should handle the possibility of lost events
       gracefully.

       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.

       If monitoring an entire directory subtree, and a new subdirectory is
       created in that tree, be aware that by the time you create a watch for the
       new subdirectory, new files may already have been created in the
       subdirectory.  Therefore, you may want to scan the contents of the
       subdirectory immediately after adding the watch.

BUGS         top

       In kernels before 2.6.16, the IN_ONESHOT mask flag does not work.

       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.

SEE ALSO         top

       inotify_add_watch(2), inotify_init(2), inotify_init1(2),
       inotify_rm_watch(2), read(2), stat(2),
       Documentation/filesystems/inotify.txt.

COLOPHON         top

       This page is part of release 3.41 of the Linux man-pages project.  A
       description of the project, and information about reporting bugs, can be
       found at http://www.kernel.org/doc/man-pages/.

Linux                               2012-04-26                          INOTIFY(7)

HTML rendering created 2012-05-11 by Michael Kerrisk, author of The Linux Programming Interface, maintainer of the Linux man-pages project

free hit counters