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Linux::Inotify2 - scalable directory/file change notification


Callback Interface

 use Linux::Inotify2;

 # create a new object
 my $inotify = new Linux::Inotify2
    or die "unable to create new inotify object: $!";
 # add watchers
 $inotify->watch ("/etc/passwd", IN_ACCESS, sub {
    my $e = shift;
    my $name = $e->fullname;
    print "$name was accessed\n" if $e->IN_ACCESS;
    print "$name is no longer mounted\n" if $e->IN_UNMOUNT;
    print "$name is gone\n" if $e->IN_IGNORED;
    print "events for $name have been lost\n" if $e->IN_Q_OVERFLOW;
    # cancel this watcher: remove no further events

 # integration into AnyEvent (works with EV, Glib, Tk, POE...)
 my $inotify_w = AE::io $inotify->fileno, 0, sub { $inotify->poll };

 # manual event loop
 $inotify->poll while 1;

Streaming Interface

 use Linux::Inotify2;

 # create a new object
 my $inotify = new Linux::Inotify2
    or die "Unable to create new inotify object: $!";

 # create watch
 $inotify->watch ("/etc/passwd", IN_ACCESS)
    or die "watch creation failed";

 while () {
   my @events = $inotify->read;
   printf "mask\t%d\n", $_->mask foreach @events;


This module implements an interface to the Linux 2.6.13 and later Inotify file/directory change notification system.

It has a number of advantages over the Linux::Inotify module:

   - it is portable (Linux::Inotify only works on x86)
   - the equivalent of fullname works correctly
   - it is better documented
   - it has callback-style interface, which is better suited for

As for the inotify API itself - it is a very tricky, and somewhat unreliable API. For a good overview of the challenges you might run into, see this LWN article:

The Linux::Inotify2 Class

my $inotify = new Linux::Inotify2

Create a new notify object and return it. A notify object is kind of a container that stores watches on file system names and is responsible for handling event data.

On error, undef is returned and $! will be set accordingly. The following errors are documented:

 ENFILE   The system limit on the total number of file descriptors has been reached.
 EMFILE   The user limit on the total number of inotify instances has been reached.
 ENOMEM   Insufficient kernel memory is available.


   my $inotify = new Linux::Inotify2
      or die "Unable to create new inotify object: $!";
$watch = $inotify->watch ($name, $mask[, $cb])

Add a new watcher to the given notifier. The watcher will create events on the pathname $name as given in $mask, which can be any of the following constants (all exported by default) ORed together. Constants unavailable on your system will evaluate to 0.

"file" refers to any file system object in the watched object (always a directory), that is files, directories, symlinks, device nodes etc., while "object" refers to the object the watcher has been set on itself:

 IN_ACCESS            object was accessed
 IN_MODIFY            object was modified
 IN_ATTRIB            object metadata changed
 IN_CLOSE_WRITE       writable fd to file / to object was closed
 IN_CLOSE_NOWRITE     readonly fd to file / to object closed
 IN_OPEN              object was opened
 IN_MOVED_FROM        file was moved from this object (directory)
 IN_MOVED_TO          file was moved to this object (directory)
 IN_CREATE            file was created in this object (directory)
 IN_DELETE            file was deleted from this object (directory)
 IN_DELETE_SELF       object itself was deleted
 IN_MOVE_SELF         object itself was moved
 IN_ALL_EVENTS        all of the above events

 IN_ONESHOT           only send event once
 IN_ONLYDIR           only watch the path if it is a directory
 IN_DONT_FOLLOW       don't follow a sym link (Linux 2.6.15+)
 IN_EXCL_UNLINK       don't create events for unlinked objects (Linux 2.6.36+)
 IN_MASK_ADD          not supported with the current version of this module

 IN_MOVE              same as IN_MOVED_FROM | IN_MOVED_TO

$cb is a perl code reference that, if given, is called for each event. It receives a Linux::Inotify2::Event object.

The returned $watch object is of class Linux::Inotify2::Watch.

On error, undef is returned and $! will be set accordingly. The following errors are documented:

 EBADF    The given file descriptor is not valid.
 EINVAL   The given event mask contains no legal events.
 ENOMEM   Insufficient kernel memory was available.
 ENOSPC   The user limit on the total number of inotify watches was reached or the kernel failed to allocate a needed resource.
 EACCESS  Read access to the given file is not permitted.

Example, show when /etc/passwd gets accessed and/or modified once:

   $inotify->watch ("/etc/passwd", IN_ACCESS | IN_MODIFY, sub {
      my $e = shift;
      print "$e->{w}{name} was accessed\n" if $e->IN_ACCESS;
      print "$e->{w}{name} was modified\n" if $e->IN_MODIFY;
      print "$e->{w}{name} is no longer mounted\n" if $e->IN_UNMOUNT;
      print "events for $e->{w}{name} have been lost\n" if $e->IN_Q_OVERFLOW;


Returns the file descriptor for this notify object. When in non-blocking mode, you are responsible for calling the poll method when this file descriptor becomes ready for reading.


Similar to fileno, but returns a perl file handle instead.

$inotify->blocking ($blocking)

Clears ($blocking true) or sets ($blocking false) the O_NONBLOCK flag on the file descriptor.

$count = $inotify->poll

Reads events from the kernel and handles them. If the notify file descriptor is blocking (the default), then this method waits for at least one event. Otherwise it returns immediately when no pending events could be read.

Returns the count of events that have been handled (which can be 0 in case events have been received but have been ignored or handled internally).

Croaks when an error occurs.

@events = $inotify->read

Reads events from the kernel. Blocks when the file descriptor is in blocking mode (default) until any event arrives. Returns list of Linux::Inotify2::Event objects or empty list if none (non-blocking mode or events got ignored).

Croaks on error.

Normally you shouldn't use this function, but instead use watcher callbacks and call ->poll.

$inotify->on_overflow ($cb->($ev))

Sets the callback to be used for overflow handling (default: undef): When read receives an event with IN_Q_OVERFLOW set, it will invoke this callback with the event.

When the callback is undef, then it broadcasts the event to all registered watchers, i.e., undef is equivalent to:

   sub { $inotify->broadcast ($_[0]) }
$inotify->broadcast ($ev)

Invokes all registered watcher callbacks and passes the given event to them. Most useful in overflow handlers.

The Linux::Inotify2::Event Class

Objects of this class are handed as first argument to the watcher callback. It has the following members and methods:


The watcher object for this event, if one is available. Generally, you cna only rely on the value of this member inside watcher callbacks.


The path of the file system object, relative to the watched name.


Returns the "full" name of the relevant object, i.e. including the name member of the watcher (if the watch object is on a directory and a directory entry is affected), or simply the name member itself when the object is the watch object itself.

This call requires $event->{w} to be valid, which is generally only the case within watcher callbacks.


The received event mask. In addition to the events described for $inotify->watch, the following flags (exported by default) can be set:

 IN_ISDIR             event object is a directory
 IN_Q_OVERFLOW        event queue overflowed

 # when any of the following flags are set,
 # then watchers for this event are automatically canceled
 IN_UNMOUNT           filesystem for watched object was unmounted
 IN_IGNORED           file was ignored/is gone (no more events are delivered)
 IN_ONESHOT           only one event was generated
 IN_Q_OVERFLOW        queue overflow - event might not be specific to a watcher

Returns a boolean that returns true if the event mask contains any events specified by the mask. All of the IN_xxx constants can be used as methods.


The event cookie to "synchronize two events". Normally zero, this value is set when two events relating to the same file are generated. As far as I know, this only happens for IN_MOVED_FROM and IN_MOVED_TO events, to identify the old and new name of a file.

Note that the inotify API makes it impossible to know whether there will be a IN_MOVED_TO event - you might receive only one of the events, and even if you receive both, there might be any number of events in between. The best approach seems to be to implement a small timeout after IN_MOVED_FROM to see if a matching IN_MOVED_TO event will be received - 2ms seem to work relatively well.

The Linux::Inotify2::Watch Class

Watcher objects are created by calling the watch method of a notifier.

It has the following members and methods:


The name as specified in the watch call. For the object itself, this is the empty string. For directory watches, this is the name of the entry without leading path elements.


The mask as specified in the watch call.

$watch->cb ([new callback])

The callback as specified in the watch call. Can optionally be changed.


Cancels/removes this watcher. Future events, even if already queued queued, will not be handled and resources will be freed.


AnyEvent, Linux::Inotify.


 Marc Lehmann <>