<s>
dnotify	B-Application
is	O
a	O
file	B-Application
system	I-Application
event	O
monitor	O
for	O
the	O
Linux	B-Operating_System
kernel	I-Operating_System
,	O
one	O
of	O
the	O
subfeatures	O
of	O
the	O
fcntl	O
call	O
.	O
</s>
<s>
It	O
was	O
introduced	O
in	O
the	O
2.4	O
kernel	B-Operating_System
series	O
.	O
</s>
<s>
It	O
has	O
been	O
obsoleted	O
by	O
inotify	B-Application
,	O
but	O
will	O
be	O
retained	O
for	O
compatibility	O
reasons	O
.	O
</s>
<s>
Its	O
function	O
is	O
essentially	O
an	O
extension	O
to	O
filesystems	B-Application
to	O
notice	O
changes	O
to	O
the	O
filesystem	B-Application
,	O
and	O
report	O
those	O
changes	O
to	O
applications	O
.	O
</s>
<s>
Instead	O
of	O
application	O
checking	O
for	O
changes	O
to	O
filesystem	B-Application
,	O
application	O
can	O
register	O
to	O
be	O
notified	O
by	O
kernel	B-Operating_System
when	O
changes	O
to	O
filesystem	B-Application
occur	O
.	O
</s>
<s>
One	O
major	O
use	O
is	O
in	O
desktop	B-Application
search	I-Application
utilities	O
like	O
Beagle	B-Language
,	O
where	O
its	O
functionality	O
permits	O
reindexing	B-Application
of	O
changed	O
files	O
without	O
scanning	O
the	O
filesystem	B-Application
for	O
changes	O
every	O
few	O
minutes	O
,	O
which	O
would	O
be	O
very	O
inefficient	O
.	O
</s>
<s>
By	O
being	O
told	O
that	O
a	O
file	O
has	O
changed	O
directly	O
by	O
the	O
kernel	B-Operating_System
,	O
rather	O
than	O
actively	O
looking	O
,	O
Beagle	B-Language
and	O
such	O
utilities	O
can	O
achieve	O
change-to-reindexing	O
times	O
of	O
only	O
about	O
a	O
second	O
,	O
with	O
very	O
small	O
performance	O
hits	O
(	O
dnotify	B-Application
therefore	O
enables	O
the	O
use	O
of	O
such	O
programs	O
in	O
a	O
sensible	O
manner	O
;	O
daemons	B-Operating_System
are	O
generally	O
not	O
accepted	O
by	O
distributors	O
if	O
they	O
drain	O
system	O
performance	O
noticeably	O
to	O
provide	O
userland	O
functionality	O
)	O
.	O
</s>
<s>
dnotify	B-Application
can	O
only	O
watch	O
directories	O
.	O
</s>
<s>
dnotify	B-Application
requires	O
maintaining	O
an	O
open	O
file	O
descriptor	O
to	O
the	O
directory	O
that	O
the	O
user	O
wants	O
to	O
watch	O
.	O
</s>
