<s>
A	O
virtual	B-Application
device	I-Application
in	O
Unix	B-Application
is	O
a	O
file	O
such	O
as	O
:/dev/null	O
or	O
:/dev/urandom	O
,	O
that	O
is	O
treated	O
as	O
a	O
device	O
,	O
as	O
far	O
as	O
user	O
level	O
software	O
is	O
concerned	O
,	O
but	O
is	O
generated	O
by	O
the	O
kernel	B-Operating_System
without	O
reference	O
to	O
hardware	O
.	O
</s>
<s>
For	O
instance	O
when	O
/dev/null	O
is	O
written	O
to	O
,	O
the	O
kernel	B-Operating_System
tells	O
the	O
program	O
it	O
wrote	O
everything	O
to	O
it	O
(	O
without	O
actually	O
writing	O
it	O
anywhere	O
)	O
,	O
and	O
when	O
read	O
from	O
,	O
the	O
reading	O
program	O
is	O
told	O
that	O
it	O
has	O
reached	O
the	O
end	O
of	O
the	O
file	O
.	O
</s>
<s>
DOS-	O
,	O
Windows	B-Application
-	O
and	O
OS/2	B-Application
-like	O
operating	B-General_Concept
systems	I-General_Concept
define	O
the	O
NUL	O
device	O
that	O
performs	O
a	O
similar	O
function	O
(	O
but	O
is	O
implemented	O
as	O
part	O
of	O
the	O
file	O
name	O
processing	O
no	O
actual	O
file	O
exists	O
by	O
that	O
name	O
)	O
.	O
</s>
