<s>
Overlapped	B-Operating_System
I/O	I-Operating_System
is	O
a	O
name	O
used	O
for	O
asynchronous	B-Architecture
I/O	I-Architecture
in	O
the	O
Windows	B-Library
API	I-Library
.	O
</s>
<s>
It	O
was	O
introduced	O
as	O
an	O
extension	O
to	O
the	O
API	O
in	O
Windows	B-Device
NT	I-Device
.	O
</s>
<s>
Utilizing	O
overlapped	B-Operating_System
I/O	I-Operating_System
requires	O
passing	O
an	O
OVERLAPPED	O
structure	O
to	O
API	O
functions	O
that	O
normally	O
block	O
,	O
including	O
ReadFile( )	O
,	O
WriteFile( )	O
,	O
and	O
Winsock	B-Application
's	O
WSASend( )	O
and	O
WSARecv( )	O
.	O
</s>
<s>
The	O
caller	O
may	O
optionally	O
specify	O
a	O
Win32	B-Library
event	B-Application
handle	O
to	O
be	O
raised	O
when	O
the	O
operation	O
completes	O
.	O
</s>
<s>
Alternatively	O
,	O
a	O
program	O
may	O
receive	O
notification	O
of	O
an	O
event	B-Application
via	O
an	O
I/O	B-Library
completion	I-Library
port	I-Library
,	O
which	O
is	O
the	O
preferred	O
method	O
of	O
receiving	O
notification	O
when	O
used	O
in	O
symmetric	B-Operating_System
multiprocessing	I-Operating_System
environments	O
or	O
when	O
handling	O
I/O	O
on	O
a	O
large	O
number	O
of	O
files	O
or	O
sockets	B-Protocol
.	O
</s>
<s>
The	O
third	O
and	O
the	O
last	O
method	O
to	O
get	O
the	O
I/O	O
completion	O
notification	O
with	O
overlapped	B-Operating_System
IO	I-Operating_System
is	O
to	O
use	O
ReadFileEx( )	O
and	O
WriteFileEx( )	O
,	O
which	O
allow	O
the	O
User	O
APC	B-Device
routine	O
to	O
be	O
provided	O
,	O
which	O
will	O
be	O
fired	O
on	O
the	O
same	O
thread	O
on	O
completion	O
(	O
User	O
APC	B-Device
is	O
the	O
thing	O
very	O
similar	O
to	O
Unix/POSIX	O
signal	B-Operating_System
,	O
with	O
the	O
main	O
difference	O
being	O
that	O
the	O
signals	O
are	O
using	O
signal	B-Operating_System
numbers	O
from	O
the	O
historically	O
predefined	O
enumeration	O
,	O
while	O
the	O
User	O
APC	B-Device
can	O
be	O
any	O
function	O
declared	O
as	O
"	O
void	O
f( void*	O
context	O
)	O
"	O
)	O
.	O
</s>
<s>
Asynchronous	B-Architecture
I/O	I-Architecture
is	O
particularly	O
useful	O
for	O
sockets	B-Protocol
and	O
pipes	B-Operating_System
.	O
</s>
<s>
Unix	B-Application
and	O
Linux	B-Application
implement	O
the	O
POSIX	O
asynchronous	B-Architecture
I/O	I-Architecture
API	O
(	O
AIO	O
)	O
.	O
</s>
