What is Winsock Overlapped I/O?

Posted on 2009-03-29
Last Modified: 2013-11-13

What is Overlapped I/O as available for sockets in Winsock?

How does it work and why is it more efficient than other i/o models?
Question by:nanderEire
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Accepted Solution

Marcjev earned 125 total points
ID: 24014711
You should see overlapped I/O as a kind of assynchronious IO. In your program you start a function call and it returns immediatly.
So WSASend en WSAReceive will return immediatly. You will be able to receive and event when the opperation is complete.
As you can do other things in the time the system is normally in the blocking call, you cal do more efficient programming, like handling the data received in a previous opperation.
But although it has to potential to be more efficient, it's up to you, the programmer, to actually achieve the gain, by programming a more complex program.
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Expert Comment

ID: 24014717
Also, when handling multiple connections, you could handle more connections as they are non blocking. So you could use the cpu more effectively. If you have a one to one connection the gain is not as much as having a many to one connection.

Author Comment

ID: 24016611

So what is the difference between the operation of an i/o overlapped and a socket placed in non-blocking mode?
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Assisted Solution

migoEX earned 125 total points
ID: 24016755
With non-blocking socket your "send/recv" will fail with WOULD_BLOCK error code, and you'll have to retry the operation later (probably using some sort of "select"). Buth the asynchronous function continues in the background, and you can later get a notification that it has completed/failed (in form of Event or callback).

Author Comment

ID: 24016929
So you can just keep queueing up the sends without checking the return value to see if each was successful?

Expert Comment

ID: 24017355
Yes. I assume there's some limit, of cause, but in theory you can sedn many requests whithout waiting for any to complete.

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