I think that Overlapped I/O was invented to be used on the server side, mainly to be able to handle thousands of clients. So I do not think that there is any reason to use it on the client side. is my assumption correct?
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This is not a client/server issue. It's a workload issue. Clients just normally don't have any reason to keep many IOs outstanding at the same time. That's the main use case for async IO.
A port scanner would be an example of a good case for async IO on the client.
usr
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Are you assuming that a client only needs 1 connection at a time? A "simple" browser could have 10 pages open, downloading 10 files, playing 10 videos, etc. Overlapped I/O would be a great way for a client to remain "responsive".
franji1
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The code required to write an IOCP based server is 95% the same as the code required to write a client. The only differences are connection establishment, ConnectEx vs AcceptEx.
IMHO there's no reason not to use IOCP for client side communications.
Len Holgate
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[Overlapped I/O](https://msdn.microsoft.com/en-us/library/aa365683.aspx) and [I/O Completion Ports](https://msdn.microsoft.com/en-us/library/aa365198.aspx) are not the same thing. You can - and often do - perform overlapped I/O with synchronization objects other than IOCPs. – IInspectable Mar 20 '15 at 21:16
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True, but that doesn't invalidate my answer at all. – Len Holgate Mar 20 '15 at 21:34
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The question asks specifically about asynchronous I/O. Your answer only addresses this in so far as IOCPs are **one** way to synchronize asynchronous I/O. It doesn't address the question directly. – IInspectable Mar 20 '15 at 21:39
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IOCP based I/O is asynchronous, it's as useful in clients as it is in servers. That's what my answer says. You may view it as an incomplete answer, but I don't view the alternative asynchronous I/O methods as being something worth using. – Len Holgate Mar 20 '15 at 21:45