CODEWITHSUNDEEP
csocketswinapiwinsockwinsock2
Amir
Amir

Reputation: 820

select() equivalence in I/O Completion Ports

I am developing a proxy server using WinSock 2.0 in Windows. If I wanted to develop it in blocking model, select() was the way to wait for client or remote server to receive data from. Is there any applicable way to do this so using I/O Completion Ports?

I used to have two Contexts for two directions of data using I/O Completion Ports. But having a WSARecv pending couldn't receive any data from remote server! I coudn't find the problem.

Thanks in advance.

EDIT. Here's the WorkerThread Code on currently developed I/O Completion Ports. But I am asking about how to implement select() equivalence.

Upvotes: 3

Views: 759

Answers (1)

Len Holgate
Len Holgate

Reputation: 21636

I/O Completion Ports provide an indication of when an I/O operation completes, they do not indicate when it is possible to initiate an operation. In many situations this doesn't actually matter. Most of the time the overlapped I/O model will work perfectly well if you assume it is always possible to initiate an operation. The underlying operating system will, in most cases, simply do the right thing and queue the data for you until it is possible to complete the operation.

However, there are some situations when this is less than ideal. For example you can always send to a socket using overlapped I/O. You can do this even when the remote peer is not reading and the TCP stack has started to use flow control and has filled the TCP window... This simply uses resources on your local machine in a completely uncontrolled manner (not entirely uncontrolled, but controlled by the peer, which is not ideal). I write about this here and in many situations you DO need to actively manage this kind of thing by tracking how many outstanding I/O write requests you have and using that as an indication of 'readiness to send'.

Likewise if you want a 'readiness to recv' indication you could issue a 'zero byte' read on the socket. This is a read which is issued with a zero length buffer. The read returns when there is data to read but no data is returned. This would give you the indication that there is data to be read on the connection but is, IMHO, pointless unless you are suffering from the very unlikely situation of hitting the I/O page lock limit, as you may as well read the data when it becomes available rather than forcing multiple kernel to user mode transitions.

In summary, you don't really need an answer to your question. You need to look at how the API works and write your code to work with it rather than trying to force the API to work in a way that other APIs that you are familiar with work.

Upvotes: 5

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