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    <p>What I did was to force the timeout/completion notifications to enter a critical section in the socket object. Once in, the winner can set a socket state variable and perform its action, whatever that might be. If the I/O completion gets in first, the I/O buffer array is processed in the normal way and any timeout is directed to restart by the state-machine. Similarly if the timeout gets in first, the I/O gets CancelIOEx'd and any later queued completion notification is discarded by the state-engine. Because of these possible 'late' notifications, I put released sockets onto a timeout queue and only recycle them onto the socket object pool after five minutes, in a similar way to how the TCP stack itself puts its sockets into 'TIME_WAIT'.</p> <p>To do the timeouts, I have one thread that operates on FIFO delta-queues of timing-out objects, one queue for each timeout limit. The thread waits on an input queue for new objects with a timeout calculated from the smallest timeout-expiry-time of the objects at the head of the queues.</p> <p>There were only a few timeouts used in the server, so I used queues fixed at compile-time. It would be fairly easy to add new queues or modify the timeout by sending appropriate 'command' messages to the thread input queue, mixed-in with the new sockets, but I didn't get that far.</p> <p>Upon timeout, the thread called an event in the object which, in case of a socket, would enter the socket object CS-protected state-machine, (these was a TimeoutObject class which the socket descended from, amongst other things).</p> <p>More:</p> <p>I wait on the semaphore that controls the timeout thread input queue. If it's signaled, I get the new TimeoutObject from the input queue and add it to the end of whatever timeout queue it asks for. If the semaphore wait times out, I check the items at the heads of the timeout FIFO queues and recalculate their remaining interval by sutracting the current time from their timeout time. If the interval is 0 or negative, the timeout event gets called. While iterating the queues and their heads, I keep in a local the minimum remaining interval before the next timeout. Hwn all the head items in all the queues have non-zero remaining interval, I go back to waiting on the queue semaphore using the minimum remaining interval I have accumulated.</p> <p>The event call returns an enumeration. This enumeration instructs the timeout thread how to handle an object whose event it's just fired. One option is to restart the timeout by recalcuating the timeout-time and pushing the object back onto its timeout queue at the end.</p> <p>I did not use RegisterWaitForSingleObject() because it needed .NET and my Delphi server was all unmanaged, (I wrote my server a long time ago!).</p> <p>That, and because, IIRC, it has a limit of 64 handles, like WaitForMultipleObjects(). My server had upwards of 23000 clients timing out. I found the single timeout thread and multiple FIFO queues to be more flexible - any old object could be timed out on it as long as it was descended from TimeoutObject - no extra OS calls/handles needed. </p>
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    1. POHow to detect WinSock TCP timeout with BindIoCompletionCallback
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    1. USMartin James
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    1. COHmm...so you use a dedicated thread and WaitForSingleObject on some Events sequentially ? isn's that what RegisterWaitForSingleObject is designed for ?
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    2. COI only ever waited on one synchro object - the semaphore that held the count for the timeout thread input queue. I edited my answer to add some more details.
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    3. COHmm... I'll read your update carefully later, but RegisterWaitForSingleObject is Win32 API and not depends on .NET :)
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