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    <p>That's not how IOCPs work.</p> <p>If you have pending operations that you want to cancel then close the corresponding socket and the operations will either complete or fail and all of the completions (including the failures) will come out of the IOCP eventually.</p> <p>You need to wait for that to occur and once it has then you are good to shut down.</p> <p>What I tend to do is have a 'per connection' structure which contains the socket and which is used as the completion key. I then have "per operation" structures which include the <code>OVERLAPPED</code> and which also include details of which operation type, the I/O buffer used and other stuff. Both of these structures is reference counted.</p> <p>When an operation is initiated you increment the reference count on both the connection object and the operation object. When you get a completion you process it and then decrement the counts. When the counts reach 0 you're not doing any work with the objects and they can be recycled to the pool for reuse.</p> <p>To aid in clean shutdown I have a counter that I can wait on that tracks the number of 'active' 'per connection' objects (sockets). </p> <p>To shut down you abort all connections and then wait for the connection counter to hit zero. At that point all of your objects are either destroyed or in your pools and you can clean up.</p> <p>I have some example code, <a href="http://www.serverframework.com/products---the-free-framework.html" rel="nofollow">here</a>, which is a set of full featured IOCP server examples which may help - it's working code that you can step through and get ideas from if nothing else.</p>
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    1. POIOCP With WSASend and OVERLAPPED pool
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    1. USLen Holgate
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    1. COThe reference counter is per-OVERLAPPED structure, I mean, every OVERLAPPED structure must have its own, and it must be atomic, since IOCP works with multiple threads, right? My concern was about that incrementing/decrementing many atomic counters too much times could impact performance, and threads' scalability. May this happen, or its just a baseless worry?
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      1. USMarco Pagliaricci
    2. COIt can do, and it can be worse on NUMA hardware; as always, profile and see. I'm adjusting my framework at the moment so that we don't use the ref count during actual IOCP dispatch (i.e. in normal operations there is no need to ref count as much as we were). I'm still keeping it as it makes for efficient server design when you want to keep the buffers around for longer than just a single IOCP operation, but for simpler servers (read, parse message, write response over inbound message, reply) there's no ref manipulation at all any more.
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      1. USLen Holgate
 

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