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    <p>For <code>GetIoCompletionPort</code> to work correctly, you need to specify a non-null pointer to a <code>ULONG_PTR</code> for it to write the 'key' value to:</p> <pre><code>ULONG_PTR key; GetQueuedCompletionStatus( completion_port, &amp;numBytes, &amp;key, &amp;po, INFINITE ); </code></pre> <p>To use <code>GetOverlappedResult</code> successfully, I believe you need to specify an event handle in the <code>OVERLAPPED</code> structure (strongly recommended in any case):</p> <pre><code>o.hEvent = CreateEvent(NULL, FALSE, TRUE, NULL); </code></pre> <p>Calling the two in succession as you were doesn't really accomplish much -- they both tell you about the same things. Though if you do call both in succession, you'll need to change the Event to be a manual-reset by changing the third parameter to <code>CreateEvent</code> to TRUE. My guess is that you were just trying both to see if you could get one to work. All things considered, I'd probably just use <code>GetQueuedCompletionStatus</code>, and leave it at that. Of course, you'll usually do more than call it once and quit. You normally call it in a loop, processing the current buffer you've read, then calling <code>ReadFile</code> again to read another buffer of info, something like this:</p> <pre><code>DWORD numBytes; LPOVERLAPPED po; while (GetQueuedCompletionStatus(completion_port, &amp;numBytes, &amp;key, &amp;po, INFINITE)) { std::cout &lt;&lt; "\rRead: " &lt;&lt; numBytes; // just to show it's set correctly. process(buffer); po-&gt;offset += sizeof(buffer); ReadFile(file, buffer, sizeof(buffer), NULL, po); } </code></pre> <p>At least in a quick test on my machine, this showed the number of bytes read correctly (<code>sizeof(buffer)</code> up to the last packet, then the remaining size of the file).</p>
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    1. USJerry Coffin
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    1. COYes, you were right about the pointer to the key. You were also right in your assumption that I tried those 2 functions to see if any of them works :-). Thanks.
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      1. USAram Hăvărneanu
 

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