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I've got a Qt app (Qt 4.8.1) that's doing some Windows serial port tasks. I'm finding that occasionally the CreateFileA call that I do to open the serial port is taking up to 30 seconds to complete! Obviously I'm doing something to trigger this odd behavior, and I want to know what it is I might be doing to cause this. m_portHand = CreateFileA( portDevice.c_str(), GENERIC_READ | GENERIC_WRITE, 0, // must be opened with exclusive-access NULL, // default security attributes OPEN_EXISTING, // must use OPEN_EXISTING FILE_FLAG_OVERLAPPED, // overlapped I/O NULL ); // hTemplate must be NULL for comm devices m_portHand is a HANDLE, and portDevice is an std::string and contains "COM5". This call is triggered by a button push in the main thread of my app. At the time it happens the app has at most one other thread, but those threads (if any) are idle. The only major thing going on in the system is a VM running Linux, but the system is a quad-core and 3 of the cores are as near to idle as you see on a Windows box, with only one doing anything with the VM. The serial ports are on an 8 port USB serial box, could that be related? Is this related to the Overlapped IO in some way? In response to comments: Port is not open by another app. Port was previously open by a previous invocation of this app, which was properly closed, and the port closed with 'CloseHandle'. I have not been able to determine any correlations between it taking 30 seconds and not - sometimes I start the app up, click the button and we're off to the races, sometimes it takes up to 30 seconds. The VM is intercepting some other USB devices on the same serial box. Other than the serial box (with the VM polling 4 ports

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