In one our Projects at company, we used WM_DEVICECHANGE message to detect the arrival and removal of USB devices. The application was supposed to run on a machine in which will be running along with several other utility applications. Some months after the project delivery, client reported that, sometimes even if a USB device is plugged in, it is not detected. Problem problem.....
After some investigations we found that if we make the window a top-level window (HWND_TOPMOST), there is no such problems. But what’s the relation between the z-order and the above problem. Later we got few more points to identify the relation.
1. The WM_DEVICECHANGE is send to all the top-level windows according to their z-order. That is the window at the top most position receives the message first, then only the window below it and so one.
After some investigations we found that if we make the window a top-level window (HWND_TOPMOST), there is no such problems. But what’s the relation between the z-order and the above problem. Later we got few more points to identify the relation.
1. The WM_DEVICECHANGE is send to all the top-level windows according to their z-order. That is the window at the top most position receives the message first, then only the window below it and so one.
2. If a window got a WM_DEVICECHANGE and if it didn’t process the message with in a period (in my machine I think it was 20 secs), the message will not reach to the subsequent windows under it.
Now everything became clear to us, some window whose z-order is greater than our application window might have hung or didn’t process the message within 20 seconds. Thus our window didn’t get a chance. Also when we made our window top level it will be one of the first window's to get WM_DEVICECHANGE message so reducing the number of windows above it to get hung. How ever we didn't find a proper solution for this and moved to an alternative method to detect device arrival and removal.
I think windows might not be using the BroadcastSystemMessage() function with the BSF_FORCEIFHUNG flag, while sending the WM_DEVICECHANGE message.
Now everything became clear to us, some window whose z-order is greater than our application window might have hung or didn’t process the message within 20 seconds. Thus our window didn’t get a chance. Also when we made our window top level it will be one of the first window's to get WM_DEVICECHANGE message so reducing the number of windows above it to get hung. How ever we didn't find a proper solution for this and moved to an alternative method to detect device arrival and removal.
I think windows might not be using the BroadcastSystemMessage() function with the BSF_FORCEIFHUNG flag, while sending the WM_DEVICECHANGE message.
