FindWindow error 183

Does anybody know what would cause the FindWindow function to return the error:

ALREADY_EXISTS error (183)

I could understand a FILE_NOT_FOUND (2), but why would it return a 183?开发者_StackOverflow

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MSDN says, that FindWindowand FindWindowEx return NULL if the function fails and that you should check GetLastError. It seems that this documentation is wrong. Take this code fragment:

SetLastError(12345);
HWND h = FindWindow(L"class_name_that_does_not_exist", nullptr);
cout << h << ' ' << GetLastError() << endl;

It will output

00000000 12345

So as you can see FindWindow fails to set the last error. In your case this means that the ERROR_ALREADY_EXISTS was the last error set before FindWindow was called.