WindowsConsoleIO fileno() gets a file descriptor on demand by calling _open_osfhandle. As a low I/O function this expects _open flags, but currently the code is passing 'rb' (int 0x7262) and 'wb' (int 0x7762). It should use _O_RDONLY | _O_BINARY and _O_WRONLY | _O_BINARY. 

Actually, _open_osfhandle isn't actually opening the file, so it only cares about a few flags. Specifically, in lowio\osfinfo.cpp I see that it looks for _O_APPEND, _O_TEXT, and _O_NOINHERIT. So in this case passing 0 for the flags would also be ok.