Why does the RegQueryValueEx() function return ERROR_FILE_NOT_FOUND while trying to read from a registry key?
You have to escape the slashes, just like you did in the first line...
if( RegOpenKeyEx( HKEY_LOCAL_MACHINE, TEXT("HARDWARE\\DEVICEMAP\\SERIALCOMM"), 0, KEY_QUERY_VALUE, &hKey ) == ERROR_SUCCESS ) { auto ret = RegQueryValueEx( hKey, TEXT("\\Device\\Serial0"), 0, &dwType, (LPBYTE)buf, &dwBufSize ); // ret always == 2 for key with slashesIf you don't, the RegQueryValueEx function can't find the specified key, and it returns ERROR_FILE_NOT_FOUND (== 2).
But there's another problem. You should be declaring the buffer array as type wchar_t (or TCHAR), rather than char:
TCHAR buf[255] = {0};Otherwise, the RegQueryValueEx function is going to attempt to fill the array with a Unicode string read from the specified registry key, and you're going to get something unreadable.