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You must avoid using these names as a file name suffix or file name body, so you have to avoid names such as aux.c, file.aux or NUL.txt, because those names are reserved cause they are 'DOS' device names.

DOS device names are reserved words, and cannot be used as folder or file names. When parsing a reference to a file or folder, Windows correctly checks for the case in which a single DOS device name is used in the path, and treats it as invalid.