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In Windows, DOS device names are reserved names such as CON, NUL, and COM1. They refer to devices or device paths rather than ordinary files, so Windows filename guidance says not to use them for files or folders. Adding an extension does not make one valid: NUL.txt is still treated as NUL.
What are DOS device names in Windows?
DOS device names are names Windows recognizes for device access and compatibility. Microsoft describes device names as symbolic links in the DosDevices namespace; examples include COM1 for a serial port and C: for a drive. In path handling, names such as CON, COM1, and LPT1 can be handled as device paths rather than ordinary file paths.
That special meaning is why Windows reserves these names for ordinary file and directory naming. They are not simply unusual labels that happen to be disallowed.
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Microsoft’s ordinary Windows file-naming guidance lists these reserved names:
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CON,PRN,AUX, andNULCOM1throughCOM9LPT1throughLPT9COM¹,COM²,COM³,LPT¹,LPT², andLPT³
Microsoft says not to use these reserved names for a file name. The same restriction applies when naming a directory.
Does adding an extension make a reserved name valid?
No. An extension does not bypass the reservation when the reserved name is the filename base. Microsoft gives NUL.txt and NUL.tar.gz as examples treated as equivalent to NUL. Avoid those names just as you would avoid NUL itself.
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Why do some lists also include CONIN$ and CONOUT$?
Microsoft’s ordinary filename guidance and its documentation for the RtlIsDosDeviceName_U function serve different purposes. The function-specific documentation includes CONIN$ and CONOUT$ along with names such as CON, NUL, COM and LPT forms. It also says that an optional trailing colon is ignored by the function. That API list describes what the function recognizes; it should not be silently substituted for Microsoft’s ordinary reserved-filename list.
Other Windows filename rules that can cause problems
Reserved device names are only one part of Windows naming guidance. For ordinary Windows filenames, Microsoft also says to avoid:
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- Characters such as
<,>,:,",/,,|,?, and* - The NUL character and, generally, control characters numbered 1 through 31
- A space or period at the end of a file or directory name
Microsoft notes that a filesystem may support names ending in a space or period even though the Windows shell and user interface do not. Filesystems can also have different rules for component names, so do not assume every filesystem behaves identically. Windows naming guidance also advises against relying on letter case to distinguish names such as OSCAR, Oscar, and oscar.
How to choose a safe Windows filename
- Choose a descriptive base name that is not on the reserved list, including the superscript-digit forms.
- Check the whole name, not just its extension. A base such as
NULremains reserved inNUL.txt. - Remove prohibited characters and avoid ending the name with a space or period.
- If software is using a Windows path API, consult that API’s documentation for device-path behavior; function-specific recognition rules may cover additional names.
Microsoft’s file and path naming guidance is the reference for ordinary Windows filenames. Its .NET path-format documentation describes legacy DOS device paths and how they are handled.
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