DOS device names are reserved Windows names that refer to devices or special system destinations rather than ordinary files. The ordinary filename list includes CON, PRN, AUX, NUL, COM1 through COM9, and LPT1 through LPT9, along with certain superscript-digit variants. Adding an extension does not make one valid: Microsoft treats NUL.txt and NUL.tar.gz as equivalent to NUL.
What does “DOS device name” mean?
Windows maintains a device-naming namespace for compatibility and access to devices. In Microsoft’s description, a named device object created by a non-WDM driver can have an MS-DOS device name represented by a symbolic link under \DosDevicesDosDeviceName. Examples include COM1 for a serial port and C: for a drive. These names therefore have a system meaning beyond their appearance as text in a filename.
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Microsoft’s .NET path documentation likewise describes legacy DOS device paths such as CON, COM1, and LPT1 as device paths. See File path formats on Windows systems and Microsoft’s explanation of named device objects.
Which names are reserved for ordinary filenames?
Microsoft’s Windows file-naming guidance says not to use these names as file or directory names:
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CON,PRN,AUX, andNULCOM1throughCOM9LPT1throughLPT9COM¹,COM²,COM³,LPT¹,LPT², andLPT³
The superscript forms are easy to overlook, but Microsoft includes them in its reserved-name guidance. The full guidance is in Naming Files, Paths, and Namespaces.
Does adding an extension make a reserved name usable?
No. A reserved device name remains reserved when an extension follows it. Microsoft gives NUL.txt and NUL.tar.gz as examples treated equivalently to NUL. Renaming a file by adding .txt, another extension, or multiple extensions does not turn the device name into an ordinary filename.
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Why do some lists also include CONIN$ and CONOUT$?
Those names appear in API-specific documentation and should not be silently added to the ordinary filename list. Microsoft’s documentation for RtlIsDosDeviceName_U says that the function recognizes LPTn, COMn, PRN, AUX, NUL, CON, CONIN$, and CONOUT$ when n is a digit; it also says an optional trailing colon is ignored. That describes names recognized by this function, not a replacement for the general filename guidance. See RtlIsDosDeviceName_U function.
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Device names are only one part of Windows filename rules. Microsoft also identifies these separate restrictions and cautions:
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<,>,:,",/,,|,?, or*in a filename. - The NUL character and, generally, control characters 1 through 31 are not allowed.
- Do not end a file or directory name with a space or period. An underlying filesystem may support such a name even though the Windows shell and user interface do not.
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OSCAR,Oscar, andoscaras the same.
Filesystem rules can differ, so these are Windows naming rules rather than a claim that every filesystem uses identical restrictions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How should you choose a safe filename?
For a name intended to work as a normal file or folder in Windows, avoid the reserved bases regardless of capitalization or extension. Also check for prohibited characters and trailing spaces or periods. If an application or API handles device paths, consult its documentation separately: the API’s device-name recognition can be broader than the ordinary filename list.
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