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Posted 5 Oct 2002

Validating file names

, 9 Oct 2002
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Function to determine if a file name is valid.


The Windows file system has documented set of rules for creating file and directory names. IsValidFileName() provides functions to check a filename against this set. They also return error codes enabling the developer to display more specific codes for the user. (These functions do NOT do a comprehensive search of the global name space to ensure there are no other failure modes with the file name in a subsequent file and/or directory creation. For example, the directory could already exist.)

The File Naming Rules

The file system has the following rules for creating names for files or directories:

  • All characters greater than ASCII 31 to be used except for the following: "*/:<>?\|
  • The name may not be only dots
  • The following device names cannot be used for a file name nor may they be used for the first segment of a file name (that part which precedes the first dot):
    		LPT1, LPT2, LPT3, LPT4, LPT5, LPT6, LPT7, LPT8, LPT9
  • Device names are case insensitive. (i.e. aux, AUX, Aux, etc. are identical.)
  • Names cannot be longer than 255 characters unless UNICODE paths are used

(There are additional rules for naming file paths, but they are of no concern here; refer to the WIN32 SDK documentation under "File Name Conventions" for additional information.)

The Prototypes

int IsValidFileName(const char *pFileName);
int IsValidFileName(const wchar_t *pFileName);

const TCHAR *pInvalidFileNameErrStr[];
const TCHAR *GetIsValidFileNameErrStr(int err);


IsValidFileName() will validate a file name against the above rules. It is implemented as two overloaded functions, one which accepts a const char * and one which accepts a const wchar_t *. The function will return zero on success and non-zero on failure. A non-zero return value indicates what the error was. The possible values are listed in the following table:

Return Value Description
0 Success
>0 An illegal character was encountered. The return value is the illegal character. If the return value is a dot ('.', 46) the file name was nothing but dots.
-1 A NULL or zero length file name was passed.
<-1 A device name was used. The value corresponds to the INVALID_FILENAME_... series of enumerations. You can pass this value to GetIsValidFileNameErrStr() to obtain a pointer to the name of this device.


GetIsValidFileNameErrStr() is provided to facilitate reporting errors. This function accepts a negative value in the range of the INVALID_FILENAME_... series of enumerations and returns the device name associated with that value.

The array of strings used for this function is exposed as const TCHAR *pInvalidFileNameErrStr[]"


The source code is commented and self explanatory.

The .cpp does not include a precompiled header. If you are using this in a project with precompiled headers, just add the proper name or include the .cpp file in one of the project's cpp files.

The source may look inefficient and confusing, but it is highly optimized. It is more than 11.5 times faster than if a scanning method using _strnicmp was used.


This article has no explicit license attached to it but may contain usage terms in the article text or the download files themselves. If in doubt please contact the author via the discussion board below.

A list of licenses authors might use can be found here


About the Author

Joe Woodbury
Software Developer (Senior)
United States United States
Joe is one of those software engineers with a film degree. His first paid programming job (what, you think film is a good way to make a living?) was writing games for Apple II's using 6502 assembly. He soon moved to 80x86 assembly, C, C++ (for a long time), C# and then back to C++ with some work in C#.

He first wrote software for Windows 3.0 in 1990. Save for some continued work in DOS and a horrid, and mercifully brief, foray into OS/2, he has concentrated on designing and writing software for all versions and types of Windows except RT.

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Comments and Discussions

GeneralGood Stuff Pin
Larry Antram4-Nov-03 18:57
memberLarry Antram4-Nov-03 18:57 
GeneralRe: Good Stuff Pin
flyingxu15-Nov-05 14:49
memberflyingxu15-Nov-05 14:49 
GeneralThink again... Pin
Andreas Saurwein7-Oct-02 1:18
memberAndreas Saurwein7-Oct-02 1:18 
GeneralRe: Think again... Pin
Joe Woodbury7-Oct-02 4:40
memberJoe Woodbury7-Oct-02 4:40 
GeneralRe: Think again... Pin
Andreas Saurwein7-Oct-02 6:24
memberAndreas Saurwein7-Oct-02 6:24 
GeneralRe: Think again... Pin
Joe Woodbury7-Oct-02 7:23
memberJoe Woodbury7-Oct-02 7:23 
GeneralRe: Think again... Pin
Gabriel 210-Oct-02 3:48
memberGabriel 210-Oct-02 3:48 
GeneralRe: Think again... Pin
Andreas Saurwein10-Oct-02 4:04
memberAndreas Saurwein10-Oct-02 4:04 
GeneralRe: Think again... Pin
Joe Woodbury10-Oct-02 5:32
memberJoe Woodbury10-Oct-02 5:32 
GeneralRe: Think again... Pin
Philippe Lhoste15-Oct-02 3:06
memberPhilippe Lhoste15-Oct-02 3:06 

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