Validating file names

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Introduction

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):
  CLOCK$, AUX, CON, NUL, PRN, COM1, COM2, COM3, COM4, COM5, COM6, COM7, COM8, COM9, 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()

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:

GetIsValidFileNameErrStr()

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[]"

Notes

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.