Win32 Windows Volume Program and Code Example 16






Editing Drive Letter Assignments Program Example


The following code example shows you how to add or remove persistent drive letter assignments. These drive letter assignments persist through system shutdown. You are not encouraged to try this code on your personal PC!

The code example uses the following functions: DefineDosDevice(), DeleteVolumeMountPoint(), GetVolumeNameForVolumeMountPoint(), and SetVolumeMountPoint().

Create a new Win32 console application project and give a suitable project name.

Add the source file and give a suitable name.


Editing Drive Letter Assignments Program Example - creating a new Win32 console project


Add the following source code.



AddRemoveDrive  -- Drive Letter Assignment Editor

Platforms: This program requires Windows 2000 or later.


Command-line syntax:

   AddRemoveDrive <drive letter> <device name>      -- Adds a drive letter

   AddRemoveDrive -r <drive letter>                 -- Removes a drive letter


Command-line examples:

   If E: refers to the CD-ROM drive, use the following commands to

   make F: point to the CD-ROM drive instead.


   AddRemoveDrive -r E:\

   AddRemoveDrive F:\ \Device\CdRom0





   This program will change drive letter assignments, and the   

   changes persist through reboots. Do not remove drive letters 

   of your hard disks if you do not have this program on floppy 

   disk or you might not be able to access your hard disks again!



// Windows Server 2003, Windows XP, change accordingly


#define _WIN32_WINNT 0x0501


#include <Windows.h>

#include <stdio.h>


// For debug info

#if defined (DEBUG)

   static void DebugPrint (LPCTSTR pszMsg, DWORD dwErr);

   #define DEBUG_PRINT(pszMsg, dwErr) DebugPrint(pszMsg, dwErr)


   #define DEBUG_PRINT(pszMsg, dwErr) NULL



// Disable the warning 4800 (forcing value to 'true' or 'false' (performance warning)).

#pragma warning (disable : 4800)

// Other examples:

// Displaying the warning only once - #pragma warning ( once : 4800 ) 

// Apply the warning level (1-4) to the specified warning message(s) - #pragma warning ( 3 : 4800 )     

// Report the specified warnings as errors - #pragma warning ( error: 4800 )


// Function prototype

void PrintHelp(LPCTSTR pszAppName);



The main function is the main routine. It parses the command-line

arguments and either removes or adds a drive letter.


   argc -  Count of the command-line arguments

   argv -  Array of pointers to the individual command-line arguments


void wmain(int argc, WCHAR *argv[])


   WCHAR * pszDriveLetter, * pszNTDevice, * pszOptions;

   WCHAR szUniqueVolumeName[MAX_PATH];

   WCHAR szDriveLetterAndSlash[4];

   WCHAR szDriveLetter[3];

   BOOL  fRemoveDriveLetter;

   BOOL  fResult;


   if (argc != 3)






   // Use the command line to see if user wants to add or remove the

   // drive letter. Do this by looking for the -r option.

   fRemoveDriveLetter = !lstrcmpi (argv[1], L"-r");


   if (fRemoveDriveLetter)


      // User wants to remove the drive letter. Command line should

      // be: dl -r <drive letter>

      pszOptions       = argv[1];

      pszDriveLetter   = argv[2];

      pszNTDevice      = NULL;




      // User wants to add a drive letter. Command line should be:

      // dl <drive letter> <NT device name>

      pszOptions       = NULL;

      pszDriveLetter   = argv[1];

      pszNTDevice      = argv[2];



   // GetVolumeNameForVolumeMountPoint, SetVolumeMountPoint, and

   // DeleteVolumeMountPoint require drive letters to have a trailing

   // backslash. However, DefineDosDevice requires that the trailing

   // backslash be absent. So, use:


   //    szDriveLetterAndSlash     for the mounted folder functions

   //    szDriveLetter             for DefineDosDevice


   // This way, command lines that use a: or a:\

   // for drive letters can be accepted without writing back

   // to the original command-line argument.

   szDriveLetter[0] = pszDriveLetter[0];

   szDriveLetter[1] = ':';

   szDriveLetter[2] = '\0';


   szDriveLetterAndSlash[0] = pszDriveLetter[0];

   szDriveLetterAndSlash[1] = ':';

   szDriveLetterAndSlash[2] = '\\';

   szDriveLetterAndSlash[3] = '\0';


   // Now add or remove the drive letter.

   if (fRemoveDriveLetter)


      fResult = DeleteVolumeMountPoint (szDriveLetterAndSlash);


      if (!fResult)

         wprintf(L"error %lu: couldn't remove %s\n", GetLastError(), szDriveLetterAndSlash);




      // To add a drive letter that persists through reboots, use

      // SetVolumeMountPoint. This requires the volume GUID path

      // of the device to which the new drive letter will refer.

      // To get the volume GUID path, use

      // GetVolumeNameForVolumeMountPoint; it requires the drive

      // letter to already exist. So, first define the drive

      // letter as a symbolic link to the device name. After 

      // you have the volume GUID path the new drive letter will

      // point to, you must delete the symbolic link because the

      // mount manager allows only one reference to a device at a

      // time (the new one to be added).

      fResult = DefineDosDevice (DDD_RAW_TARGET_PATH, szDriveLetter, pszNTDevice);


      if (fResult)


          // If GetVolumeNameForVolumeMountPoint fails, then

          // SetVolumeMountPoint will also fail. However,

          // DefineDosDevice must be called to remove the temporary symbolic link.

          // Therefore, set szUniqueVolume to a known empty string.

         if (!GetVolumeNameForVolumeMountPoint (szDriveLetterAndSlash, szUniqueVolumeName, MAX_PATH))


            DEBUG_PRINT("GetVolumeNameForVolumeMountPoint failed", GetLastError());

            szUniqueVolumeName[0] = '\0';



         fResult = DefineDosDevice (


                      DDD_EXACT_MATCH_ON_REMOVE, szDriveLetter,



         if (!fResult)

            DEBUG_PRINT("DefineDosDevice failed", GetLastError());


         fResult = SetVolumeMountPoint (szDriveLetterAndSlash, szUniqueVolumeName);


         if (!fResult)

            wprintf(L"error %lu: could not add %s\n", GetLastError(), szDriveLetterAndSlash);






The PrintHelp function prints the command-line usage help.

Parameters: pszAppName

      The name of the executable. Used in displaying the help.


void PrintHelp (LPCTSTR pszAppName)


   wprintf(L"---Adds/removes a drive letter assignment for a device---\n\n");

   wprintf(L"Usage: %s <Drive> <Device name> add a drive letter\n", pszAppName);

   wprintf(L"       %s -r <Drive>            remove a drive letter\n\n", pszAppName);

   wprintf(L"Example: %s e:\\ \\Device\\CdRom0\n", pszAppName);

   wprintf(L"         %s -r e:\\\n", pszAppName);



#if defined (DEBUG)


The DebugPrint function prints a string to STDOUT.




      The string to be printed to STDOUT.


      The error code; usually obtained from GetLastError. If dwErr is

      zero, no error code is added to the error string. If dwErr is

      nonzero, the error code will be printed in the error string.


void DebugPrint (LPCTSTR pszMsg, DWORD dwErr)


   if (dwErr)

      wprintf(L"%s: %lu\n", pszMsg, dwErr);


      wprintf(L"%s\n", pszMsg);




Build and run the project. The following screenshot is an output sample. This sample program should not be tested on the production machine.


Editing Drive Letter Assignments Program Example - a sample output without any argument




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