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NTFS Alternate Streams

Links, Junctions, Shortcuts

NTFS Sparse Files



      
 

 

NTFS Hard Links, Directory Junctions, and Windows Shortcuts

A link is basically an alternate name for a file or a directory. In many cases creating a link is preferrable to making several copies of the file. Links save disk space and more important, if the file should be changed afterwards, you will not have to edit a bunch of copies - all the links point to the same file.

If you are familiar with Unix-family operating systems, you must have known what the ln command is. Any BSD or Linux disk contains a lot of links, and the ln command is one of the most commonly used. Windows also supports both hard and soft links, but surprisingly, it provides no command line tool for creating links.

UPDATE: Windows 7 includes mklink command-line utility for creating links.

In this article we will show how to create symbolic and hard links programmatically. If you are not interested in programming and just want to get the utility, skip directly to the download section.


NTFS Hard Links

Hard link uses the same MFT entry as the original file. Adding a hard link creates a new name attribute and increases the hard link count (for a newly created file this count equals to one). Deleting a hard link removes the appropriate name and decreases the hard link count. When the count goes to zero, the system deletes the file, freeing up its allocated disk space and releasing its MFT record. All the name attributes are independent, so deleting, moving, or renaming the file don't affect other hard links.

In order to get the value of the hard link count, open the file in the FlexHEX editor and select the File Properties page in the Streams pane (the lower left one). If there is any hard link to the file, the Hard Links value will be greater than one (the normal directory record is also counted, so one means no hard links).

Because all the links must refer to the same MFT entry, you cannot create a hard link on a different drive. Another limitation is that only files can be hard-linked, not directories (however soft links to directories are allowed).

Programming Considerations

To get the number of links use the Win32 API function GetFileInformationByHandle. The nNumberOfLinks member of the returned BY_HANDLE_FILE_INFORMATION structure contains the total number of links (a value of 1 means there are no hard links).

Creating Hard Link

Creating a hard link is simple - just call the Win32 API function CreateHardLink (this function is available in Windows 2000 and later systems).

BOOL CreateHardLink(LPCTSTR lpFileName,               // Link path
                    LPCTSTR lpExistingFileName,
                    LPSECURITY_ATTRIBUTES lpSecurityAttributes);

While the first two arguments are fairly obvious, the third one may be somewhat misleading. Security descriptors belong to files, not to hard links, so if you specify a security attributes in a CreateHardLink call, this will change security settings for the file itself (and for all the file links). You should also keep in mind that calling CreateHardLink with a non-NULL lpSecurityAttributes argument most likely will fail if the user is not the owner of the file or a system administrator. It is not clear what reasons did Microsoft have for adding that argument - modifying file security the usual way, using the SetSecurityInfo function, would be a better practice.

Deleting Hard Link

In order to delete a hard link, call the Windows API function DeleteFile, specifying the link path. The DeleteFile function removes the link and decreases the link count in the file's MFT record. If the link count becomes zero, the system deletes the file, that is frees its MFT entry and disk space.

Note that there is no difference between the original file name and an additional hard link. Both are just pointers to the file's MFT entry.


Reparse Points and Directory Junctions

Reparse points provide another way of creating links. If a file or a directory has a reparse point attached, the system calls a file system filter, indicated by the reparse point tag. The filter may implement any method of accessing the actual data, including quite complicated ones. HSM (Hierarchical Storage Management) is a good example how useful reparse points can be.

Unfortunately, although we can link to directories using the reparse points mechanism (such links are called junctions), there is no way of linking to files short of writing a custom file system filter driver. Even if using reparse points is a natural way of implementing soft links, the gain is hardly worth the pain.

However reparse points are not all that useless for our purpose: junctions can supplement NTFS file-only hard links. Just keep in mind that they are, in fact, soft links, and if you move or delete the referred directory, the junction will point to nowhere. On the other hand, being a soft link, a junction is not limited to objects residing on the same drive; it can point to a directory on a different drive, or even to the drive itself (although a junction is not allowed to point to a mapped network object).

Programming Considerations

In order to improve readability, the code examples below don't include any error processing. You should add some error checks if you want to use this code in your program. See the sources in the download section for an example of error handling.

Aside from a couple of overview articles, reparse points are very scantily documented, and the Platform SDK does not include the required definitions:

#define REPARSE_MOUNTPOINT_HEADER_SIZE   8

typedef struct {
  DWORD ReparseTag;
  DWORD ReparseDataLength;
  WORD Reserved;
  WORD ReparseTargetLength;
  WORD ReparseTargetMaximumLength;
  WORD Reserved1;
  WCHAR ReparseTarget[1];
} REPARSE_MOUNTPOINT_DATA_BUFFER, *PREPARSE_MOUNTPOINT_DATA_BUFFER;

The header file WinNT.h included in the VC++ 6.0 distributive contains another definition (Microsoft removed it in the later versions of the Platform SDK):

typedef struct _REPARSE_DATA_BUFFER {
    DWORD  ReparseTag;
    WORD   ReparseDataLength;
    WORD   Reserved;
    union {
        struct {
            WORD   SubstituteNameOffset;
            WORD   SubstituteNameLength;
            WORD   PrintNameOffset;
            WORD   PrintNameLength;
            WCHAR PathBuffer[1];
        } SymbolicLinkReparseBuffer;
        struct {
            WORD   SubstituteNameOffset;
            WORD   SubstituteNameLength;
            WORD   PrintNameOffset;
            WORD   PrintNameLength;
            WCHAR PathBuffer[1];
        } MountPointReparseBuffer;
        struct {
            BYTE   DataBuffer[1];
        } GenericReparseBuffer;
    };
} REPARSE_DATA_BUFFER, *PREPARSE_DATA_BUFFER;

#define REPARSE_DATA_BUFFER_HEADER_SIZE   \
           FIELD_OFFSET(REPARSE_DATA_BUFFER, GenericReparseBuffer)

We will use the former structure as defined in the Junctions example on the Systems Internals Web site.

Helper Functions

We will need a couple of helper functions. First one opens a directory and returns its handle:

// Returns directory handle or INVALID_HANDLE_VALUE if failed to open.
// To get extended error information, call GetLastError.

HANDLE OpenDirectory(LPCTSTR pszPath, BOOL bReadWrite) {
// Obtain backup/restore privilege in case we don't have it
  HANDLE hToken;
  TOKEN_PRIVILEGES tp;
  ::OpenProcessToken(::GetCurrentProcess(), TOKEN_ADJUST_PRIVILEGES, &hToken);
  ::LookupPrivilegeValue(NULL,
                         (bReadWrite ? SE_RESTORE_NAME : SE_BACKUP_NAME),
                         &tp.Privileges[0].Luid);
  tp.PrivilegeCount = 1;
  tp.Privileges[0].Attributes = SE_PRIVILEGE_ENABLED;
  ::AdjustTokenPrivileges(hToken, FALSE, &tp, sizeof(TOKEN_PRIVILEGES), NULL, NULL);
  ::CloseHandle(hToken);

// Open the directory
  DWORD dwAccess = bReadWrite ? (GENERIC_READ | GENERIC_WRITE) : GENERIC_READ;
  HANDLE hDir = ::CreateFile(pszPath, dwAccess, 0, NULL, OPEN_EXISTING,
                     FILE_FLAG_OPEN_REPARSE_POINT | FILE_FLAG_BACKUP_SEMANTICS, NULL);

  return hDir;
}

The second helper function checks if the directory has an associated junction point:

#define DIR_ATTR  (FILE_ATTRIBUTE_DIRECTORY | FILE_ATTRIBUTE_REPARSE_POINT)

BOOL IsDirectoryJunction(LPCTSTR pszDir) {
  DWORD dwAttr = ::GetFileAttributes(pszDir);
  if (dwAttr == -1) return FALSE;  // Not exists
  if ((dwAttr & DIR_ATTR) != DIR_ATTR) return FALSE;  // Not dir or no reparse point

  HANDLE hDir = OpenDirectory(pszDir, FALSE);
  if (hDir == INVALID_HANDLE_VALUE) return FALSE;  // Failed to open directory

  BYTE buf[MAXIMUM_REPARSE_DATA_BUFFER_SIZE];
  REPARSE_MOUNTPOINT_DATA_BUFFER& ReparseBuffer = (REPARSE_MOUNTPOINT_DATA_BUFFER&)buf;
  DWORD dwRet;
  BOOL br = ::DeviceIoControl(hDir, FSCTL_GET_REPARSE_POINT, NULL, 0, &ReparseBuffer,
                                      MAXIMUM_REPARSE_DATA_BUFFER_SIZE, &dwRet, NULL);
  ::CloseHandle(hDir);
  return br ? (ReparseBuffer.ReparseTag == IO_REPARSE_TAG_MOUNT_POINT) : FALSE;
}

Creating Directory Junction

The first step is creating a directory if it does not exists. If the directory exists, it must be empty - you cannot associate a reparse point with a non-empty directory.

::CreateDirectory(szJunction, NULL)

The next step is obtaining the directory handle:

// Open for reading and writing (see OpenDirectory definition above)
HANDLE hDir = OpenDirectory(szJunction, TRUE);

And the last step is the black magic:

// Take note that buf and ReparseBuffer occupy the same space
BYTE buf[sizeof(REPARSE_MOUNTPOINT_DATA_BUFFER) + MAX_PATH * sizeof(WCHAR)];
REPARSE_MOUNTPOINT_DATA_BUFFER& ReparseBuffer = (REPARSE_MOUNTPOINT_DATA_BUFFER&)buf;

// Prepare reparse point data
memset(buf, 0, sizeof(buf));
ReparseBuffer.ReparseTag = IO_REPARSE_TAG_MOUNT_POINT;
int len = ::MultiByteToWideChar(CP_ACP, 0, szTarget, -1,
                                ReparseBuffer.ReparseTarget, MAX_PATH);
ReparseBuffer.ReparseTargetMaximumLength = (len--) * sizeof(WCHAR);
ReparseBuffer.ReparseTargetLength = len * sizeof(WCHAR);
ReparseBuffer.ReparseDataLength = ReparseBuffer.ReparseTargetLength + 12;

// Attach reparse point
DWORD dwRet;
::DeviceIoControl(hDir, FSCTL_SET_REPARSE_POINT, &ReparseBuffer,
         ReparseBuffer.ReparseDataLength+REPARSE_MOUNTPOINT_HEADER_SIZE,
         NULL, 0, &dwRet, NULL);

Note that szTarget string must contain the path prefixed with the "non-parsed" prefix "\??\", and terminated with the backslash character, for example "\??\C:\Some Dir\".

You can save some effort by using the CreateJunction function you can find in the download section.

Deleting Directory Junction

If the directory is empty, you can use the RemoveDirectory function to remove both the directory and the associated junction point. Another way is to use DeviceIoControl, which does not require the directory to be empty and removes the junction only, leaving the directory and its content intact.

if (!IsDirectoryJunction(szJunction)) {
  // Error: no junction here
}

// Open for reading and writing (see OpenDirectory definition above)
HANDLE hDir = OpenDirectory(szJunction, TRUE);

BYTE buf[REPARSE_MOUNTPOINT_HEADER_SIZE];
REPARSE_MOUNTPOINT_DATA_BUFFER& ReparseBuffer = (REPARSE_MOUNTPOINT_DATA_BUFFER&)buf;
DWORD dwRet;

memset(buf, 0, sizeof(buf));
ReparseBuffer.ReparseTag = IO_REPARSE_TAG_MOUNT_POINT;

if (::DeviceIoControl(hDir, FSCTL_DELETE_REPARSE_POINT, &ReparseBuffer,
                          REPARSE_MOUNTPOINT_HEADER_SIZE, NULL, 0, &dwRet, NULL)) {
  // Success
  ::CloseHandle(hDir);
}
else {  // Error
  DWORD dr = ::GetLastError();
  ::CloseHandle(hDir);
  // Some error action (throw or MessageBox)
}

Querying Directory Junction

The following code returns the path the junction point is linked to:

TCHAR szPath[MAX_PATH];  // Buffer for returned path

if (!IsDirectoryJunction(szJunction)) {
  // Error: no junction here
}

// Open for reading only (see OpenDirectory definition above)
HANDLE hDir = OpenDirectory(szJunction, FALSE);

BYTE buf[MAXIMUM_REPARSE_DATA_BUFFER_SIZE];  // We need a large buffer
REPARSE_MOUNTPOINT_DATA_BUFFER& ReparseBuffer = (REPARSE_MOUNTPOINT_DATA_BUFFER&)buf;
DWORD dwRet;

if (::DeviceIoControl(hDir, FSCTL_GET_REPARSE_POINT, NULL, 0, &ReparseBuffer,
                                 MAXIMUM_REPARSE_DATA_BUFFER_SIZE, &dwRet, NULL)) {
  // Success
  ::CloseHandle(hDir);

  LPCWSTR pPath = ReparseBuffer.ReparseTarget;
  if (wcsncmp(pPath, L"\\??\\", 4) == 0) pPath += 4;  // Skip 'non-parsed' prefix
  ::WideCharToMultiByte(CP_ACP, 0, pPath, -1, szPath, MAX_PATH, NULL, NULL);
}
else {  // Error
  DWORD dr = ::GetLastError();
  ::CloseHandle(hDir);
  // Some error action (throw or MessageBox)
}

Windows Shortcuts

A more commonly used name for Windows symbolic link is shortcut. Shortcut is a much more advanced version of Unix soft link - in addition to the basic link-to-file relationship you can also set a description, an icon, and so on. In addition, shortcuts can point to non-file objects like printers.

A shortcut is actually a file, usually with the .lnk extension (any other extension can also be registered as a shortcut). Like any other file it can be copied, renamed, or deleted; file operations on a shortcut don't affect the object being pointed to.

Programming Considerations

In order to improve readability, the code examples below don't include any error processing. You should add some error checks if you want to use this code in your program. See the sources in the download section for an example of error handling.

Creating New Shortcut

In order to set up a Windows shortcut programmatically first create it using the IShellLink interface (don't forget to call CoInitialize before calling any COM function).

IShellLink *pShellLink;
::CoCreateInstance(CLSID_ShellLink, NULL, CLSCTX_ALL,
                   IID_IShellLink, (void**)&pShellLink);
pShellLink->SetPath(szPath);  // Path to the object we are referring to

If you want to create a shortcut for a non-file object, use the SetIDList function instead of SetPath. In this case you should use a pointer to the object's ITEMIDLIST structure, or PIDL. PIDL is a more universal way of object addressing so you can use the SetIDList function for both file and non-file objects. You can obtain PIDLes for the standard shell objects by calling the SHGetFolderLocation or SHGetSpecialFolderLocation function. To convert a PIDL to the corresponding file path use the shell function SHGetPathFromIDList.

To set additional parameters like a descripton or an icon location use the appropriate IShellLink functions:

pShellLink->SetDescription(szDescr);

When the shortcut is set up completely, save it using the IPersistFile interface:

IPersistFile *pPersistFile;
pShellLink->QueryInterface(IID_IPersistFile, (void**)&pPersistFile);
pPersistFile->Save(wszLinkPath, TRUE);

Please note that unlike IShellLink::SetPath, and like most COM/OLE functions, IPersistFile::Save requires a UNICODE path string.

As a final stroke, commit I/O by releasing the interface pointers, and the newly created shortcut is here!

pPersistFile->Release();
pShellLink->Release();

Modifying Existing Shortcut

An existing shortcut can be modified using the same code; the only difference is that you should first read the shortcut using the IPersistFile::Load call. For example, a function for changing the path to the shortcut target would look as follows:

//  wszLinkPath - a UNICODE string containing the path to the shortcut (for
//      example 'C:\Documents and Settings\All Users\Desktop\MyShortcut.lnk')
//
//  szNewTarget - an ANSI string containing a new path to the target.

IShellLink *pShellLink;
IPersistFile *pPersistFile;

// Obtain IShellLink and IPersistFile pointers
::CoCreateInstance(CLSID_ShellLink, NULL, CLSCTX_ALL,
                   IID_IShellLink, (void**)&pShellLink);
pShellLink->QueryInterface(IID_IPersistFile, (void**)&pPersistFile);

// Load existing shortcut
pPersistFile->Load(wszLinkPath, STGM_READWRITE | STGM_SHARE_EXCLUSIVE);

// Modify shortcut (set a new target path)
pShellLink->SetPath(szNewTarget);

// Save the modified shortcut
pPersistFile->Save(wszLinkPath, TRUE);

// Commit I/O and release interface pointers
pPersistFile->Release();
pShellLink->Release();

Deleting Shortcut

Simply delete the shortcut file using the standard DeleteFile API function.

Desktop? Where Is It?

You will probably want to place your shiny new shortcut on the desktop or in the Start menu. However the directories associated with the standard locations may vary for different Windows versions and localizations. If you want your code to work on different computers, you should never assume that the Desktop directory is C:\Document and Settings\All Users\Desktop or the Programs directory is C:\Program Files.

The Windows Shell API function SHGetSpecialFolderPath and the newer SHGetFolderPath retrieve the path of a special folder. However if a compatibility with older Windows versions is important, use the older function SHGetSpecialFolderLocation, which is supported by all Windows versions and does not require a redistributable.

TCHAR szDesktopPath[MAX_PATH];
LPITEMIDLIST pidl;
::SHGetSpecialFolderLocation(NULL, CSIDL_DESKTOPDIRECTORY, &pidl);
::SHGetPathFromIDList(pidl, szDesktopPath);

Remember to free the PIDL when it is no more needed:

LPMALLOC pMalloc;
::SHGetMalloc(&pMalloc);
pMalloc->Free(pidl);
pMalloc->Release();

Notifying the Explorer

After you created or deleted a shortcut, you may need to use the SHChangeNotify function to notify the Windows Explorer about the change in the shortcut directory and to ensure the directory view is properly updated.


Downloads

All the content is provided on the "as is" basis and without any warranty, express or implied. You can use the supplied tools and examples for any commercial or non-commercial purpose without charge. You can copy and redistribute these tools and examples freely, provided that you distribute the original umodified archives.

ln.zip - a command-line utility, similar to the Unix ln command. See the enclosed readme.txt file for a description and usage examples.

links.zip - The complete Visual C++ source code for the ln command-line utility. These sources supplement the Programming Considerations sections and can be used as reference code.


Comments? Suggestions? Please feel free to let us know.

 
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