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  · DriverStudio          Understanding NT Symbolic Links
  · DriverBundle
  · Previews              Many device drivers communicate with applications.
  · Compatibility         Typically, an application uses CreateFile to open a
 [Downloads]  ·           device, and then uses ReadFile, WriteFile, and
                          DeviceIoControl to exchange information with the
 Wizards                  driver. The name that an application passes to
  · Utilities             CreateFile is neither the name of the driver nor the
  · NT source             name of the device object to be opened. Instead, it
 examples                 is the name of a symbolic link to the device object,
  · VxD source            with a special prefix to designate it as such.
 examples
  · WDM source            To understand what a symbolic link is, you need to
 examples                 know that Windows NT maintains a hierarchical
 [Resources]  ·           directory of objects. Device objects are just one
 Technical papers         type of object in the hierarchy, and reside in the
  · Useful links          \Device branch of the object directory. For example,
  · Technical tips        \Device\Serial0 names the device object created by
 [Support]  ·             SERIAL.SYS to control COM1. The system uses the
                          object directory structure as a repository for
 Support                  information that is common to a variety of different
  · Knowledge base        kinds of objects, such as security data and reference
  · Problem               counts.
 submission
  · Product               A symbolic link is a special type of object whose
 registration             purpose is to refer to another object in the object
  · Release notes         directory. The system stores symbolic links to
 [Shop NuMega]  ·         devices that applications may open in branch \?? of
 Buy it!                  the object directory. Prior to NT 4.0, the branch was
  · Price list            named \DosDevices, which is still valid. Each
  · How to buy            symbolic link contains a string that specifies
  · Sales offices         another object in the hierarchy. Most often, that
                          object is a device object. For example, \??\COM1
                          refers to \Device\Serial0.
 [Y2K Compliance]
                          Sometimes a symbolic link refers to another symbolic
                          link. For example, \??\PRN is an alias for \??\LPT1.
 [More information]
                          When calling CreateFile, an application tells the
                          system to search for a device in the \?? branch of
                          the object directory by prefixing the symbolic link
                          name with the special string "\\.\". In C/C++, the
                          quoted string looks like "\\\\.\\". For example, if
                          the symbolic link name is MyDevice0, then the name
                          passed to CreateFile is "\\\\.\\MyDevice0". An
                          application can't open a device for which there is no
                          symbolic link in the \?? branch.

                          You can download symlinks.zip (68kb), a very simple
                          utility program that quickly shows you the symbolic
                          links currently defined on your system. To view the
                          entire object directory, get Winobj from the NT
                          Internals web site.

                          Creating Symbolic Links
                          There are three ways to create symbolic links:

                             * By adding special registry entries under
                               \HKLM\CurrentControlSet\Control\Session
                               Manager\DOS. This approach is rarely used, and
                               not recommended.
                             * By calling DefineDosDevice from an application.
                               This API is documented in the Win32 SDK.
                             * By calling IoCreateSymbolicLink or
                               IoCreateUnprotectedSymoblicLink from a kernel
                               mode driver.

                          If you are using DriverWorks, the constructor for
                          class KDevice automatically creates the symbolic link
                          if a link name is supplied. The link name is the
                          third parameter to the constructor of KDevice. Here's
                          an example, the constructor for class RamdiskDevice,
                          which implements a ram disk:

                          RamdiskDevice::RamdiskDevice(
                                  PCWSTR DriveLetter,
                                  ULONG nRootDirs,
                                  ULONG nSectorsPerCluster,
                                  ULONG DiskSize
                                  ) :

                                  KDevice(
                                          L"RAMDISK0",  // Device name
                                          FILE_DEVICE_VIRTUAL_DISK,
                                          DriveLetter,  // Symbolic Link name
                                          FILE_REMOVABLE_MEDIA,
                                          DO_DIRECT_IO
                                          ),

                                  m_Image(NULL),
                                  m_OpenCount(0)
                          {
                                  . . .

                          Note: In the above that string DriveLetter passed to
                          the constructor of RamdiskDevice becomes the symbolic
                          link to the device object. In other words, the device
                          is known to applications by its drive letter, not by
                          "RAMDISK0", its actual name. DriverWorks
                          automatically prefixes \Device and \?? to the device
                          and link names, respectively.

                          Back to technical tip start page.

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