How To Export Data from a DLL or an Application

• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.1, 3.5, 3.51, 4.0
      - Microsoft Windows 95 version 4.0
  

SUMMARY

It is possible for a Win32-based application to be able to address DLL global variables directly by name from within the executable. This is done by exporting global data names in a way that is similar to the way you export a DLL function name. Use the following steps to declare and utilize exported global data.

1. Define the global variables in the DLL code. For example:

         int i = 1;
         int *j = 2;
         char *sz = "WBGLMCMTP";
   

2. Export the variables in the module-definition (DEF) file. With the 3.1 SDK linker, use of the CONSTANT keyword is required, as shown below:

   EXPORTS

         i  CONSTANT
         j  CONSTANT
         sz CONSTANT
   

   With the 3.5 SDK linker or the Visual C++ linker, use of the DATA keyword is required, as shown below

   EXPORTS

         i  DATA
         j  DATA
         sz DATA
   

   Otherwise, you will receive the warning

         warning LNK4087: CONSTANT keyword is obsolete; use DATA
   

   Alternately, with Visual C++, you can export the variables with:

         _declspec( dllexport ) int i;
         _declspec( dllexport ) int *j;
         _declspec( dllexport ) char *sz;
   

3. If you are using the 3.1 SDK, declare the variables in the modules that will use them (note that they must be declared as pointers because a pointer to the variable is exported, not the variable itself):

         extern int *i;
         extern int **j;
         extern char **sz;
   

   If you are using the 3.5 SDK or Visual C++ and are using DATA, declare the variables with _declspec( dllimport ) to avoid having to manually perform the extra level of indirection:

         _declspec( dllimport ) int i;
         _declspec( dllimport ) int *j;
         _declspec( dllimport ) char *sz;
   

4. If you did not use _declspec( dllimport ) in step 3, use the values by dereferencing the pointers declared:

         printf( "%d", *i );
         printf( "%d", **j );
         printf( "%s", *sz );
   

   It may simplify things to use #defines instead; then the variables can be used exactly as defined in the DLL:

         #define i *i
         #define j *j
         #define sz *sz
   

         extern int i;
         extern int *j;
         extern char *sz;
   

         printf( "%d", i );
         printf( "%d", *j );
         printf( "%s", sz );
   

MORE INFORMATION

NOTE: This technique can also be used to export a global variable from an application so that it can be used in a DLL.

REFERENCE

For more information on the use of EXPORTS and CONSTANT in the Module Definition File (DEF) file for the 3.1 SDK, see Chapter 4 of the Win32 SDK "Tools" manual.