How To Export Data from a DLL or an Application

http://support.microsoft.com/kb/90530

HOWTO: How To Export Data from a DLL or an Application

View products that this article applies to.

This article was previously published under Q90530

Expand all | Collapse all

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   warning LNK4087: CONSTANT keyword is obsolete; use DATA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 );

Back to the top