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

SUMMARY

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   Otherwise, you will receive the warning warning LNK4087: CONSTANT keyword is obsolete; use DATA Alternately, with Visual C++, you can export the variables with: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 );