Microsoft KB Archive/90530

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

Q90530

The information in this article applies to:

Microsoft Win32 Application Programming Interface (API), used with:
- the operating system: Microsoft Windows NT, versions 3.1, 3.5, 3.51, 4.0
- Microsoft Windows 95
- the operating system: Microsoft Windows 2000

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.

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

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

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warning LNK4087: CONSTANT keyword is obsolete; use DATA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

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.

REFERENCES

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.

For more information regarding _declspec(dllexport), or the EXPORT def file keyword search the Visual C++ documentation or your vendor's compiler documentation regarding exporting objects.

Additional query words: 3.10 3.50

Keywords : kbDLL kbKernBase kbOSWinNT310 kbOSWinNT350 kbOSWinNT351 kbOSWinNT400 kbOSWin2000 kbOSWin95 kbDSupport kbGrpDSKernBase
Issue type : kbhowto
Technology : kbAudDeveloper kbWin32sSearch kbWin32API