INFO: Exporting Callback Functions Not Required in Win32 Apps

• Microsoft Visual C++, 32-bit Edition, versions 1.0, 2.0, 2.1, 4.0, 5.0

SUMMARY

In the Win32 operating system, an application is not required to export callback functions. In 16-bit Windows 3.1 and prior versions, an application must export callback functions primarily to allow them to fix up references to global data and to ensure that any required EMS memory is available. Neither of these situations applies to applications running under the Windows NT 32-bit operating system.

MORE INFORMATION

It is necessary to export a function to make it available to the GetProcAddress() function (for dynamic linking) or to list it in an import library (for static linking). Either of these linking methods require information about the function name or ordinal number. Exporting the function places the name or ordinal number into the application's exported entry points table and allows Windows to determine the associated address at run time.

The loader performs load-time linking by looking up the imported entry points an application requires to load. Typically, it looks up this information by ordinal number. An application performs dynamic linking with the GetProcAddress() function which instructs the system to look up the desired function in the desired dynamic-link library (DLL) exports table by ordinal number or by name. It is not necessary to export functions in the Win32 application programming interface (API).

In 16-bit Windows, the linker automatically fixes exported entries to load the appropriate value into the DS register. Exporting an entry in Win32 simply adds the entry to the application's exported names and ordinal numbers table; the linker need not "fix" them. To maintain source code compatibility with 16-bit Windows, you may want to continue using the MakeProcInstance() function and exporting all callback functions.

The following table summarizes this information:

Function Type      Windows 3.x (16-bit Windows)     Windows NT (Win32)

Callback           Exported or MakeProcInstance     Address of fn() *
GetProcAddress     Must be exported                 Must be exported
Static linking     Must be exported                 Must be exported