Q102871: Callback Functions in Multiple Instance Applications

Article: Q102871
Product(s): Microsoft Windows Software Development Kit
Version(s): WINDOWS:3.0,3.1
Operating System(s): 
Keyword(s): kb16bitonly
Last Modified: 06-NOV-1999

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The information in this article applies to:

- Microsoft Windows Software Development Kit (SDK) versions 3.0, 3.1 
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SUMMARY
=======

The following are some requirements for callback functions in multiple instance
applications when compiling with Microsoft C version 7.0 or C version 8.0 for
protected-mode-only Win16:

- Callback functions must be branded with the "__export" keyword. The compiler
  switches -GA -GEf force all far functions in a module to be __export. The
  optimum method is to add the __export keyword to the particular callback
  function and not use the -GEf switch. Example:

        BOOL CALLBACK __export CallBackProc

- Callback functions called via MakeProcInstance() must load DS from the value
  in AX. Use the compiler switch -GA with the -GEa switch to generate prolog
  code to load DS from the value in AX on all __export functions. The default
  for -GA is load DS from SS, which is not a valid assumption in callbacks. The
  code when loaded into memory should have three NOPs instructions at __export
  function entry points. Check with your favorite debugger that can show
  mixed/asm view once the module is loaded into memory.

- Callback functions must have an export record in the EXE header. The classic
  method is to list the function in the EXPORTS section of the module's .DEF
  file. The callback's module can be built with the - GA -GEe switches to place
  an export record in the .OBJ at compile time for all __export functions.

- Callback functions must assume SS != DS, DS not loaded on function entry.
  Compile with -Aw memory model customizer to have correct code generated.

- Callback functions must not call C Run-time library code contained in the
  application (xLIBCyW.LIB) libraries. They assume SS == DS.

Additional query words: 3.10

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Keywords          : kb16bitonly 
Technology        : kbAudDeveloper kbWin3xSearch kbSDKSearch kbWinSDKSearch kbWinSDK300 kbWinSDK310
Version           : WINDOWS:3.0,3.1

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