Explanation of Exporting Functions in Windows

The information in this article applies to:

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.0 and 3.1

SUMMARY

Exporting functions under Windows is one of the least understood but most important concepts to understand. If string constants are garbage or global variables contain the wrong information, then exported functions are probably not doing their job. Exported functions simply establish the correct data segment (DS) value on function entry.

MORE INFORMATION

Function entry point code, referred to as "prolog" code, does the work of setting up all the registers on function entry. Exported prolog code works with the Windows loader to support all the different techniques of establishing DS. There are three types of exported prolog code: loading DS from the value in AX, loading DS from the value in SS, and loading DS from a hard-coded value fixed up by the loader. The first two techniques are used in applications, the last is used by dynamic-link libraries.

Loading DS from the Value in AX

This is the classic application exporting technique. This works with the MakeProcInstance function, which loads the correct DS value into AX before calling the exported function's prolog code. As an application is loaded, all exported functions built with the -Gw switch or -GA -GEa pair are modified so that the first 3 bytes of prolog code are NOPs. The following prolog code sets up the stack frame and finally loads DS from the value in AX. In mixed mode, it would appear as follows:

  {
     nop
     nop
     nop
     push   bp
     mov    bp,sp
     push   ds
     mov    ds,ax

Loading DS from the Value in SS

This is a more recent technique that assumes that the SS register contains the correct DS value on function entry. For applications that don't switch their stack segment nor are called from other applications, this is a valid assumption. The -GA default code loads DS from the value in SS without the loader required to fix up the prolog code. In mixed mode, it would appear as follows:

  {
     mov    ax,ss
     push   bp
     mov    bp,sp
     push   ds
     mov    ds,ax

Loading DS from a Hard-Coded Value

This is the classic dynamic-link library exported function prolog code. Because libraries are single instance, it's possible to get away with hard-coded values to establish DS. As a library is loaded, all exported functions built with -Gw or -GD are fixed up to load DS from a hard-coded value. In mixed mode, it appears as follows:

  {
     mov    ax,????
     push   bp
     mov    bp,sp
     push   ds
     mov    ds,ax

   void CALLBACK __export TheExportedFunction()