Creating Import Libraries with the __export Modifier

ID: Q88278

The information in this article applies to:

Microsoft Windows Software Development Kit (SDK) 3.1

SUMMARY

The __export function modifier, widely advertised in version 7.0 of the Microsoft C/C++ Optimizing Compiler, can be used in conjunction with the /GD option switch to export a function from a dynamic-link library (DLL) for the Microsoft Windows operating system without listing the name of the function in the EXPORTS section of the DLL's module definition (.DEF) file.

To create an import library for a DLL that uses this technique, the filename of the DLL must be listed as a parameter to the IMPLIB function. Otherwise, IMPLIB does not place the functions marked with __export into the import library.

For more information on exporting functions, query on the following words in the Microsoft Knowledge Base:

__export and keyword and def and prolog

MORE INFORMATION

The first argument to the IMPLIB command is the name of the import library to create. The second parameter is the name of the .DEF file, the name of the .DLL file, or both. The syntax listed in the IMPLIB online help file is as follows:


   IMPLIB [options] implibname {dllfile... | deffile...}

If a .DEF file is listed as the second parameter, IMPLIB places all functions listed in the EXPORTS section into the import library. This is the traditional method of creating a DLL import library and it applies to versions of the Microsoft C Compiler prior to version 7.0.

If a DLL file is listed as the second parameter, IMPLIB places all functions marked with the __export keyword in the DLL source code in the import library. This method is required when the DLL source code contains exported functions that are not listed in the .DEF file. Otherwise, these functions would be excluded from the import library.

In addition, if the DLL is developed in C++, the names of exported functions are generally not available to place in the .DEF file because C++ decorates function names. To create an import library for exported C++ functions, IMPLIB must read the function names from the DLL.

To create an import library that contains only a subset of the functions marked __export, create a .DEF file containing only the names of the functions to include in the import library and specify the name of the .DEF file as an argument to IMPLIB.

Note that any function that is marked with the __export keyword is compiled with the default DLL export attributes in the import library. To modify any of the attributes (for example, to add the NODATA or RESIDENTNAME attributes to the exported function), you must list the function name in the EXPORTS section of a .DEF file.

An example of a function that must be listed in the .DEF file is a WEP (Windows exit procedure) function. Because the WEP must have the RESIDENTNAME attribute, the following information must appear in the EXPORTS section of the DLL's .DEF file:


   SEGMENTS 'WEP_TEXT' FIXED PRELOAD
   EXPORTS
      WEP  @1  RESIDENTNAME

The SEGMENTS declaration tells Windows to load the WEP_TEXT segment into a fixed memory location as it loads the DLL into memory.

To place both the functions marked as __export and the WEP into the import library, specify both the .DEF file and the .DLL file as arguments to the IMPLIB command.

For more information on using Microsoft C/C++ 7.0 to create a WEP function compatible with Windows 3.1, see pages 980 and 981 of the Microsoft Windows Software Development Kit (SDK) "Programmer's Reference, Volume 2: Functions" manual for version 3.1 and the DETAILS.TXT file distributed with Microsoft C/C++.

For more information on using the /GD option switch with the __export keyword, see page 515 of the Microsoft C/C++ "Environment and Tools" manual for version 7.0, or query in the Microsoft Knowledge Base on the word "/GD."

Additional query words: 3.10

Keywords : kb16bitonly
Version : WINDOWS:3.1
Platform : WINDOWS
Issue type :