PRB: Error LNK2001 Generated When Porting 16-bit Code to Win32

• Microsoft Visual C++, 32-bit Editions, versions 2.0, 2.1, 4.0, 5.0, 6.0

SYMPTOMS

Porting your 16-bit application to Win32 results in LINK error LNK2001 (unresolved externals), if a __cdecl function is exported using a module- definition (.DEF) file. The error specifies the function as defined in the .DEF file, with the leading underscore (_) in the name.

CAUSE

In 16-bit code, functions are either declared with the __cdecl, __pascal, or __fastcall conventions. The linker should not resolve a call to a __cdecl function with a __pascal function, therefore, the compiler decorates __cdecl names with a leading underscore, converts __pascal names to all caps, and decorates __fastcall functions with a leading at symbol (@), so the linker will not make this mistake. It is a requirement that you put the decorated name in the .DEF file.

In 32-bit code for the x86, functions are either declared with the __cdecl, __stdcall, or __fastcall conventions. The compiler decorates __cdecl names with a leading underscore, __stdcall names with a leading underscore and an appended @N (where N is the number of bytes in the parameter list), and __fastcall names with a leading at symbol (@) and an appended @N (where N is the number of bytes in the parameter list).

The 32-bit linker uses a different method to resolve function calls, so a single .DEF file can be compatible across all Win32 platforms, including RISC, where the names are not decorated in a similar manner. You specify the undecorated name in the .DEF file and pass the object file(s) to the linker when building the import library. The linker will do a fuzzy lookup, comparing the undecorated names to the decorated names in the object file to find a match.

In the case mentioned in the Symptoms section of this article, the function name already has the leading underscore, whereas the linker is expecting an undecorated name. Suppose the function is MyFunc and the .DEF file contains _MyFunc. The decorated name is _MyFunc, but the linker searches for __MyFunc, __MyFunc@N, and @_MyFunc@N and cannot resolve the reference.

RESOLUTION

To work around the problem, update your .DEF file by removing the underscore at the beginning of the file name. If you are maintaining the same sorce code with both Visual C++ for Windows and Visual C++ 32-bit Edition, you must use two .MAK (project) files and two .DEF files, one for each environment. However, the one 32-bit .DEF file will work across all Win32 platforms.

STATUS

This behavior is by design.

Additional query words:

Keywords : kbtshoot kbVC200 kbVC210 kbVC400 kbVC500 kbVC600 LinkIss
Version : 2.0 2.1 4.0 5.0
Platform : NT WINDOWS
Issue type : kbprb