How to export PASCAL-like symbols in a 32-bit dll file in Visual C++

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Article ID: 140485 - View products that this article applies to.
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SUMMARY

There is no _pascal keyword in the 32-bit editions of Visual C++. Instead the Windef.h header file has PASCAL defined as __stdcall. This creates the correct style calling convention for the function (the called function cleans up the stack) but decorates the function name differently. So, when __declspec(dllexport) is used (in a .dll file, for example), the decorated name is exported instead of the desired PASCAL style name, which is undecorated and all uppercase.

MORE INFORMATION

PASCAL name decoration is simply the undecorated symbol name in uppercase letters. __stdcall name decoration prefixes the symbol name with an underscore (_) and appends the symbol with an at sign (@) character followed by the number of bytes in the argument list (the required stack space). So, the function when declared as:
   int  __stdcall func (int a, double b)
				
is decorated as:
   _func@12
				
The C calling convention (__cdecl) decorates the name as _func. Whereas the desired PASCAL style name is FUNC.

To get the decorated name set the Generate Mapfile option in the Linker General category setting.

Use of __declspec(dllexport) does the following:
  • If the function is exported with C calling convention (_cdecl), it strips the leading underscore (_) when the name is exported.
  • If the function being exported does not use the C calling convention (for example, __stdcall ), it exports the decorated name.
So to simulate PASCAL name decoration and calling conventions, you must have the "Called Function stack clean-up" provided by using __stdcall and the undecorated uppercase name.

Because there is no way to override who does the stack clean up, you must use __stdcall. To undecorate names with __stdcall, you must specify them by using aliases in the EXPORTS section of the .def file. This is shown below for the following function declaration:
   int  __stdcall MyFunc (int a, double b);
   void __stdcall InitCode (void);
				
In the .def file:
   EXPORTS
      MYFUNC=_MyFunc@12
      INITCODE=_InitCode@0
				
For .dll files to be called by programs written in the 32-bit versions of Visual Basic (versions 4.0 and above), the alias technique shown in this article is needed in the .def file. If alias is done in the Visual Basic program, use of aliasing in the .def file is not necessary. It can be done on the Visual Basic program by adding an Alias clause to the Declare statement as shown here:
Declare Function MyFunc Lib "dlllibname" Alias "_MyFunc@12"  (...)
   As Integer
				
The complete syntax for the Visual Basic Declare statement follows:
   [Public | Private ] Declare Function name Lib
   "libname" [Alias "aliasname" ] [([arglist])][As type]
				

References

For more information, query the MSDN compact disc using these keywords:
VB alias DLL
NOTE: A very good discussion (with example code) of calling a C .dll file from Visual Basic can be found in the file Vb4dll.txt in the Visual Basic directory. If you can not locate the Vb4dll.txt file, please see the following article in the Microsoft Knowledge Base:
150705 Using VB4DLL.TXT File to Develop DLLs for Visual Basic

Properties

Article ID: 140485 - Last Review: June 2, 2005 - Revision: 3.0
APPLIES TO
  • Microsoft Visual C++ 2.0 Professional Edition
  • Microsoft Visual C++ 2.1
  • Microsoft Visual C++ 2.2
  • Microsoft Visual C++ 4.0 Standard Edition
  • Microsoft Visual C++ 5.0 Enterprise Edition
  • Microsoft Visual C++ 6.0 Enterprise Edition
  • Microsoft Visual C++ 5.0 Professional Edition
  • Microsoft Visual C++ 6.0 Professional Edition
  • Microsoft Visual C++, 32-bit Learning Edition 6.0
Keywords: 
kbhowto kbinfo kbcompiler KB140485

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