In order for Visual Basic applications (or
applications in other languages such as Pascal or Fortran)
to call functions in a C/C++ DLL, the functions must be exported using the
correct calling convention without any name decoration done by the compiler.
__stdcall creates the
correct calling convention for the function (the called function cleans up the
stack and parameters are passed from right to left) but decorates the function
name differently. So, when __declspec(dllexport) is used on an
exported function in a DLL, the decorated name is exported.
The __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
.
To get the
decorated name, use /MAP. Use of __declspec(dllexport) does the following:
·
If the function is exported with the 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.
Because there is no way to override where
the stack cleanup occurs, 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 DLLs to be called by programs written
in Visual Basic, the alias technique shown in this article is needed in the
.DEF file. If the alias is done in the Visual Basic program, use of aliasing in
the .DEF file is not necessary. It can be done in the Visual Basic program by
adding an alias clause to the DECLARE statement.