stdcall name mangling using extern c and dllexport vs module definitions (msvc++)

Question

I was trying to export a simple test function for a dll to work with an application (fyi: mIRC) that specifies the calling convention as:

int __stdcall test_func(HWND mWnd, HWND aWnd, char *data, char *parms, BOOL show, BOOL nopause)

Now, to call this from the application, I'd be using test_func but I have noticed due to name mangling it is not as simple as I'd thought.

Through similar topics here I have come to the understanding that using extern "C" in combination with __declspec(dllexport) is an equivelant (somewhat) method of removing mangling to module definitions (.def). However, when using the extern/dllexport method my function (as an example) is always _test_func@numbers whereas the .def removed all mangling as required for use with the application i needed to export to.

Could someone please explain why this is? I'm just curious about the two methods. Thanks!

Answer 1

extern "C" has nothing to do with stdcall: it only declares that C++ name mangling (aka type-safe linkage; inclusion of type information in symbol name) is disable. You need to use it independent of whether you use C calling convention or stdcall calling convention.

In stdcall calling convention, the callee removes the parameters from the stack. To make that safe, the exported name contains the number of bytes that the callee will remove from the stack.

If the application you are exporting to requires that no @number suffix is added to the name, it probably means that it expects C calling convention. So you should stop declaring the function as __stdcall. When you the declare it as declspec(dllexport), you should get an undecorated name in the DLL.

In the DEF file, you can call the function whatever you want; no additional checking is performed.

Answer 2

dllexport/import are designed to be loaded back by themselves, not an old C library using GetProcAddress. The mangling you have seen is what all Microsoft compilers have done for a long time for __stdcall functions. Most likely, your target either expects a __cdecl function, not __stdcall, but if not, you will need to use a .def file to specifically un-mangle the name.