exporting C++ dll to Delphi program

Question

I have a dll with exporting function

extern "C" __declspec(dllexport) IDriver * __stdcall GetDriver()

There is a programm which is written on Delphi. It can't see the function GetDriver(). It's double awful because I can not get and modify sources of this program.

What may be the reason of successful loading my dll (according log file) and failed call exporting function? Thank you.

Window 7 x64, Visual Studio 2010, C++ project for x86 target

is this declaration from .h file or .cpp file ? do declarations from those files match letter to letter ? — Arioch 'The, Oct 26 '12 at 08:16

score 4 · Accepted Answer · edited May 23 '17 at 11:55

4

The most likely explanation is that the function will have been exported with a decorated name. I'd expect it to have been exported with the name GetDriver@0. So you could import it like this:

function GetDriver: IDriver; stdcall; external 'DllName.dll' name 'GetDriver@0';

Use a tool like Dependency Walker to check the exact name used to export the function.

If you cannot modify the Delphi code, then you'll need to make your C++ DLL match. Do that by using a .def file which allows you control over the exported name.

The other problem you will face is that Delphi's ABI for return values differs from that used by most other tools on the Windows platform. Specifically a return value is semantically a var parameter. On the other hand, your C++ compiler will regard the return value as an out parameter. My question on Delphi WideString return values covers exactly this issue.

Because of this, I'd expect the function declaration above to lead to access violations. Instead you should declare the return value to be a Pointer and cast it to an interface reference in your Delphi code. You'll need to double check and make sure that the reference counting is handled appropriately.

Again, if you cannot modify the Delphi code, you need to make the C++ code match. A Delphi interface return value is implemented as an additional var parameter following the other parameters. So, to make your C++ function match, declare it like this:

void __stdcall GetDriver(IDriver* &retVal);

edited May 23 '17 at 11:55

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answered Oct 26 '12 at 07:57

David Heffernan

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Checking export table would never harm, yet how can he end with mangled name after specifying `extern "C"` ? – Arioch 'The Oct 26 '12 at 08:14
@Arioch'The It's not a mangled name. It's a decorated name. `extern "C"` suppresses mangling, but does not suppress decoration. – David Heffernan Oct 26 '12 at 08:19
@David Heffernan Thank you for answering. I can't access to Delpi code. The exporting function GetDriver() returns pointer to the object. Do you mean that I need return this pointer as out parameter of function? – Myosotis Oct 26 '12 at 08:27
@Myosotis If you can't change the Delphi program then you are stuck with how it is. I think you may have problems, but I'm not 100% on that. – David Heffernan Oct 26 '12 at 08:31
@David there is a difference? http://en.wikipedia.org/wiki/Name_mangling#C_name_decoration_in_Microsoft_Windows – Arioch 'The Oct 26 '12 at 08:33
Delphi program collapses when my dll returns pointer. 100% problem %)) Anyway thank you a lot, @David Heffernan – Myosotis Oct 26 '12 at 08:34
@Arioch'The Yes, there is a difference. – David Heffernan Oct 26 '12 at 08:48
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@Myosotis I'll update the answer to help you out. I think I can solve it. – David Heffernan Oct 26 '12 at 08:48

exporting C++ dll to Delphi program

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