Importing objects from a DLL

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I'm unable to access some const objects from a DLL despite the fact that they are exported and other items from the DLL, functions and classes are importing correctly.

Is their some rule against this or am I just being thick again and not spotting the obvious mistake?

//DLL header
extern __QOR_INTERFACE( __ARCHQOR ) const CGPReg eax;

//DLL cpp
__QOR_INTERFACE( __ARCHQOR ) const CGPReg eax( nsCodeQOR::_Initialize(), REG_EAX );

In the debugger I can hit the line above proving that eax gets initialised on load of the DLL.

Depends shows the export

?eax@nsx86@nsArch@@3VCGPReg@12@B

at ordinal 1003 but building the exe which imports the symbol gives the error:

TestArchQOR.obj : error LNK2001: unresolved external symbol "class nsArch::nsx86::CGPReg const nsArch::nsx86::eax" (?eax@nsx86@nsArch@@3VCGPReg@12@B)

The nsArch::nsx86::CGPReg type is also exported and given that the export and the claimed missing import symbols match I am properly confused.com

Posted 13-Jun-13 10:00am

Matthew Faithfull

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nv3 13-Jun-13 16:29pm

Sorry for the probably silly question, but have you specified the lib file in the linker inputs of your TestArchQOR project? Just asking to make sure you did. -- Ah, you said, other items of the DLL were imported correctly. So disregard my question.

Matthew Faithfull 13-Jun-13 16:36pm

Good point but the answer is yes.
There are plenty of imports working for example the dword_ptr function was the last thing I fixed, a header inline that was using a function I'd forgotten to import/export.

__QOR_INTERFACE( __ARCHQOR ) CMem _MemPtrBuild( const CGPReg& base, Cmp_int_ptr disp, Cmp_unsigned__int32 ptrSize ) __QCMP_THROW;

Given that this fix worked I know that build is not an issue, everything is in the right place and any changes I make will affect the build outcome.

nv3 13-Jun-13 16:39pm

I have never tried to export a const object. What if you remove the const?

Matthew Faithfull 13-Jun-13 16:44pm

I don't know. This is code I'm recycling so not mine originally. I don't know why const would be important in this case. I'll try removing it and see what happens...

...The good news is nothing further breaks but the bad news is that the linker error is still the same. constness is not the determining factor in this case.

zlogdan 13-Jun-13 16:59pm

Although the DLL mechanism permits that we export C++ classes, I personally prefer to export ANSI C functions, although most of the DLL internals I write in C++. Not sure if this will fix your compilation problems though.

Matthew Faithfull 13-Jun-13 17:07pm

Usually I do the same. In this case I'm converting someone else's code so to some extent I'm stuck with what I have. A redesign to use these objects via function calls would be a serious amount of work. I've considered it and I'm fairly sure it would work but it's a last resort.

chaau 13-Jun-13 19:19pm

Just wondering what happens if you export the class as a fully qualified name together with the namespace. Also, it sounds a bit silly, but can you rename it to something not related to asm reserved names?

Matthew Faithfull 13-Jun-13 19:51pm

Interesting ideas.
I'm pretty sure there's nothing wrong with the CGPReg class because CGPReg GeneralPurposeRegister; works correctly in the .exe code.

An asm reserved name issue is a possibility. It would be a problem with the original code as well and a real pain to change but I'll try one case to rule it out.

No sadly that's not it either. The error now reads:

TestArchQOR.obj : error LNK2001: unresolved external symbol "class nsArch::nsx86::CGPReg nsArch::nsx86::eax_reg" (?eax_reg@nsx86@nsArch@@3VCGPReg@12@A)

zlogdan 17-Jun-13 14:09pm

I bet you already did it, but it is worth asking: have you checked calling conventions on your project that imports the DLL? Also, have you used a .def exporting file or the __QOR_INTERFACE( __ARCHQOR ) macro calls the exporting __dllexport__dllimport macros ?

Matthew Faithfull 17-Jun-13 16:47pm

Yes it's worth checking. Calling conventions are OK, C++ at both ends and the mangled names match which confirms it.
__QOR_INTERFACE( __ARCHQOR ) expands to __declspec( __dllexport ) on one end and __declspec( __dllimport ) on the other which is working because the functions and classes that use the same macro work fine.

I've worked round the problem by adding accessor functions that return references to the objects and moved them inside an exported class as static members. Quite a bit of work and it makes the client code more verbose but probably a better design anyway.
However I still don't understand why it didn't work in the first place so I'll leave this question open for suggestions. Thanks.

The_Inventor 20-Jun-13 6:24am

You have:
extern __QOR_INTERFACE( __ARCHQOR ) const CGPReg eax;
might you try:
__QOR_INTERFACE( __ARCHQOR ) const CGPReg eax;

I am sure you have it, however I will ask, at the top of the .cpp file did you write:
#include *.h ???!!!???

Matthew Faithfull 20-Jun-13 6:37am

The include is fine or it wouldn't build. I had a similar idea about the extern, that it might be interfering with the import. I tried without it and it didn't make any difference :-(

The_Inventor 20-Jun-13 7:11am

then how about:
//DLL cpp
extern __QOR_INTERFACE( __ARCHQOR ) const CGPReg eax( nsCodeQOR::_Initialize(), REG_EAX );

Matthew Faithfull 20-Jun-13 14:05pm

You can't initialize an extern, not in C++ anyway. The declaration and the initialization need to be kept separate especially as I'm trying to share the instance between modules.

The_Inventor 21-Jun-13 3:53am

I know that this will seem somewhat like a hit in the head, however if you can compare what you have, line by line, to the example in the link, using a decision tree logic, like: 'Well if he did that there, did I do the same in my case ...' realizing the defining differences between the concepts covered in the example and what is going on in yours. Then the irritation code object line will expose itself to you. It is a very basic example, but complete in what it is attempting to demonstrate.

http://www.codeproject.com/Articles/6351/Regular-DLL-Tutor-For-Beginners

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