global variable access, from c to c++

[manav]

i have a c file which contains some global variables.
and i have one cpp file which needs to access these global variables.
it did the following but failed:

/* file.c */
int value;

// file.cpp
extern "C" {
extern int value;
}

i got a linker error saying _value is unresolved symbol.

i am using MSVC .Net 2003, but i need to be able to compile this on gcc as well. so a portable solution is needed here

[Magos]

How do you compile these? With one C and one C++ file I'm sure you don't compile into one single binary?

[manav]

Hi Magos!

The .c file ends up in a .dll file, while .cpp file is used for building .exe file which then depends on .dll file as well.

I tried DUMPBIN on the .dll as follows:
DUMPBIN /SYMBOLS cfile.dll

but nothing was output!

File Type: DLL

  Summary

       4A000 .data
        2000 .idata
       30000 .rdata
        F000 .reloc
      154000 .text

[vart]

you have to declare value as dllexport...

But I personally prefer not to export globals from dll - instead I provide some reasonable API

[Magos]

http://support.microsoft.com/kb/90530
http://msdn2.microsoft.com/en-us/library/9h658af8.aspx

[manav]

Thanks Magos! Thanks vart!
The above solution would be MS only, i need to do the same in gcc also.

I tried to use 2 functions instead of globals, one function will set the values, other will get the values.

But again same linker error of unresolved symbols

vart what do you suggest as a reasonable API here?

[Magos]

DLL's are windows-specific anyways

[manav]

Yeah Magos, but as you asked:
"How do you compile these? With one C and one C++ file I'm sure you don't compile into one single binary?"
So i told you what the files end up as. It may not be in .dll or .so file on Linux, maybe different situation on Mac
But the c file and cpp file, both are needed for final .exe file. it's just that c file happens to be compiled in a .dll on windows, on linux it will be different.

but why i am unable to access global variables or functions from my c file?

[vart]

vart what do you suggest as a reasonable API here?

I do not know what you want to do with this global var...

One of the ways...

Provide Init that will dynamically allocate all neded data and return handle to this memory
Every other funtion in Dll will receive the handle - cast it to the real data type and perform required actions

End function will release the allocated memory making handle invalide

In such approach DLL could be used in MT environment without big problems, and also work on several instancies of the data simulteneously...

[manav]

vart, the c file contains some low level audio stuff, cpp file is using Qt to make gui over the low level stuff in c file. so using gui controls i want to set some values in the c file. my first thought was making the values global in c file and using them in cpp file. but then i made a set/get pair of functions to do the same. both approaches give me unresolved symbol linker error.

[vart]

have you used __declspec(dllexport) declarations as suggested by the links provided?

[manav]

No vart, that will work with MSVC only. I need to compile on Windows, MacOS & Linux.
Isn't there a portable approach?
I thought that at least in case of functions I will be able to access them from my c file, regardless of whether the c file ends up as a .dll or .so or anything.

[vart]

you cannot compile dll on other OSes... so use #ifdef

Or - do not use dll at all, compile it as static library

[CornedBee]

Isn't there a portable approach?

No. You need OS-specific code.

[matsp]

So what is the reason you can't pass the variable from one side to the other?

Global variables are a nuisance, and it gets worse when you have to pass them across shared library/DLL boundaries.

--
Mats