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__declspec(dllexport)

 
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PostPosted: Fri Aug 20, 2004 1:42 pm    Post subject: __declspec(dllexport) Reply with quote


Hi everybody...

I am triing to create shared libraries with MSC++.net from C++ sources
currently compiled with gcc on Linux.

So, I must use '__declspec(dllexport)' in my code in order to export
some classes/functions/variables

Then, I have some problems to export variables. I read on the web that,
to export a variable, it must be declared as extern (no problem !).

First, I wrote the following code in order to create the shared library
foo.dll :

Quote:
foo.hpp

class Foo {
public :
Foo() ;
~Foo() ;
void bar() ;
private :
int bar_ ;
}

__declspec(dllexport) Foo* e_foo ;

Quote:
foo.cpp

Foo::Foo(){ ... }
Foo::~Foo(){ ... }
void Foo::bar(){ ... }

Foo* e_foo = new Foo ;

Second, I wrote the main program which is linked with foo.dll :

Quote:
main.cpp

int main( int argc, char* argv[] ) {
e_foo->bar() ;
return 0 ;
}

When I run MSCV.net, I have a link error during the main compilation :
'unresolved symbol e_foo'

To compile without link error, I tried the following solution which is
in the standard :

Quote:
foo.hpp

class Foo {
public :
Foo() ;
~Foo() ;
void bar() ;
private :
int bar_ ;
}

__declspec(dllexport) Foo* e_foo = new Foo ;

Quote:
foo.cpp

Foo::Foo(){ ... }
Foo::~Foo(){ ... }
void Foo::bar(){ ... }

Quote:
main.cpp

int main( int argc, char* argv[] ) {
e_foo->bar() ;
return 0 ;
}

But, it is a Windows solution : even if my pointer is same in all
libraries, I can link this solution on Linux ('multiple definition of')
and the compilation give me a warning.

So, my question is : how can I export my extern variable (both on
Windows and Linux) with the declaration in the header file, and its
instanciation in a source file in order to have the same pointer in all
my programm ?

Thanks,
Mathieu.

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Victor Bazarov
Guest
PostPosted: Sat Aug 21, 2004 3:29 am    Post subject: Re: __declspec(dllexport) Reply with quote


news user wrote:
Quote:
[...] So, my question is : how can I export my extern variable (both on
Windows and Linux) with the declaration in the header file, and its
instanciation in a source file in order to have the same pointer in all
my programm ?

AFAIK, since "exporting a variable" is an OS-specific thing, you will
need to have conditional compilation sections in your header, something
like

#ifdef WIN32
#ifdef EXPORT_THIS // this is to keep the same header for DLL and EXE
// only define EXPORT_THIS when building the DLL
#define DLL_THING __declspec(dllexport)
#else
#define DLL_THING __declspec(dllimport)
#endif
#else
#define DLL_THING // you may not need to do anything here -- check
// with a Linux programming newsgroup
#endif

DLL_THING Foo* e_foo = new Foo;

I've seen people referring to a command-line option for g++ to export all
global symbols. Check with g++ newsgroup for recommendations how to do
that for individual symbols.

To ALL: sorry for the partially OT solution.

Victor

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