library compilation undefined reference to extern function

library compilation undefined reference to extern function

[Azatyan, Anushavan]

Hi,

I am compiling a huge library, which is written and compiled under Ubuntu, I have fixed some problems to get it compiled in cygwin.
The main() function is inside library. I stopped in some point which goes different in cygwin.
I have some functions which are declared as - extern void func(void);   Linker gives error - undefined reference to `func()'

then I tried with this to compare differences.. 

#include <iostream>
using namespace std;
extern void foo(void);
int main()
{
        cout<<" Print ";
        foo();
        return 0;
}

a very simple example I can compile in linux (Ubuntu) without any problem, but in cygwin it goes with linker errors - undefined reference, in both systems I use  

$ gcc -c -Wall -fpic filename.cc
$ gcc -shared -o libfoo.so filename.o

commands.
I could not find solution of this anywhere.

Thank you, 
Best regards, Anushavan

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Re: library compilation undefined reference to extern function

[René Berber]

On 3/6/2016 4:27 PM, Azatyan, Anushavan wrote:

[snip]
> #include <iostream>
> using namespace std;
> extern void foo(void);
> int main()
> {
>         cout<<" Print ";
>         foo();
>         return 0;
> }
> 
> a very simple example I can compile in linux (Ubuntu) without any
problem, but in cygwin it goes with linker errors - undefined reference,
in both systems I use
> 
> $ gcc -c -Wall -fpic filename.cc
> $ gcc -shared -o libfoo.so filename.o

Besides the obvious (dynamic libraries in Windows do not have a .so
extension, but a .dll ext.) the linker in Windows does not allow
undefined references, so your second command should look like:

$ gcc -shared -o libfoo.dll filename.o -lstdc++
filename.o:filename.cc:(.text+0x23): undefined reference to `foo()'
collect2: error: ld returned 1 exit status

obviously you still have an undefined foo().  You have to provide it,
wherever it may be.
-- 
René Berber



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