Linking error (undefined reference to...)

I am facing a problem related to linking.  Since my source code and Makefile is very big,
I have tried to cut down all other details.  I have global variable defined
in one .cpp file and am trying to use it in another .cpp file using extern.


#include "commonevents.cpp"
int testvariable = 0;
...
Using the variable in /src/libraryfunction.cpp
In src/ libraryfunction.cpp

extern testvariable;
.....

I have one Makefile that resides in the /src directory, I am able to run the "make" and all the .cpp files
compiles successfully. This is generating both "libraryfunction.o" (and other .o file of other .cpp files) and the libraryfile
libmystuff.so.1 which has the listing of all the .o file.


I have another Makefile in the main directory that links all the libraries, when I run this makefile it generates the
error:
"undefined reference to `testvariable'"

On googling, I came to know that it is linking error.  I used the "nm" tool to get
the symbol reference.

bash-3.00$  nm libmystuff.so.1 | grep testvariable
         U testvariable
         
I ran this command within my src directory.


I have couple of questions in this regard:

1.  is the output of nm command (i.e U testvariable ) expected ?   It says that testvariable is
undefined, is this correct?

2. If it is a linker error, which make file should I modify, the one within my src directory (which generated library file)
or the main Makefile which links all the .o 's and libraryfiles ?

Thanks,
joy