mingw DLL problem

I still can't figure out how to make it work.

Consider the file  "foo.cc"
=========
extern int bar();
int foo(int q)
{
  return q*bar();
}
=========


On Linux, to make a native .so I can do:
        g++ -fPIC --shared foo.cc
and it builds an appropriate file, such that "bar" will be 
resolved later.


The problem is if I do:
        i586-mingw32msvc-g++  --shared foo.cc
It responds with:
========
/tmp/cch3JG6k.o:foo.cc:(.text+0x7): undefined reference to 
`bar()'
collect2: ld returned 1 exit status
========

With a real file, I get a flood of these messages.

What am I doing wrong?