Using "weak externals" with ld on cygwin?

[Brooks Moses <bmoses at stanford dot edu>]

I've been pounding my head all day against trying to use weak symbols in 
libraries on Cygwin, and have finally gotten to the point where I need 
to admit defeat and ask for help.

In the online documentation for ld, version 2.16 [1], I find the 
following statement at the bottom of the page: "The Windows object 
format, PE, specifies a form of weak symbols called weak externals. When 
a weak symbol is linked and the symbol is not defined, the weak symbol 
becomes an alias for some other symbol."

My understanding of this is that I should be able to create a libA.dll 
file with some code in it that references a function foo(), and define 
foo() as a weak symbol aliased to bar() such that, if an executable 
defines its own foo() or links to another dll that defines it, the libA 
code will use that definition of foo(), but otherwise it will use bar().

Is that actually a correct understanding?  Or am I missing something 
somewhere?

I've been trying, with a copy of the release version of gcc 4.1.0 and 
the latest Cygwin version of ld (which reports GNU ld version 2.16.91 
20050610 as its version), to write some code that will do that. 
Everything I try seems to either give me an error like "Cannot export 
.weak.__Z3foov.__Z3barv: symbol not found" or else always uses bar() 
regardless of whether I supply a foo() elsewhere.  I'm not sure I've got 
a very good idea at all how to do it correctly, though; the ld manual 
seems to provide no useful details beyond the above statement, and I 
haven't found much helpful in the gcc manual either.

If this is possible on Cygwin, can anyone provide a simple but complete 
example of how to make it work?  Ideally a C++ example, but I'll be 
happy with anything I can compile and run.  :)

Thanks muchly,
- Brooks

[1] http://sourceware.org/binutils/docs-2.16/ld/WIN32.html