[C++] GCC tree linkage types

[Chris Lattner]

Ian Lance Taylor writes:

> A weak undefined symbol is like an ordinary undefined symbol, except
> that it is not an error if the symbol is never defined (and in that
> case the symbol is implicitly defined with the value zero).

What are weak undefined symbols used for?  If they are referenced but
never defined, how does the program link?

-Chris

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