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> Now serious, can someone enlighten me about why RTTI needs this global
> flag?

c++ defines some implicit structures for types to support RTTI such as
type_info instances and implicit entries in an object's virtual
function table.  These structures are implicitly defined in *all*
libraries that refer the type.  If different libraries link to their
*own* instance of such structures then RTTI fails, for example a
dyamic_cast will always fail between objects created in different
libraries.  Not specifying RTLD_GLOBAL will keep these implicit
structures local to the loaded library and the linker selects the
local instance instead of the first global instance that it

A public c++ module interface should ideally not rely on RTTI.
However, it is not always feasible to rewrite interfaces since there's
often a *large* existing code base that entirely depends on RTTI...

On Windows the story is completely different since dll's are already
linked.  The type info structures are explicitly placed in a shared
administration to make RTTI work.