[Bug dynamic-link/19884] Document libdl API in glibc to describe loading process in detail.

["carlos at redhat dot com" <sourceware-bugzilla at sourceware dot org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=19884

--- Comment #11 from Carlos O'Donell <carlos at redhat dot com> ---
(In reply to Nathaniel J. Smith from comment #2)
> *If* the same library is loaded twice, then having the two copies share
> state makes perfect sense. The problem here is that according to all the
> library lookup rules, we're actually loading two different libraries that
> live in two different files at two different locations, yet they still end
> up sharing state...
> 
> Yes, the two different libraries have the same SONAME, but there's no
> guarantee that all libraries with a given SONAME will be interchangeable.

That is wrong. That's exactly what having the same SONAME means.

> "interchangeable" means fully backwards *and* forward compatible, and that's
> not even true for basic libraries like glibc or libgcc.

It is true, but the story is more complicated.

With the advent of symbol versioning the state became a tuple { SONAME,
<Versioned symbols> }, where symbol versions extended the SONAME described
interface.

> (Actually, I'm not even sure how much state the two copies share -- since
> they're in different scopes they get different relocation processing, right?

No. The DSO is mapped only once and has relocations processed only once. The
first loaded library wins.

> I guess those are in a different segment or something, so the two copies
> share .data but not relocations or something?)

No.

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