Re: Q: library dependencies

[Nick Bowler]

On 2020-09-24, Oleg Smolsky <osmolsky@netskope.com> wrote:
> Hello, I'm working on an autotools-based build system and finally decided to
> look into these messages:
>
>   CXXLD    libtop.la
> copying selected object files to avoid basename conflicts...
>
> It looks like libtool links (copies?) .o files when it finds a dupe in the
> dependency DAG. Here is the situation I have w.r.t. the dependencies:
>
> libtop.la: liba.la, libx.la
> liba.la: libb.la libc.la libx.la
>
> All these libraries are static, noinst_LILIBRARIES and the dependencies are
> expressed via _LIBADD syntax:
>
> libtop_la_LIBADD = liba.la libx.la
>
> I do this to get transitive dependencies where the top-level executable just
> needs to link to link libtop.la.
>
> I think the issue is with libx.la as it is present in two different branches
> of the dependency DAG. This kind of thing is common in large projects and
> seems strange that libtool is trying to be smart about it.
>
> Is there a way to tell automake/libtool to disable this copying and just do
> straightforward linking? I wonder if the current behavior results in
> unnecessary bloat?.. Or perhaps I am saved by the linker that would dedup?

Unless I misunderstand something, what you describe is the documented
and expected behaviour.

When using primaries such as noinst_LTLIBRARIES, Automake will generate
rules that create libtool convenience libraries.  Such libraries are
basically intended to be a convenient shorthand for directly listing
the libtool objects that make up the convenience library whenever you
use libtool to link with it.

So when you create another libtool convenience library, liba.la, linking
with libx.la, since libx.la is a shorthand for directly listing all
its objects this creates a new library that contains all the objects
that made up libx.la.

Now when creating the libtop.la convenience library, you link against
_both_ convenience libraries liba.la and libx.la.  This directly includes
all the objects of liba.la and all the objects of libx.la.  But as the
objects of libx.la are duplicated in liba.la, this means some objects
are to be included twice into libtop.la.  Libtool has to rename one of
the objects to make this work (hence the "copying ... to avoid basename
conflicts").

I assume this is not the desired result.  Probably simplest to just
arrange for libx.la to either be a dependency of libtop.la OR liba.la,
but not both.

Cheers,
  Nick