Weird object file link issue

[Tom Kacvinsky]

Answering my own question.  See below...

On Tue, Nov 22, 2022 at 3:16 PM Tom Kacvinsky <tkacvins@gmail.com> wrote:

> Hi,
>
> Using binutils 2.39 and GCC 12.1, built from source on CentOS 7
>
> This is a really weird one.  I have a simple "hello world" C program.
> When I link this against a third party object file foo.o, symbols from
> foo.o are in my hello world main program, even though the main
> program does not have any symbol dependencies from foo.o.
>
> Sample link driver:
>
> gcc -fuse-ld=gold -o bar bar.c foo.o
>
> I would have thought that foo.o would just have been discarded from the
> final link, but using -Wl,--trace or -Wl,-Map=link shows it is indeed
> used in the final link.  This is happening with both the gold and bfd
> linkers. Any ideas on what I should do to diagnose this?  Completely new
> to me - I have always seen unused object files and archives discarded
> from the file link.
>
>
So, on a lark, I added foo.o to an archive "ar cru libfoo.a foo.o" and
linked
against libfoo.a.  libfoo.a was discarded as no symbols from it were used.

I guess I was mistaken in thinking that the symbols from a lone object
file, if
not used, would lead to the object file being discarded from the final link.

Clarification on the matter would be appreciated.  A +1 would be a reference
to a linker option that would enable the behavior I'd like, if one exists.
I've
not found one yet.

I'd like to file an issue with the third party company I am working with,
> but
> I want to go to them with concrete data as to why this is happening, in the
> hopes they can fix it.
>
> Yes, I know I can just drop the object file from the link line as it is not
> really needed.  That poses a problem from how we build with cmake.
> That and I am curious as to the black magic that is causing this.
>

If there is no link option I can use, I'll just do the static archive trick
above
and be on my merry way.

Sorry for the noise.

Tom