Re: Unhelpful automatic 3rd-party library linkage

[Bob Friesenhahn]

On Tue, 29 Jun 2021, Oleg Smolsky wrote:
> It looks like GCC9 references come from libzmq:
>
> $ ldd /opt/3p/lib/libzmq.so | grep libstd
>        libstdc++.so.6 => /opt/gcc-9/lib/../lib64/libstdc++.so.6
> (0x00007f95f8d9f000)
>
> Obviously the 3rd-party library was built a while ago with GCC9. At the
> time it was linked to the compiler's runtime... but now the main
> application has moved to GCC11 and I'm linking to the runtime that is
> correct right now.
>
> It looks like automake/libtool try to be helpful and check the library's
> dependencies... but that gets in the way as the new libstdc++ is a strict
> superset of the old one. They maintain ABI compatibility and so scenarios
> like these are supported.

Are you absolutely sure that the above is true?  You specified c++17 
when compiling your application.  Are the libstdc++ ABI's the same 
across GCC versions and C++ language versions?

> Is there a way to suppress dependency tracking for the 3rd-party libraries?
> I wish Libtool/automake was not trying to be smart and simply passed
> "-lzmq" directly to the linker. Yet instead, the actual .so file is
> discovered and then its libstdc++.so is linked. This is just wrong for the
> scenario at hand.

Assuming that the whole system does not have these directories in the 
default search path (e.g. via ldconfig), it appears that this is a 
recorded implicit dependency which is encoded in the library itself. 
The only way to remove such an implicit dependency is to rebuild the 
library (e.g. libzmq.so) with different options.

If the persons who delivered the compilers to you expected that the 
C++ library was truely reusable, then they would not have have put 
everything under /opt/gcc-foo directories (also suggesting that these 
directories are removable).  Instead they would have put the C++ 
run-time libraries in a standard system location.  For example, under 
Ubuntu Linux, I see that libstdc++.so.6 is at 
/usr/lib/x86_64-linux-gnu/libstdc++.so.6 which is a common system 
directory.

As far as I am aware, there is no option to request that libtool 
not perform the full linkage that it does.  A common work-around is to 
remove the ".la" files that libtool produces and installs.

It is possible that GCC itself is pre-programmed (e.g. via the spec 
file) to record this information when it links with the C++ standard 
library.

Bob
--
Bob Friesenhahn
bfriesen@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
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