Relation between gcc version and libstdc++ version

[Anton Wöllert]

Hi Jonathan,

thank you for your reply!

On Tue, 2022-08-30 at 17:09 +0100, Jonathan Wakely wrote:
> 
> 
> On Tue, 30 Aug 2022, 15:48 Anton Wöllert via Gcc, <gcc@gcc.gnu.org>
> wrote:
> > If this libstdc++ is
> > newer than the one one the target, I get undefined references
> > (because
> > there are some newer implementation details and things like that). 
> 
> Then you're not telling the executable how find the new libstdc++.
>  
>
https://gcc.gnu.org/onlinedocs/libstdc++/faq.html#faq.how_to_set_paths
> 
> 

I tried to do that.  If I let the toolchain use it's own libstdc++.so,
then I get at runtime unresolved symbols due to the version mismatch
(these GLIBCXX errors). This is clear.

If I force the toolchain to link against the older target libstdc++,
then I get undefined symbols at compile time, because it is still using
it's own shipped headers for the newer libstdc++, which have
implementation details that use newer "functions/symbols" that are not
available in the old libstdc++.

If I furthermore remove the shipped headers and force it to include the
c++ headers from the older libstdc++, then everything works out.

But this whole "patching" seems very hacky.

> >  Is
> > it possible to tell G++/GCC to use the libstdc++.so from the target
> > and
> > also to use the C++ headers (like iostream) from the target?
> 
> It's possible, but unsupported and probably won't work.

So it seems to be indeed possible, but not intended.

> 
> > If not, is there any reason this is hard-coded?
> 
> The libstdc++ headers are tightly coupled to the GCC version, so
> headers from a given GCC release might not even compile with a newer
> or older GCC.

I would see an argument if you're trying to compile an newer libstdc++
with an older gcc - but why not the other way around?  C++ in general
tries to be very good in backward compatibility.
This essentially means that you can't use newer compilers with more
features/bugfixes to compile software for older targets.
Is there any obvious reason this is not supported?  Clang, for example,
also seems to be able to compile/link against different libstdc++
versions.  I'm just wondering.

Best,
Anton