libpodioDict.so is not always properly linked about podio

[andresailer]

The difference between podioDict linked and not is probably the linker version and the existence or default for the "--as-needed" option. If there is no symbol from podioDict used in the RootIO the linker chooses not to link the library when "--as-needed" is enabled.
I tried with the ubuntu container, adding -Wl,--no-as-needed before the podioDict library and the library was linked.

[andresailer]

In both instances the linker has the "as-needed" option, so maybe in one case it can better figure out that podioDict is needed, or the compiler is doing some optimisation ...
In #91 I add no-as-needed around podioDict when it is linked into podioRootIO, which solves the problem for ubuntu.

[tmadlener]

It is definitely strange in my opinion. Apart from the possible (small) differences in the paths to root, gcc, etc. the output of make VERBOSE=1 is exactly the same in both containers. But I also couldn't really find anything about the default values of the linker flags, especially since they are passed to it through gcc.

But as already mentioned in #91 it seems to be solved by adding --no-as-needed around podioDict

[andresailer]

So it is maybe this: gcc on ubuntu is configured to ad "as-needed" for the linker
https://askubuntu.com/questions/1077276/why-do-gccs-linker-options-as-per-dumpspec-contain-as-needed

[tmadlener]

Yes, I think that is very likely to be what is causing our problem. I am wondering how other projects handle this, since we can't be the only ones that want to link a root dictionary into a library, right?