Re: if I linked a static lib do I need to...

[Xi Ruoyao <ryxi at stu dot xidian dot edu dot cn>]

On 2017-03-13 23:04 +0000, lejeczek via gcc-help wrote:

> if I linked a static lib and compilation rendered a dynamic lib,
> now when I further compile and use that dyn lib, do I need 
> to include that static lib(s) I linked to the first time?

It depends on whether you linked the symbols in the static lib
into the dyn lib. If you built the dyn lib with

  cc -shared -fPIC libdyn_1.o libdyn_2.o -o a/libdyn.so -lstatic

then the required symbols in libstatic.a would be added into
libdyn.so. Then you can use -ldyn without -lstatic:

  cc main.c -La -ldyn

However, if libdyn.so was built

  cc -shared -fPIC libdyn_1.o libdyn_2.o -o b/libdyn.so

(without -lstatic) then the required symbols in libstatic.a
would be undefined in libdyn.so. You would have to use
-ldyn -lstatic:

  cc main.c -Lb -ldyn -lstatic

If you objdump -t b/libdyn.so, you'll find symbols in
libstatic.a which are "UND". And if you forgot -lstatic,
the linker would complain like "undefined reference to
<symbol name>".

> I probably also make the question vague, if I could give you 
> a bit insight into a scenario... that could be one of those 
> (common?) where someone uses those specialised/optimized 
> math libraries.

Then I think your dyn lib is like b/libdyn.so. For math libs,
some developers intend to use static library since "-fPIC"
would slow down the target code (a little). So the code in
your static library is likely no PIC. They can't be linked into
shared objects like we did for a/libdyn.so. So it's likely you
have to use -ldyn -lstatic.

On the other hand, if your static library is PIC, it's likely
the shared version of this library is also existing. In this
situation I suggest to use the shared version since shared
librarys have many advantages (unless you want to ship
only one shared object file without more dependencies).

Best wishes.
-- 
Xi Ruoyao <ryxi@stu.xidian.edu.cn>
School of Aerospace Science and Technology, Xidian University