Mixing static libraries and shared libraries

Question

I have a project where I have one static library libhelper.a and another with my actual shared object library, libtestlib.so. My goal is to link libhelper.a into libtestlib.so. Is that possible on Linux/BSD? When I tried and created a test program I got the following errors:

./prog1:/usr/local/lib/libtestlib.so.1.0: undefined symbol ''

My guess is that this is occurring because libhelper.a was not compiled with -fPIC while libtestlib.so was. What is the proper way to build programs that use shared libraries that also have dependancies on static libraries?

Thanks!

Answer 1

My goal is to link libhelper.a into libtestlib.so. Is that possible on Linux?

Sure. This should do:

gcc -shared -fPIC -o libtestlib.so $(OBJS) \
  -Wl,--whole-archive -lhelper -Wl,--no-whole-archive

libhelper.a was not compiled with -fPIC

It's best to rebuild libhelper.a with -fPIC. If that's not possible, above command will still work on Linux/ix86, but not on e.g. Linux/x86_64.

What is the proper way to build programs that use shared libraries that also have dependancies on static libraries?

If you include libhelper.a into libtestlib.so as above, then simple:

gcc main.c -ltestlib

is all you need. If you insist on linking with libhelper.a, then you must tell the end-user that he must link with e.g.

gcc main.c -ltestlib -lhelper

There is no way to specify that libtestlib.so depends on libhelper.a.