Question

I use log4cxx logging library. I need to link with its static version to avoid additional binary dependencies. I use it in my dynamic library. Default build of log4cxx produces static library but I cannot link with it because it was compiled w/o -fPIC flag. So I changed log4cxx bulding as:

CPPFLAGS="-fPIC -static" ./configure
make

As a result I received a liblog4cxx.a that I can link with my .so library. Linking was done by Cmake, something like:

target_link_libraries(my_dynamic_lib log4cxx)
link_directories(relative_path_to_dir_where_liblog4cxx.a_lives)

Everything looked fine until runtime. I cannot load my_dynamic_lib.so because of undefined symbol "logger"

Please explain me what’s wrong and how to resolve this problem.

thanks

Answer 1

You can verify whether the shared library contains the symbol by using

nm -g my_dynamic_lib.so | grep logger

If it is shown with symbol type U it means it’s undefined.

Normally a shared library will not resolve all the symbols it needs until run-time, so it is possible (and perfectly normal) to link a shared library with missing symbols.

If you put -llog4cxx at the start of the linker command line for my_dynamic_lib.so then it won’t link to any of the code in there, and will leave the logger symbol unresolved until run-time. To force it to use the symbols in the static library make sure you list the static library after the objects that need it:

g++ -fPIC -shared -o my_dynamic_lib.so obj1.o obj2.o -llog4cxx ...

I don’t know how to do that with cmake, but it looks as though your CMakefile only links to log4cxx when linking the main executable, not the dynamic library.