Having troubles with mixing library types (static vs. dynamic)

Question

After battling my makefile woes I'm now onto problems with how the two libraries are supposed to interact. So, this is on Linux (CentOS 6.2 - 6.4, not that that seems to make much difference in terms of tools). The project, on the whole, is to be deployed in two flavors

• A C++ Static library which is used for linking with other C++ appications (a *.a file)
• A shared *.so which is deployed through python using Boost.python

I have everything compiling, but I'm not linking correctly somehow. In the case of the static library, it is built in two ways:

• If meant to be linked with other C++ code, -fPIC is not used
• If meant to be linked into the python module, use -fPIC

I control this through passing parameters to the make program. Presently, I'm trying to build the boost python module because the static stuff compiles just fine. So, I have dependencies on the boost libraries and zlib. The final linking command looks like this:

g++ -o pythonmod.so -L/boost/boost_libs -L/zlibs -lz -lboost_python -lboost_thread -lboost_regex -lboost_system /path/to/static.a -fPIC -shared [many_objects]

The "many_objects" comes from the various wrappers, and other code, that wraps the "pure" C++ from the boost.python layer in the code. Each of these object files are compiled with -fPIC as well. Their compiled with:

g++ -I/boost/boost_1_47 -I/usr/include/python2.6 -D _linux -MMD -std=c++0x -c -m32 -fPIC <input> -o <output>

The lines compiling the object files for the archive file look quite similar to the above only they do not include the python include directory.

I've found other links here to similar problems and tried the solutions but to no avail thus far. For example, this link uses -Wl,--whole-archive ... -Wl,--no-whole-archive . I tried this solution when I was trying to link in the static library archive before I was compiling it with -fPIC. Now that I'm doing that, I've tried this solution but also to no avail. In each case, every time I load up python and import the module, I get some sort of undefined symbol error --> something went wrong during linking.

How should I mix these libraries together to make the python module work?

Answer 1

After joining the gcc-help mailing list I received the pointer I needed to resolve the issue. The problem turned out to the ordering of the libraries used for linking on the build command line. Basically, the object files generated during the build for the *.so needed to be placed first. Then, the references to the boost and other libraries. Reordering the object files to reference the objects built for wrapping the static library before the other libraries was the key. No longer am I seeing weird "unresolved objects" when loading my python module.

After 4 or 5 years of using Visual Studio, my gcc knowledge had become sufficiently rusty that I'd forgotten the importance of ordering when it comes to linking.