[CMake] Combining several static libraries into a single static library

[Johan Laneau]

Hi,

I am in the process of deploying CMake into the build process of our department. I got already quite a lot of it working but I get stuck at one point: I can't see how we can combine several static libraries into a single static library with CMake.

I'll give some background first, to help you understand my problem. Our code is split up in more than 100 projects. Echo of those project is built into static libraries. In our department we work with static libraries because we target embedded systems that don't have support shared libraries, so shared libraries are not allowed. The static libraries can have dependencies on other projects. We build these project on many different platforms so CMake will definitely help in our build system.
With our current build system, we combine these static libraries into  "Release libraries". "Release libraries" are a combination of all required static libraries (no dependencies left anymore) into a single static library without any new code being added. This makes it very easy to release the combined functionality to a 3rd Party (such as another department or customer). We have a lot of "Release Libraries" with different combinations of the projects because we target different applications.

So I could simplify our build structure as below:
/Project01 (Static library)
/Project02 (Static library)
/Project03 (Static library)
...
/Project99 (Static library)
/ReleaseLibrary01 (Static library links e.g. Project01, Project02 and Project03)
/ReleaseLibrary02 (Static library links e.g. Project01, Project02 and Project04)
/ReleaseLibrary02 (Static library links e.g. Project05, Project06 and Project99)
/...

We already succeeded in building all our projects into static libraries with CMake. We also succeeded in building our test applications based upon those static libraries including the resolution of the dependencies (CMake is easy for this).
But we are not able to build our "Release Libraries". Simply adding a target with add_library for the "Release Library" does not work.

What I already read so far is that there are several workarounds: (e.g. rebuild the "Release Library" with the source of all dependent projects, unpack all libraries and repack again with platform specific commands, etc.). Some references I found are: https://svn.jmodelica.org/FMILibrary/trunk/Config.cmake/mergestaticlibs.cmake, https://code.google.com/p/toadlet/source/browse/cmake/Modules/MergeStaticLibraries.cmake, http://www.cmake.org/pipermail/cmake/2011-February/043002.html. I also saw that CMake supports an OBJECT library which might be helpful.

All of this seems very clumsy to me and I can't find any convergence on the best way to approach this. Is there any good reference where it is guided how to combine static libraries into a single static library with CMake ?

Thanks already for the support.

Johan Laneau
Software Architect, NXP Software

Interleuvenlaan 80, B-3001 Leuven, Belgium
Phone: +32 495 354 839, Fax: +32 16 390 855
E-mail: johan.laneau at nxp.com<mailto:johan.laneau at nxp.com>, http://www.nxpsoftware.com


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