[CMake] linking static libs into shared libs on Linux

[Alexander Neundorf]

On Tuesday, July 07, 2015 14:36:25 Steve Borho wrote:
> Hello, I am the technical lead for x265 (HEVC encoder) and we have used
> cmake successfully since the beginning of the project.  There are a few
> rough edges that we've had to work around (uninstall rules, yasm support
> in MSVC, etc) but in general it has worked pretty well.
> 
> x265 can support a number of bit-depth options, but these are compile
> time decisions. You must chose at build time whether the encoder will
> use 8bit, 10bit, or 12bit color samples internally.  Recently we've
> resolved the naming conflicts and thus allow these multiple builds of
> libx265 to be linked together into a single binary.
> 
> The way the multilib build process currently works is it creates 8bit/
> 10bit/ and 12bit/ folders, then runs cmake in each seperately (with
> different options) so there are three distinct build folders. It builds
> 10bit and 12bit first, outputting only static libraries, then finally it
> builds the 8bit folder and adds the 10bit and 12bit static libraries to
> the 8bit shared library link target.
> 
> This works well on Windows and Mac but on x86_64 Linux I get this error:
> 
> /bin/ld: common/CMakeFiles/common.dir/x86/pixel-a.asm.o: relocation
> R_X86_64_PC32 against symbol `x265_pw_1' can not be used when making a
> shared object; recompile with -fPIC
> 
> (in a nutshell, the 10bit and 12bit static libs are no longer position
> independent)
> 
> I figure there are multiple potential solutions for this and would
> welcome pointers for any of them
> 
> 1 - is there a way for the project to re-include itself? If so we could
>   have the 8bit build simply build the 10 and 12bit object libraries and
>   link with those directly and bypass the creation of static libraries.
>   This would perhaps be optimal since you would end up with a single
>   Makefile driving the whole build.
> 
> 2 - is there some invocation that will force cmake to build the static
>   libraries such that they are still safe for linking into a shared
>   library?
> 
>   I've already tried these unsuccessfully:
>     set(CMAKE_POSITION_INDEPENDENT_CODE TRUE)
>     set_target_properties(x265-static PROPERTIES POSITION_INDEPENDENT_CODE
> TRUE) set_target_properties(x265-static PROPERTIES LINK_FLAGS "-fPIC")
> 
> 3 - is there another intermediate format that cmake can output that
>   preserves -fPIC, or some way to access the object libraries of the
>   other external builds?
> 
> 4 - is there any post-processing that can be done on the static libs to
>   make them safe for shared libs again (guessing this is not likely)

did you already have a look at the "object libraries" (created using 
add_library(... OBJECT...) ? Those basically just build the object files, and 
can later be used to link into other targets.

Alex