[CMake] Best way to combine generated static libraries into a single static library

[Ke Gao]

Thank you all for the help.

I finally use a way quite similar to David's first approach. I first
generate all sub-projects into object libraries using add_library(lib1
OBJECT SOURCES). Then in the final library, I use
add_library(single_static_lib STATIC SOURCES) and target_link_libraries(
single_static_lib lib1 lib2 ...). Note that I didn't use "
$<TARGET_OBJECTS:lib1>" in the final "add_library" and also didn't use
"PUBLIC" keyword in the final "target_link_libraries". It works on CMake
v3.12.2 and gives me a single static lib which combines all the objs I
want.

But currently I still have problems of further combining third party static
libraries into the final generated static single_static_lib. Can anybody
provide a solution for this?

Thank you very much.

Ke

On Fri, Sep 21, 2018 at 6:15 AM Deniz Bahadir <dbahadir at benocs.com> wrote:

> Am 21.09.2018 um 09:33 schrieb David Jobet:
> > Hello,
> >
> > I had a similar issue lately and wanted to "pack" several static libs
> > into a dynamic one. (Not even talking about an INTERFACE lib since I
> > really wanted that .so)
> > I made it work with 3 different solutions, none of them being "clean"
> > from my point of view.
> >
> > 1- OBJECT libs on sub projects : add_library(lib1 OBJECT SOURCES) and
> > for the single static lib : add_library(single_static_lib STATIC
> > $<TARGET_OBJECTS:lib1> ...)
> > Problem I faced : since OBJECT libs do not support
> > target_link_libraries(), I had to remove the existing one and move
> > them instead to the target_include_directories() using generators.
>
> This is no longer true. Since CMake 3.12 `target_link_libraries` fully
> supports OBJECT libraries. You just need to pay attention to the special
> case of linking an OBJECT library with keyword "PUBLIC". (Object-files
> are always private and inherited object-files are therefore never
> further inherited. See:
>
> https://cmake.org/cmake/help/v3.12/command/target_link_libraries.html#linking-object-libraries
> )
>
> > e.g : target_include_directories(lib1 PUBLIC
> > $<TARGET_PROPERTY:another_lib,INCLUDE_DIRECTORIES>)
> > Because I had a dependency to a protobuf generated lib, I also had to
> > add manual add_dependencies to respect proper build order.
> > Not clean at all
> >
> > 2- add_library(mysharedlib STATIC CMakeLists.txt)
> > target_linked_libraries(mysharedlib PUBLIC lib1 ...)
> > Maybe the cleanest way I found.
> > The trick with CMakeLists.txt is that add_library needs at least one
> > source file. You can put any kind of files you want. CMakeLists.txt is
> > not compilable, so no extra compilation step, no need for dummy empty
> > source file and add_library is happy.
> > It did not work in my case because of problems related to how our .so
> > are used/generated. (problems at runtime with duplicated symbols in
> > protobufs)
> >
> > 3- a variation around 1
> > instead of defining OBJECT libs, define a variable holding all the
> > sources for lib1, another for lib2, ...
> > then just do add_library(mysharedlib STATIC ${SOURCES_FOR_lib1}
> > ${SOURCES_FOR_lib2})
> > It works a little bit like 1) but does not have any of its problems
> > (target_link, add_dependencies, generators, ...)
> > It has new problems of its own though : if your libs live in different
> > subfolders, the variables might not be visible from your
> > add_library(mysharedlib...) call.
> > To work around that, you can use PARENT_SCOPE (not sure if that works
> > accross several levels), or includes (defines those variables into
> > separate files living in each subfolders and include them in the
> > parent CMakeLists).
> >
> > Hope this helps (and I'd be happy to know of other ways)
> >
> > David
> > On Thu, Sep 20, 2018 at 5:45 PM Ke Gao <ke.gao.ut at gmail.com> wrote:
> >>
> >> Hi,
> >>
> >> I have a project which includes many sub-projects. Each sub-project
> generates a static library. In the main project, I want to combine the
> generated objs, generated static libraries from other sub-projects, and
> some other third party static libraries together into a single static
> library. Is there an elegant way to do this, or maybe an existing function?
> >>
> >> Thank you very much.
> >>
> >> --
> >>
> ..............................................................................................................................................
> >> Ke Gao
>
>
> Hope that information was of value,
> Deniz
>
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-- 
..............................................................................................................................................
Ke Gao
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