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

Deniz Bahadir dbahadir at benocs.com
Fri Sep 21 14:33:51 EDT 2018


Am 21.09.2018 um 18:38 schrieb Ke Gao:
> Hi David,
> 
> It didn't give me error info. It can produce the final static library, 
> but the contents of third party static libraries are not included.

Some clarification regarding STATIC and OBJECT library targets and how 
they interact with each other using `target_link_libraries`:


A STATIC library target A can be linked (via `target_link_libraries`) to 
another STATIC library target B.

   `target_link_libraries(A STATIC PUBLIC B)`

However, the resulting library file libB does not contain the 
information (aka object-files) of libA.
Instead, CMake makes sure to link both static library files libB and 
libA when e.g. creating an executable exeC1 using target C1 which just 
links (via `target_link_libraries`) to target B.

   `target_link_libraries(C1 EXECUTABLE PUBLIC B)`

(Instead of an executable exeC1 you could have created a shared library 
libC1 instead. The behavior is the same.)


If you instead want to create a combined, single STATIC library libC2, 
which contains the object-files archived in both static library files 
libA and libB created from targets A and B, then you somehow need to 
extract the archived object-files from libA and libB. I am not sure if 
CMake provides a (simple) way to do this.



For OBJECT libraries the behavior is like this:


An OBJECT library target X can be linked (via `target_link_libraries`) 
to another OBJECT library target Y.

   `target_link_libraries(Y OBJECT PUBLIC X)`

This, however, only transports the usage-requirements 
(preprocessor-defines, include-paths etc.) from X to Y. The object-files 
of X are in no way referenced by Y.

So, if you then create a STATIC target Z1 that links (via 
`target_link_libraries`) to Y,...

   `target_link_libraries(Z1 STATIC PUBLIC Y)`

... Z1 will reference/contain the usage-requirements of X and Y and the 
object-files of Y, but not the object-files of X. (The static library 
file libZ1 resulting from Z1 will contain the object-libraries of Y but 
not of X.)


What you should do instead is not to link the OBJECT targets X and Y 
together but instead only link them (explicitly) to the STATIC target Z2:

   `target_link_libraries(Z2 STATIC PUBLIC Y X)`

Z2 then references the usage-requirements as-well-as object-files of 
both OBJECT targets X and Y and the resulting static library file libZ2 
will then contain the object-files of both library targets, X and Y.


> 
> Also, sorry for the mistake I have made in my previous email. Deniz is 
> right, the keyword "PUBLIC" should be used in target_link_librarie().
> 
> Thanks

You're welcome.

Deniz


> 
> On Fri, Sep 21, 2018 at 10:06 AM David Jobet <djobet at tower-research.com 
> <mailto:djobet at tower-research.com>> wrote:
> 
>     Hello,
> 
>     glad that could help you.
>     For your newer problem, you don't describe them, so it's tough to know
>     what kind of problems you're facing.
>     Maybe a small example of your CMakeLists.txt + a capture of the error
>     cmake gives you could help ?
> 
>     David
>     On Fri, Sep 21, 2018 at 4:52 PM Ke Gao <ke.gao.ut at gmail.com
>     <mailto:ke.gao.ut at gmail.com>> wrote:
>      >
>      > 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 <mailto: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
>     <mailto: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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> -- 
> ..............................................................................................................................................
> Ke Gao
> 
> 


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