[CMake] Using cmake to build a static library

[David Cole]

Michael's correct.

If CMake generates the build instructions for the final executable,
then all is well, because CMake knows all the dependencies from the
target_link_libraries commands, and it can produce the linker line
containing all the right lib references.

If all you are doing is building static libraries and then feeding
them to some other build system, you also have to feed it all the
libraries you depend on too, because that information is simply NOT
encoded into static libraries.

The best way, as always, to depend on "3rd party" stuff, is simply to
avoid those dependencies whenever possible. 2nd best is to incorporate
their source directly into your library. After that, you just have to
list them explicitly by hand somewhere...


HTH,
David


On Thu, Sep 15, 2011 at 3:12 AM, Michael Wild <themiwi at gmail.com> wrote:
> On 09/15/2011 08:49 AM, Denis Carniel wrote:
>> Hi,
>>
>> I came across cmake few weeks ago as a very interesting way to build
>> projects for multiple platforms. It allows us at work to use a common
>> code base for multiple platforms without actual duplication which is
>> really neat.
>>
>> Though we're facing an issue related to the fact cmake isn't the only
>> tool involved in our build process. We have split our code between
>> the core logic which is compiled in C++ for multiple platforms
>> (windows x86 and x86_64, Mac OSX and in the future linux) then based
>> on that core logic we would like to produce SDKs for each platform
>> using their specific features (for easier integration in projects).
>>
>> The problem is the following: The core logic is meant to be a static
>> library (.lib on windows, .a on Mac / Linux) and then another project
>> should pick up that library and include it into an executable. That
>> final project is (a) not linked with the original cmake project (b)
>> potentially not even built by cmake. For those reasons we'd like that
>> the generated static library include its own dependencies in order to
>> avoid dragging a long list of dependencies, unfortunately that does
>> not seem to be be case in the current state of things.
>>
>> If I set the type of my output to "SHARED" (through the "ADD_LIBRARY"
>> command), I see all dependencies appear in the Visual Studio project
>> and the produced DLL seems correct. Though if the type is set to
>> "STATIC" (the one we want), the dependencies are simply not passed to
>> the Visual Studio project (AdditionalDependencies &
>> AdditionalLibraryDirectories are not set for VCLibrarianTool).
>>
>> Hence my overall question: How can I have cmake produce a static
>> library in which dependencies are linked ?
>>
>> Thanks in advance for your help. Denis
>>
>> PS: I know DLLs work on windows but those are not an option, because
>> of deployment constraints we need to produce a single executable file
>> in the end.
>
> Well, static libraries just don't give you that. They are essentially
> glorified archive files (think "zip-file") containing the object files.
> When you do a TARGET_LINK_LIBRARIES(myStaticLib someOtherLib), CMake
> remembers that dependency internally and will add someOtherLib to all
> the link-lines where myStaticLib is used, but it can't possibly embed
> that information into the static library.
>
> If your downstream projects are CMake, you can tell CMake to "export"
> the myStaticLib target along with its dependencies to a special file
> which then can be imported by the downstream project. If it isn't, on
> Linux Mac and other Unix-ish platforms (MinGW, Cygwin, etc.) you would
> write a pkg-config file which contains that information. With pure MSVC
> I don't think that this is possible, so IMHO you'll have to add the
> dependencies manually.
>
> Michael
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