[CMake] How to build and link Externa Project with exported target

[Michael Wild]

On 17. Mar, 2010, at 14:37 , Luigi Calori wrote:

> Hi Michael and Nicola,
> I'm in a similar situation as I' m trying to build up a cmake based dependency builder for OpenSceneGraph based projects:
> So I' m really interested in any "best practice" advice
> 
> Michael Wild wrote:
>> On 17. Mar, 2010, at 13:17 , Nicola Brisotto wrote:
>> 
>>  
>>> Hi!
>>> I'm building a project that require a 3rd party library libqxmpp. Both project uses cmake I want to build libqxmpp with ExternalProject_add, this the code I use:
>>> 
>>> ExternalProject_add(
>>> 	libqxmpp
>>> 	#no download, i'm using git submodule
>>> 	DOWNLOAD_COMMAND ""
>>> 	CMAKE_ARGS -DQT_QMAKE_EXECUTABLE=/devel/BIN/v4.6.1-git-phonon-dbg-release/bin/qmake -DCMAKE_INSTALL_PREFIX=${CMAKE_BINARY_DIR}
>>> 	SOURCE_DIR ${CMAKE_SOURCE_DIR}/src/3rdparty/qxmpp-read-only-git/source
>>> )
>>>    
> Means there is a way to instruct
> 
> ExternalProject_add
> 
> to download from GIT repos? I was not aware of and starting to hack it to add... let me know if one is available:
> I have already added Bazaar download similarly to SVN ...  so if there is a way to share hacking on it

He's not downloading with CMake, he just told git that in this directory is a submodule. The user has then to fetch it himself with one command:

http://git.wiki.kernel.org/index.php/GitSubmoduleTutorial

[...]
>> 
>> 2) also build your main project wit a ExternalProject_Add and drive the whole thing from a "master-CMakeLists.txt"
>> 
>> The first one is probably easier to set up, but requires you to guess the installation names and paths correctly. The second option requires you to restructure your whole build system and adds considerable complexity due to the communication between your master-project and the external projects. For this communication I'd try the following:
>> 
>> - In the master project do all the feature-detection and setting of cache variables (such as options etc)
>> - Write a cache-initializer script to the binary tree
>> - Do all the ExternalProject_Add calls and specify the cache-initializer script with the -C option in CMAKE_ARGS
>>  
> This suggestion is really interesting:    the purpouse is to let any config  options in the "master"  projects to be passed to the "slaves"?
> Have you any examples?

You could pass every single cache variable using -D in CMAKE_ARGS, but that becomes pretty tedious very soon... Currently I don't have any example, sorry.

> 
> 
> I came up with a schema like 2:
> any project is built as external, dependencies are resolved by  ExternalProject_Add and I have used CMAKE_ARGS to communicate settings:
> As most of cmake projects were based on FindXXX stuff for finding deps, I have overridden the necessary modules in order to make the projects find the good components at configure time.

That's what you could do with the cache-initialization script, no need to override the FindXXX modules. If more than one of the external projects use the same FindXXX module, do it in the master and then communicate the results using the initializer script. This way the user can easily override the results during the configure-step and get consistent results in all the sub-projects. Only thing that has me worried is when the user aborts the build, changes the settings in the master cache and then restarts the build. You'd need to somehow set up correct dependencies...

If sub-project B does a FIND_PACKAGE(A REQUIRED) and A is built as another sub-project, then you can specify A_DIR to the B project containing the path to the directory containing its A-config.cmake.


Michael