[cmake-developers] Introducing targets in FindPkgConfig

[Daniele E. Domenichelli]

Hello Eike,

On 22/12/14 16:39, Rolf Eike Beer wrote:
> What I would like to get is the following: when a list of libraries and 
> directories is returned by PkgConfig then for every of these libraries the 
> absolute path to the library is determined using find_library() using the given 
> directories as hints.
> [...]
> Since this would introduce new behavior people would need to adapt 
> their CMakeLists.txt to this new feature. Therefore I think it would be a good 
> idea to expose this new information not by new variables, but using targets.


We have a macro to do exactly this and a few other things (actually 2 macros,
that diverged slightly I will make them one again as soon I as I have
some time), you can find them here:

  https://github.com/robotology/yarp/blob/master/conf/MacroStandardFindModule.cmake
  https://github.com/robotology/ycm/blob/master/modules/StandardFindModule.cmake

The macro tries first to find a CMake config file (some recent version
of some package installs cmake config files, but we still need to
support older versions, so that's the reason for this check). 
If the config files were not found, it uses pkg-config to detect the
package, and find the libraries as you want to do, but instead of
creating targets it sets some variables (see the documentation in the
files).

It's probably not perfect, but with this, creating a "FindXXX.cmake"
for a software using pkg-config becomes trivial, for example:

  https://github.com/robotology/yarp/blob/master/conf/FindGtkDataboxMM.cmake

and then you just use find_package() to locate your package and to
get the variables.


I agree that using targets would be a great thing, but at the time I
didn't find a way generic enough to do all of this in the macro, since
it's not always easy to understand which one is the "main" library
"add_library(UNKNOWN IMPORTED)" and which ones should be in the
"INTERFACE_LINK_LIBRARIES" property. Moreover we depend on CMake 2.8.9
that does not support all the features that we would need for targets.
Perhaps using a recent CMake version you can use
"add_library(INTERFACE IMPORTED)" and put all of them in the
INTERFACE_LINK_LIBRARIES?



> So if I search for OpenSSL using PkgConfig I would get a target 
> PkgConfig::OpenSSL that I can link to, and that has the libraries, compile 
> flags, include dirs and whatnot.
> 
> Sadly, I don't quite understand all the macros in FindPkgConfig.cmake. Can you 
> either point me to the right place where I would hook up something like that 
> or try doing it?

You can start from the macros I linked before for the pkg-config part, and
then have a look at the CMake modules that create imported targets
(FindQt4, FindGTK2, FindZLIB, FindGLUT, FindGLEW) for the targets part
(all the variables you need should be there already).
Let me know if you do it, I'd like to do it too one day...


About the FindPkgConfig macros documentation, what is that is not clear?

  http://www.cmake.org/cmake/help/git-master/module/FindPkgConfig.html

It looks well documented to me, but if there is something that is not
clear we could try to improve it...


I hope this helps. Let me know if I can help you more specifically with
this...


Cheers,
 Daniele