[CMake] When should I use add_subdirectory and when ExternalProject?

[David Cole]

-----Original Message-----
From: Alexander Neundorf <a.neundorf-work at gmx.net>
To: cmake <cmake at cmake.org>
Sent: Tue, Feb 5, 2013 1:09 pm
Subject: Re: [CMake] When should I use add_subdirectory and when ExternalProject?


On Monday 04 February 2013, David Cole wrote:
> The OpenChemistry project ( https://github.com/OpenChemistry/openchemistry
> ) is a very good example of a SuperBuild project that builds all of its
> external dependencies via ExternalProject, and then all of its git
> submodules ("internal dependencies" perhaps ?) also via ExternalProject.
> 
> They use CMAKE_PREFIX_PATH both as a place to install all of the built
> components, and as a place for find_package to find stuff that was built
> by a previous ExternalProject_Add call.
> 
> Study how they do stuff in their CMakeLists files and then get back to us
> here with more questions.

one thing I had trouble with is related to the install step.

With externalProject the install step happens at build time.
If shared libraries are involved, the install destination should be the 
correct and final install destination, so that the RPATH of using projects 
will point to the correct location.
This means that at build time I need to have the permissions to write to the 
final install destination (e.g. /opt/), i.e. typically root.

Is this correct or am I missing something ?

Alex
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You are correct, but you are coming at it from a Linux-biased perspective.

For developers on the project, this technique works as-is, in the build tree,
and in the super-build's install tree, which is in a non-system, i.e., user writable
location. That's true on all platforms.

If you are building the openchemistry projects for packaging on a Linux box,
then you will use all the "system" packages, and not even build them as part
of the super build. Each one has a flag for whether or not to use the system one
or to build it.

For Mac and Windows installers, this technique works well, since all the built
pieces of the super build are bundled up together as relocatable app installations.

All that said, it's still evolving, and may need changes before becoming part of
any Linux distributions.


Cheers,
D


 
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