[CMake] Parent component options passed to ExternalProject

[Dustyn Blasig]

Thanks for the help, I will give the FetchContent approach a shot and see
how things go. I was actually in the process of trying to do a custom
command using the built-in "cmake -E tar xvf" approach to roll a custom
FetchContent that doesn't need such a new CMake, but I can't get it to work
with zip files even though it seems like it should at this point. I'm
hoping the fetch and include as subdirectory will be a more stable approach.

On Tue, Mar 26, 2019 at 4:39 PM Craig Scott <craig.scott at crascit.com> wrote:

>
>
> On Tue, Mar 26, 2019 at 4:02 AM Dustyn Blasig <dustyn at blasig.us> wrote:
>
>> Hi All,
>>
>> I'm really struggling to use ExternalProject_Add() in our CMake project.
>>
>> Based on the documentation, I was expecting the simplest Git-based
>> external project with all defaults to act just like using a Git submodule
>> locked at a specific commit.
>>
>> include(ExternalProject)ExternalProject_Add(foobar  GIT_REPOSITORY    git at github.com:FooCo/FooBar.git  GIT_TAG           origin/release/1.2.3)
>>
>> The documentation states "The default configure command runs CMake with
>> options based on the main project.", so I was expecting any options we pass
>> to the owning project such as CMAKE_INSTALL_PREFIX and other variables
>> would get propagated to the ExternalProject. However, that doesn't seem to
>> be the case. Do I need to explicitly apply every variable through the
>> "CMAKE_ARGS" options? If so,
>>
>
> Some basic details are passed down to the external project, but I don't
> recall exactly which variables. I don't think there are a lot of them and I
> do tend to find that I need to pass down some vars in my own projects. You
> may have to dig into the ExternalProject module's source to get
> confirmation for yourself, possibly also look into what the
> external_process() command's implementation does as well.
>
>
>
>> Also, unless I specify a custom CONFIGURE_COMMAND, I can't figure out how
>> to make the configure step happen during the parent projects configure
>> step. Should that be happening? If not, is there a simple way to force it
>> to happen without having to duplicate everything the default options do?
>> I'm seeing mixed results with online resources, along with many older
>> examples that just don't work anymore.
>>
>
> The external project's configure step happens during the main project's
> build stage, just like every other step of ExternalProject_Add(). This will
> be the case whether you override the CONFIGURE_COMMAND or not. If you
> really need the configure step to occur during the main project's configure
> stage, then you have at least a couple of options:
>
>    - Consider using the FetchContent module instead and bring the
>    external project into your main build directly. If this suits your
>    scenario, then I'd recommend this method. I use this a lot and it seems to
>    be growing in popularity. It requires CMake 3.11 or later.
>    - Go for a pure superbuild approach where your main project does
>    nothing more than call various ExternalProject functions to tie together
>    different external projects (i.e. your main project doesn't add any
>    targets, etc. of its own). This will mean that even though the configure
>    steps all occur during the main project's build step, you can control the
>    dependencies between the different external projects' steps so that they
>    can find what they need from each other (I'm guessing here that you want
>    the external project's configure stage to run during your main project's
>    build stage so that something later in your main project's configure stage
>    can make use of something the external project's configure stage does). You
>    will still have to explicitly specify a number of variables to be passed
>    through to each external project's configure stage though, so maybe this
>    doesn't solve the problem you are facing.
>
>
> --
> Craig Scott
> Melbourne, Australia
> https://crascit.com
>
> Get the hand-book for every CMake user: Professional CMake: A Practical
> Guide <https://crascit.com/professional-cmake/>
>
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