[CMake] ExternalProject_Add and Dependent Targets

[David Cole]

On Thu, Sep 22, 2011 at 10:13 AM, Lori Pritchett-Sheats
<lpritch at lanl.gov> wrote:
>
>
> David Cole wrote:
>>
>> On Wed, Sep 21, 2011 at 11:45 AM, Lori Pritchett-Sheats
>> <lpritch at lanl.gov> wrote:
>>
>>>
>>> Reading the documentation it is not clear how the DEPENDS option works in
>>> ExternalProject_Add.
>>>
>>> For example, I add HDF5 to my build and HDF5 depends on zlib the
>>> following
>>> doesn't work
>>>
>>> find_package(ZLIB)
>>> add_library(zlib UNKNOWN IMPORTED)
>>> set_target_properties(zlib PROPERTIES IMPORTED_LOCATION ${ZLIB_LIBRARY})
>>>
>>> ExternalProject_Add(hdf5 DEPENDS zlib ......)
>>>
>>> throws an error with the message 'No rule to make /zlib-done', needed by
>>> external_projects/src/hdf5-stamp/hdf5-configure
>>>
>>> For ExternalProject_Add, are the targets listed under the DEPENDS option
>>> only targets that will be built with another ExternalProject_Add, not
>>> existing libraries?
>>>
>>>
>>
>> That's correct.
>>
>> The first argument to ExternalProject_Add does become the name of a
>> custom target, though. So after the ExternalProject_Add call, you can
>> do:
>>
>> ExternalProject_Add(hdf5 ...)
>>
>> add_dependencies(hdf5 zlib)
>>
>> However, in this particular case, you are finding an existing zlib and
>> then using an imported target to say where it is *for other
>> non-external project targets in the same CMake build*... For the
>> ExternalProject build of hdf5, you're going to have to pass down in
>> the location of zlib via some arguments, because the ExternalProject
>> build knows nothing about the context of what else is built in the
>> current CMake build...
>>
>> So in this particular case, the hdf5 custom target does not actually
>> depend (in a build order sense) on the zlib target -- because the zlib
>> target doesn't build anything, it's just imported. So you don't
>> actually need the dependency... you just need to tell hdf5 where zlib
>> is.
>>
>> Does that make sense?
>>
>>
>> HTH,
>> David
>>
>>
>
> I think so. The HDF5 configuration has switch options to pass in the zlib
> include and library path, both are available once I call find_package(ZLIB).
> But if I'm building a library that depends on HDF5 I need to include the
> zlib target in the dependency definitions, correct?
>
> If I'm understanding this correctly, after the ExternalProject_Add call, I
> need to define HDF5_INCLUDE_PATH, HDF5_LIBRARIES, since the project will not
> exist when CMake runs. Then when I create a library or executable that needs
> the HDF5 libraries for linking, I would do the following:
>
> add_library(mylib ${some_source})
> target_link_libraries(mylib ${HDF5_LIBRARIES} zlib)
>
>

That looks right to me. Let us know if you run into problems with it.

Thanks,
David