[CMake] add_subdirectory inheritance

[Kent Williams]

I think I understand what you're trying to do better now.  It's a bit
off the beaten path for CMake, and has more moving parts than is
strictly necessary.

It could be set up as a nested suite of ExternalProjects, and avoid
the definitions inheritance issue you're running into.

You could also manage this by having a flag that is defined if this is
a top-level build and not defined in a standalone build, and using
Source Properties to properly set the definitions for the files that
need different flags.

The problem is that ADD_DEFINITIONS is global for the current project,
 It is also -- for that very reason -- if not a deprecated CMake
feature, one whose use is discouraged.  You're much better off using
SET_SOURCE_FILE_PROPERTIES to handle this kind of configuration trick,
since the properties are tied to a particular source file.  You can
use macros and FOREACH to avoid having to call
SET_SOURCE_FILE_PROPERTIES on every file explicitly.

You have become invested in the idea that it can only be done by
changing CMake, but you can do it without changing CMake if you get
creative with the tools already available to you.

On Tue, Apr 17, 2012 at 5:02 PM, irukandji <irukandji at voidptr.eu> wrote:
> Hi,
>
> Maybe i can try to reexplain, from head, ignore syntactic errors and
> oversimplifying...
> **********************************
> /sources/cmakelist.txt
>
> /sources/binaries/helloworld
> ..................cmakelists.txt
>
> /sources/binaries/hellocmake
> ..................cmakelists.txt
>
> /sources/libraries/stdio
> ..................cmakelists.txt
> ..................description.txt
>
> /sources/libraries/gfx
> ..................cmakelists.txt
> ..................description.txt
> **********************************
>>>
>>> helloworld: <<
>
> project(helloworld)
> include("${libs}/stdio/description.txt")
> include("${libs}/network/description.txt")
> file(glob da_files "*.*")
> add_executable( helloworld da_files )
> helper_add_stdio()
> **********************************
>>>
>>> stdio description <<
>
> function (helper_add_stdio)
> include_directory( "${sources}/libraries/stdio" )
> target_add_library( ${CMAKE_PROJECT_NAME} "${BINDIR}/whatever/stdio.lib" )
> add_definition( -DUSING_STDIO )
> ...
> execute(grab_me_a_beer_and_add_some_more_flags)
> ...
> if(not exists "${BINDIR}/whatever/stdio.lib")
> add_subdirectory( "/sources/libraries/stdio")
> endif()
> endfunction()
> **********************************
>>>
>>> hellocmake <<
>
> project(hellocmake)
> include("${libs}/gfx/description.txt")
> include("${libs}/network/description.txt")
> file(glob da_files "*.*")
> add_executable( hellocmake da_files )
> helper_add_gfx()
> helper_add_stdio()
> **********************************
>>>
>>> gfx description <<
>
> function (helper_add_gfx)
> include_directory( "${sources}/libraries/gfx" )
> target_add_library( ${CMAKE_PROJECT_NAME} "${BINDIR}/whatever/gfx.lib" )
> add_definition( -DUSING_GFX )
> ...
> if(not exists "${BINDIR}/whatever/gfx.lib")
> add_subdirectory( "${sources}/libraries/gfx" "${BINDIR}/whatever/gfx.lib" )
> endif()
> endfunction()
> **********************************
>>>
>>> sources/cmakelist.txt <<
>
> add_subdirectory( "/sources/binaries/helloworld" )
> add_subdirectory( "/sources/binaries/hellocmake" )
> **********************************
>
> * If i run cmake on helloworld, lets say for windows, i get solution and
> vcprojs
> of helloworld with dependancy to generated vcproj for stdio. Stand alone.
>
> * If i run cmake on hellocmake, lets say for windows, i get solution and
> vcprojs
> of hellocmake with dependancy to generated vcproj for stdio and gfx. Stand
> alone.
>
> * If i run cmake on /sources/cmakelist.txt i get full build.
>
> * The author(s) of helloworld doesnt need to have a clue about stdio library
> its paths
> or whatever as the author(s) of stdio/description.txt has prepared
> everything for him
> (in a perfect world :) ). Same goes for hellocmake.
>
> * The full build doesnt need to understand what is behind the helloworld and
> hellocmake as
> they both will take care about building the dependancies on their own.
>
> * The coupling is minimized.
>
> * The description.txt can serve as a hook in case of changing whatever
> settings for
> the libraries (from renaming them, to changing directory tree, changing
> compiler flags etc)
>
> This is what i would like to achieve. But if you look at >> hellocmake <<,
> the
> stdio cmakelists will inherit the include path from gfx library which you
> really
> dont want. It will also inherit -DUSING_GFX even if it has nothing to do
> with it.
>
> Regards,
> irukandji
>
>
>
> On 2012-04-17 22:56, irukandji wrote:
>>>
>>> I don't know how you'd ever maintain a sane overall project if it
>>> depends on each subdirectory having conflicting compiler flags.
>>
>>
>> Well here is the fun, there is not something like "you", we are taling
>> about
>> over 100 developers and if everyone is handling his own garden, this is
>> not
>> a problem. I cant tell you the details (the lawyers stuff) but believe me
>> it
>> is as sane as it can be in regards of doing insane things :)
>>
>> I'll check your suggestions tomorrow, thank you.
>>
>> Regards,
>> irukandji
>>
>> On 2012-04-17 20:54, Kent Williams wrote:
>>>
>>> I think then that you shouldn't use add_subdirectory.
>>>
>>> I'd suggest using the ExternalProject module in this case, because it
>>> uncouples the subdirectory's project from the parent project.  In that
>>> case, each subdirectory can be its own project and maintain private
>>> configurations.
>>>
>>> You can manage dependencies between ExternalProjects with the DEPENDS
>>> keyword.
>>>
>>> I think that what you're describing doesn't really make any sense to
>>> me.  I don't know how you'd ever maintain a sane overall project if it
>>> depends on each subdirectory having conflicting compiler flags.
>>>
>>> Another way you can manage this sort of thing is to use Source file
>>> properties -- see SET_SOURCE_FILE_PROPERTIES in the CMake
>>> documentation and the "Properties on Source Files" section as well.
>>>
>>>
>>> On Tue, Apr 17, 2012 at 1:27 PM, irukandji <irukandji at voidptr.eu> wrote:
>>>>
>>>> Oh, hi :)
>>>>
>>>> Well, the add_subdirectory takes all the preprocessor defines and
>>>> include/library
>>>> paths defined before calling it into the added "subdirectory"
>>>> cmakelists.txt.
>>>>
>>>> If cmakelists.txt A defines -DWhatever and calls add_subdirectory(/B)
>>>> where
>>>> the
>>>> cmakelists.txt for building library B resides then the library B is
>>>> going to
>>>> be
>>>> built with -DWhatever. This is probably great behaviour for some cases
>>>> but
>>>> can
>>>> also be undesired:
>>>> in my case, each library has its own description file, residing in same
>>>> directory
>>>> as its cmakelists.txt, with only one function, which is included and
>>>> called
>>>> with few
>>>> parameters to specify runtime etc. by each binary which has a dependancy
>>>> to
>>>> library.
>>>> The description file function sets the -I to itself for caller, the -l
>>>> to
>>>> its output
>>>> and, if the output file is not found, also calls add_subdirectory on its
>>>> own
>>>> directory.
>>>>
>>>> This is a perfect situation for developers as we can just do out of
>>>> source
>>>> configuration
>>>> for whatever directory (where project resides) and only the dependancy
>>>> tree
>>>> for that
>>>> particular binary is built.
>>>>
>>>> It also brings additional benefit that the team which takes care about
>>>> specific
>>>> library also takes care for description file and needed defines which
>>>> splits
>>>> the
>>>> ownership of projects and also presents a single point of inclusion for
>>>> that
>>>> particular
>>>> library, meaning only one file has to be changed for library
>>>> specializations.
>>>>
>>>> For the nightly builds, only executables are added to the build while
>>>> everything
>>>> else is built only if actually used.
>>>>
>>>> It works great, but the add_subdirectory is propagating the settings
>>>> from
>>>> executable
>>>> to dependant libraries and messes the build.
>>>>
>>>> The question actually is: can i workaround this behaviour and if not,
>>>> can
>>>> you please
>>>> include the parameter to add_subdirectory function to NOT propagate the
>>>> settings.
>>>>
>>>> Regards,
>>>> Irukandji
>>>>
>>>>
>>>> On 2012-04-17 17:58, Kent Williams wrote:
>>>>>
>>>>>
>>>>> Frankly, I don't entirely understand what the problem is, or what your
>>>>> proposed solution is.
>>>>>
>>>>> What is it that you don't want the subdirectory context to inherit?
>>>>>
>>>>> On Tue, Apr 17, 2012 at 10:30 AM, irukandji <irukandji at voidptr.eu>
>>>>> wrote:
>>>>>>
>>>>>>
>>>>>> Hi,
>>>>>>
>>>>>> (as no one answered to my previous email, let me add this:
>>>>>> multiplatform
>>>>>> project with few million lines
>>>>>> of code, sheer size of the project is not allowing to turn around
>>>>>> whole
>>>>>> directory tree as price / performance
>>>>>> is a very relevant factor and even rewriting makefiles/vcprojs to
>>>>>> cmake
>>>>>> will
>>>>>> take months)
>>>>>>
>>>>>> The add_subdirectory inheritance striked which is practically a show
>>>>>> stopper, it was already discussed here
>>>>>> http://www.mail-archive.com/cmake@cmake.org/msg34291.html What was
>>>>>> proposed
>>>>>> (DONT_INHERIT) was a great idea
>>>>>> and improvement to versability of CMake and also harmless for backward
>>>>>> compatibility so i dont really
>>>>>> understand why forcing subproject to inherit all the settings.
>>>>>>
>>>>>> Yes, what is added with add_subdirectory is not a child but a sibling
>>>>>> or
>>>>>> even some other part of the tree
>>>>>> but the point is that it doesnt stop the cmake from building it
>>>>>> correctly
>>>>>> but this inheritance is a problem
>>>>>> as it adds include directories to wrong versions of headers in public
>>>>>> libs
>>>>>> (due to bugs in different versions,
>>>>>> affecting different platforms,... the unification is impossible), the
>>>>>> flags
>>>>>> are used on wrong places etc.
>>>>>>
>>>>>> Is there a way to workaround it?
>>>>>>
>>>>>> Thank you.
>>>>>>
>>>>>> --
>>>>>>
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>>>>
>>>>
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>
>
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