Fwd: KF5 CMake usage question

Aleix Pol aleixpol at kde.org
Sun May 21 22:27:38 BST 2017

On Sat, May 20, 2017 at 7:41 PM, Shaheed Haque <srhaque at theiet.org> wrote:
> Actually, there is one thing about "target CMake"-based KF5 that I
> don't quite understand: is there a way to get to the C++ compile flags
> needed from CMake? That is, the modern equivalent of Foo_COMPILE_FLAGS
> but for target Foo? Even if the general answer is "no", I'm interested
> in at least the CMake variables/properties/commands needed to get to
> "-fPIC" and "-std=gnu++14".
> I'm aware of the target properties
> POSITION_INDEPENDENT_CODE and CXX_STANDARD but none of these seem to
> be set on targets I have tried.
> Perhaps these are only set if somehow the compiler name etc. is specified?
> Thanks, Shaheed
> On 18 May 2017 at 18:04, Shaheed Haque <srhaque at theiet.org> wrote:
>> Hi,
>> On 18 May 2017 at 12:51, Andreas Hartmetz <ahartmetz at gmail.com> wrote:
>>> On Samstag, 13. Mai 2017 23:48:33 CEST Shaheed Haque wrote:
>>>> Hi,
>>>> On 13 May 2017 at 22:04, Sven Brauch <mail at svenbrauch.de> wrote:
>>>> > Hi,
>>>> >
>>>> > On 05/13/2017 06:06 PM, Shaheed Haque wrote:
>>>> >> The printed output shows that the variable KF5KIO_INCLUDE_DIRS is
>>>> >> not
>>>> >> set. In poking around, I see references to a (new-to-me)
>>>> >> target-based
>>>> >
>>>> >> system, and using that like this:
>>>> > The question is, why do you need to do that?
>>> The idea with the target based system aka "Modern CMake" is that you say
>>> you want to compile against a library, and that is all you have to do. A
>>> library requires you to add an include path for its own headers, include
>>> paths for headers of one of its dependencies, and link against a bunch
>>> of libraries? All covered by target properties.
>>> If for some reason (e.g. handover to an external tool) you need those
>>> properties, you can still query them. Under enforced names nonetheless,
>>> unlike FOO_INCLUDE_DIR or was it FOO_INCLUDE_DIRS?
>> Ack. The problem from the point of view of an automated tool which starts
>> with a component called Foo arises ONLY because the target(s) of Foo are
>> called FooFoo and FooBar. CMake does not - AFAICS - allow one to query Foo
>> and somehow find FooFoo and FooBar inorder to look up FooFoo_INCLUDE_DIRS
>> etc.
>>>> I'm continuing to experiment with trying to build Python bindings for
>>>> KF5. As part of that, the SIP tooling creates C++ wrapper code which
>>>> must be compiled for each framework, and for that I need to know the
>>>> header file directories. So far, I have simply been hardcoding the
>>>> needed paths on my own system, but I now want to move to using
>>>> standard CMake-based logic instead.
>>>> (In some sense, this might be seen as similar to the stuff that was
>>>> added to ECM, but I'm trying for a significantly more automated
>>>> approach).
>>>> Also, I am trying to feel my way towards a Pythonic build system for
>>>> the KF5 bindings (as, roughly speaking, PyQt seems to be doing): in
>>>> other words I'm interested in using CMake as a stepping stone, not the
>>>> actual build system.
>>> I would recommend against that unless you really need to have heavy
>>> logic in the build system. A build system's main job is to "solve" a
>>> dependency tree - that is the difference between a build system and a
>>> script that runs the compiler. The dependency tree enables cheap
>>> incremental builds and correct parallel builds. Maybe not that important
>>> for bindings, admittedly.
>>> Your advantage from using Python must be larger than the overhead from
>>> doing your own dependency resolution plus the overhead from the CMake-
>>> Python interfacing plus the build-related facilities that CMake has and
>>> Python doesn't. Or were you considering scons?
>>> PyQt may have chosen Python because qmake sucks, and it needs Python
>>> anyway, so it avoids any extra dependencies. I know from experience that
>>> you really want to avoid extra dependencies in commercial products.
>> /me nods vigourosly.
>> I'm not (yet) familair with all the intricacies of the Python build system
>> (or CMake for that matter!), but I do see that PyQt has to work quite hard
>> to keep its build system working as a Python user might expect. Further, the
>> system I am seeking to build has to support more than KF5 (or even KDE). So,
>> roughly speaking, the split I am going for is:
>> - Keep all platform and system independent code in Python
>> - Isolate all platform and system independent logic in CMake
>> As I say, I am feeling my way a bit here, but this seems like a
>> philosophically justifiable separation. Oh, and to solve the problem of
>> finding the targets, I resorted to parsing the CMake files (!!). I can live
>> with that hack precisely because by having the split, users of this code who
>> are not using it against KF5 will need to replace this CMake part with their
>> own anyway.
>> (At this point, abstracting CMake away entirely is a minor detail).
>> Thanks for the helpful remarks.
>> Shaheed
>>>> Thus, I'm after the moral equivalents of:
>>>> Thanks, Shaheed
>>>> > The usual way is to simply call
>>>> >
>>>> > target_link_libraries(mybinary KF5::KIOCore)
>>>> >
>>>> > and include paths etc. will be set up for your target automatically.
>>>> >
>>>> > Best,
>>>> > Sven
>>> Cheers,
>>> Andreas M9

You can easily see how it works in extra-cmake-modules code.


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