Fwd: KF5 CMake usage question

[Albert Astals Cid]

El dimarts, 23 de maig de 2017, a les 2:04:54 CEST, Aleix Pol va escriure:
> On Mon, May 22, 2017 at 7:54 PM, Shaheed Haque <srhaque at theiet.org> wrote:
> > On 21 May 2017 at 22:27, Aleix Pol <aleixpol at kde.org> wrote:
> >> 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
> >>> COMPILE_FLAGS/OPTIONS/DEFINITIONS/OPTIONS as well as
> >>> 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:
> >>>>>> 
> >>>>>> Foo_INCLUDE_DIRS
> >>>>>> Foo_COMPILE_FLAGS
> >>>>>> 
> >>>>>> 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.
> > 
> > I don't think it is quite so simple. First, for "-std=gnu++14",the ECM
> > code just hardcodes it (with a comment that hints at difficulties
> > extracting this value). Second, it grabs INTERFACE_COMPILE_DEFINITIONS
> > and INTERFACE_COMPILE_OPTIONS of which the latter seems to be needed
> > in order to get stuff from come KF5 cmake file which set -fexception
> > (in some case, for example).
> > 
> > But what sets "-fPIC"? It seems to be inherited in some way from
> > Qt5::Core INTERFACE_COMPILE_OPTIONS, but how can I get to that
> > programmatically?
> > 
> > Thanks, Shaheed
> > 
> >> Aleix
> 
> I don't understand what you're trying to do.
> You are not supposed to change the definitions Qt5::Core suggests to
> the compiler.

As far as i understand he wants to know what Qt5::Core actually expands to so 
he can pass it to some other tool.

Cheers,
  Albert

> 
> Aleix