[CMake] Cross-compilation vs FindQt4.cmake

[Alexander Neundorf]

On Monday 31 May 2010, Andrey. wrote:
> >> I would like to know whether there is some progress on making
> >> FindQt4.cmake friends with cross-compilation.
> >
> > Not much, unfortunately.
>
> Not a piece of good news.
>
> >> Writing to you, as you seems to be a maintainer of FindQt4.cmake in both
> >> Cmake and KDE repositories.
> >
> > Well, Clinton Stimpson is the maintainer of FindQt4.cmake in CMake, and I
> > mostly merge the fixes there to the version in KDE.
> > So actually he's the maintainer, but I'm also not the completely wrong
> > person here :-)
>
> Sorry, I misunderstood the commit history a little.
>
> >> Also, I'm interested to know, why FindQt4.cmake doesn't utilize
> >> pkg-config for most introspection of Qt. Qt4 provides nice .pc files.
> >> pkg-config has some built-in support for cross-compilation through
> >> PKG_CONFIG_SYSROOT_DIR and PKG_CONFIG_LIBDIR while qmake doesn't. Simply
> >> compiling qmake to be used during cross-compilation and compiling qmake
> >> to return correct include/library paths are two different tasks. In
> >> first case, qmake can be used for multiple cross-compilation
> >> environments.
> >
> > In general, CMake tries not to rely on any additional external tools, and
> > pkg-config is one of those tools.
>
> Instead of blindly relying on pkg-config, it may be used as a hint in some
> complex situations like cross-compiling. And anyway, in case of Qt, CMake
> relies on an external qmake, which is not itself required to be present at
> compile time (in contrast to moc/uic).
>
> > I have (until now) never cross compiled anything with Qt.
> > How does a typical Qt installation for cross compiling look like ?
> > I guess it has native tools (qmake, moc, uic), etc.
>
> I think, the installation heavily differs depending on cross-compilation
> environment used (buildroot, scratchbox, etc). 

If I remember correctly, scratchbox means running a virtual machine, so from 
this POV building inside scratchbox is not cross compiling, and in that 
regard it shouldn't need anything special from cmake.

Buildroot doesn't do that, right ?

> Currently, I'm using some
> hand-written environment, mostly resembling buildroot. It consists of:
> 1) host system with general native libraries and utilities;
> 2) cross-compiled libraries and executables for target system placed at
> some prefix; 3) special tools, required for cross-compilation and related
> stuff (cross-toolchain, qmake, image/firmware generators go here).
>
> Tools at item 3) usually must match specific requirements on version used.
> In case of Qt, I have statically compiled qmake, moc, uic, lrelease, etc
> against exactly the same version of Qt used for target system.

So this seems to be the actual crosscompiling case.
If you want to use a specific version of Qt, you have to make sure cmake finds 
the qmake for the Qt you want to use.
This can be done by
- setting CMAKE_PREFIX_PATH so that it points to the base directory where that 
qmake is installed
- setting QT_QMAKE_EXECUTABLE directly when running cmake:
cmake -DQT_QMAKE_EXECUTABLE=/your/qmake

How far do you get with this ?


> > When FindQt4.cmake is executed, it queries qmake for information.
> > Does this also work for a cross-compilation installation of Qt ?
> > What are the results of a FindQt4.cmake with a cross-compiling qmake ?
>
> If no special care is taken, FindQt4 finds qmake from host system that
> returns information about host library ignoring CMAKE_FIND_ROOT_PATH (and
> that's wrong). If PATH variable is modified appropriately, FindQt4 finds
> specially compiled qmake with good version. It's possible to compile qmake
> to return information, suitable for cross-compilation. But it's then
> required to recompile qmake if a change prefix somewhere.
>
> >> Currently, I'm going to use simple FindQt4.cmake with some hardcoded
> >> paths, that will override system wide one.
>
> It's worth mentioning, that I managed to successfully compile a qt4-based
> application with such a hacked FindQt4.cmake with bunch of set() statements
> and a couple of include() directives.

Yes, what makes FindQt4.cmake big is all the flexibility to find everything 
for the different versions on the different OSs. It started small...

Alex