[cmake-developers] [PATCH 3/3] FindQt4: document cross compilation

[Pascal Bach]

Hi Clint

Am 29.07.2015 um 15:45 schrieb Clinton Stimpson:
> Hi Pascal,
>
> Thanks for the patches.
>
> Can you share with us why setting CMAKE_FIND_ROOT_PATH does not work, or how 
> this new method compares?
>
> For example, in the toolchain file:
> SET(CMAKE_FIND_ROOT_PATH  /path/to/Qt ...)

The problem is how FindQt4 does find the locations using qmake.

let's assume there are two sysroots one is /sysroots/x86_64 which contains binaries usable on the host machine, and there is /sysroots/arm which contains libraries for the target system.
this are both set via:
set( CMAKE_FIND_ROOT_PATH /sysroots/arm /sysroots/x86_64    )
set( CMAKE_FIND_ROOT_PATH_MODE_PACKAGE ONLY )
set( CMAKE_FIND_ROOT_PATH_MODE_PROGRAM ONLY )
set( CMAKE_FIND_ROOT_PATH_MODE_LIBRARY ONLY )
set( CMAKE_FIND_ROOT_PATH_MODE_INCLUDE ONLY )

Here is what happens when only setting CMAKE_FIND_ROOT_PATH:

1. FindQt4 is trying to find the qmake executable. As there is no qmake in /sysroots/arm (it would not be runnable on the host) it will find qmake from the /sysroots/x86_64 directory.
2. FindQt4 is asking qmake for the path to the Qt installation. As the qmake found is the one from /sysroots/x86_64 it will point to the libraries from /sysroots/x86_64 which will obviously not run on arm!

Currently I didn't find a way to override this behaviour of FindQt4, except by setting all the variables usually set by FindQt4 manually.
It is usually accepted to set some variables manually when doing cross compilation. With this patchset I tried to reduce this variable to a minumum and let FindQt4 figure out the rest.

The minimum variables required with the patch are:

QT_BINARY_DIR is necessary to find the correct qmake
QT_LIBRARY_DIR is necessary to find the library directory
QT_INCLUDE_DIR is necessary to find the include directory (might be possible to figure this out from QT_INCLUDE_DIR if some assumptions are made)
QT_MKSPECS_DIR necessary to have the correct prefixes (E in the case of qt4-embedded).


I hope my explanation is clear. If there is a better way to achive this I'm all ears.

Pascal

>
> Clint
>
> On Wednesday, July 29, 2015 02:32:48 PM Pascal Bach wrote:
>> ---
>>  Modules/FindQt4.cmake | 20 ++++++++++++++++++++
>>  1 file changed, 20 insertions(+)
>>
>> diff --git a/Modules/FindQt4.cmake b/Modules/FindQt4.cmake
>> index 9d03378..64c06e1 100644
>> --- a/Modules/FindQt4.cmake
>> +++ b/Modules/FindQt4.cmake
>> @@ -29,6 +29,26 @@
>>  #  for a particular executable, set the ``QT4_NO_LINK_QTMAIN`` target
>>  #  property to ``TRUE`` on the executable.
>>  #
>> +# Cross Compile
>> +# ^^^^^^^^^^^^^
>> +#
>> +# To find Qt in a cross compile environment set the following variables in
>> your toolchain file: +#
>> +#
>> +# ``QT_BINARY_DIR``  => Path to native Qt binary directory
>> +# ``QT_LIBRARY_DIR`` => Path to target Qt library directory
>> +# ``QT_INCLUDE_DIR`` => Path to target Qt include directory
>> +# ``QT_MKSPECS_DIR`` => Path to target Qt mkspecs directory
>> +
>> +# example
>> +#
>> +# ::
>> +#   set( QT_BINARY_DIR   /sysroots/x86_64-linux/usr/bin )
>> +#   set( QT_LIBRARY_DIR  /sysroots/arm/usr/lib )
>> +#   set( QT_INCLUDE_DIR  /sysroots/arm/usr/include/qtopia )
>> +#   set( QT_MKSPECS_DIR  /sysroots/arm/usr/share/qtopia/mkspecs )
>> +#
>> +#
>>  # Qt Build Tools
>>  # ^^^^^^^^^^^^^^
>>  #