Policies/CMake and Source Compatibility

Introduction

All KDE 4 releases need to keep binary and source compatibility with KDE 4.0.0. This doesn't affect only the C++ code, but also the CMake modules installed with kdelibs (also the cmake modules of the other KDE modules ?).

What does CMake source compatibility mean?

A program which was developed against KDE 4.0.0, must also build against let's say KDE 4.5.9. The CMakeLists.txt may look like that (ignoring KDE specific stuff):

find_package(FooBar)

set(MySources main.cpp)
set(MyLinkLibs ...)

if(FOOBAR_FOUND)
   add_definitions(-D_USE_FOOBAR ${FOOBAR_DEFINITIONS} )
   set(MySources ${MySources} foobarwidget.cpp)
   include_directories(${FOOBAR_INCLUDES})
   set(MyLinkLibs ${MyLinkLibs} ${FOOBAR_LIBRARIES})
   
   foobar_do_stuff()
endif(FOOBAR_FOUND)

...
add_executable(kapp ${MySources})
...

target_link_libraries(kapp ${MyLinkLibs})

So here we have a CMakeLists.txt which uses the library/package "FooBar", you can exchange FooBar with ANY CMake module, e.g. KDE4, Qt4, GIF, Freetype, etc.

Source compatibility rules

In order to stay source compatible, the following has to be kept:

• 1. The CMake module FindFooBar.cmake must stay available

Otherwise CMake will fail with "could not find FindFooBar.cmake". This means either it has to stay in the KDE module, or in the case that KDE would require a newer version of CMake which comes with a (for KDE) usable version of that module, that the minimum required version of CMake must ship this module (which must be fully compatible).

• 2. The file name of the CMake module must stay exactly the same

E.g. in KDE 4.0.0 we have a FindFreetype.cmake installed by kdelibs. This filename must stay exactly the same, also the case must not change (e.g. FindFreeType.cmake). Otherwise the same as in 1) will happen.

• 3. Macros coming with the module must stay compatible

So if FindFooBar.cmake in kdelibs 4.0.0 has a macro FOOBAR_DO_STUFF(), this macro must be available in all KDE4 versions, with exactly the same name and the arguments supported in KDE 4.0.0 must be supported in all KDE4 versions. It is possible to add new or alternative arguments to a macro, but the old behaviour must stay supported.

• 4. Variables coming with the module must stay compatible

In the example above several variables from FindFooBar.cmake are used: FOOBAR_DEFINITIONS, FOOBAR_INCLUDES, FOOBAR_LIBRARIES and FOOBAR_FOUND. At least all "public" variables (i.e. those documented in the header of the cmake module) must be available from this module in all releases of KDE4. This means

• their case must not change
• they must have equivalent values

This means that e.g. FOOBAR_LIBRARIES must always contain all libraries required to link to in order to use the package FooBar. Maybe this would be in KDE 4.0.0 something like "-lfoobar -lz". This could be changed later on to the recommended format: "/usr/lib/libfoobar.so;/usr/lib/libz.so" It must not be changed in a way so that then additional variables would be required for successful linking. E.g. the following change would be wrong:

FOOBAR_LIBRARIES="/usr/lib/libfoobar.so"
FOOBAR_EXTRA_LIBRARIES="/usr/lib/libz.so"

What changes are possible?

• 1. New modules can be added

• 2. New variables can be added to existing modules

It has to be made sure that the existing ones stay working, s.a.

• 3. New macros can be added

It has to be made sure that the existing ones stay working, s.a. If a new and better alternative to an existing macro is added, the old one may be marked as deprecated like this:

macro(OLD_SIMPLE_MACRO args...)
   message(STATUS "OLD_SIMPLE_MACRO() is deprecated, 
                   use NEW_FANCY_MACRO() instead")
   # ...keep the macro implementation here, 
   # so the macro keeps working
endmacro(OLD_SIMPLE_MACRO)

macro(NEW_FANCY_MACRO args...)
   # ... new fancy implementation
endmacro(NEW_FANCY_MACRO)

• 4. More arguments can be added to existing macros

It has to be made sure that the macro stays working as it did before with the old set of arguments.

If nothing else helps...

If you are developing an application with the KDE 4, but don't trust the KDE developer team that they will be able to keep the one CMake module you are interested in compatible, there is still an option left for you.

We take the same example as above, i.e. your application uses package FooBar. If you want to make sure that one specific version of FindFooBar.cmake is used, just add this module to your application, put it e.g. in myapp/cmake/FindFooBar.cmake. To make CMake look in this directory if it searches the file FindFooBar.cmake, you have to adjust the CMAKE_MODULE_PATH CMake variable, so that the top of your top level CMakeLists.txt will look something like this:

set(CMAKE_MODULE_PATH ${CMAKE_CURRENT_SOURCE_DIR}/cmake 
                      ${CMAKE_MODULE_PATH})

find_package(KDE4)

find_package(FooBar)

...