Difference between revisions of "Policies/CMake and Source Compatibility"

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(initial version -- pasted from Alex's original draft email)
 
(What does that mean?)
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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):
 
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):
  
 +
<code>
 
find_package(FooBar)
 
find_package(FooBar)
  
Line 26: Line 27:
  
 
target_link_libraries(kapp ${MyLinkLibs})
 
target_link_libraries(kapp ${MyLinkLibs})
 +
</code>
  
  
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with ANY CMake module, e.g. KDE4, Qt4, GIF, Freetype, etc.
 
with ANY CMake module, e.g. KDE4, Qt4, GIF, Freetype, etc.
  
In order to stay source compatible, the following has to be kept:
+
==In order to stay source compatible, the following has to be kept ==
  
1) The CMake module FindFooBar.cmake must stay available
+
===The CMake module FindFooBar.cmake must stay available===
  
 
Otherwise CMake will fail with "could not find FindFooBar.cmake".
 
Otherwise CMake will fail with "could not find FindFooBar.cmake".
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2) The file name of the CMake module must stay exactly the same.
+
===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  
 
E.g. in KDE 4.0.0 we have a FindFreetype.cmake installed by kdelibs. This filename must stay  
Line 48: Line 50:
  
  
3) Macros coming with the module must stay compatible
+
===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
+
So if FindFooBar.cmake in kdelibs 4.0.0 has a macro <tt>FOOBAR_DO_STUFF()</tt>, this macro
 
must be available in all KDE4 versions, with exactly the same name and the arguments  
 
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.
 
supported in KDE 4.0.0 must be supported in all KDE4 versions.
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4) Variables coming with the module must stay compatible
+
===Variables coming with the module must stay compatible===
  
 
In the example above several variables from FindFooBar.cmake are used:
 
In the example above several variables from FindFooBar.cmake are used:
FOOBAR_DEFINITIONS, FOOBAR_INCLUDES, FOOBAR_LIBRARIES and FOOBAR_FOUND.  
+
<tt>FOOBAR_DEFINITIONS, FOOBAR_INCLUDES, FOOBAR_LIBRARIES</tt> and <tt>FOOBAR_FOUND</tt>.  
 
At least all "public" variables (i.e. those documented in the header of the cmake
 
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.
 
module) must be available from this module in all releases of KDE4.
 
This means
 
This means
-their case must not change
+
* their case must not change
-they must have equivalent values
+
* they must have equivalent values
  
This means that e.g. FOOBAR_LIBRARIES must always contain all libraries required to link to
+
This means that e.g. <tt>FOOBAR_LIBRARIES</tt> must always contain all libraries required to link to in order to use the package FooBar.  
in order to use the package FooBar.  
+
 
Maybe this would be in KDE 4.0.0 something like "-lfoobar -lz".
 
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"
 
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.
 
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:
 
E.g. the following change would be wrong:
 +
<pre>
 
FOOBAR_LIBRARIES="/usr/lib/libfoobar.so"
 
FOOBAR_LIBRARIES="/usr/lib/libfoobar.so"
 
FOOBAR_EXTRA_LIBRARIES="/usr/lib/libz.so"
 
FOOBAR_EXTRA_LIBRARIES="/usr/lib/libz.so"
 
+
</pre>
  
 
== What changes are possible? ==
 
== What changes are possible? ==

Revision as of 00:06, 8 January 2008

Contents

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 that 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.

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

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).


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.


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.


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.


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