[sldev] Re: CMake 2.4.8 vs 2.6 (Re: Missing cares, apr in Mac build)

[Aimee Walton]

On Jul 18, 2008, at 18:51, Rob Lanphier wrote:
>
> Does anyone know if there a fundamental incompatibility between  
> CMake 2.4.8 and 2.6, or is CMake 2.6 support something that can be  
> made to work without too much hassle?  This seems like a FAQ in the  
> making.

This is the problem ...



aimee$ ./cmake --help-policy CMP0003

cmake version 2.6-patch 0
------------------------------------------------------------------------ 
------


   CMP0003
        Libraries linked via full path no longer produce linker  
search paths.

        This policy affects how libraries whose full paths are NOT  
known are
        found at link time, but was created due to a change in how  
CMake deals
        with libraries whose full paths are known.  Consider the code

          target_link_libraries(myexe /path/to/libA.so)

        CMake 2.4 and below implemented linking to libraries whose  
full paths
        are known by splitting them on the link line into separate  
components
        consisting of the linker search path and the library name.  The
        example code might have produced something like

          ... -L/path/to -lA ...

        in order to link to library A.  An analysis was performed to  
order
        multiple link directories such that the linker would find  
library A in
        the desired location, but there are cases in which this does  
not work.
        CMake versions 2.6 and above use the more reliable approach  
of passing
        the full path to libraries directly to the linker in most  
cases.  The
        example code now produces something like

          ... /path/to/libA.so ....

        Unfortunately this change can break code like

          target_link_libraries(myexe /path/to/libA.so B)

        where "B" is meant to find "/path/to/libB.so".  This code is  
wrong
        because the user is asking the linker to find library B but  
has not
        provided a linker search path (which may be added with the
        link_directories command).  However, with the old linking
        implementation the code would work accidentally because the  
linker
        search path added for library A allowed library B to be found.

        In order to support projects depending on linker search paths  
added by
        linking to libraries with known full paths, the OLD behavior  
for this
        policy will add the linker search paths even though they are not
        needed for their own libraries.  When this policy is set to  
OLD, CMake
        will produce a link line such as

          ... -L/path/to /path/to/libA.so -lB ...

        which will allow library B to be found as it was previously.   
When
        this policy is set to NEW, CMake will produce a link line  
such as

          ... /path/to/libA.so -lB ...

        which more accurately matches what the project specified.

        The setting for this policy used when generating the link  
line is that
        in effect when the target is created by an add_executable or
        add_library command.  For the example described above, the code

          cmake_policy(SET CMP0003 OLD) # or cmake_policy(VERSION 2.4)
          add_executable(myexe myexe.c)
          target_link_libraries(myexe /path/to/libA.so B)

        will work and suppress the warning for this policy.  It may  
also be
        updated to work with the corrected linking approach:

          cmake_policy(SET CMP0003 NEW) # or cmake_policy(VERSION 2.6)
          link_directories(/path/to) # needed to find library B
          add_executable(myexe myexe.c)
          target_link_libraries(myexe /path/to/libA.so B)

        Even better, library B may be specified with a full path:

          add_executable(myexe myexe.c)
          target_link_libraries(myexe /path/to/libA.so /path/to/libB.so)

        When all items on the link line have known paths CMake does  
not check
        this policy so it has no effect.

        Note that the warning for this policy will be issued for at  
most one
        target.  This avoids flooding users with messages for every  
target
        when setting the policy once will probably fix all targets.

        This policy was introduced in CMake version 2.6.0.  CMake  
version 2.6
        warns when the policy is not set and uses OLD behavior.  Use the
        cmake_policy command to set it to OLD or NEW explicitly.

telstar-airport:/Applications/Development/CMake 2.6-0.app/Contents/ 
bin aimee$