[CMake] Relink on library rebuild dilema

[Michael Hertling]

On 11/29/2010 05:05 AM, Bill Hoffman wrote:
> On 11/28/2010 3:19 AM, Sebastian Schaetz wrote:
>> Michael Hertling<mhertling at ...>  writes:
>>
>>> 1) In the top-level CMakeLists.txt, you might say
>>>
>>> SET_SOURCE_FILES_PROPERTIES(
>>>      main.cpp PROPERTIES OBJECT_DEPENDS
>>>      ${CMAKE_CURRENT_BINARY_DIR}/kernels/libkernel_executable.a
>>> )
>>>
>>> When libkernel_executable.a has changed, this results in recompiling
>>> main.cpp - a penalty - and the desired relinking of main_target. If
>>> main.cpp's recompilation is expensive you may possibly add an empty
>>> cpp file to main_target's sources and impose the property on that.
>>>
>>
>> Wow, those suggestions all sound great and I'm completely unfamiliar with them
>> (so I think there's a good chance they will work). I will try them as soon as
>> possible! Thanks a lot for the suggestions.
>>
> 
> No need for all the complication...
> 
> If you already know where the library is going to be:
> 
>   ${CMAKE_CURRENT_BINARY_DIR}/kernels/libkernel_executable.a
> 
> Then link directly to the full path:
> 
> target_link_libraries(main_target
>   ${CMAKE_CURRENT_BINARY_DIR}/kernels/libkernel_executable.a)
> 
> Now the makefiles will depend on that file and it will relink when that 
> .a changes.   This is why link_directories/*link_libraries without a 
> full path to the library is almost always a bad idea...

Oops, the simplest solution at all. ~8) Thanks for pointing it out, Bill.

Perhaps, this is a good opportunity to ask a related question: Some
time ago, there has been an issue with special linker scripts which
are part of a project. Of course, one would like to see the linker
triggered when these scripts change, and the best solution I have
found so far is - just like suggested to the OP -

- a dummy source file to impose the OBJECT_DEPENDS property on, or
- a dummy static library that is invalidated when the scripts change
  and (mis)used to force a dependency of the link line on the scripts.

Both alternatives are not really satisfying, so I wonder if there is any
smarter and more direct possibility to establish a file-level dependency
of the link line on an arbitrary file that's not mentioned as a library
in TARGET_LINK_LIBRARIES(). For Makefiles, this involves just a single
additional line.

Regards,

Michael