[CMake] Touching files on cache var changes

[Michael Hertling]

On 05/17/2011 07:46 PM, David Cole wrote:
> On Tue, May 17, 2011 at 1:30 PM, Michael Hertling <mhertling at online.de>wrote:
> 
>> On 05/17/2011 05:45 PM, Robert Bielik wrote:
>>> Hi all,
>>>
>>> I'm wondering if there's a way to touch files on cached var changes.
>> Let's say I have an option to enable or disable a feature
>>> in my application, and depending on its setting a preprocessor macro is
>> defined (or not defined) in the CMakeLists.txt file:
>>>
>>> OPTION(FEATURE_X "Check to enable feature X" OFF)
>>>
>>> IF(FEATURE_X)
>>> ADD_DEFINITIONS(-DUSE_FEATURE_X=1)
>>> ELSE(FEATURE_X)
>>> ADD_DEFINITIONS(-DUSE_FEATURE_X=0)
>>> ENDIF(FEATURE_X)
>>>
>>> and in "features.c":
>>>
>>> ...
>>> #if USE_FEATURE_X
>>> // Code for feature X
>>>
>>> #endif
>>> ...
>>>
>>> The source code file "features.c" that have code depending on the
>> preprocessor macro needs to be recompiled each time the option changes,
>> which does not automatically
>>> happen if the macro is defined like above (and I _really_ need to have it
>> defined, either to zero or one...).
>>>
>>> So I pretty much need to hook a dependency of "features.c" to change of
>> cached cmake options/vars...
>>>
>>> Ideas?
>>> TIA
>>> /Rob
>>
>> AFAICS, with 2.8.4, the concerned files *are* recompiled:
>>
>> # CMakeLists.txt:
>> CMAKE_MINIMUM_REQUIRED(VERSION 2.8.4 FATAL_ERROR)
>> PROJECT(RECOMPILE C)
>> SET(CMAKE_VERBOSE_MAKEFILE ON)
>> OPTION(FEATURE_X "Check to enable feature X" OFF)
>> IF(FEATURE_X)
>> ADD_DEFINITIONS(-DUSE_FEATURE_X=1)
>> ELSE(FEATURE_X)
>> ADD_DEFINITIONS(-DUSE_FEATURE_X=0)
>> ENDIF(FEATURE_X)
>> ADD_EXECUTABLE(main main.c)
>>
>> /* main.c: */
>> int main(void){return 0;}
>>
>> After reconfiguring the project with an altered FEATURE_X value,
>> you will see main.c being recompiled. This is due to the line
>>
>> CMakeFiles/main.dir/main.c.o: CMakeFiles/main.dir/flags.make
>>
>> in ${CMAKE_BINARY_DIR}/CMakeFiles/main.dir/build.make with the
>> ${CMAKE_BINARY_DIR}/CMakeFiles/main.dir/flags.make containing:
>>
>> C_DEFINES = -DUSE_FEATURE_X=...
>>
>> It's this latter file which is modified when the preprocessor
>> definitions change, and the object file's dependency provides
>> for the recompilation. Can you confirm that?
>>
>> Regards,
>>
>> Michael
> 
> That works with makefile generators, but we trust Visual Studio and Xcode to
> do their own dependency analysis, and with those generators, I don't think
> that the main.c will be recompiled after changing the FEATURE_X value in
> CMake...

Ahh, thanks for this hint! Indeed, with VS, nothing happens. :-(

Regards,

Michael

> However, if you use a configured header file, then it will.
> 
> HTH,
> David