[CMake] Touching files on cache var changes

[Michael Hertling]

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