[CMake] Making a variable a dependency...

[Oliver kfsone Smith]

Michael Hertling said the following on 2/6/2012 6:39 PM:
> On 02/06/2012 10:56 PM, Alexander Neundorf wrote:
>> On Saturday 04 February 2012, Oliver Smith wrote:
>>> My CMakeLists uses the Subversion repository information in a couple of
>>> places (it configures a file revision.h and it uses it for the CPack
>>> package name).
>>>
>>> The problem is that this variable is cached and retained until the cache
>>> is rebuilt, instead of being calculated or evaluated per make. So if I
>>> do a build, then do an svn update and pull some changes, it will build a
>>> new executable but it will stamp it with the revision number from when
>>> CMake last regenerated the make files...
>>>
>>> Is there a way to mark a variable as volatile or something so that CMake
>>> will always recalculate it and check if it has changed?
>> Would it be acceptable if cmake would rerun after every build ?
>> You could enforce that e.g. with a add_custom_command( POST_BUILD ... ) which
>> could e.g. touch CMakeCache.txt or something.
>>
>> Better ideas ?
> Delay the generation of the revision.h header until build phase
> via a custom command; look at the following exemplary project:
>
> # CMakeLists.txt:
> CMAKE_MINIMUM_REQUIRED(VERSION 2.8 FATAL_ERROR)
> PROJECT(P C)
> SET(CMAKE_VERBOSE_MAKEFILE ON)
> ADD_CUSTOM_COMMAND(OUTPUT dummy revision.h
>      COMMAND ${CMAKE_COMMAND}
>          -DBD=${CMAKE_BINARY_DIR}
>          -DWC=${CMAKE_SOURCE_DIR}
>          -P ${CMAKE_SOURCE_DIR}/revision.cmake)
> FILE(WRITE ${CMAKE_BINARY_DIR}/main.c
> "#include<stdio.h>
> #include \"revision.h\"
> int main(void)
> {
>      printf(\"%d\\n\",REVISION); return 0;
> }
> ")
> ADD_EXECUTABLE(main main.c revision.h)
>
> # revision.cmake:
> FIND_PACKAGE(Subversion)
> Subversion_WC_INFO(${WC}@HEAD P)
> FILE(WRITE ${BD}/revision.h.in "#define REVISION @P_WC_REVISION@\n")
> CONFIGURE_FILE(${BD}/revision.h.in ${BD}/revision.h @ONLY)
>
> A "make" run rebuilds the main target whenever the repository's head
> revision has changed - possibly inappropriate for actual usage, just
> for demonstration purposes; adapt the revision.cmake script to suit
> the needs.
>
> BTW, can anybody confirm that the above-noted example doesn't work
> if the order of "dummy" and "revision.h" in the custom command is
> reversed? AFAICS, revision.h is removed after being generated in
> this case, so the subsequent compilation fails.
>
Unless I'm missing something, that won't work because P_WC_REVISION gets 
cached in the CMakeCache.txt -- or does using -P cause it to skip the 
caching?

- Oliver