Upper level seeing a lower level option - cmake

Question

I have a lower level project that builds several libraries. The upper level takes the libs and links them into many executables. The lower level has this option:
option(TOOLA_SUPPORT "Ena/Disable toolA support" OFF)
I'd like to get the upper level build to see the TOOLA_SUPPORT flag from the lower level build. As it is now, there are 2 TOOLA_SUPPORT flags. One at the lower, the other at the upper level builds. This is a hassle as they both need to be in the same state for a successful build.
I'm not sure how to get one CMakeLists.txt file to inherit a flag from another (cache). (Only want this one flag, not the whole lower CMakeLists.txt/cache settings.) What commands might help, or where to look for info?
Edit_1:
dirUpper/src/CMakeLists.txt (Does NOT contain dirLower) I'd like this cmakelists to "inherit" the cache var TOOLA_SUPPORT from dirLower/src/CMakeLists.txt
option(TOOLA_SUPPORT "" OFF)
add_subdirectory(${dirLower}/src ${dirLower}/lib EXCLUDE_FROM_ALL)
if(TOOLA_SUPPORT)
link_directories(blah bla bla)
else()
link_directories(blah bla)
endif()
dirLower/src/CMakeLists.txt (Is not part of the dirUpper tree) I'd like to pass TOOLA_SUPPORT from this cmakelists, to dirUpper/src/cmakelists.txt
IF(NOT (${COMPILER_FLAGS} MATCHES "-Wall")) #this only applies to dirLower
MESSAGE(FATAL_ERROR "compiler flag not properly set") #will crash the dirUpper 'config'
ENDIF()
OPTION(TOOLA_SUPPORT "" OFF)
IF(TOOLA_SUPPORT)
INCLUDE_DIRECTORIES(some/dir/include)
SET(TOOLA_SUPPORT ON CACHE "" FORCE PARENT_SCOPE)
ELSE()
MESSAGE("TOOLA is not supported")
SET(TOOLA_SUPPORT OFF CACHE "" FORCE PARENT_SCOPE)
ENDIF()
When I run the dirLower file, I get a cmake warning telling me there is no parent for it. (Not part of the dirUpper tree)
When I run the dirUpper file, I get a fatal error due to the COMPILER_FLAG test in the dirLower file. When I bypass this issue, Im not seeing the TOOLA_SUPPORT setting being inherited from the dirLower.

Answer 1

Assuming the lower level is "included" in the upper level with ADD_SUBDIRECTORY(), cache variables should already be shared between upper and lower. You can use a command-line option to set the value, or use SET( ... CACHE ... FORCE) to be sure the value gets set as you actually intend. Then just reference it from the other level.
If instead you want regular variables, it appears that regular variables set before the ADD_DIRECTORY() are passed down to the lower level. And to pass a regular variable UP a level, use SET(... PARENT_SCOPE).
EDIT:
Writing out in code my top suggest:
In file ~/test/CMakeLists.txt:
OPTION(TOOLA_SUPPORT "" OFF)
ADD_SUBDIRECTORY(test2)
MESSAGE(STATUS "${TOOLA_SUPPORT}")
In file ~/test/test2/CMakeLists.txt:
SET(TOOLA_SUPPORT ON CACHE BOOL "" FORCE)
The output is "ON".
I think the trouble you're running into is that once a cache value is set, it doesn't get changed, so the top-level value holds. That's where the FORCE comes in.
If you're still having difficulty, please post a minimum code sample that demonstrates what's not working.