[Cmake] Setting Variables in CMAKE

[Brad King]

Linton, Tom wrote:
> I have a source tree with a top level CMakeLists.txt file that contains
> something like:
> 
>     INCLUDE(${CMAKE_ROOT}/Modules/CMakeDetermineSystem.cmake)
>     SET(MYFLAG "defaultValue")
>     IF(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
>        SET(MYFLAG "linuxValue")
>     ENDIF(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
>     PROJECT(TRY)
>     SUBDIRS(a b)
> 
> The behavior of CMAKE is that (1) in the top-level CMakeLists.txt the variable
> "MYFLAG" gets set to "linuxValue" wherease (2) in the subdirectories
> "MYFLAG" is left at "defaultValue". 
> 
> I tried changing the SET command like: 
>     SET(MYFLAG "defaultValue" CACHE INTERNAL "myflag")
> but this had no effect.
> 
> I want to set variables at top-level based on the system name and have these
> variables propagate to the subdirectories and it seems like what I'm doing is
> correct, according to section 3.4 in the CMake book, which say that variable
> scope is limited to a CMakeList file and any subdirectory CMakeList files.
> 
> This seems like a really basic thing to want to do. Any suggestions on
> what's going wrong?

The CMakeDetermineSystem.cmake module makes use of the EXEC_PROGRAM 
command, which is not inherited by subdirectories.  Therefore it will 
never set CMAKE_SYSTEM_NAME correctly when CMake is configuring a 
subdirectory.  However, module is included automatically by the PROJECT 
command, and CMAKE_SYSTEM_NAME is automatically set.  Change your code 
to look like this:

PROJECT(TRY)
SET(MYFLAG "defaultValue")
IF(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
   SET(MYFLAG "linuxValue")
ENDIF(${CMAKE_SYSTEM_NAME} MATCHES "Linux")
SUBDIRS(a b)

The top level CMakeLists.txt file should always have the PROJECT command 
first except in a few special cases.

-Brad