[CMake] handling global state

[Brad King]

Brandon J. Van Every wrote:
> I need some stylistic advice on handling global state.  I'm starting to 
> break my monolithic CMakeLists.txt up into subdirectories.  I'm building 
> the Chicken compiler and I need to do staged builds.  A lot of 
> essentially the same operations, done on itself multiple times so that 
> it's compiled up to its own most current version.
> 
> I realize I can use the cache to pass global state.

Before CMake 2.4 the only way to get information from subdirectories was 
to set it as INTERNAL cache variables and then configure again.  Now you 
can get variable settings from subdirectories using GET_DIRECTORY_PROPERTY.

However it looks like you want to pass information from the top-level 
down, which is trivial.

 > However, there's
> often global state that I don't want the user to modify.  Nor do I 
> really want to clutter the output of CMakeSetup with such extraneous 
> state.  I could mark those variables as advanced, which reduces the 
> immediately visible clutter, but I don't really want clutter in the 
> advanced variables either.  Also they can still modify them, which is 
> not what I want.  For instance I've got certain constants like the 
> minimum Chicken build number, and nothing good can come from someone 
> changing them.  I've also got a lot of -D flags from probing the system, 
> that go into ADD_DEFINITIONS.  I don't want the user to mess with 'em, 
> or run the probes multiple times.  I just want to determine them at the 
> toplevel and then pass them down to subdirectories.
> 
> I could include constants via an INCLUDE(global.cmake) file, but I'd 
> rather have them in the toplevel CMakeLists.txt.  Right at the beginning 
> of it, where a maintainer can easily see what's going on and nothing is 
> obfuscated.
> 
> So how do you pass read-only global state to subdirectory consumers?

There is no such thing as a "read only" variable setting except by 
convention.  If you set a variable in the top level then all 
subdirectories will get that value.  However, if one subdirectory 
changes it the other children will not see the altered value.

-Brad