What is CMakeCache.txt and why it overrides my variables

Question

As I am using cmake after editing CMakeLists.txt some variables wouldn't be loaded. If I had something defined with CACHE STRING it wouldn't let me to change it without forcing it or deleting cache.

So then why we have this CMakeCache.txt file. Is it even needed?

Answer 1

Yes, it's certainly needed. CMake uses the cache when it's re-running itself during a build because a CMakeList file changed, or when you make rebuild_cache. It also loads the cache at start of a normal configure run.

The standard workflow for using CMake is as follows:

1. Run CMake in an empty binary directory to generate the initial version of the project & cache
2. In CMake GUI or ccmake or similar, inspect the cache variables set up by the initial run and modify them as you see fit.
3. Re-run CMake (or just its Configure step if your UI offers that).
4. Repeat steps 2&3 until you're satisfied with the configuration
5. If you were only running Configure in 3, run Generate

You now have a buildsystem configured according to your taste.

For the above to work, user changes in the cache must take precedence over default cache values specified in CMakeLists.txt. Otherwise, the user changes from point 2 would be lost at next configure, overwritten by the project-specified defaults again.

That's why CMake commands set(var ... CACHE) do not modify the cache variable var if it already exists. Normally, your project should treat setting up the cache as providing user-tweakable defaults.

If you really need to override user choices in your project, you can:

• add FORCE to the set command, or
• use set without CACHE to set non-cache variables. Non-cache variables take precedence over cache variables of the same name.