moving CMake options from the command line into the top-level CMakeLists.txt

Question

I'm building DCMTK using the command line, using the following command:

cmake -DBUILD_APPS=BOOL:OFF ../latest_dcmtk

I want to move the BUILD_APPS setting from the command line into the top-level CMakeLists.txt configuration file. No matter how I attempt set the BUILD_APPS option in the top level configuration file, I never get the correct behavior unless I explicitly set the BUILD_APPS option from the command line.

Is there a way to explicitly set command line options from the top-level CMakeLists.txt file?

I've borrowed this from the DCMTK sources & it doesn't work:

SET (BUILD_APPS OFF CACHE BOOL "" FORCE)                                                             
MESSAGE ("-- BUILD_APPS is set to " ${BUILD_APPS})

When I look at the CMakeCache.txt file that's generated by the SET command above, I see the option set correctly:

./CMakeCache.txt:BUILD_APPS:BOOL=OFF

Unfortunately, the option doesn't work.

Because DCMTK is a decent sized open-source project, I won't be able to strip the code down to a minimal example. At best, I could provide the top-level CMake configuration file. Even if I gave you that file, you'd need a Mac to build the equivalent project. — russes, Oct 29 '15 at 18:19
the example just has to consist of a stripped down CMakeLists.txt file with that option which is not working — m.s., Oct 29 '15 at 18:42
Option set with command `option(BUILD_APPS "" OFF)` or `set(BUILD_APPS OFF CACHE BOOL "")` (FORCE is not needed) is equivalent for option set with `cmake` command line option `-DBUILD_APPS=BOOL:OFF`. If the second way works, but the first one doesn't, then `CMakeLists.txt` does something very special. — Tsyvarev, Oct 29 '15 at 18:43
Just to be sure, I have given it a try again and the `-D` does force the Cache entry. So there should be no difference between using `-D` command line option and `set(... FORCE)`. I agree something is weird and suggest adding `variable_watch(BUILD_APPS)` at the top of the main `CMakeLists.txt` to find the codepart that is hiding/overwriting the variable. For more details see e.g. [here](http://stackoverflow.com/questions/31037882/whats-the-cmake-syntax-to-set-and-use-variables). — Florian, Oct 29 '15 at 21:25
Where do you set the variable? At the very beginning or towards the end? — usr1234567, Oct 29 '15 at 21:48

score 0 · Answer 1 · edited May 23 '17 at 12:14

Generally speaking, there are two things you have to keep in mind when using CMake:

CMake is a script language, so all code is processed sequential
The variable cache of CMake is there to persist values between two runs

So I agree with @Tsyvarev and @usr1234567 that this is most likely is an ordering issue:

You have to do the set(BUILD_APPS OFF CACHE BOOL "" FORCE) before you/the code does check for if (BUILD_APPS)
You have to check that there are no normal variable set(BUILD_APPS ...) afterwards in your main CMakeLists.txt hiding your cached variable
Whatever is in the CMakeCache.txt file is only the state of those variables after all processing of your code has finished (at the end of your main CMakeLists.txt)

I have given it a try again and the -D does force the Cache entry. So there should be no difference between using -D command line option and set(... CACHE ... FORCE). Just you can't use them in combination (the later will always overwrite any command line given option).

Please try adding variable_watch(BUILD_APPS) at the top of the main CMakeLists.txt to find the codepart that is hiding/overwriting the variable.

References

moving CMake options from the command line into the top-level CMakeLists.txt

1 Answers1