Install path for cmake modules - cmake

Question

So if I'm making a package with cmake/cpack, there must be a variable already set in cmake that tells it where to put the *.cmake files that will be used by find_package for my module...right?
What's that variable?

Answer 1

According to the documentation of find_package, it is either CMAKE_MODULE_PATH, but I would not recommend that, otherwise you can put your <name>-config.cmake files anywhere, as long as you specify the path in your find_package call.
Otherwise system-wide paths might be set, being CMAKE_SYSTEM_PREFIX_PATH, CMAKE_SYSTEM_FRAMEWORK_PATH andCMAKE_SYSTEM_APPBUNDLE_PATH.
from the documentation (removed unnecessary information):
find_package(<package> ...
[CONFIG]
...
[NAMES name1 [name2 ...]]
[CONFIGS config1 [config2 ...]]
[HINTS path1 [path2 ... ]]
[PATHS path1 [path2 ... ]]
[PATH_SUFFIXES suffix1 [suffix2 ...]]
...)
It will search for <NAMES>-config.cmake in all PATHS and all HINTS (in that order), as well as predefines CMAKE Paths.
The exact search order (from the documentation cited above):
Search paths specified in cmake-specific cache variables. These are intended to be used on the command line with a -DVAR=value. This can be skipped if NO_CMAKE_PATH is passed
Search paths specified in cmake-specific environment variables. These are intended to be set in the user’s shell configuration. This can be skipped if NO_CMAKE_ENVIRONMENT_PATH is passed
Search paths specified by the HINTS option. These should be paths computed by system introspection, such as a hint provided by the location of another item already found. Hard-coded guesses should be specified with the PATHS option.
Search the standard system environment variables. This can be skipped if NO_SYSTEM_ENVIRONMENT_PATH is passed. Path entries ending in /bin or /sbin are automatically converted to their parent directories

Answer 2

The Variable
I've found the variable named CMAKE_ROOT to include the path to the default cmake modules:
CMAKE_ROOT=/usr/share/cmake-3.10
This path is what it is set to under Ubuntu 18.04 which is correct compared to the cmake version on that platform:
$ cmake --version
cmake version 3.10.2
CMake suite maintained and supported by Kitware (kitware.com/cmake).
How do you find such hidden variables?
A while back, I stole a piece of code I found on stackoverflow which I can use to display all the variables at a given point in the processing of my CMakeLists.txt files. I create a command I called DumpCMakeVariables.
To use it (assuming the Modules directory is somehow accessible):
find_package(DumpCMakeVariables)
DumpCMakeVariables()
If you have an idea of the variable name, then you can include a valid regular expression as the parameter of that function. To only list variables that start with "CMAKE_" write:
DumpCMakeVariables("^CMAKE_")
Then re-run your configuration process, something like this:
cmake <path to source> | less -S
and search for a specific variable or what your think the value should be. In out case here:
/usr.share.cmake
That will eventually point us to CMAKE_ROOT.
Other Installation Path
Note that I also install files directly under:
/usr/share/cmake/<name>/<name>Config.cmake
And if you want to include a version:
/usr/share/cmake/<name>/<name>ConfigVersion.cmake
where <name> is generally Camel Case.
Having such a directory allows you to have many sub-.cmake files that your main configuration uses to find this or that. So it is a way to make it much cleaner if you need many files instead of just one.
That being, that /usr/share/cmake directory works great. However, certain types of modules should certainly be installed under CMAKE_ROOT to 100% work as expected.
Catch2 also uses that scheme:
# This variable is used in some subdirectories, so we need it here, rather
# than later in the install block
set(CATCH_CMAKE_CONFIG_DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/Catch2")