[cmake] What use is find_package() if you need to specify CMAKE_MODULE_PATH anyway?

I'm trying to get a cross-plattform build system working using CMake. Now the software has a few dependencies. I compiled them myself and installed them on my system.

Some example files which got installed:

-- Installing: /usr/local/share/SomeLib/SomeDir/somefile
-- Installing: /usr/local/share/SomeLib/SomeDir/someotherfile
-- Installing: /usr/local/lib/SomeLib/somesharedlibrary
-- Installing: /usr/local/lib/SomeLib/cmake/FindSomeLib.cmake
-- Installing: /usr/local/lib/SomeLib/cmake/HelperFile.cmake

Now CMake has a find_package() which opens a Find*.cmake file and searches after the library on the system and defines some variables like SomeLib_FOUND etc.

My CMakeLists.txt contains something like this:

set(CMAKE_MODULE_PATH "/usr/local/lib/SomeLib/cmake/;${CMAKE_MODULE_PATH}")
find_package(SomeLib REQUIRED)

The first command defines where CMake searches after the Find*.cmake and I added the directory of SomeLib where the FindSomeLib.cmake can be found, so find_package() works as expected.

But this is kind of weird because one of the reasons why find_package() exists is to get away from non-cross-plattform hard coded paths.

How is this usually done? Should I copy the cmake/ directory of SomeLib into my project and set the CMAKE_MODULE_PATH relatively?

If you are running cmake to generate SomeLib yourself (say as part of a superbuild), consider using the User Package Registry. This requires no hard-coded paths and is cross-platform. On Windows (including mingw64) it works via the registry. If you examine how the list of installation prefixes is constructed by the CONFIG mode of the find_packages() command, you'll see that the User Package Registry is one of elements.

Brief how-to

Associate the targets of SomeLib that you need outside of that external project by adding them to an export set in the CMakeLists.txt files where they are created:

add_library(thingInSomeLib ...)
install(TARGETS thingInSomeLib Export SomeLib-export DESTINATION lib)

Create a XXXConfig.cmake file for SomeLib in its ${CMAKE_CURRENT_BUILD_DIR} and store this location in the User Package Registry by adding two calls to export() to the CMakeLists.txt associated with SomeLib:

export(EXPORT SomeLib-export NAMESPACE SomeLib:: FILE SomeLibConfig.cmake) # Create SomeLibConfig.cmake
export(PACKAGE SomeLib)                                                    # Store location of SomeLibConfig.cmake

Issue your find_package(SomeLib REQUIRED) commmand in the CMakeLists.txt file of the project that depends on SomeLib without the "non-cross-platform hard coded paths" tinkering with the CMAKE_MODULE_PATH.

When it might be the right approach

This approach is probably best suited for situations where you'll never use your software downstream of the build directory (e.g., you're cross-compiling and never install anything on your machine, or you're building the software just to run tests in the build directory), since it creates a link to a .cmake file in your "build" output, which may be temporary.

But if you're never actually installing SomeLib in your workflow, calling EXPORT(PACKAGE <name>) allows you to avoid the hard-coded path. And, of course, if you are installing SomeLib, you probably know your platform, CMAKE_MODULE_PATH, etc, so @user2288008's excellent answer will have you covered.

You don't need to specify the module path per se. CMake ships with its own set of built-in find_package scripts, and their location is in the default CMAKE_MODULE_PATH.

The more normal use case for dependent projects that have been CMakeified would be to use CMake's external_project command and then include the Use[Project].cmake file from the subproject. If you just need the Find[Project].cmake script, copy it out of the subproject and into your own project's source code, and then you won't need to augment the CMAKE_MODULE_PATH in order to find the subproject at the system level.

How is this usually done? Should I copy the cmake/ directory of SomeLib into my project and set the CMAKE_MODULE_PATH relatively?

If you don't trust CMake to have that module, then - yes, do that - sort of: Copy the find_SomeLib.cmake and its dependencies into your cmake/ directory. That's what I do as a fallback. It's an ugly solution though.

Note that the FindFoo.cmake modules are each a sort of a bridge between platform-dependence and platform-independence - they look in various platform-specific places to obtain paths in variables whose names is platform-independent.