What use is find_package() when you need to specify CMAKE_MODULE_PATH? - cmake

Question

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?

Answer 1

Command find_package has two modes: Module mode and Config mode. You are trying to
use Module mode when you actually need Config mode.
Module mode
Find<package>.cmake file located within your project. Something like this:
CMakeLists.txt
cmake/FindFoo.cmake
cmake/FindBoo.cmake
CMakeLists.txt content:
list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}/cmake")
find_package(Foo REQUIRED) # FOO_INCLUDE_DIR, FOO_LIBRARIES
find_package(Boo REQUIRED) # BOO_INCLUDE_DIR, BOO_LIBRARIES
include_directories("${FOO_INCLUDE_DIR}")
include_directories("${BOO_INCLUDE_DIR}")
add_executable(Bar Bar.hpp Bar.cpp)
target_link_libraries(Bar ${FOO_LIBRARIES} ${BOO_LIBRARIES})
Note that CMAKE_MODULE_PATH has high priority and may be usefull when you need to rewrite standard Find<package>.cmake file.
Config mode (install)
<package>Config.cmake file located outside and produced by install
command of other project (Foo for example).
foo library:
> cat CMakeLists.txt
cmake_minimum_required(VERSION 2.8)
project(Foo)
add_library(foo Foo.hpp Foo.cpp)
install(FILES Foo.hpp DESTINATION include)
install(TARGETS foo DESTINATION lib)
install(FILES FooConfig.cmake DESTINATION lib/cmake/Foo)
Simplified version of config file:
> cat FooConfig.cmake
add_library(foo STATIC IMPORTED)
find_library(FOO_LIBRARY_PATH foo HINTS "${CMAKE_CURRENT_LIST_DIR}/../../")
set_target_properties(foo PROPERTIES IMPORTED_LOCATION "${FOO_LIBRARY_PATH}")
By default project installed in CMAKE_INSTALL_PREFIX directory:
> cmake -H. -B_builds
> cmake --build _builds --target install
-- Install configuration: ""
-- Installing: /usr/local/include/Foo.hpp
-- Installing: /usr/local/lib/libfoo.a
-- Installing: /usr/local/lib/cmake/Foo/FooConfig.cmake
Config mode (use)
Use find_package(... CONFIG) to include FooConfig.cmake with imported target foo:
> cat CMakeLists.txt
cmake_minimum_required(VERSION 2.8)
project(Boo)
# import library target `foo`
find_package(Foo CONFIG REQUIRED)
add_executable(boo Boo.cpp Boo.hpp)
target_link_libraries(boo foo)
> cmake -H. -B_builds -DCMAKE_VERBOSE_MAKEFILE=ON
> cmake --build _builds
Linking CXX executable Boo
/usr/bin/c++ ... -o Boo /usr/local/lib/libfoo.a
Note that imported target is highly configurable. See my answer.
Update
Example

Answer 2

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.

Answer 3

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.

Answer 4

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.