How to include CMake files from source tree of included project - cmake

Question

I'm setting up a CMake based build system for an old framework, written in C. It consists of a single binary, some core libraries and many dynamically linked libraries, which are built against the core libraries. My goal is to structure those libraries in several CMake projects ("base framework", "extension libraries", …), supporting two scenarios:
Independent build of extension libraries against base framework's build tree:
- base_framework/
- cmake/
- functions.cmake
- core_libraries/
- CMakeLists.txt
- extension_libs/
- lib1/
- lib2/
- CMakeLists.txt
For this scenario, I use
export(EXPORT foo
FILE FooFrameworkConfig.cmake)
and
find_package(FooFramework)
and add the build tree to CMAKE_PREFIX_PATH when configuring the extension_libs project.
Using base framework and extension libraries as sub-projects (e.g. as git submodules) of an application specific project:
- my_project/
- base_framework/
- cmake/
- functions.cmake
- core_libraries/
- CMakeLists.txt
- extension_libs/
- lib1/
- lib2/
- CMakeLists.txt
- my_lib1/
- CMakeLists.txt
For this scenario, I use add_subdirectory() for both, the base framework and the extension libs.
(A third scenario would be building the extension libraries against an install tree of the base framework. Unfortunately, this scenario is currently prevented by other CMake problems.)
Now, I want to include the base_framework/cmake/functions.cmake file, containing some custom CMake functions for all libraries, into all projects' top-level CMakeLists.txts in both scenarios.
For the second scenario, I simply set a cached CMake variable in base_framework/CMakeLists.txt:
set(BASE_FRAMEWORK_DIR ${CMAKE_CURRENT_SOURCE_DIR}
CACHE PATH "" FORCE)
and use this variable for including the functions file in extension_libraries/CMakeLists.txt as well as my_project/CMakeLists.txt:
include(${BASE_FRAMEWORK_DIR}/cmake/functions.cmake)
But how do I find the function.cmake file in the base framework's source tree from extenstion_libs/cmake in the first scenario? All the directories (base_framework, extension_libs, base framework's build tree) may be anywhere on my computer.

Answer 1

To expand on my comment regarding your first scenario, it looks like you can set one of the properties of your exported core_libraries targets to contain the path to the base_framework/cmake modules. So you can set this in the base_framework/CMakeLists.txt file, let's set it in the LABELS target property. Then, we tell CMake to export the LABELS property using EXPORT_PROPERTIES:
Yourbase_framework/CMakeLists.txt file:
# Define the path to 'base_framework/cmake'
set(BASE_FRAMEWORK_CMAKE_MODULES "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
add_library(CoreLibrary1 ...)
# Set the LABELS property, and tell CMake to export it.
set_target_properties(CoreLibrary1 PROPERTIES
LABELS "${BASE_FRAMEWORK_CMAKE_MODULES}"
EXPORT_PROPERTIES "LABELS"
)
# Define the export 'foo'.
install(TARGETS CoreLibrary1 EXPORT foo DESTINATION /path/to/install/targets)
export(EXPORT foo FILE FooFrameworkConfig.cmake)
This will ensure the path to the base_framework CMake modules gets exported explicitly, and can be accessed when imported by another project:
Your extension_libs/CMakeLists.txt file:
find_package(FooFramework)
# Get the LABELS property from the imported target.
get_target_property(BASE_FRAMEWORK_MODULE_DIR CoreLibrary1 LABELS)
message(STATUS "BASE_FRAMEWORK_MODULE_DIR: ${BASE_FRAMEWORK_MODULE_DIR}")
With a debug message(), we can verify the CMake modules directory was imported:
BASE_FRAMEWORK_MODULE_DIR: /your/path/to/base_framework/cmake
EDIT: Instead of hijacking the LABELS target property, you could also use define_property() to define your own custom property for the target.