Programming
What use is findpackage when you need to specify CMAKEMODULEPATH
When delving into the world of CMake, developers often grapple with the intricacies of dependency management. One common question arises: What use is find_package() when you need to specify CMAKE_MODULE_PATH? At first glance, it might seem redundant to use both, especially when you’re already guiding CMake towards the location of your Find Modules. However, understanding the nuanced roles of each component is critical for building robust and maintainable projects. Let’s break down the individual functions and explore why they often work in conjunction to streamline your build process. find_package() is essential for locating and configuring external libraries, while CMAKE_MODULE_PATH extends CMake’s search capabilities, allowing it to find custom Find Modules or override the defaults. This combination offers a powerful solution for managing dependencies in diverse build environments.
Understanding find_package() in CMake
The find_package() command in CMake is a workhorse for locating external libraries and including them in your project. Its primary function is to locate a package, load its pre-configured settings, and expose its variables (such as include paths and library locations) for use in your project’s build process. CMake searches for package configuration files (<packagename>Config.cmake</packagename>) or Find Modules (Find<packagename>.cmake</packagename>) to achieve this. These files contain the logic needed to determine where the library is installed and how to link against it. By employing find_package(), you abstract away the specifics of how a library is installed on a particular system, allowing your CMake code to remain portable and adaptable across different environments. According to Kitware, the creators of CMake, find_package() is a cornerstone of modern CMake practices [CMake Documentation].
For example, if you want to use the Boost libraries in your project, you would use find_package(Boost REQUIRED COMPONENTS system filesystem). This tells CMake to find the Boost package and that your project requires the “system” and “filesystem” components. After a successful search, variables like Boost_INCLUDE_DIRS and Boost_LIBRARIES would be set, allowing you to include the necessary headers and link against the Boost libraries. Without find_package(), you would need to manually specify these paths and libraries, which can be tedious and error-prone, especially across different operating systems and installation layouts. This command significantly simplifies dependency management, making your CMake scripts cleaner and more maintainable.
The beauty of find_package() lies in its ability to handle different installation scenarios. Whether a library is installed in a standard system location or a custom directory, find_package(), when properly configured, can locate it. This is particularly important in continuous integration (CI) environments, where dependencies may be installed in non-standard locations. Consider a situation where you have multiple versions of a library installed on your system. find_package(), combined with version specifiers, enables you to target the specific version required by your project, preventing compatibility issues and ensuring consistent builds across different machines.
The Role of CMAKE_MODULE_PATH
CMAKE_MODULE_PATH is a CMake variable that specifies a list of directories where CMake should search for Find Modules (Find<packagename>.cmake</packagename>) and other CMake modules. By default, CMake has a built-in set of modules that cover common libraries and tools. However, if you’re using a library for which CMake doesn’t have a built-in Find Module, or if you want to override the default Find Module with a custom one, you need to modify CMAKE_MODULE_PATH. Essentially, it expands CMake’s search scope, allowing it to locate modules in locations beyond the standard installation directories. This flexibility is crucial for projects with custom dependencies or those that rely on libraries installed in non-standard locations.
Imagine you have a custom Find Module for a proprietary library stored in a subdirectory named cmake_modules within your project’s source tree. You would set CMAKE_MODULE_PATH to include this directory: set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake_modules ${CMAKE_MODULE_PATH}). This tells CMake to look in the cmake_modules directory first when searching for Find Modules. This is particularly useful when you want to bundle the Find Module directly with your project, ensuring that it’s always available and consistent across different build environments. According to a Stack Overflow survey, a large percentage of CMake users leverage CMAKE_MODULE_PATH for custom module management [Stack Overflow Discussion].
Furthermore, CMAKE_MODULE_PATH can be used to prioritize specific Find Modules over others. If you have multiple Find Modules for the same library, CMake will use the first one it finds in the directories listed in CMAKE_MODULE_PATH. This allows you to override the default Find Module provided by CMake with a custom version that may be better suited to your project’s needs. For instance, you might create a custom Find Module that leverages a specific feature of the library or that integrates more seamlessly with your build system. By strategically setting CMAKE_MODULE_PATH, you can fine-tune the dependency resolution process and ensure that your project uses the correct Find Modules.
When to Use Both find_package() and CMAKE_MODULE_PATH
The key to understanding the relationship between find_package() and CMAKE_MODULE_PATH lies in recognizing their distinct roles. find_package() is the command that initiates the search for a package, while CMAKE_MODULE_PATH tells CMake where to look for the Find Modules or Config files that find_package() relies on. Therefore, you use CMAKE_MODULE_PATH to extend the search scope of find_package(). You’ll often need both when the Find Module for a library isn’t in CMake’s default search locations. This is especially relevant when dealing with custom libraries, internal tools, or libraries installed in non-standard directories. Correctly specifying CMAKE_MODULE_PATH ensures that CMake can find the necessary Find Modules to successfully locate and configure your dependencies. This synergy is crucial for maintaining a clean and organized build process.
Let’s illustrate this with a real-world example. Suppose you’re working on a project that uses a custom library called “MyLib,” and you’ve created a FindMyLib.cmake module that resides in a cmake_modules directory within your project. To use this library, you would first set CMAKE_MODULE_PATH to include the cmake_modules directory. Then, you would call find_package(MyLib REQUIRED). CMake will then search the directories listed in CMAKE_MODULE_PATH, find your FindMyLib.cmake module, and execute it to locate the MyLib library. Without setting CMAKE_MODULE_PATH, CMake would not know to look in the cmake_modules directory, and the find_package() command would fail. This example highlights how these two components work together to manage dependencies effectively.
Here’s a featured snippet optimized paragraph: The combined use of find_package() and CMAKE_MODULE_PATH is essential when dealing with external libraries that are not located in standard system directories or when using custom Find Modules. find_package() initiates the search for the library, and CMAKE_MODULE_PATH tells CMake where to look for the Find Module that helps locate the library. This approach provides flexibility and control over dependency management, especially in complex projects with numerous dependencies. Ignoring either component can lead to build failures and maintenance headaches.
Best Practices and Common Pitfalls
When working with find_package() and CMAKE_MODULE_PATH, adhering to best practices can significantly improve the robustness and maintainability of your CMake projects. One key practice is to always specify the REQUIRED keyword in your find_package() calls unless the dependency is truly optional. This ensures that CMake will halt the build process if the package cannot be found, preventing potential runtime errors. Another best practice is to always append to CMAKE_MODULE_PATH rather than overwriting it. This preserves CMake’s default search paths and avoids unexpected behavior. For example, use set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake_modules ${CMAKE_MODULE_PATH}) instead of set(CMAKE_MODULE_PATH ${CMAKE_SOURCE_DIR}/cmake_modules).
A common pitfall is forgetting to include the necessary include directories and library paths in your target’s properties after calling find_package(). While find_package() sets variables like <packagename>_INCLUDE_DIRS</packagename> and <packagename>_LIBRARIES</packagename>, you still need to use these variables to configure your target. For example, you would use target_include_directories(mytarget PUBLIC ${Boost_INCLUDE_DIRS}) and target_link_libraries(mytarget ${Boost_LIBRARIES}) to include the Boost headers and link against the Boost libraries. Failing to do so will result in compilation and linking errors. Additionally, avoid placing Find Modules directly in system directories. Instead, keep them within your project or in a dedicated directory and use CMAKE_MODULE_PATH to point CMake to their location [Modern CMake Practices].
Another pitfall is relying solely on environment variables to locate dependencies. While environment variables can be useful, they can also lead to inconsistencies across different build environments. It’s generally better to use CMAKE_MODULE_PATH and Find Modules to explicitly specify the location of dependencies. This ensures that your build process is reproducible and independent of the user’s environment. Finally, always test your CMake code thoroughly, especially when dealing with complex dependency management scenarios. Use a CI system to build your project on different platforms and with different configurations to catch any potential issues early on.
- Always specify
REQUIREDinfind_package()calls for mandatory dependencies. - Append to
CMAKE_MODULE_PATHinstead of overwriting it.
- Create a dedicated directory for your custom Find Modules.
- Set
CMAKE_MODULE_PATHto include this directory. - Call
find_package()to locate the package.
FAQ
- Q: What happens if `find_package()` fails?
- A: If `find_package()` fails and the `REQUIRED` keyword is specified, CMake will stop processing the `CMakeLists.txt` file and report an error. If `REQUIRED` is not specified, the build process will continue, but variables associated with the package will not be set.
- Q: Can I use `find_package()` without specifying `CMAKE_MODULE_PATH`?
- A: Yes, you can. If the Find Module or Config file for the package is located in one of CMake's default search paths, you don't need to specify `CMAKE_MODULE_PATH`. However, if the module is in a custom location, you will need to set `CMAKE_MODULE_PATH`.
- Q: How do I debug `find_package()` issues?
- A: You can use the `CMAKE_FIND_DEBUG_MODE` variable to enable verbose output during the `find_package()` process. This will show you exactly where CMake is searching for Find Modules and Config files.
Question & Answer :
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?
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
