cmake

Use find_package() on external project

I have an External project called messages. I am using ExternalProject_Add in order to fetch and build the project.

If i use find_package(messages REQUIRED) in top level CMakeLists.txt the cmake .. fails because it couldn't find the package installation files, which is logical as they are only build during make command invocation.

I am not sure, if there is way use find_package() on ExternalProjects. If so, point me to an example.

Solution

  • You have misunderstood how ExternalProject is supposed to work. You cannot find_package(messages REQUIRED) because it hasn't been built yet. ExternalProject merely creates the build steps necessary to build the subproject.

    You have two options:

    1. Use add_subdirectory or FetchContent in place of ExternalProject. In this case, you don't need a find_package call. This effectively adds the sub-project to the main build and imports the subproject's targets.
    2. Use two ExternalProject calls: one for messages and another for main_project, which depends on messages. If messages uses the export(EXPORT) function, you can point CMAKE_PREFIX_PATH or messages_ROOT to the build directory. Otherwise you'll need to run the install step for messages and set up an install prefix inside your build directory. Then the find_project(messages REQUIRED) call inside main_project will succeed. This will likely require re-structuring your build.

    Generally speaking, ExternalProject is only useful for defining super-builds, which are chains of CMake builds that depend on one another. And super builds are only useful when you need completely different configure-time options, like different toolchains (eg. you're cross compiling, but need a code generator to run on the build machine). If that's not the case, prefer FetchContent or add_subdirectory with a git submodule.

    It is best to use FetchContent with CMake 3.14+ since it adds the FetchContent_MakeAvailable macro that cuts down on boilerplate.

    Docs:

    https://cmake.org/cmake/help/latest/module/ExternalProject.html https://cmake.org/cmake/help/latest/module/FetchContent.html