find_package() ignores <PackageName>

In my CMake script I append the path to a folder containing <PackageName>Config.cmake to <PackageName>_ROOT and then call find_package(<PackageName> REQUIRED) but it can't find my package.

When I use CMAKE_FIND_DEBUG_MODE I see my folder listed in <PackageName>_ROOT CMake variable [CMAKE_FIND_USE_PACKAGE_ROOT_PATH] (it is actually the only folder there), but it is not listed after find_package considered the following locations for the Config module:.

Why am I getting such behaviour? I use CMake 3.18.1 from Android Studio. The package I'm trying to find is OpenCV.

Relatively to my CMakeLists.txt file OpenCV is installed in ../../build/opencv-build<custom suffix>, so my code for finding it is:

get_filename_component(OPENCV_BUILD_DIRS_ROOT ../../build REALPATH)
file(GLOB OPENCV_BUILD_DIRS ${OPENCV_BUILD_DIRS_ROOT}/opencv-build*)
list(APPEND OpenCV_ROOT ${OPENCV_BUILD_DIRS})

find_package(OpenCV ${OPENCV_VERSION} REQUIRED ${OPENCV_PUBLIC_LIBRARIES} ${OPENCV_PRIVATE_LIBRARIES})

When I run it on Linux (WSL actually), everything works fine and CMake finds OpenCV succesfully in ../../build/opencv-build (or <project path>/build/opencv-build in the form of absolute path).

But when I try to build the project from Android Studio it doesn't, and I get this output from CMAKE_FIND_DEBUG_MODE:

CMake Debug Log at external/opencv/CMakeLists.txt:12 (find_package):
  find_package considered the following paths for OpenCV.cmake

    C:/Users/<username>/AppData/Local/Android/Sdk/cmake/3.18.1/share/cmake-3.18/Modules/FindOpenCV.cmake

  The file was not found.

  <PackageName>_ROOT CMake variable [CMAKE_FIND_USE_PACKAGE_ROOT_PATH].

    <project path>/build/opencv-build

  

  CMAKE_PREFIX_PATH variable [CMAKE_FIND_USE_CMAKE_PATH].

    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64

  

  CMAKE_FRAMEWORK_PATH and CMAKE_APPBUNDLE_PATH variables
  [CMAKE_FIND_USE_CMAKE_PATH].

  

  Env variable OpenCV_DIR [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  CMAKE_PREFIX_PATH env variable [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  CMAKE_FRAMEWORK_PATH and CMAKE_APPBUNDLE_PATH env variables
  [CMAKE_FIND_USE_CMAKE_ENVIRONMENT_PATH].

    none

  Paths specified by the find_package HINTS option.

    none

  Standard system environment variables
  [CMAKE_FIND_USE_SYSTEM_ENVIRONMENT_PATH].

    <a lot of unrelated directories>

  

  CMake User Package Registry [CMAKE_FIND_USE_PACKAGE_REGISTRY].

    none

  CMake variables defined in the Platform file
  [CMAKE_FIND_USE_CMAKE_SYSTEM_PATH].

    C:/Users/<username>/AppData/Local/Android/Sdk/cmake/3.18.1
    <other unrelated directories>

  

  CMake System Package Registry
  [CMAKE_FIND_PACKAGE_NO_SYSTEM_PACKAGE_REGISTRY].

    none

  Paths specified by the find_package PATHS option.

    none

  find_package considered the following locations for the Config module:

    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/OpenCVConfig.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/opencv-config.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/OpenCVConfig.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/opencv-config.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/usr/lib/i686-linux-android/21/OpenCVConfig.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/usr/lib/i686-linux-android/21/opencv-config.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/usr/local/OpenCVConfig.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/usr/local/opencv-config.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/usr/OpenCVConfig.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/usr/opencv-config.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/OpenCVConfig.cmake
    C:/Users/<username>/AppData/Local/Android/Sdk/ndk/23.1.7779620/toolchains/llvm/prebuilt/windows-x86_64/sysroot/opencv-config.cmake

  The file was not found.

CodePudding user response：

Since you are providing a config file, you should set <PackageName>_DIR to where you have the config, not <PackageName>_ROOT (which is a hint for find modules and irrelevant when you have a path to a config).

If you don't know the path exactly, or want to use multiple paths you should consult the docs regarding the search procedure as there are multiple ways to influence where CMake would search for the config file (that includes <PackageName>_ROOT, HINTS, CMAKE_PREFIX_PATH and others).

But if your config can't be located by the defaults and you still need to make hints I don't see any reason why you can't just set the <PackageName>_DIR variable. You know what package you want, you know where it is and it is a single path, there is no reason to use any other variable.

The only reason might be if you have an expected folder structure with many libraries but this tree isn't located at the default location, in this case I'd modify CMAKE_PREFIX_PATH because with just one modification you get all you libs found.