[CMake] ExternalProject_Add

[Nicholas Braden]

The way to solve this is to use a superbuild project layout - you use
ExternalProject_Add to build your dependencies AND your own project, using
the DEPENDS option to control build order. Thus by the time it gets around
to configuring your project, the dependencies have already been built and
installed fully.

On Sun, Jan 8, 2017 at 3:49 PM, Saad Khattak <saadrustam at gmail.com> wrote:

> Hello,
>
> I have an external project glfw that I added to my project like this:
>
> ==========
> include(ExternalProject)
> ExternalProject_Add(glfw
>   GIT_REPOSITORY "https://github.com/glfw/glfw.git"
>   GIT_TAG "master"
>
>   SOURCE_DIR "${CMAKE_SOURCE_DIR}/dep/glfw"
>   CMAKE_ARGS -DGLFW_BUILD_DOCS=OFF -DGLFW_BUILD_TESTS=OFF
> -DGLFW_BUILD_EXAMPLES=OFF -DCMAKE_INSTALL_PREFIX=${
> CMAKE_SOURCE_DIR}/install/glfw/ -DCMAKE_DEBUG_POSTFIX=_d
>
>   TEST_COMMAND ""
>   )
> set(GLFW_INCLUDE_DIR "${CMAKE_SOURCE_DIR}/install/glfw/include/")
> set(GLFW_LIBRARY_DIR "${CMAKE_SOURCE_DIR}/install/glfw/lib/")
> ==========
>
> Then I include it in my project like so:
>
> ==========
> find_library(GLFW_LIB_D  glfw3_d ${GLFW_LIBRARY_DIR})
> find_library(GLFW_LIB    glfw3   ${GLFW_LIBRARY_DIR})
>
> include_directories(${GLFW_INCLUDE_DIR})
>
> add_executable(vk_test
>   src/vulkan_test.cpp
>   )
> target_link_libraries(vk_test debug ${GLFW_LIB_D} optimized ${GLFW_LIB})
> add_dependencies(vk_test glfw)
> ==========
>
> As you can see, I depend on the libraries compiled by the external project
> glfw. Unfortunately, when I first configure the project, CMake complains
> that it cannot find the libraries specified by ${GLFW_LIB_D} and
> ${GLFW_LIB} variables.
>
> Of course, this is because CMake did not begin cloning, configuring and
> building the glfw project. That only happens AFTER my project has been
> configured and starts building. This becomes a chicken and the egg problem.
>
> Currently, my solution is to add dummy .lib files so that I can at least
> configure and generate my project. My question is, am I approaching this
> problem in the wrong way? If yes, what is the correct way to add a
> dependency to an external project and have it clone/configure/build BEFORE
> the "find_library" call?
>
>
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