Integrate pre-compiled libraries into C++ codebase with CMake ExternalProject

Question

I want to integrate CasADi into a CMake-based C++ codebase as an ExternalProject. For this purpose, I would like to use pre-compiled libraries because building from source is not recommended. So far, I have only managed to write the following:

ExternalProject_Add(
  casadi-3.5.5
  URL https://github.com/casadi/casadi/releases/download/3.5.5/casadi-linux-py39-v3.5.5-64bit.tar.gz
  CONFIGURE_COMMAND ""
  BUILD_COMMAND ""
  INSTALL_COMMAND ""
  PREFIX ${CMAKE_BINARY_DIR}/external/casadi)

and I noticed that all the binaries are correctly downloaded in the specified folder. However, I do not know how to link my targets to CasADi, nor how to find the package.

Answer 1

There is a natural problem with ExternalProject_Add:

ExternalProject_Add executes commands only on build.

Hence, download will not happen at the configure stage of your project which makes it difficult to use find_package, because the files cannot be found during your first configure run.

Take this CMakeLists.txt:

cmake_minimum_required(VERSION 3.21)
project(untitled)

set(CMAKE_CXX_STANDARD 17)

add_executable(untitled main.cpp)

include(ExternalProject)
ExternalProject_Add(
        casadi-3.5.5
        URL https://github.com/casadi/casadi/releases/download/3.5.5/casadi-linux-py39-v3.5.5-64bit.tar.gz
        CONFIGURE_COMMAND ""
        BUILD_COMMAND ""
        INSTALL_COMMAND ""
        PREFIX ${CMAKE_BINARY_DIR}/external/casadi)

find_package(casadi HINTS ${CMAKE_BINARY_DIR}/external/casadi/src/casadi-3.5.5/casadi)

target_link_libraries(untitled casadi)

In order to use it you have to do the following:

1. Configure your project

mkdir build
cd build
cmake ..

2. Build (download) casadi-3.5.5

cmake --build . --target casadi-3.5.5

3. Reconfigure your project, because now find_package will find the needed files

cmake ..

4. Build your targets

cmake --build .

---

If you want a one step build, there are ways to get around this problem

• Use FetchContent
• Create a sub-cmake-project in a subfolder with all the ExternalProject_Add commands and execute the approriate build (download) steps manually in your own CMakeLists.txt via execute_process calls: https://stackoverflow.com/a/37554269/8088550

---

Here is an example for the second option, which might be better since FetchContent doesn't have the full functionality of ExternalProject.

• main.cpp

#include <casadi/casadi.hpp>

int main()
{
    casadi_printf("This works!");
    return 0;
}

• CMakeLists.txt

cmake_minimum_required(VERSION 3.20)
project(untitled)

set(CMAKE_CXX_STANDARD 17)

# some default target
add_executable(untitled main.cpp)

# Configure and build external project
file(MAKE_DIRECTORY ${CMAKE_BINARY_DIR}/external)
execute_process(
        COMMAND ${CMAKE_COMMAND} ${CMAKE_SOURCE_DIR}/external
        WORKING_DIRECTORY ${CMAKE_BINARY_DIR}/external
)
execute_process(
        COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR}/external
)

# find and link externals
find_package(casadi REQUIRED HINTS ${CMAKE_BINARY_DIR}/external/external/casadi/src/casadi-3.5.5/casadi)
target_link_libraries(untitled casadi)

• external/CMakeLists.txt

cmake_minimum_required(VERSION 3.20)
project(external)

include(ExternalProject)
ExternalProject_Add(
        casadi-3.5.5
        URL https://github.com/casadi/casadi/releases/download/3.5.5/casadi-linux-py39-v3.5.5-64bit.tar.gz
        CONFIGURE_COMMAND ""
        BUILD_COMMAND ""
        INSTALL_COMMAND ""
        PREFIX ${CMAKE_BINARY_DIR}/external/casadi)

The point is to have another cmake project under external/CMakeLists.txt, which gets configured and build via execute_process calls from the main cmake project. Do note, that you can now have find_package(casadi REQUIRED ...) at configure stage, because the download will happen just before.