What is the correct usage of CMake EXTERNALPROJECT_ADD with a Git repository?

Question

I would like to learn how to download and compile external libraries using the cmake external project module.

For example, lets say that I wanted to download the source

Answer 1

Perhaps your variables do not contain the values you think they contain... Double check the value of your sfml_* variables. Also double check that the CMake variable GIT_EXECUTABLE has the value you expect after including ExternalProject...

The following CMakeLists.txt file works for me on my Mac using CMake 2.8.5:

cmake_minimum_required(VERSION 2.8)
project(SfmlBuilder)
include(ExternalProject)

set(sfml_PREFIX "${CMAKE_CURRENT_BINARY_DIR}/sfml")
set(sfml_INSTALL_DIR "${CMAKE_CURRENT_BINARY_DIR}/sfml")
set(sfml_CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=${sfml_INSTALL_DIR})

message("sfml_PREFIX='${sfml_PREFIX}'")
message("sfml_INSTALL_DIR='${sfml_INSTALL_DIR}'")
message("sfml_CMAKE_ARGS='${sfml_CMAKE_ARGS}'")
message("GIT_EXECUTABLE='${GIT_EXECUTABLE}'")

ExternalProject_Add(sfml
  PREFIX ${sfml_PREFIX}
  GIT_REPOSITORY https://github.com/LaurentGomila/SFML.git
  INSTALL_DIR ${sfml_INSTALL_DIR}
  CMAKE_ARGS ${sfml_CMAKE_ARGS}
)

It fails during the install for me with a permission denied because I did not run "make" as sudo, and it tries to install into the absolute path "/Library/Frameworks/sndfile.framework"

One other piece of advice, too. I notice you're installing "/Applications/CMake 2.8-5.app/Contents/share/cmake-2.8/Modules/FindSFML.cmake" directly into the CMake installation... That is generally discouraged, as that modification to the CMake installation is likely to disappear if the user uninstalls and re-installs CMake. Or simply upgrades to another CMake. Or uses a 2nd or 3rd CMake that is also installed on the computer.

You should instead create a project config file in your own installation, that CMake can find with it's built-in rules for finding packages at standard locations. Read the fine print of the CMake find_package documentation for the full details on project config files:

http://cmake.org/cmake/help/cmake-2-8-docs.html#command:find_package