How do I use CMake ExternalProject_Add or alternatives in a cross-platform way?

Question

I would like to build a third-party project that already has CMake as part of my project\'s CMake strips. ExternalProject_Add is for this purpose, but I have found it can on

Answer 1

CMake ExternalProject_Add calls work cross-platform by default and will only fail to do so if one uses particular commands that are only available on a subset of operating systems.

Typically, CMAKE_ARGS is used to pass information to each superbuild unit within an external project build. The CMakeLists.txt files that control each miniature part of the overall build use CMake's declarative syntax (e.g., "add_library(library_name SHARED filename1.hpp filename1.cpp). CMake will convert such syntax to the commands that are specific to the particular build system you wish to use (e.g., make and Ninja).

The sample above re: zlib fails to be cross-platform in part because the ExternalProject_Add_Step contains "COMMAND cd && make install", which necessarily only works in situations where invoking "cd" is actually the correct way to change directories, and where invoking "make" is actually the correct way to build software.

CMake's -E option provides a way to invoke basic operations like changing/copying/making/removing directories without making such assumptions.

(By the way, if you're using IDEs such as Visual Studio or Xcode, you'll likely want to invoke one or more IDE generators when using CMake. For instance, setting

-G "Eclipse CDT4 - Unix Makefiles" -DCMAKE_ECLIPSE_GENERATE_SOURCE_PROJECT=TRUE

will cause Eclipse projects to be generated in each build area, and also in the source code area that is shared for all builds. Of course, if you are using Xcode or Visual Studio, you'll have to substitute the appropriate flag for those IDEs. Alternatively, you could consider using Eclipse with Ninja on all platforms, though at the time of writing, I am not completely certain that Ninja is ready for prime-time on non-Linux, non-Windows operating systems.)