Cross Compilation for ARM on Linux
==================================

Through cross compilation, on Linux it is possible to build CoreCLR for arm or arm64.

Requirements
------------

You need a Debian based host and the following packages needs to be installed:

    $ sudo apt-get install qemu qemu-user-static binfmt-support debootstrap

In addition, to cross compile CoreCLR the binutils for the target are required. So for arm you need:

    $ sudo apt-get install binutils-arm-linux-gnueabihf

and conversely for arm64:

    $ sudo apt-get install binutils-aarch64-linux-gnu


Generating the rootfs
---------------------
The `cross\build-rootfs.sh` script can be used to download the files needed for cross compilation. It will generate an Ubuntu 14.04 rootfs as this is what CoreCLR targets.

    Usage: build-rootfs.sh [BuildArch]
    BuildArch can be: arm, arm64

The `build-rootfs.sh` script must be run as root as it has to make some symlinks to the system, it will by default generate the rootfs in `cross\rootfs\<BuildArch>` however this can be changed by setting the `ROOTFS_DIR` environment variable.

For example, to generate an arm rootfs:

    $ sudo ./cross/build-rootfs.sh arm

and if you wanted to generate the rootfs elsewhere:

    $ sudo ROOTFS_DIR=~/coreclr-cross/arm ./build-rootfs.sh arm

Cross compiling CoreCLR
-----------------------
Once the rootfs has been generated, it will be possible to cross compile CoreCLR. If `ROOTFS_DIR` was set when generating the rootfs, then it must also be set when running `build.sh`.

So, without `ROOTFS_DIR`:

    $ ./build.sh arm debug verbose clean cross

And with:

    $ ROOTFS_DIR=~/coreclr-cross/arm ./build.sh arm debug verbose clean cross

As usual the resulting binaries will be found in `bin/Product/BuildOS.BuildArch.BuildType/`