How to build Emacs for an embedded ARM Linux system

Question

I am trying to build Emacs for an embedded ARM Linux system. Unfortunately, it is proving to be more difficult than I anticipated. I have the device mounted at /mnt/my_device. Now I am not sure exactly how to go about building Emacs from source.

I know I can set the prefix of the install via --prefix=/mnt/my_device/usr/local. However, beyond that, things get more difficult. How can I configure my Emacs build for my ARM system? After configuration, how do I compile Emacs? Using an ARM toolchain in QEMU or something? I know that there are cross-compiler tools that run on x86 systems, but should I use those given that configure may get the wrong info if I run it on my x86 system? Also, I need this installation of Emacs to be quite minimal, as this is an embedded system and there's not a lot of room.

Any tips that would help cohere and streamline this build process would be most appreciated.

Answer 1

First: do you need to run Emacs on that system? Emacs can access remote files easily. Typically you just run it on your local machine. If you only ever access the system remotely, it probably doesn't need Emacs. ARM covers a very wide range of system sizes, from appliances with only a few megabytes of memory to equivalents of a 10-year old PC such as the latest Raspberry Pi model.

Second: do you need to compile Emacs yourself? There are plenty of Linux distributions that have ARM binaries and include Emacs. Distributions intended for smaller systems usually don't have it, but see “First” above. On “larger” distributions such as Debian or Arch, just install the package.

If you determine that you do need to compile Emacs, then you have a choice of cross-compiling or not. Emacs is written in C, and C compilers can work in relatively small amounts of memory. For example, you could compile on something like a Pi — but even a slightly aging PC will do it faster. If the device is powerful enough to cross-compile, then you'll probably want to run a distribution that has an Emacs package anyway.

If you do want to cross-compile, you'll need a cross-compiler. This can be a bit painful to set up sometimes. However, the distribution you're running on the device probably already has a cross-compiler setup. This is your most promising route: use whatever toolchain the distribution uses for its own packages.

Regarding Emacs itself, there are a few instructions in the INSTALL file. You'll need to pass the --host option to ./configure. The argument depends on the exact platform, depending on whether it has floating point registers and which standard library it runs. It might look something like arm-linux-gnueabihf (with GNU libc and Hardware Floating point). Usually the argument is the same name that the toolchain is installed under; for example a compiler for the arm-linux-gnueabihf target is typically called arm-linux-gnueabihf-gcc.

Answer 2

try chrooting to the root filesystem - assuming it has a build environment... you can then use the arm compiler.... something like:-

cd arm/rootfs
sudo su
cp /usr/bin/qemu-aarch64-static  usr/bin
cp /run/systemd/resolve/stub-resolv.conf run/resolvconf/resolv.conf

mount /sys ./sys -o bind
mount /proc ./proc -o bind
mount /dev ./dev -o bind

sudo LC_ALL=C chroot . /bin/bash
# we are not in kansas anymore!

root@jw-XPS-13-9370:/# gcc -dumpmachine
aarch64-linux-gnu