Have you ever wanted to run a program in some device without needing to follow a complicated, possibly slow build process? In many cases it can be simpler to use zig cc to compile a static binary and call it a day.

Zig What?

The Zig programming language is a new(ish) programming language that may not get as much rep as others like Rust, but the authors have had one of the greatest ideas ever: reuse the chops that Clang has as a cross-compiler, throw in a copy of the sources for the Musl C library, and provide an extremely convenient compiler driver that builds the parts of the C library needed by your program on-demand.

Try the following, it just works, it’s magic:

cat > hi.c <<EOF
#include <stdio.h>

int main() {
    puts("That's all, folks!");
    return 0;
}
EOF
zig cc --target=aarch64-linux-musl -o hi-arm hi.c

Go on, try it. I’ll wait.

All good? Here is what it looks like for me:

% uname -m
x64_64
% file hi-arm
hi-arm: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, with debug_info, not stripped
% ls -lh hi-arm
-rwxrwxr-x+ 1 aperez aperez 82K nov 24 16:08 hi-arm*
% llvm-strip -x --strip-unneeded hi-arm
% ls -lh hi-arm
-rwxrwxr-x+ 1 aperez aperez 8,2K nov 24 16:11 hi-arm*
% 

A compiler running on a x86_64 machine produced a 64-bit ARM program out of thin air (no cross-sysroot, no additional tooling), which does not have any runtime dependency (it is linked statically), weighting 82 KiB, and can be stripped further down to only 8,2 KiB.

While I cannot say much about Zig the programming language itself because I have not had the chance to spend much time writing code in it, I have the compiler installed just because of zig cc.

A Better Example

One of the tools I often want to run in a development board is drm_info, so let’s cross-compile that. This little program prints all the information it can gather from a DRM/KMS device, optionally formatted as JSON (with the -j command line flag) and it can be tremendously useful to understand the capabilities of GPUs and their drivers. My typical usage of this tool is knowing whether WPE WebKit would run in a given embedded device, and as guidance for working on Cog’s DRM platform plug-in.

It is also a great example because it uses Meson as its build system and has a few dependencies—so it is not a trivial example like the one above—but not so many that one would run into much trouble. Crucially, both its main dependencies, json-c and libdrm, are available as WrapDB packages that Meson itself can fetch and configure automatically. Combined with zig cc, this means the only other thing we need is installing Meson.

Well, that is not completely true. We also need to tell Meson how to cross-compile using our fancy toolchain. For this, we can write the following cross file, which I have named zig-aarch64.conf:

[binaries]
c = ['/opt/zigcc-wrapper/cc', 'aarch64-linux-musl']
objcopy = 'llvm-objcopy'
ranlib = 'llvm-ranlib'
strip = 'llvm-strip'
ar = 'llvm-ar'

[target_machine]
system = 'linux'
cpu_family = 'aarch64'
cpu = 'cortex-a53'
endian = 'little'

While ideally it should be possible to set the value for c in the cross file to zig with the needed command line flags, in practice there is a bug that prevents the linker detection in Meson from working correctly. At least until the patch which fixes the issue is not yet part of a release, the wrapper script at /opt/zigcc-wrapper/cc will defer to using lld when it notices that the linker version is requested, or otherwise call zig cc:

#! /bin/sh
set -e
target=$1
shift

if [ "$1" = '-Wl,--version' ] ; then
    exec clang -fuse-ld=lld "$@"
fi
exec zig cc --target="$target" "$@"

Now, this is how we would go about building drm_info, note the use of --default-library=static to ensure that the subprojects for the dependencies are built accordingly and that they can be linked into the resulting executable:

git clone https://gitlab.freedesktop.org/emersion/drm_info.git
meson setup drm_info.aarch64 drm_info \
  --cross-file=zig-aarch64.conf --default-library=static
meson compile -Cdrm_info.aarch64

A short while later we will be kicked back into our shell prompt, where we can check the resulting binary:

% file drm_info.aarch64/drm_info
drm_info.aarch64/drm_info: ELF 64-bit LSB executable, ARM aarch64, version 1 (SYSV), statically linked, with debug_info, not stripped
% ls -lh drm_info.aarch64/drm_info
-rwxr-xr-x 1 aperez aperez 1,8M nov 24 23:07 drm_info.aarch64/drm_info*
% llvm-strip -x --strip-unneeded drm_info.aarch64/drm_info
% ls -lh drm_info.aarch64/drm_info
-rwxr-xr-x 1 aperez aperez 199K nov 24 23:09 drm_info.aarch64/drm_info*
%

That’s a ~200 KiB drm_info binary that can be copied over to any 64-bit ARM-based device running Linux (the kernel). Magic!