An example repository showing how to build Rust code for Android and embed it in an app. Implements a very basic Reverse Polish Notation calculator.

This README gives a quick rundown of the repo. If you want some more information, check out the post on my blog.

NOTE: If you want a quick and easy solution, there is a Rust plugin for Gradle which makes the whole process rather trivial. For an example of how to use the plugin, you can check out the example repo by ssrlive.

This repository contains a more low-level approach, where C and Rust compilers are directly called and all the stuff related to setting up cross-compilation, compiler flags, linker flags etc. is handled explicitly. Gradle is used pretty much only to glue everything together into an .apk.

To build this app, you'll need the following:

• Android Software Development Kit
• Android Native Development Kit r21
• bash
• curl
• gcc
• Java Runtime Environmennt
• m4
• make
• Rust with cross-compilers
• tar

You can download the Android tools from the Android developer portal.

• SDK - you will only need the command-line tools, not the whole Android Studio package.
• NDK - make sure to grab the r21 LTS release, not the latest r22 release.

The app has a dependency on the GNU Multiple Precision Arithmetic Library, or GMP, for short. To build GMP, you'll need some Linux build tools.

On Debian/Ubuntu, you can use the following:

apt install -y curl gcc m4 make tar

On Fedora/CentOS, you can use:

dnf install --assumeyes curl gcc m4 make tar

To install Rust, you can use rustup-init.

Once you have Rust installed, you can use rustup to install support for Android targets.

rustup target add aarch64-linux-android
rustup target add armv7-linux-androideabi
rustup target add x86_64-linux-android

For building the .apk, this repo uses Gradle, which requires a JRE to run. While Android Studio offers you to download a JRE and Gradle can be configured to run with that, this repo just tries to use the your system's default JRE.

On Debian/Ubuntu, you can use the following command to install one:

apt install -y openjdk-11-jre-headless

On Fedora/CentOS, you can use:

dnf install --assumeyes java-11-openjdk-headless

Before you can trigger a build, you'll need to set up some environment variables.

• ANDROID_SDK_ROOT - should point to the directory where you installed the Android SDK.
• ANDROID_NDK_ROOT - should point to the directory where you installed the Android NDK r21.
• ANDROID_API - an integer specifying which Android API level you want to target. Suggested value is 29.

The build-all.sh script can be used to perform all the steps and build the .apk.

build-all.sh [--clean] [--release]

Options:
  --clean  Remove all leftovers from previous builds before starting.
           Results in a clean build (i.e. "from scratch").
  --release  Build in release mode. When not specified, debug mode is used.

The resulting .apk files will be placed in the build/ directory (which will be created, if it doesn't exist) right next to this script. Building GMP and Rust code will happen inside said directory, too.

This script can be used to build only the GMP library.

build-libgmp.sh --arch ARCH --build-dir BUILD_DIR --build-mode MODE
                --libs-dir LIBS_DIR [--clean]

Options:
  --arch ARCH  Specifies which architecture to build for.
               Valid values are 'aarch64', 'armv7' and 'x86_64'.
  --build-dir BUILD_DIR  Specifies the directory to use for building.
  --build-mode   Specifies the build mode.
                 Must be either 'debug' or 'release'.
  --clean  Perform a clean build (before starting,
           remove any files left over from previous builds).
  --libs-dir LIBS_DIR  Specified the directory where built libraries
                       should be copied over to.

This script can be used to build only the Rust code. Note that you must build GMP first, and that GMP's .so files must be present inside LIBS_DIR; otherwise, linking will fail.

build-librpn.sh --arch ARCH --build-dir BUILD_DIR --build-mode MODE
                --libs-dir LIBS_DIR [--clean]

Options:
  --arch ARCH  Specifies which architecture to build for.
               Valid values are 'aarch64', 'armv7' and 'x86_64'.
  --build-dir BUILD_DIR  Specifies the directory to use for building.
  --build-mode   Specifies the build mode.
                 Must be either 'debug' or 'release'.
  --clean  Perform a clean build (before starting,
           remove any files left over from previous builds).
  --libs-dir LIBS_DIR  Specified the directory where built libraries
                       should be copied over to. GMP libraries
                       should also be located here.

This script can bse used to build only the .apk. Note that you should build GMP and the Rust code first. While the app will build without native libraries, it will crash at runtime.

build-apk.sh --build-dir BUILD_DIR --build-mode MODE
             --libs-dir LIBS_DIR [--clean]

Options:
  --build-dir BUILD_DIR  Specifies the directory where to put
                         the resulting .apk files.
  --build-mode   Specifies the build mode.
                 Must be either 'debug' or 'release'.
  --clean  Perform a clean build (before starting,
           remove any files left over from previous builds).
  --libs-dir LIBS_DIR  Specified the directory where native libraries
                       can be copied from.

The main directory contains bash scripts used to perform the build and glue eveything together.

Contains the Android app files, such as the AndroidManifest.xml file, an Activity with some basic UI, and "glue" classes for interfacing with the Rust code.

Contains the Rust code. It is split into three parts:

• android.rs - JNI glue code
• lib.rs - the actual RPN implementation
• main.rs - command-line program

It is possible to build the app as a standalone executable, in which case it will take input from stdin, one line at a time, evaluate it, and print the answer.

The contents of this repository are subject to the zlib licence. For the full text of the licence, consult the LICENCE file.