Compile Rust library for Android targets

There seems to be no good tutorial online on how to compile Rust library to Android target. So here’s one after a few hours of struggling.

Requirements

cargo
rustc
Android NDK

Download Android NDK

To start with, Android NDK is required. It could be downloaded from here: https://developer.android.com/ndk/downloads

We are going to show the steps for Linux in this tutorial.

wget https://dl.google.com/android/repository/android-ndk-r26b-linux.zip
unzip android-ndk-r26b-linux.zip

To compile Rust code into Android targets, we need the toolchain shipped with Android NDK located at android-ndk-r26b/toolchains/llvm/prebuilt/linux-x86_64/bin. This directory should be added to PATH environment variable.

export PATH=$PATH:/path/to/android-ndk-r26b/toolchains/llvm/prebuilt/linux-x86_64/bin

Compile Rust Library

To compile Rust library to Android, we need two things from the toolchain: ar and clang. Where ar is target architecture/version-independent and clang is architecture/version-dependent.

Take aarch64 as an example, Rust will try to run aarch64-linux-android-ar and aarch64-linux-android-clang to compile the library. So we need to make sure these two commands are available in PATH. From the Android NDK, we can find no aarch64-linux-android-ar but only llvm-ar. So we need to create a soft link to llvm-ar or copy it to aarch64-linux-android-ar.

cp /path/to/llvm-ar /path/to/aarch64-linux-android-ar

In terms of the clang, we can specify it in ~/.cargo/config.toml. The content of the file should be like this:

[target.aarch64-linux-android]
linker = "aarch64-linux-android34-clang"

[target.x86_64-linux-android]
linker = "x86_64-linux-android34-clang"

[target.i686-linux-android]
linker = "i686-linux-android34-clang"

[target.armv7a-linux-androideabi]
linker = "armv7a-linux-androideabi34-clang"

There’s one more thing to do before we could compile the library: we need to add the target to Rust. For example, if we want to compile the library to aarch64-linux-android, we need to run the following command:

rustup target add aarch64-linux-android

Now we are ready to compile the library. The command is as simple as:

cargo build --target=aarch64-linux-android --release --manifest-path /path/to/Cargo.toml

And it should work magically!

Troubleshooting

error occurred: Command “aarch64-linux-android-clang” … Permission denied

Basically it says it cannot run aarch64-linux-android-clang. You might want to double check you specified the correct linker and it is in PATH.

error occurred: Command “aarch64-linux-android-ar” … Permission denied

Basically it says it cannot run aarch64-linux-android-ar. You might want to double check you copied llvm-ar to aarch64-linux-android-ar and it is in PATH.

As of why they are Permission denied not Command not found, I have no idea.