+++
title = "Building on Android"
weight = 3
aliases = ["second-edition/extra/building-on-android/index.html"]
+++

I finally managed to get `blog_os` building on my Android phone using [termux](https://termux.com/). This post explains the necessary steps to set it up.

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<img src="building-on-android.png" alt="Screenshot of the compilation output from android" style="height: 50rem;" >


### Install Termux and Nightly Rust

First, install [termux](https://termux.com/) from the [Google Play Store](https://play.google.com/store/apps/details?id=com.termux) or from F-Droid. After installing, open it and perform the following steps:

- Install fish shell, set as default shell, and launch it:
    ```
    pkg install fish
    chsh -s fish
    fish
    ```

    This step is of course optional. However, if you continue with bash you will need to adjust some of the following commands to bash syntax.

- Install some basic tools:
    ```
    pkg install wget tar
    ```

- Add the [community repository by its-pointless](https://wiki.termux.com/wiki/Package_Management#By_its-pointless):
    ```
    wget https://its-pointless.github.io/setup-pointless-repo.sh
    bash setup-pointless-repo.sh
    ```

- Install cargo and a nightly version of rustc:
    ```
    pkg install rustc cargo rustc-nightly
    ```

- Prepend the nightly rustc path to your `PATH` in order to use nightly (fish syntax):
    ```
    set -U fish_user_paths $PREFIX/opt/rust-nightly/bin/ $fish_user_paths
    ```

Now `rustc --version` should work and output a nightly version number.

### Install Git and Clone blog_os

We need something to compile, so let's download the `blog_os` repository:

- Install git:
    ```
    pkg install git
    ```

- Clone the `blog_os` repository:
    ```
    git clone https://github.com/phil-opp/blog_os.git
    ```

If you want to clone/push via SSH, you need to install the `openssh` package: `pkg install openssh`.

### Install Xbuild and Bootimage

Now we're ready to install `cargo xbuild` and `bootimage`

- Run `cargo install`:
    ```
    cargo install cargo-xbuild bootimage
    ```

- Add the cargo bin directory to your `PATH` (fish syntax):
    ```
    set -U fish_user_paths ~/.cargo/bin/ $fish_user_paths
    ```

Now `cargo xbuild` and `bootimage` should be available. It does not work yet because `cargo xbuild` needs access to the rust source code. By default it tries to use rustup for this, but we have no rustup support so we need a different way.

### Providing the Rust Source Code

The Rust source code corresponding to our installed nightly is available in the [`its-pointless` repository](https://github.com/its-pointless/its-pointless.github.io):

- Download a tar containing the source code:
    ```
    wget https://github.com/its-pointless/its-pointless.github.io/raw/master/rust-src-nightly.tar.xz
    ```

- Extract it:
    ```
    tar xf rust-src-nightly.tar.xz
    ```

- Set the `XARGO_RUST_SRC` environment variable to tell cargo-xbuild the source path (fish syntax):
    ```
    set -Ux XARGO_RUST_SRC ~/rust-src-nightly/rust-src/lib/rustlib/src/rust/src
    ```

Now cargo-xbuild should no longer complain about a missing `rust-src` component. However it will throw an I/O error after building the sysroot. The problem is that the downloaded Rust source code has a different structure than the source provided by rustup. We can fix this by adding a symbolic link:

```
ln -s ~/../usr/opt/rust-nightly/bin ~/../usr/opt/rust-nightly/lib/rustlib/aarch64-linux-android/bin
```

Now `cargo xbuild --target x86_64-blog_os.json` and `bootimage build` should both work!

I couldn't get QEMU to run yet, so you won't be able to run your kernel. If you manage to get it working, please tell me :).
