Android startup tour - init

I’ve been working on the Android Go team for close to two months now, our goal is to make Android run well on sub-$100 phones.

As part of my work, I have to jump between different parts of Android - some days are spent profiling application code, other days digging into frameworks, sometimes looking at the kernel.

One of my investigations recently was around Android startup, so the goal of this series of post is to summarize and highlight interesting/important points of the process.

Android’s init

If you’ve used Unix-like OS, you’ve probably heard of the init process. This is pid 1 on the OS. It is started by the kernel and runs as a daemon. Some init processes include launchd (on Mac OS) and systemd (on various Linux distributions like Ubuntu and Debian).

Android has its own init process ¹, the source is found here.

rc files

The README goes into a lot more details about how it works, especially around the rc files, but here are the main points:

1. Configuration for init is defined in rc files
2. These files contain actions and services
3. An action is a trigger and a list of commands
4. Commands look like shell commands, e.g. mkdir, chown, start
5. A service is a program, such a binary, that is started by init

It’s probably instructive to look at some example rc files: this folder contains some rc files for Android that are copied to the root of the Android device, and hence the folder name rootdir.

init.rc

The primary file is init.rc. It isn’t particularly long, at 700+ lines, and it’s possible to trace through it to figure out what exactly it’s doing. But here are high level overview:

1. creates directories
2. mounts device onto filesystem
3. writes files the rest of the system needs
4. changes ownership of files and directories

other rc files

Looking at the top of init.rc we can see some import statements.

import /init.environ.rc
import /init.usb.rc
import /init.${ro.hardware}.rc
import /vendor/etc/init/hw/init.${ro.hardware}.rc
import /init.usb.configfs.rc
import /init.${ro.zygote}.rc

The import strategy adopted by init is described in the README, and the simplified description is that it recursively traverses all the imported rc files to extend the configuration.

The init.${ro.hardware}.rc file is a vendor specific file, for example marlin (the Google Pixel XL), has its rc file here.

There are also other rc files located in system/core, for example lmkd, the low memory killer, has a rc file here, which describes how to start lmkd, what class it is, what user group it should be started as, etc.

Starting up

So here is a summary of what init process looks like:

1. Run init
2. Parse init.rc files
3. Parse imported rc files recursively
4. Triggers a bunch of actions
5. Starts a bunch of services

In another blog post we will look at a very important service called zygote.