Init of Android The /init process in Android is different than a typical Linux system. It has two main parts - a boot part and a daemon part. The boot part sets up directories and parses the /init.rc file. The daemon part runs in a forever loop handling events from four file descriptors for devices, properties, signals, and keychords. It is responsible for device management, property updates, restarting services, and special key combinations. The /init process is statically linked so it can run independently of the system.

1. Init of Android
2010.12.10
Tetsuyuki Kobayashi
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2. Who am I?
 20+ years involved in embedded systems
 10 years in real time OS, such as iTRON
 10 years in embedded Java Virtual Machine
 Now GCC, Linux, QEMU, Android, …
 Blogs
 http://d.hatena.ne.jp/embedded/ (Personal)
 http://blog.kmckk.com/ (Corporate)
 Twitter
 @tetsu_koba
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3. Today's topic
 /init
 Boot part of init
 Daemon part of init
 Check it out using debugger
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4. /init
 Quite different from usual Linux
system
 Add ”init=/init” in kernel boot
parameter. Kernel searches only the
following init
 /sbin/init
 /etc/init
 /bin/init
 /bin/sh

5. /init
 Android init is static linked. So it can
run standalone
 (Linux /sbin/init is dynamically
linked. If some trouble in dynamic
link system, the whole system won't
work)

6. Component of init
 See the source code of init
 system/core/init
 Init is made of 2 parts: boot part
and daemon part
 At first, boot part is executed
once
 After that daemon part executed
repeatedly

7. int main(int argc, char **argv)
{
/* Boot part */
for (;;) {
/* Daemon part */
}
}

8. Boot part
 Setup the directories
 /dev
 /proc
 /sys
 Parse /init.rc and execute it
 Yokohama Android Platform club
has a lot of information for this
topic (written in Japanese)

9. Daemon part
 Waiting for 4 fds by poll system
call and handle these in forever
loop
 device_fd
 property_set_fd
 signal_recv_fd
 keychord_fd

10. device_fd
 Receiving the device status
notification from kernel and create
or remove device files under /dev
 In usual Linux system, udevd do
this.

11. property_set_fd
 Waiting for write request for
system property
 Init process maintains master data
of system property. The other
processes share it in read only
mode by android ashmem
mechanism.
 (Let's talk this in other session)

12. signal_recv_fd
 Waiting for exiting child process
and finish them by waitpid system
call
 If the child process is registered as
service, restart it as registered
info.

13. keychord_fd
 Monitoring special key comination
and start the associated service
 Effective only if system property
”ro.debuggable” == ”1” ||
“init.svc.adbd” == “running”
 No services use this in default
init.rc

14. See also
 KMC blog ”init of Android”
 http://blog.kmckk.com/archives/3137191.html
 This page have links of Yokohama Android
Platform club, too.
 Written in Japanese

15. Q&A
Thank you for listening!
Any comments to blogs are welcome.
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