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Inter-process communicationof Android

Tetsuyuki Kobayashi

2011.6.3 updated

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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)

http://kobablog.wordpress.com/(English)

Twitter

@tetsu_koba

new

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Today's topic

Inter process communications in Android

Inter process method invocation using Binder

Ashmem

System Properties

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Processes running on Android$ ps ...root 37 1 248 156 c00aef2c 0000875c S /sbin/ueventdsystem 42 1 768 260 c022950c afd0b6fc S /system/bin/servicemanagerroot 43 1 3824 564 ffffffff afd0bdac S /system/bin/voldroot 44 1 3796 560 ffffffff afd0bdac S /system/bin/netdroot 45 1 628 264 c02588c0 afd0c0cc S /system/bin/debuggerdradio 46 1 4336 672 ffffffff afd0bdac S /system/bin/rildroot 47 1 62224 27576 c00aef2c afd0b844 S zygotemedia 48 1 16828 3736 ffffffff afd0b6fc S /system/bin/mediaserverbluetooth 49 1 1216 572 c00aef2c afd0c59c S /system/bin/dbus-daemonroot 50 1 776 316 c02a8424 afd0b45c S /system/bin/installdkeystore 51 1 1704 432 c02588c0 afd0c0cc S /system/bin/keystoreshell 52 1 696 336 c0050934 afd0c3ac S /system/bin/shroot 53 1 3356 160 ffffffff 00008294 S /sbin/adbdsystem 67 47 172464 32596 ffffffff afd0b6fc S system_serversystem 115 47 80028 20728 ffffffff afd0c51c S com.android.systemuiapp_24 124 47 80732 20720 ffffffff afd0c51c S com.android.inputmethod.latinradio 135 47 87848 20324 ffffffff afd0c51c S com.android.phoneapp_18 144 47 89136 24160 ffffffff afd0c51c S com.android.launcherapp_7 165 47 86136 22736 ffffffff afd0c51c S android.process.acoreapp_0 197 47 73996 17472 ffffffff afd0c51c S com.android.deskclockapp_14 208 47 75000 18464 ffffffff afd0c51c S android.process.mediaapp_3 219 47 72228 17652 ffffffff afd0c51c S com.android.bluetoothapp_25 234 47 85336 17836 ffffffff afd0c51c S com.android.mmsapp_26 254 47 74656 19080 ffffffff afd0c51c S com.android.emailapp_27 266 47 74912 18100 ffffffff afd0c51c S com.android.providers.calendarapp_1 285 47 71616 16280 ffffffff afd0c51c S com.android.protipsapp_19 293 47 72184 16572 ffffffff afd0c51c S com.android.musicapp_21 301 47 74728 17208 ffffffff afd0c51c S com.android.quicksearchboxapp_28 311 47 75408 18040 ffffffff afd0c51c S com.cooliris.mediashell 323 52 856 316 00000000 afd0b45c R ps$

More than 30 processes (200+ threads).

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ActivityManager

WindowManager

AlarmManager

Activity

Kernel

Inter Process Communication

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Abstraction of Inter Process Communication

Binder

AIDL

IntentMore abstract

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Inter Process Communication of Android

Intent

The highest level abstraction

Inter process method invocationAIDL: Android Interface Definition Language

binder

kernel driver

ashmem

shared memory

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Intent (1)

The highest level abstraction IPC in Android

Requests are queued and handled sequentially.

Powerful enough between applications

Implemented using inter process method invocation

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Intent (2)

Loosely coupling

The sender need not choose the receiver

The sender can specify ”action”. Then proper receiver is chosen and handles it.VIEW + http://... → Browser opens the url.VIEW + geo:0,0?q=Tokyo → Map app shows a map of Tokyo.

CALL + tel:01234567 → Phone app dials the number.

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Inter process method invocation

used for system services

needs multi-thread programming

wrapped by AIDL to hide complexity

implemented using binder

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Invoking method via interface

caller

callee

In the same process

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Inter-process invocation

caller

callee

callee

caller

interface

interface

interface

How?

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Inter-process invocation

caller

callee

Binder in kernel

callee

caller

Proxy

Binder Thread

Stub

interface

interface

interface

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android.os.Parcel

”flatten” ”unflatten”

transmit

Delivering arguments of method

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<<interface>>

Proxy Stub

Describing in UML ...

implements

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<<interface>>

Proxy Stub

Describing in UML ...

caller

callee

calls

implements

extends

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<<interface>>

Proxy Stub

AIDL

caller

callee

Auto generated from .aidl file

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ActivityManager

Kernel

Binder Thread #1

Main Thread

Looperqueue

3:”onPause”is called in mainthread

1:Call ”schedulePauseActivity”across process

Use case: Who calls ”onPause” in Activity?

2:Send message

by Handler

Activity

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Binder

/dev/binder

Not for general purpose. Tuned for specific transaction.

Multi-thread awareHave internal status per thead

(CF. Socket have internal status per fd.)

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Comparing binder with socket

socket binder

internal status

associated to FD

associated to PID(FD can be shared among threads in the same process)

read & write operation

as a stream done at once by ”ioctl”

network transparency

Yes Noexpects local only

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Transaction of Binder (1)

if (ioctl(fd, BINDER_WRITE_READ, &bwt ) >= 0) err = NO_ERROR;else err = -errno;

write buffer

read buffer

write_size

write_consumed

write_buffer

read_size

read_consumed

read_buffer

binder_write_read

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Transaction of Binder (2)

Process A

Process B

Binder

Process A

Binder

Process B

Copy memory by copy_from _user

Copy memory by copy_to_user

Then, wake up process B

Process A and B have different memory space.They can not see each other.

Kernel

Kernel

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AshmemAndroid / Anonymous SHared MEMory

subsystem

$(ANDROID_TOP)/system/core/cutils/ashmem.hint ashmem_create_region(const char *name, size_t size) → returns

fd

int ashmem_set_prot_region(int fd, int prot)

int ashmem_pin_region(int fd, size_t offset, size_t len)

int ashmem_unpin_region(int fd, size_t offset, size_t len)

kernel reclaim not ‘pin’ ed memory

Similar to weak reference of Java. useful to implement cache.

android.os.MemoryFile from Java program

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System Properties of Android

'Key-Value store' type data base for basic system information.

Shared among all processes.

Init process has the master data.

The other processes have read-only copy.

Init process handles write request to the master data.

implemented using ashmem before Gingerbread.

At Gingerbread changed not to use ashmem.

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Example of System Properties ...[ro.bt.bdaddr_path]: [/sys/module/board_mahimahi/parameters/bdaddr][ro.ril.def.agps.mode]: [2][ro.ril.fast.dormancy.rule]: [1][ro.ril.fd.scron.timeout]: [4][ro.ril.fd.scroff.timeout]: [3][ro.ril.gprsclass]: [10][init.svc.adbd]: [running][ril.reload.count]: [1][init.svc.bootanim]: [stopped][hw.keyboards.0.devname]: [mahimahi-keypad][hw.keyboards.65537.devname]: [mahimahi-keypad][hw.keyboards.131075.devname]: [h2w headset][net.hostname]: [android_200142d4dfd0d25c][dev.bootcomplete]: [1][gsm.version.ril-impl]: [HTC-RIL 2.2.1001G (Jul 16 2010,21:52:13)][gsm.sim.operator.numeric]: [][gsm.sim.operator.alpha]: [][gsm.sim.operator.iso-country]: [][gsm.sim.state]: [ABSENT][gsm.current.phone-type]: [1][wlan.driver.status]: [ok][init.svc.wpa_supplicant]: [running] ...

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Special Key of System Properties

Starts from ”ro.” is read-only.

”ctl.start” / ”ctl.stop” is command to start/stop the service.

property_set("ctl.stop", "bootanim"); stops boot animation service

Starts from ”init.svc.” shows the state of the service.

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Reading System Property

Kernel

init Other processes

/dev/__properties__(on tmpfs)

mmap (read/write) mmap (read only)

Just read the property in each process.

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Writing System Property

Kernel

init Other processes

/dev/__properties__(on tmpfs)

mmap (read/write)

mmap (read only)

Send write request to init through socketsocket

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Actual code of initialize System Properties

fd = open("/dev/__properties__", O_RDWR | O_CREAT, 0600); if (fd < 0) return -1;

if (ftruncate(fd, size) < 0) goto out;

data = mmap(NULL, size, PROT_READ | PROT_WRITE, MAP_SHARED, fd, 0); if(data == MAP_FAILED) goto out;

close(fd);

fd = open("/dev/__properties__", O_RDONLY); if (fd < 0) return -1;

unlink("/dev/__properties__");

Give this fd to other processes.

No longer access by file name.

$(ANDROID_TOP)/system/core/init/property_service.c

/dev is a tmpfs

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Resources

3rd session of Yokohama Android Platform clubhttp://www.yokohama.android-pf.org/study/dai-san-kai-benkyou-kai

1st session of Android SDK work grouphttp://www.android-group.jp/index.php?%CA%D9%B6%AF%B2%F1%2FAndroid ...

KMC bloghttp://blog.kmckk.com/archives/3611344.html

http://blog.kmckk.com/archives/3613707.html

http://blog.kmckk.com/archives/3627888.html

http://blog.kmckk.com/archives/3645070.html

http://blog.kmckk.com/archives/3647635.html

http://blog.kmckk.com/archives/3617229.html

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Q&A and demo

http://www.kmckk.co.jp/eng/kzma9/http://www.kmckk.co.jp/eng/jet_index.html