Introduction to the Android NDK

Sebastian Mauer GDG Aachen

CodeFest Karlsruhe February 20th, 2014

Who am I?Sebastian MauerGDG Aachen Co-Lead CS StudentSoftware Engineer I don’t work for Google…yet

Part I: NDK? What is that?

Android Platforms

MIPS x86ARM

Two kinds of Apps

Apps in the DalvikVM(that’s the kind of apps you probably know)

Native Apps(created using the NDK)

One VM to rule them all

• Dalvik is a Virtual Machine (VM)

• A VM is a common abstraction across different hardware platforms

• „Translates“ VM Bytecode to platform specific instructions

It just works™

• The DalvikVM is already optimized for the x86 Platform

• Apps relying on the Android SDK / Dalvik Bytecodewill automatically benefit from Platform-specific Optimizations (like SSE & Co.)

From Source to BytecodeJava

Sourcecode

Java Bytecode (.class)

Dalvik Bytecode (.dex)

JAR Archive

Dalvik VM

Java VM

Android VM

Hardware

Linux Kernel

Dalvik VM App

App

App

App

App

App

ONE DOES NOT SIMPLY

RUN CODE ON MULTIPLE PLATFORMS

NOT SURE IF NDK CAN HELP ME RUN CODE FASTER

OR MAKE THINGS EVEN MORE COMPLICATED

Part II: Going native. The NDK.

What’s the NDK?• NDK stands for Native Development Kit

• Allows to compile C/C++ code to native (read: platform specific) executables/libraries.

• Build scripts/toolkit to incorporate native code in Android apps via the Java Native Interface (JNI)

• Has to be compiled for every platform you want to support

But why?

• Performancee.g., complex algorithms, multimedia applications, games

• Differentiation app that takes advantage of direct CPU/HW accesse.g., using SSSE3 for optimization

• Fluid and lag-free animations

• Software code reuse

Why not?

What could possibly go wrong?

• Performance improvements are not guaranteed

• In fact, you could make it worse (read: slower).

• Added complexity (Java/C++ Interop, Multiple platforms)

• Somewhat harder to debug

What’s an NDK app?It’s an Android application that uses native libraries. !Libraries are .so files, usually found inside libs/CPU_ABI/. !These libs can be generated from native sources inside jni folder, game engines, or required by other 3rd party libraries. !There is no 100% native application. Even an application purely written in C/C++, using native_app_glue.h, will be executed in the context of the Dalvik Virtual Machine.

NDK Development in a NutshellC/C++Code Makefile ndk-build Java* calls GDB debug

Java Framework

Java Application

SDK APIs

JNI

Native Libs

Android* Applications

NDK APIs

Bionic C Library

APP_ABI := all or APP_ABI := x86

through jni

NDK Anatomy

Native Platform

Dalvik VMDalvik Bytecode

NDK compiled binary

Java Native Interface

(JNI)

Compatibility with Standard C/C++

• Bionic C Library: Lighter than standard GNU C LibraryNot POSIX compliantpthread support included, but limitedNo System-V IPCs Access to Android* system properties

• Bionic is not binary-compatible with the standard C library

• It means you generally need to (re)compile everything using the Android NDK toolchain

Pick One

Runtime Exceptions RTTI STL

system No No No

gabi++ Yes Yes No

stlport Yes Yes Yes

gnustl Yes Yes Yes

• By default, libstdc++ is used. It lacks: Standard C++ Library support (except some headers) C++ exceptions supportRTTI support

• Fortunately, you have other libs available with the NDK:

Compile for all the platforms.If you have the source code of your native libraries, you can compile it for several CPU architectures by setting APP_ABI to all in the Makefile “jni/Application.mk”:! APP_ABI=all

The NDK will generate optimized code for all target ABIs You can also pass APP_ABI variable directly to ndk-build, and specify each ABI: ndk-build APP_ABI=x86

ARM v7a libs are built

ARM v5 libs are built x86 libs are built

mips libs are built

Put APP_ABI=all inside Application.mkRun ndk-build…

Fat Binaries

Use lib/armeabi libraries

Use lib/armeabi-v7a libraries

Use lib/x86 libraries

libs/armeabi-v7a

libs/x86

libs/armeabi

APK file

…

Q&A

Thanks for your attention.