Introduction to the Android NDK
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This document provides an introduction to the Android NDK (Native Development Kit). It discusses that the NDK allows developing parts of Android apps using native code written in C/C++ for performance reasons or hardware access. Using native code adds complexity since it must be compiled for each hardware platform and interfaced with Java code. The document outlines NDK development, including compiling native libraries for different ABIs (CPU architectures), debugging native code, and considerations for standard C/C++ compatibility on Android.
Introduction to the Android NDK
- 1. Introduction to the Android NDK Sebastian Mauer GDG Aachen CodeFest Karlsruhe February 20th, 2014
- 2. Who am I? Sebastian Mauer GDG Aachen Co-Lead Software Engineer CS Student I don’t work for Google…yet
- 3. Part I: NDK? What is that?
- 5. Two kinds of Apps Apps in the DalvikVM (that’s the kind of apps you probably know) Native Apps (created using the NDK)
- 6. 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
- 7. It just works™ • The DalvikVM is already optimized for the x86 Platform • Apps relying on the Android SDK / Dalvik Bytecode will automatically benefit from Platform-specific Optimizations (like SSE & Co.)
- 8. From Source to Bytecode Java Sourcecode Java Bytecode (.class) Dalvik Bytecode (.dex) JAR Archive Dalvik VM Java VM
- 9. Android VM Dalvik VM App App App App App App Linux Kernel Hardware
- 10. ONE DOES NOT SIMPLY RUN CODE ON MULTIPLE PLATFORMS
- 11. NOT SURE IF NDK CAN HELP ME RUN CODE FASTER OR MAKE THINGS EVEN MORE COMPLICATED
- 12. Part II: Going native. The NDK.
- 13. 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
- 14. But why? • Performance e.g., complex algorithms, multimedia applications, games • Differentiation app that takes advantage of direct CPU/HW access e.g., using SSSE3 for optimization • Fluid and lag-free animations • Software code reuse
- 15. Why not?
- 17. 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
- 19. 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.
- 20. NDK Development in a Nutshell C/C++Code Makefile ndk-build APP_ABI := all or APP_ABI := x86 SDK APIs Java* calls GDB debug through jni Android* Applications Java Framework Java Application JNI Native Libs Bionic C Library NDK APIs
- 21. NDK Anatomy Dalvik VM Dalvik Bytecode Java Native Interface (JNI) NDK compiled binary Native Platform
- 22. Compatibility with Standard C/C++ • Bionic C Library: Lighter than standard GNU C Library Not POSIX compliant pthread support included, but limited No 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
- 23. Pick One • By default, libstdc++ is used. It lacks: Standard C++ Library support (except some headers) C++ exceptions support RTTI support • Fortunately, you have other libs available with the NDK: Runtime Exceptions RTTI STL system No No No gabi++ Yes Yes No stlport Yes Yes Yes gnustl Yes Yes Yes
- 25. 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 Put APP_ABI=all inside Application.mk Run ndk-build… ARM v7a libs are built ARM v5 libs are built x86 libs are built mips libs are built 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
- 26. Fat Binaries libs/armeabi Use lib/armeabi libraries libs/armeabi-v7a libs/x86 … APK file Use lib/armeabi-v7a libraries Use lib/x86 libraries
- 27. Q&A
- 28. Thanks for your attention.