Running native code on Android #OSDCfr 2012

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Running native code on Android OSDC.fr 2012-10-13 Cédric @deltheil C NDK ARM

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- Why using native code? -

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1. Reuse some C code

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2. Use 3rd party libraries

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3. Create portable components iOS Android ...

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4. Use hardware acceleration CPU SIMD algorithm

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5. ... because it’s fun :)

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...but How? Native Development Kit

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- Android NDK overview -

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Downloads http://developer.android.com/tools/sdk/ndk/index.html cross-platform

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Contents 1. Tools for Linux,OS X,Win to cross-compile native ARM,x86,MIPS binaries.

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Contents 2. System ﬁles and headers for Android native APIs. w/ Java Native Interface (a.k.a JNI)

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Contents 3. Easy-to-use build system: Android.mk & ndk-build (make wrapper).

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Contents 4. Documentation and code samples.

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- Steps to embed C code -

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1/3 Wrap C Java JNI C code C Java libmisc.so

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1/3 Wrap C /* 1. Wrap your C code with the JNI */ #include jstring Java_com_example_Foo_bar(JNIEnv* env, jobject thiz) { char buffer[512]; /* ... */ return (*env)->NewStringUTF(env, buffer); } namespace class method instance native interface

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2/3 Build native lib Java JNI C code C Java libmisc.so

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# 2. Generate a lib with ndk-build $ ndk-build Compile thumb : misc <= misc.c StaticLibrary : libmisc.a SharedLibrary : libmisc.so Install : libmisc.so => libs/armeabi/libmisc.so ... 2/3 Build native lib

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3/3 Java ext Java JNI C code C Java libmisc.so

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/* 3. Expose the logic via a native extension */ package com.example; public class Foo extends /* ... */ { static { System.loadLibrary("misc"); } public native String bar(); } 3/3 Java ext

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Basic Java JNI C code C Java libmisc.so

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Advanced Java JNI libA.a C Java libmisc.so A sources C code libB.a B sources 3rd party libs

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- Building 3rd party libs -

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Pro-tip Use the Standalone Toolchain.

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What? A customized install for a given platform, arch. handy

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Why? No need to write specific Android.mk makefiles: reuse existing build systems. 0 LOC Makefile autotools

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How?

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Makeﬁle [Ex. 1] jsmn JSON C parser

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[Ex. 1] jsmn 1/3 override Makefile variables

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[Ex. 1] jsmn 2/3

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[Ex. 1] jsmn 3/3 choose the proper arch... and that’s it! Full gist @ http://git.io/ndk-jsmn

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[Ex. 2] msgpack autotools binary C/C+ serialization

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[Ex. 2] msgpack 1/3

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[Ex. 2] msgpack 2/3

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[Ex. 2] msgpack 3/3 Full gist @ http://git.io/ndk-msgpack use the cross-toolchains... and that’s it!

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- Using a prebuilt library -

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Overview Android.mk Application.mk ndk-build armeabi/ libfoo.so libs/ single arch

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Overview Android.mk Application.mk armeabi-v7a/ libfoo.so ndk-build armeabi/ libfoo.so libs/ fat binary

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- Loading a native library -

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Basic Pick the right arch vs. target device

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Medium

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Custom frontal library native method (frontal lib.) Pick the right arch yourself!

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Custom: why? [2] Bypass a loadLibrary bug on ICS! [1] Properly target ARMv7 without NEON CPUs when getCpuFeatures() can’t be used at runtime (e.g. 3rd party libs) [2] see http://www.moodstocks.com/2012/03/20/ice-cream-sandwich-why-native-code-support- sucks [1] e.g. «the NVidia Tegra 2 generation SoC has a dual-core ARM Cortex-A9 CPU (lacking ARM's advanced SIMD extension—NEON)» - see http://en.wikipedia.org/wiki/Tegra

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- Quick JNI hints -

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C pointer = int ﬁeld e.g. persist a DB handle and use it throughout the JNI extensions Don’t forget to destruct it (explicitly or at finalize() time)

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C error codes C error code & string

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Resources #AltDevBlogADay NDK Part 1 & Part 2 Linux Mag., 07/11 VLC for Android Tokyo Cabinet Java Pkg

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Thanks! Questions? Comments? cedric@moodstocks.com | @deltheil