Running native code on Android #OSDCfr 2012

* cross-compiling 3rd party libraries with the Android Standalone Toolchain,
* using prebuilt libraries,
* using NEON code while still targeting all devices,
* illustrating some Java Native Interface (JNI) tips and tricks

Transcript

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

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2. - Why using native
   code? -
   

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3. 1. Reuse some C
   code
   

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4. 2. Use 3rd party
   libraries
   

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5. 3. Create portable
   components
   iOS
   Android ...
   

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6. 4. Use hardware
   acceleration
   CPU
   SIMD algorithm
   

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7. 5. ... because it’s
   fun :)
   

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8. ...but How?
   Native Development Kit
   

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9. - Android NDK
   overview -
   

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

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

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

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

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14. Contents
    4. Documentation and
    code samples.
    

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15. - Steps to embed C
    code -
    

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16. 1/3 Wrap C
    Java
    JNI
    C code
    C
    Java
    libmisc.so
    

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17. 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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18. 2/3 Build native lib
    Java
    JNI
    C code
    C
    Java
    libmisc.so
    

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19. # 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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20. 3/3 Java ext
    Java
    JNI
    C code
    C
    Java
    libmisc.so
    

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21. /* 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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22. Basic
    Java
    JNI
    C code
    C
    Java
    libmisc.so
    

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

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24. - Building 3rd
    party libs -
    

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25. Pro-tip
    Use the Standalone Toolchain.
    

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

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

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28. How?
    

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29. Makefile
    [Ex. 1] jsmn
    JSON
    C parser
    

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

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31. [Ex. 1] jsmn 2/3
    

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

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

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34. [Ex. 2] msgpack 1/3
    

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35. [Ex. 2] msgpack 2/3
    

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

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37. - Using a prebuilt
    library -
    

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

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

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40. - Loading a native
    library -
    

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41. Basic
    Pick the right
    arch vs.
    target device
    

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42. Medium
    

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

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44. 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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45. - Quick JNI hints -
    

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46. 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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47. C error codes
    C error code
    & string
    

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

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

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