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

2. - Why using native
code? -

3. 1. Reuse some C
code

4. 2. Use 3rd party
libraries

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

6. 4. Use hardware
acceleration
SIMD CPU algorithm

7. 5. ... because it’s
fun :)

8. ...but How?
Native Development Kit

9. - Android NDK
overview -

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

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

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

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

14. Contents
4. Documentation and
code samples.

15. - Steps to embed C
code -

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

17. 1/3 Wrap C
/* 1. Wrap your C code with the JNI */
#include <jni.h>
jstring
Java_com_example_Foo_bar(JNIEnv* env, jobject thiz) {
char buffer[512];
/* ... */
return (*env)->NewStringUTF(env, buffer);
}
namespace class method native interface instance

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

19. 2/3 Build native lib
# 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
...

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

21. 3/3 Java ext
/* 3. Expose the logic via a native extension */
package com.example;
public class Foo extends /* ... */ {
static {
System.loadLibrary("misc");
}
public native String bar();
}

22. Basic
Java
libmisc.so
Java
JNI
C
C code

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

24. - Building 3rd
party libs -

25. Pro-tip
Use the Standalone Toolchain.

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

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

28. How?

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

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

31. [Ex. 1] jsmn 2/3

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

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

34. [Ex. 2] msgpack 1/3

35. [Ex. 2] msgpack 2/3

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

37. - Using a prebuilt
library -

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

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

40. - Loading a native
library -

41. Basic
Pick the right
arch vs.
target device

42. Medium

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

44. Custom: why?
[1] Properly target ARMv7 without NEON CPUs
when getCpuFeatures() can’t be used at
runtime (e.g. 3rd party libs)
[2] Bypass a loadLibrary bug on ICS!
[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
[2] see http://www.moodstocks.com/2012/03/20/ice-cream-sandwich-why-native-code-support-
sucks

45. - Quick JNI hints -

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)

47. C error codes
C error code
& string

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

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