1. Android NDK Intro
Tokyo Android Meetup 21st July 2016

2. About The Presenter
Giles Payne
Android Developer - TenTen Corp
Developing for Android since Cupcake
BA Honours Mathematics and Computation, Oxford University
Country of Origin: Scotland

3. What is the Android NDK?
A set of tools for building/debugging C/C++ code to run on Android
Invoked from Java via JNI(Java Native Interface)
Available for download at https://developer.android.com/ndk/index.html

4. Android NDK - Why?
Performance
Order of magnitude speedup for performance critical code
Code reuse
Existing code C/C++ base can be ported to Android without total rewrite in Java
Large amount of libraries written in C/C++ can be reused (boost/cocos2d-x/OpenCV)
Cross platform development
iOS and other mobile OSs all support C/C++

5. NDK How To
Declare method as native with no body
public native void myNativeMethod();
Use javah to generate native stub
javah -o "MyClass.h" -classpath "/home/giles/myjavalibs" com.example.MyClass
Add implementation to stub and build C/C++ lib
extern "C" void Java_com_example_MyClass_myNativeMethod(JNIEnv* env, jobject this) {
/* do stuff */
}

6. What if I need to call back out to Java?
jclass myjclass = (*env)->GetObjectClass(env, myjobj);
jmethodID myMethodId = (*env)->GetMethodID(env, myjclass, "myMethod", "()Ljava/lang/String;");
jstring jstrResult = (jstring)(*env)->CallObjectMethod(env, myjobj, myMethodId);
Reference:
http://docs.oracle.com/javase/7/docs/technotes/guides/jni/spec/functions.html

7. Android NDK Gotchas
Managing your own reference counting
Check you have correct calls to NewStringUTF/ReleaseStringUTFChars
Create/delete global references using NewGlobalRef/DeleteGlobalRef
Library state on second run
Native libraries will not be unloaded after the calling Java activity exits!
Libraries need to be completely reinitialized when calling activity restarts. Hint:
extern int __bss_start;
extern int _end;
void reStart() {
void* bssBackup = native_backup_bss(); /* back-up somethings that need to be remembered */
memset((void*)__bss_start, 0 , _end-__bss_start); /* clear all memory that was initially zeroed */

8. Android NDK with autotools
Inspect verbose output to get the correct compiler linker flags. Might look
something like:
./configure --with-gcc=$ANDROID_NDK/toolchains/$ANDROID_NDK_TOOLCHAIN/bin/arm-linux-androideabi-gcc --with-
ld=$ANDROID_NDK/toolchains/$ANDROID_NDK_TOOLCHAIN/bin/arm-linux-androideabi-ld --build=i386-unknown-linux --
host=arm-unknown-linux CFLAGS="-w -I$ANDROID_NDK/platforms/android-18/arch-arm/usr/include -march=armv5te -
DANDROID -mtune=xscale -msoft-float -mthumb -Os -fomit-frame-pointer -fno-strict-aliasing -finline-limit=64" LDFLAGS="--
sysroot=$ANDROID_NDK/platforms/android-18/arch-arm -lgcc -no-canonical-prefixes -Wl,-z,noexecstack -lc -lm -llog"

9. NDK Chip Architecture Support
ARM - nearly all smartphones use ARM chips
x86 - recently x86 tablets becoming more common
MIPs - not widely used

10. Android NDK Debugging: The Old Way
Set up port forwarding for android emulator
adb forward tcp:1234 tcp:1234
Run gdbserver on device/emulator
gdbserver :1234 ondevicebinary args
Run gdb on PC
./arm-linux-androideabi-gdb.exe
Select file
file onPCbinary
Set shared object search path
set solib-search-path .
Run
target remote :1234

11. Android NDK Debugging: The New Way
Demo!