Android NDK IntroTokyo Android Meetup 21st July 2016

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

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

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++

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 */

}

Call System.loadLibrary to load the C/C++ lib

System.loadLibrary("myLibrary");

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

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 */native_restore_bss(bssBackup); /* restore the backed-up stuff */

}

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"

NDK Chip Architecture Support

ARM - nearly all smartphones use ARM chips

x86 - recently x86 tablets becoming more common

MIPs - not widely used

Android NDK Debugging: The Old WaySet up port forwarding for android emulatoradb forward tcp:1234 tcp:1234

Run gdbserver on device/emulatorgdbserver :1234 ondevicebinary args

Run gdb on PC./arm-linux-androideabi-gdb.exe

Select filefile onPCbinary

Set shared object search pathset solib-search-path .

Runtarget remote :1234

Android NDK Debugging: The New Way

Demo!