basic NDK Java / C

[fabiodelorenzo]

in your project, create a jni directory and make sure you have a libs directory


create the file jni/example.c

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/* File : example.c */

/* A global variable */
double Foo = 3.0;

/* Compute the greatest common divisor of positive integers */
int gcd(int x, int y) {
  int g;
  g = y;
  while (x > 0) {
    g = x;
    x = y % x;
    y = g;
  }
  return g;
}

Note:
the file name is example.c and so the module will be called example
This code exposes a global variable Foo and a function gcd.
These will be the only properties the java can call.

Based on this data, create a SWIG interface file for this C code, called jni/example.i:

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/* File : example.i */
%module example

%inline %{
extern int    gcd(int x, int y);
extern double Foo;
%}

now, create the C interface to be used in java:
$ swig -java -package org.swig.simple -outdir src/org/swig/simple -o jni/example_wrap.c jni/example.i


create the make file jni/Android.mk

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# File: Android.mk
LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := example
LOCAL_SRC_FILES := example_wrap.c example.c

include $(BUILD_SHARED_LIBRARY)

from the project root folder, build the native with:
$ ndk-build
Compile thumb : example <= example_wrap.c
Compile thumb : example <= example.c
SharedLibrary : libexample.so
Install : libexample.so => libs/armeabi/libexample.so

ndk has created libs/armeabi/libexample.so to be installed on your device

Keep in mind, by default ndk-buld will only build for ARM5 (EABI5)
if you want your native software built for any other platform, use:
ndk-build APP_ABI=armeabi armeabi-v7a x86 mips
or APP_ABI=all to build it for all the platforms

alternatively, in the same directory of the Android.mk you can add a new file:
Application.mk

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APP_ABI := all

How to call the C from JAVA:

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/** Calls into C/C++ code */
    public void nativeCall()
    {
      // Call our gcd() function
     
      int x = 42;
      int y = 105;
      int g = example.gcd(x,y);

      // Manipulate the Foo global variable
     
      // Output its current value
      double foo = example.getFoo();

      // Change its value
      example.setFoo(3.1415926);

      // Restore value
      example.setFoo(foo);
    }

    /** static constructor */
    static {
        System.loadLibrary("example");
    }

note how you never declare example as you load it in System.loadLibrary("example");
also, beside it's never defined, SWIG lets you use example.getFoo() and example.setFoo(foo)