Using cygwin with the Android NDK on Windows

This tutorial illustrates how to setup and use the Android NDK under Windows. It will use cygwin for compiling the native code. It has been tested on Windows XP and Windows 7.
This guide assumes that you have Eclipse with ADT and the Android SDK version 3 (1.5) up and running.
There are three important paths:

Eclipse Workspace       D:\Dev\workspace-android
NDK                     D:\Dev\SDKs\android-ndk-r4b
Cygwin                  C:\Cygwin

I am using these paths as they appear on my computer. Please adapt them to use system if necessary. Note that the paths MUST NOT CONTAIN SPACES.
The code for this tutorial is available here

Download NDK

Go to http://developer.android.com/sdk/ndk/index.html and download the Android NDK for Windows. At the time of writing, android android-ndk-r4b was the latest version. Copy the folder into D:\dev\SDKs\

Install Cygwin

Download setup.exe from http://cygwin.com/. The direct link is http://cygwin.com/setup.exe. Execute setup.exe and select a server to download the Cygwin files from. Then a huge list appears where you can select the components to download.
Add “Devel/make” and “Shells/bash”. Search for “make” and “shell” to find them. Press next to download. I installed all files to C:\cygwin.

Create Android Project

Create a new standard Android project. I called it HelloNDK and used the following code.

package org.sample.hellondk;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;

public class HelloNDK extends Activity {
 /** Called when the activity is first created. */
 @Override
 public void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  setContentView(R.layout.main);

  System.loadLibrary("hellondk");
  int result = sayHello();
  Log.i("HelloNDK", "" + result);
 }

 private native int sayHello();
}

Make sure to compile it using the Android SDK. Javac.exe won’t work! Running this code should fail, as there is no native library yet.

Create Java Native Interface

Make sure your PATH contains your Java SDKs /bin directory. Open a terminal (cmd.exe) and enter javah. Receiving something like this means everything is alright.

Now, in the terminal, go to the root of your project in Eclipse’s workspace D:\Dev\workspace-android\HelloNDK. Enter

javah.exe -classpath bin/classes -d jni org.sample.hellondk.HelloNDK

When no error occurs, the compilation was successful. Since some comments below report from difficulties with this step, you might want to check if HelloNDK.class is really located in bin/classes/org/pielot/hellondk/. If not, try changing bin/classes/… to bin/… or src/… . It appears to have worked for others.
Now you should find a new folder jni in your project, containing the file org_pielot_hellondk_HelloNDK.h

/* DO NOT EDIT THIS FILE - it is machine generated */
#include
/* Header for class org_sample_hellondk_HelloNDK */

#ifndef _Included_org_pielot_hellondk_HelloNDK
#define _Included_org_pielot_hellondk_HelloNDK
#ifdef __cplusplus
extern "C" {
#endif
/*
 * Class:     org_pielot_hellondk_HelloNDK
 * Method:    sayHello
 * Signature: ()I
 */
JNIEXPORT jint JNICALL Java_org_sample_hellondk_HelloNDK_sayHello
  (JNIEnv *, jobject);

#ifdef __cplusplus
}
#endif
#endif

For my convenience, I usually create a .bat file including the above javah command and put it into the project folder.

Implement the Native Interface

Now we have to provide an implementation of the generated header file. Create org_pielot_hellondk_HelloNDK.c in the same folder as org_sample_hellondk_HelloNDK.h and fill it with:

#include "org_sample_hellondk_HelloNDK.h"

JNIEXPORT int JNICALL Java_org_sample_hellondk_HelloNDK_sayHello
 (JNIEnv * env, jobject obj) {

 return 42;
}

Inform the Compiler what Files to Compile

Next, we have to inform the NDK compiler what files should be compiled. In the /jni folder we therefore create “Android.mk”:

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := hellondk
LOCAL_SRC_FILES := org_sample_hellondk_HelloNDK.c

include $(BUILD_SHARED_LIBRARY)

Compiling the Native Code

In the HelloNDK project folder create a batch file named “make.bat”. Fill it with:

@echo on

@set BASHPATH="C:\cygwin\bin\bash"
@set PROJECTDIR="/cygdrive/d/dev/workspace-android/hellondk"
@set NDKDIR="/cygdrive/d/dev/SDKs/android-ndk-r4b/ndk-build"

%BASHPATH% --login -c "cd %PROJECTDIR% && %NDKDIR%

@pause:

This file will NOT WORK WHEN EXECUTED FROM WITHIN ECLIPSE. Thus, always find it with the Windows Explorer and execute it by double-clicking.
If successful it should look like

In the HelloNDK project directory you now should find libs/armeabi/libhellondk.so created.

Loading and Testing library

If you now run the HelloNDK Activity, you should see no exceptions. In LogCat, something like

12-05 13:42:45.311: INFO/HelloNDK(24329): 42

should appear. Voila! You have done it!
One last note: if you are just working on the C code, Eclipse will not realize that the native lib has been updated. To make sure that the latest version of the lib will be used, mark the HelloNDK project in Eclipse’s project explorer and hit F5.