Building GDAL using Android NDK on Linux

Warning! this is still a work in progress!

This procedure was developed on a Linux system using Android NDK r5b and is based on information from this thread:  http://groups.google.com/group/android-ndk/browse_thread/thread/9988752f332eff45

Cross compiling GDAL

First, a standalone toolchain is created to make the configure script easier to use. (See android-ndk-r5b/docs/STANDALONE-TOOLCHAIN.html for more details) 

~$ android-ndk-r5b/build/tools/make-standalone-toolchain.sh --platform=android-8 --install-dir=android-8-toolchain

Next, the toolchain's bin directory is added to the PATH. 

~$ export PATH=$PATH:~/android-8-toolchain/bin/

From the GDAL source directory, configure for cross compiling and make. (This assumes config.guess and config.sub where updated. Latest versions can be found here:  http://git.savannah.gnu.org/cgit/config.git/tree/)

Older GDAL versions (before r21653 - includes 1.8.0)

before compiling, patch the source with the patch given at http://trac.osgeo.org/gdal/ticket/3952

Newest GDAL versions after r22958 (including 1.9)

as below, but --without-grib is not needed anymore

All GDAL versions

gdal$ CFLAGS="-mthumb" CXXFLAGS="-mthumb" LIBS="-lsupc++ -lstdc++" \
      ./configure --host=arm-linux-androideabi --without-grib --prefix=$PROJECT/external/gdal
gdal$ make
gdal$ make install

The $PROJECT variable contains the path to where you want your Android NDK modules to live.

In $PROJECT/external/gdal, an Android.mk file is created with the following contents: 

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := gdal
LOCAL_SRC_FILES := lib/libgdal.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/include
LOCAL_EXPORT_LDLIBS := -lz
include $(PREBUILT_STATIC_LIBRARY)

Using in a JNI library

Here's a HelloGdal app to test the use of the library as part of a JNI component of an Android project.

In the project's jni directory, hello-jdal.cpp was create with the following contents: 

#include <jni.h>
#include <ogr_api.h>
#include <sstream>

extern "C" {
    JNIEXPORT jstring
    JNICALL Java_org_gdal_HelloGdal_stringFromGDAL(JNIEnv*env, jobject thiz);
}

JNIEXPORT jstring
JNICALL Java_org_gdal_HelloGdal_stringFromGDAL(JNIEnv* env, jobject thiz)
{
    OGRRegisterAll();
    std::ostringstream drivers;
    drivers << "OGR Drivers:\n";
    for (int i = 0; i < OGRGetDriverCount(); ++i)
        drivers << "\t" << OGR_Dr_GetName(OGRGetDriver(i)) << "\n";

    return env->NewStringUTF(drivers.str().c_str());
}

(Yes, I know, it's called HelloGdal, yet it lists the OGR drivers. I'll leave it as an exercise to have the GDAL drivers listed)

Here's the corresponding Android.mk file: 

LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := hello-gdal
LOCAL_SRC_FILES := hello-gdal.cpp
LOCAL_STATIC_LIBRARIES := gdal
include $(BUILD_SHARED_LIBRARY)
$(call import-module,gdal)

The path to the external GDAL module is set and the library is built using the NDK's build process. 

jni$ export NDK_MODULE_PATH=$PROJECT/external
jni$ $NDK_ROOT/ndk-build

Finally, the Java portion that uses the JNI library. 

package org.gdal;

import android.app.Activity;
import android.os.Bundle;
import android.widget.TextView;


public class HelloGdal extends Activity {
	
	public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        TextView  tv = new TextView(this);
        tv.setText( stringFromGDAL() );
        setContentView(tv);
    }
	
	public native String stringFromGDAL();
	
	static {
        System.loadLibrary("hello-gdal");
    }
    	
}

Next steps