Compiling open source libraries with Android NDK: Part 2

This is a repost from my old blog.

For the second part of this series, we will deal with the compiling of open source libraries that have several makefiles.

Android NDK r5 added support for prebuilt libraries and also included standalone toolchains that could be used to compile open-source libraries for Android. These are very useful, and lets you tap into the pool of open-source goodies that desktop developers enjoy. Essentially, we will be using the same makefiles but with a cross-compiler.

I am using Mac OS X, but the steps should also work on a Linux machine. If you are a Windows user and know how to compile open-source libraries for Android on a Windows machine, please leave a comment.

Here, I will use the fftw3 library as an example. Get the library from the FFTW website.

1. Use Eclipse to create a new Android project (let’s name it FourierTest) in your workspace. This uses Android ADT to create the files required by ndk-build.
2. Unpack the fftw3 library in a directory parallel to the project directory that you have just created.

3. Inside the project directory, create a new build.sh file with the following contents:

   #!/bin/sh
   # FourierTest/build.sh
   # Compiles fftw3 for Android
   # Make sure you have NDK_ROOT defined in .bashrc or .bash_profile
   
   INSTALL_DIR="`pwd`/jni/fftw3"
   SRC_DIR="`pwd`/../fftw-3.2.2"
   
   cd $SRC_DIR
   
   export
   PATH="$NDK_ROOT/toolchains/arm-linux-androideabi-4.4.3/prebuilt/darwin-x86/bin/:$PATH"
   export SYS_ROOT="$NDK_ROOT/platforms/android-8/arch-arm/"
   export CC="arm-linux-androideabi-gcc --sysroot=$SYS_ROOT"
   export LD="arm-linux-androideabi-ld"
   export AR="arm-linux-androideabi-ar"
   export RANLIB="arm-linux-androideabi-ranlib"
   export STRIP="arm-linux-androideabi-strip"
   
   mkdir -p $INSTALL_DIR
   ./configure --host=arm-eabi --build=i386-apple-darwin10.8.0
   --prefix=$INSTALL_DIR LIBS="-lc -lgcc"
   
   make
   make install
   
   exit 0
   

   At this point, your directory structure should look like the following:

   <workspace>/
       FourierTest/
           AndroidManifest.xml
           build.sh
           ...
           src/
       fftw-3.2.2/
           configure
           ...
   

   The build.sh script given above tells the makefile to use the Android cross-compiler instead of the system's compiler. In particular,

   • INSTALL_DIR tells make to install the compiled library in our project's jni directory
   • PATH tells make to look for our tool chain in the NDK directory. Note that you might have to change this value - explore your NDK directory to make sure that the path exists
   • SYS_ROOT tells make where to look for system libraries and header files
   • ./configure --host=arm-eabi --build=i386-apple-darwin10.8.0 tells make that we are cross-compiling using a i386 architecture for an ARM architecture.

   You most likely have to change the PATH variable and the --build parameter.

4. Open up Terminal and cd to your project directory

5. Run build.sh
6. The compiled library will be installed in FourierTest/jni/fftw3. To use this library, create the following Android.mk file in FourierTest/jni/fftw3:

   # FourierTest/jni/fftw3/Android.mk
   LOCAL_PATH := $(call my-dir)
   
   include $(CLEAR_VARS)
   
   LOCAL_MODULE := fftw3
   
   LOCAL_SRC_FILES := lib/libfftw3.a
   LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/include
   
   include $(PREBUILT_STATIC_LIBRARY)