How to build multiple projects in the correct dependency order with Android NDK?

Question

I have a series of existing libraries which I need to re-use in an Android application. The layout is similar to:

\Libraries\libOne
\Libraries\libTwo [Static library]
\Libraries\libThree
\Applications\MyApplication\ [Application]

libTwo depends on libOne, and libThree depends on libTwo. How can I get the build system to build all of the libraries in the correct order? I'm trying to use Eclipse, but if necessary I can use the command line.

All of these libraries will eventually be referenced by a Java application (and use JNI to interact with them). Any clues on how I setup the Android.mk / Application.mk files?

I've tried using BUILD_STATIC_LIBRARY for libTwo, but it doesn't actually output any files! I was expecting a libTwo.a file, but nothing gets compiled or built.

Do I write one Android.mk in the application? Or an Android.mk for each project?

Answer 1

OK, now I see your edit, and this makes it possible to answer the specific question.

You must have at least one Android.mk file for your application if you want to use Android NDK to build your native library/ies. This is not a requirement, though. It is OK to build it though Cmake, or a "standalone toolchain" with "traditional" makefiles, or with a MS Visual Studio plugin, or any other way. It is the result that matters. The result is a shared object built with a compatible compiler for a bionic runtime.

It makes goode sense to put the library in ${project_root}/libs/armeabi/ directory (for ARM v6 compatible devices, other subdirectories for x86, MIPS, arm v7a) to allow the APK builder pack it correctly, to allow app installer to unpack the correct version (compatible with the device processor) into /data/data/${package_name}/lib directory on the device, and finally to be able to use System.loadLibrary(short_name) to use it from Java. But it is also quite possible to pack the so file differently, unpack it manually, and load it from any place on the device file system (provided your app has permission to write and read this file).

But if we filter out exotic cases, it is much more comfortable to have an Android.mk in the ${project_root}/jni directory. In terms of ndk-build command, each library is a separate MODULE, but all three may be defined in one Android.mk file. On the other hand, if your libraries are isolated (e.g. come from separate 3rd parties), you will probably prefer to create three Android.mk files. Luckily, ndk-build is nothing but a wrapper around gnu make, and the simple include statement in Android.mk works as in any other makefiles.

In summary, your case is probably covered by a simple Applications/MyApplication/ [Application]/jni/Android.mk file:

include ../../Libraries/libOne/Android.mk
include ../../Libraries/libTwo/Android.mk
include ../../Libraries/libThree/Android.mk

I don't know what dependency you have between libOne and libTwo, but for libOne the file Libraries/libOne/Android.mk will look like

LOCAL_PATH = $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE    := libOne
LOCAL_SRC_FILES := first.c
include $(BUILD_STATIC_LIBRARY)

and Libraries/libThree/Android.mk

LOCAL_PATH = $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE    := libThree
LOCAL_SRC_FILES := third.c
LOCAL_C_INCLUDES := $(LOCAL_PATH)/../libOne $(LOCAL_PATH)/../libTwo
LOCAL_STATIC_LIBRARIES := libOne libTwo
include $(BUILD_SHARED_LIBRARY)

You should run ndk-build from Applications/MyApplication/ [Application] directory - either from command prompt, or through Eclipse ADT plugin.

update the same may be expressed by one Android.mk file in jni directory:

LOCAL_PATH = ../../Libraries/libOne
include $(CLEAR_VARS)
LOCAL_MODULE    := libOne
LOCAL_SRC_FILES := first.c
include $(BUILD_STATIC_LIBRARY)

LOCAL_PATH = ../../Libraries/libThree
include $(CLEAR_VARS)
LOCAL_MODULE    := libThree
LOCAL_SRC_FILES := third.c
LOCAL_C_INCLUDES := $(LOCAL_PATH)/../libOne $(LOCAL_PATH)/../libTwo
LOCAL_STATIC_LIBRARIES := libOne libTwo
include $(BUILD_SHARED_LIBRARY)