NDK Prebuilt library support:
-----------------------------

Android NDK r5 introduced support for prebuilt libraries (shared and
static), i.e. the ability to include and use, in your applications,
prebuilt version of libraries.

This feature can be useful for two things:

1/ You want to distribute your own libraries to third-party NDK developers
   without distributing your sources.

2/ You want to use a prebuilt version of your own libraries to speed up
   your build.

This document explains how this support works.


I. Declaring a prebuilt library module:
---------------------------------------

Each prebuilt library must be declared as a *single* independent module to
the build system. Here is a trivial example where we assume that the file
"libfoo.so" is located in the same directory than the Android.mk below:

   LOCAL_PATH := $(call my-dir)

   include $(CLEAR_VARS)
   LOCAL_MODULE := foo-prebuilt
   LOCAL_SRC_FILES := libfoo.so
   include $(PREBUILT_SHARED_LIBRARY)

Notice that, to declare such a module, you really only need the following:

1. Give the module a name (here 'foo-prebuilt'). This does not need to
   correspond to the name of the prebuilt library itself.

2. Assign to LOCAL_SRC_FILES the path to the prebuilt library you are
   providing. As usual, the path is relative to your LOCAL_PATH.

   IMPORTANT: You *must* ensure that the prebuilt library corresponds
              to the target ABI you are using. More on this later.

3. Include PREBUILT_SHARED_LIBRARY, instead of BUILD_SHARED_LIBRARY, if
   you are providing a shared, library. For static ones, use
   PREBUILT_STATIC_LIBRARY.

A prebuilt module does not build anything. However, a copy of your prebuilt
shared library will be copied into $PROJECT/obj/local, and another will be
copied and stripped into $PROJECT/libs/<abi>.

II. Referencing the prebuilt library in other modules:
------------------------------------------------------

Simply list your prebuilt module's name in the LOCAL_STATIC_LIBRARIES or
LOCAL_SHARED_LIBRARIES declaration int he Android.mk of any module that
depends on them.

For example, a naive example of a module using libfoo.so would be:

    include $(CLEAR_VARS)
    LOCAL_MODULE := foo-user
    LOCAL_SRC_FILES := foo-user.c
    LOCAL_SHARED_LIBRARY := foo-prebuilt
    include $(BUILD_SHARED_LIBRARY)


III. Exporting headers for prebuilt libraries:
----------------------------------------------

The example above was called 'naive' because, in practice, the code in
foo-user.c is going to depend on specific declarations that are normally
found in a header file distributed with the prebuilt library (e.g. "foo.h").

In other words, foo-user.c is going to have a line like:

  #include <foo.h>

And you need to provide the header and its include path to the compiler
when building the foo-user module.

A simple way to deal with that is to use exports in the prebuilt module
definition. For example, assuming that a file "foo.h" is located under
the 'include' directory relative to the prebuilt module, we can write:

   include $(CLEAR_VARS)
   LOCAL_MODULE := foo-prebuilt
   LOCAL_SRC_FILES := libfoo.so
   LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/include
   include $(PREBUILT_SHARED_LIBRARY)

The LOCAL_EXPORT_C_INCLUDES definition here ensures that any module that
depends on the prebuilt one will have its LOCAL_C_INCLUDES automatically
prepended with the path to the prebuilt's include directory, and will thus
be able to find headers inside that.


IV. Debugging prebuilt binaries:
--------------------------------

We recommend you to provide prebuilt shared libraries that contain debug
symbols. The version that is installed into $PROJECT/libs/<abi>/ is always
stripped by the NDK build system, but the debug version will be used for
debugging purposes with ndk-gdb.


V. ABI Selection of prebuilt binaries:
--------------------------------------

As said previously, it is crucial to provide a prebuilt shared library
that is compatible with the targetted ABI during the build. To do that,
check for the value of TARGET_ARCH_ABI, its value will be:

   armeabi     => when targetting ARMv5TE or higher CPUs
   armeabi-v7a => when targetting ARMv7 or higher CPUs
   x86         => when targetting x86 CPUs

Note that armeabi-v7a systems can run armeabi binaries just fine.

Here's an example where we provide two versions of a prebuilt library
and select which one to copy based on the target ABI:

    include $(CLEAR_VARS)
    LOCAL_MODULE := foo-prebuilt
    LOCAL_SRC_FILES := $(TARGET_ARCH_ABI)/libfoo.so
    LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/include
    include $(PREBUILT_SHARED_LIBRARY)

Here. we assume that the prebuilt libraries to copy are under the
following directory hierarchy:

    Android.mk            --> the file above
    armeabi/libfoo.so     --> the armeabi prebuilt shared library
    armeabi-v7a/libfoo.so --> the armeabi-v7a prebuilt shared library
    include/foo.h         --> the exported header file

NOTE: Remember that you don't need to provide an armeabi-v7a prebuilt
      library, since an armeabi one can easily run on the corresponding
      devices.