android ndk 纵览

Introduction:
The Android NDK is a set of tools that allows Android application developersto embed native machine code compiled from C and/or C++ source files intotheir application packages.
IMPORTANT: The Android NDK can only be used to target Android system images running Cupcake (a.k.a 1.5) or later versions of the platform.
1.0 and 1.1 system images are specifically *not* supported due to subtle ABI and toolchain changes that happened for the 1.5 release.

I. Android NDK Goals:---------------------
The Android VM allows your application's source code to call methodsimplemented in native code through the JNI. In a nutshell, this means that:
- Your application's source code will declare one or more methods with the 'native' keyword to indicate that they are implemented through native code. E.g.:
native byte[] loadFile(String filePath);
- You must provide a native shared library that contains the implementation of these methods, which will be packaged into your application's .apk. This library must be named according to standard Unix conventions as lib<something>.so, and shall contain a standard JNI entry point (more on this later). For example:
libFileLoader.so
- Your application must explicitly load the library. For example, to load it at application start-up, simply add the following to its source code:
static { System.loadLibrary("FileLoader"); }
Note that you should not use the 'lib' prefix and '.so' suffix here.

The Android NDK is a complement to the Android SDK that helps you to:
- Generate JNI-compatible shared libraries that can run on the Android 1.5 platform (and later) running on ARM CPUs.
- Copy the generated shared libraries to a proper location of your application project path, so they will be automatically added to your final (and signed) .apks
- In later revisions of the NDK, we intend to provide tools that help debug your native code through a remote gdb connection and as much source/symbol information as possible.
Moreover, the Android NDK provides:
- A set of cross-toolchains (compilers, linkers, etc..) that can generate native ARM binaries on Linux, OS X and Windows (with Cygwin)
- A set of system headers corresponding to the list of stable native APIs supported by the Android platform. This corresponds to definitions that are guaranteed to be supported in all later releases of the platform.
They are documented in the file docs/STABLE-APIS.html
IMPORTANT: Keep in mind that most of the native system libraries in Android system images are not frozen and might changed drastically, or even deleted, in later updates and releases of the platform.
- A build system that allow developers to only write very short build files to describe which sources need to be compiled, and how. The build system deals with all the hairy toolchain/platform/CPU/ABI specifics. Moreover, later updates of the NDK can add support for more toolchains, platforms, system interfaces without requiring changes in the developer's build files (more on this later).

II. Android NDK Non-Goals:--------------------------
The NDK is *not* a good way to write generic native code that runs on Androiddevices. In particular, your applications should still be written in the Javaprogramming language, handle Android system events appropriately to avoid the"Application Not Responding" dialog or deal with the Android applicationlife-cycle.
Note however that is is possible to write a sophisticated application innative code with a small "application wrapper" used to start/stop itappropriately.
A good understanding of JNI is highly recommended, since many operationsin this environment require specific actions from the developers, that arenot necessarily common in typical native code. These include:
- Not being able to directly access the content of VM objects through direct native pointers. E.g. you cannot safely get a pointer to a String object's 16-bit char array to iterate over it in a loop.
- Requiring explicit reference management when the native code wants to keep handles to VM objects between JNI calls.

The NDK only provides system headers for a very limited set of nativeAPIs and libraries supported by the Android platform. While a typicalAndroid system image includes many native shared libraries, these shouldbe considered an implementation detail that might change drastically betweenupdates and releases of the platform.
If an Android system library is not explicitly supported by the NDKheaders, then applications should not depend on it being available, orthey risk breaking after the next over-the-air system update on variousdevices.
Selected system libraries will gradually be added to the set of stable NDKAPIs.

III. NDK development in practice:---------------------------------
Here's a very rough overview of how you can develop native code with theAndroid NDK:
1/ Place your native sources under $PROJECT/jni/...
2/ Write $PROJECT/jni/Android.mk to describe your sources to the NDK build system
3/ Optional: write $PROJECT/jni/Application.mk to describe your project in more details to the build system. You don't need one to get started though, but this allows you to target more than one CPU or override compiler/linker flags (see docs/APPLICATION-MK.html for all details).
4/ Build your native code by running "$NDK/ndk-build" from your project directory, or any of its sub-directories.
The last step will copy, in case of success, the stripped shared librariesyour application needs to your application's root project directory. Youwill then need to generate your final .apk through the usual means.
Now, for a few more details:

III.1/ Configuring the NDK:- - - - - - - - - - - - - -
Previous releases required that you run the 'build/host-setup.sh'script to configure your NDK. This step has been removed completelyin release 4 (a.k.a. NDK r4).

III.2/ Placing C and C++ sources:- - - - - - - - - - - - - - - - -
Place your native sources under the following directory:
$PROJECT/jni/
Where $PROJECT corresponds to the path of your Android applicationproject.
You are pretty free to organize the content of 'jni' as you want,the directory names and structure here will not influence the finalgenerated application packages, so you don't have to use pseudo-uniquenames like com.<mycompany>.<myproject> as is the case for applicationpackage names.
Note that C and C++ sources are supported. The default C++ file extensionssupported by the NDK is '.cpp', but other extensions can be handled as well(see docs/ANDROID-MK.html for details).
It is possible to store your sources in a different location by adjustingyour Android.mk file (see below).

III.3/ Writing an Android.mk build script:- - - - - - - - - - - - - - - - - - - - - -
An Android.mk file is a small build script that you write to describe yoursources to the NDK build system. Its syntax is described in details inthe file docs/ANDROID-MK.html.
In a nutshell, the NDK groups your sources into "modules", where each modulecan be one of the following:
- a static library - a shared library
You can define several modules in a single Android.mk, or you can writeseveral Android.mk files, each one defining a single module.
Note that a single Android.mk might be parsed several times by the buildsystem so don't assume that certain variables are not defined in them.By default, the NDK will look for the following build script:
$PROJECT/jni/Android.mk
If you want to define Android.mk files in sub-directories, you shouldinclude them explicitly in your top-level Android.mk. There is evena helper function to do that, i.e. use:
include $(call all-subdir-makefiles)
This will include all Android.mk files in sub-directories of the currentbuild file's path.

III.4/ Writing an Application.mk build file (optional):- - - - - - - - - - - - - - - - - - - - - - - - - - - -
While an Android.mk file describes your modules to the build system, theApplication.mk file describes your application itself. See thedocs/APPLICATION-MK.html document to understand what this file allows youto do. This includes, among others:
- The exact list of modules required by your application.
- The CPU architecture(s) to generate machine code for.
- Optional information, like whether you want a release or debug build, specific C or C++ compiler flags and others that should apply to all modules being built.
This file is optional: by default the NDK will provide one that simplybuilds *all* the modules listed from your Android.mk (and all the makefilesit includes) and target the default CPU ABI (armeabi).
There are two ways to use an Application.mk:
- Place it under $PROJECT/jni/Application.mk, and it will be picked up automatically by the 'ndk-build' script (more on this later)
- Place it under $NDK/apps/<name>/Application.mk, where $NDK points to your NDK installation path. After that, launch "make APP=<name>" from the NDK directory.
This was the way this file was used before Android NDK r4. It is still supported for compatibility reasons, but we strongly encourage you to use the first method instead, since it is much simpler and doesn't need modifying / changing directories of the NDK installation tree.
Again, see docs/APPLICATION-MK.html for a complete description of itscontent.

III.5/ Invoke the NDK build system:- - - - - - - - - - - - - - - - - -
The preferred way to build machine code with the NDK is to use the'ndk-build' script introduced with Android NDK r4. You can also usea second, legacy, method that depends on creating a '$NDK/apps' subdirectory.
In both cases, a successful build will copy the final stripped binary modules(i.e. shared libraries) required by your application to your application'sproject path (Note that unstripped versions are kept for debuggingpurposes, there is no need to copy unstripped binaries to a device).

1: Using the 'ndk-build' command: ---------------------------------
The 'ndk-build' script, located at the top of the NDK installation path can be invoked directly from your application project directory (i.e. the one where your AndroidManifest.xml is located) or any of its sub-directories. For example:
cd $PROJECT $NDK/ndk-build
This will launch the NDK build scripts, which will automatically probe your development system and application project file to determine what to build.
For example:
ndk-build ndk-build clean --> clean generated binaries ndk-build -B V=1 --> force complete rebuild, showing commands
By default, it expects an optional file under $PROJECT/jni/Application.mk, and a required $PROJECT/jni/Android.mk.
On success, this will copy the generated binary modules (i.e. shared libraries) to the appropriate location in your project tree. You can later rebuild the full Android application package either through the usual 'ant' command, or the ADT Eclipse plug-in.
See docs/NDK-BUILD.html for a more complete description of what this script does and which options it can take.

2: Using the $NDK/apps/<name>/Application.mk: ---------------------------------------------
This build method was the only one before Android NDK r4 and is only supported for compatibility reason. We strongly recommend you to migrate to using the 'ndk-build' command as soon as possible, since we may remove legacy support in a later NDK release.
It requires the following:
1. Creating a sub-directory named $NDK/apps/<name>/ under your NDK installation directory (not your project path).
Where <name> is an arbitrary name to describe your application to the NDK build system (no spaces allowed).
2. Write $NDK/apps/<name>/Application.mk, which then requires a definition for APP_PROJECT_PATH that points to your application project directory.
3. Go to the NDK installation path on the command line then invoke the top-level GNUMakefile, as in:
cd $NDK make APP=<name>
The result will be equivalent to the first method, except for the fact that intermediate generated files will be placed under $NDK/out/apps/<name>/

IV. Rebuild your application package:- - - - - - - - - - - - - - - - - - -
After generating the binaries with the NDK, you need to rebuild yourAndroid application package files (.apk) using the normal means, i.e.either using the 'ant' command or the ADT Eclipse plug-in.
See the Android SDK documentation for more details. The new .apk willembed your shared libraries, and they will be extracted automaticallyat installation time by the system when you install the package on atarget device.

V. Debugging support:- - - - - - - - - - -
The NDK provides a helper script, named 'ndk-gdb' to very easily launcha native debugging session of your applications.
Native debugging can *ONLY* be performed on production devices runningAndroid 2.2 or higher, and does not require root or privileged access, aslong as your application is debuggable.
For more information, read docs/NDK-GDB.html. In a nutshell, native debuggingfollows this simple scheme:
1. Ensure your application is debuggable (e.g. set android:debuggable to "true" in your AndroidManifest.xml)
2. Build your application with 'ndk-build', then install it on your device/emulator.
3. Launch your application.
4. Run 'ndk-gdb' from your application project directory.
You will get a gdb prompt. See the GDB User Manual for a list of usefulcommands.</pre><p> </p>

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