docs/OVERVIEW.html - platform/ndk - Git at Google

 <html><body><pre>Android NDK Overview

 Introduction:

 The Android NDK is a set of tools that allows Android application developers
 to embed native machine code compiled from C and/or C++ source files into
 their 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 methods
 implemented 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&lt;something&gt;.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 Android
 devices. In particular, your applications should still be written in the Java
 programming language, handle Android system events appropriately to avoid the
 "Application Not Responding" dialog or deal with the Android application
 life-cycle.

 Note however that is is possible to write a sophisticated application in
 native code with a small "application wrapper" used to start/stop it
 appropriately.

 A good understanding of JNI is highly recommended, since many operations
 in this environment require specific actions from the developers, that are
 not 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 native
 APIs and libraries supported by the Android platform. While a typical
 Android system image includes many native shared libraries, these should
 be considered an implementation detail that might change drastically between
 updates and releases of the platform.

 If an Android system library is not explicitly supported by the NDK
 headers, then applications should not depend on it being available, or
 they risk breaking after the next over-the-air system update on various
 devices.

 Selected system libraries will gradually be added to the set of stable NDK
 APIs.


 III. NDK development in practice:
 ---------------------------------

 Here's a very rough overview of how you can develop native code with the
 Android 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 libraries
 your application needs to your application's root project directory. You
 will 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 completely
 in 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 application
 project.

 You are pretty free to organize the content of 'jni' as you want,
 the directory names and structure here will not influence the final
 generated application packages, so you don't have to use pseudo-unique
 names like com.&lt;mycompany&gt;.&lt;myproject&gt; as is the case for application
 package names.

 Note that C and C++ sources are supported. The default C++ file extensions
 supported 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 adjusting
 your 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 your
 sources to the NDK build system. Its syntax is described in details in
 the file docs/ANDROID-MK.html.

 In a nutshell, the NDK groups your sources into "modules", where each module
 can be one of the following:

   - a static library
   - a shared library

 You can define several modules in a single Android.mk, or you can write
 several Android.mk files, each one defining a single module.

 Note that a single Android.mk might be parsed several times by the build
 system 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 should
 include them explicitly in your top-level Android.mk. There is even
 a 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 current
 build file's path.


 III.4/ Writing an Application.mk build file (optional):
 - - - - - - - - - - - - - - - - - - - - - - - - - - - -

 While an Android.mk file describes your modules to the build system, the
 Application.mk file describes your application itself. See the
 docs/APPLICATION-MK.html document to understand what this file allows you
 to 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 simply
 builds *all* the modules listed from your Android.mk (and all the makefiles
 it 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/&lt;name&gt;/Application.mk, where $NDK
     points to your NDK installation path. After that, launch
     "make APP=&lt;name&gt;" 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 its
 content.


 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 use
 a 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's
 project path (Note that unstripped versions are kept for debugging
 purposes, 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    --&gt; clean generated binaries
     ndk-build  -B V=1   --&gt; 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.


 III.6/ Specifying custom output directories:
 - - - - - - - - - - - - - - - - - - - - - - -

 By default, ndk-build places all intermediate generated files under
 $PROJECT/obj. You can however select a different location by defining
 the NDK_OUT environment variable to point to a different directory.

 When defined, this variable has two effects:

   1/ It ensures that all files that normally go under $PROJECT/obj are
      stored in $NDK_OUT instead

   2/ It tells ndk-gdb to look into $NDK_OUT, instead of $PROJECT/obj for
      any symbolized (i.e. unstripped) versions of the generated binaries.


 IV. Rebuild your application package:
 - - - - - - - - - - - - - - - - - - -

 After generating the binaries with the NDK, you need to rebuild your
 Android 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 will
 embed your shared libraries, and they will be extracted automatically
 at installation time by the system when you install the package on a
 target device.


 V. Debugging support:
 - - - - - - - - - - -

 The NDK provides a helper script, named 'ndk-gdb' to very easily launch
 a native debugging session of your applications.

 Native debugging can *ONLY* be performed on production devices running
 Android 2.2 or higher, and does not require root or privileged access, as
 long as your application is debuggable.

 For more information, read docs/NDK-GDB.html. In a nutshell, native debugging
 follows 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 useful
 commands.
 </pre></body></html>
	<html><body><pre>Android NDK Overview

	Introduction:

	The Android NDK is a set of tools that allows Android application developers
	to embed native machine code compiled from C and/or C++ source files into
	their 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 methods
	implemented 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 Android
	devices. In particular, your applications should still be written in the Java
	programming language, handle Android system events appropriately to avoid the
	"Application Not Responding" dialog or deal with the Android application
	life-cycle.

	Note however that is is possible to write a sophisticated application in
	native code with a small "application wrapper" used to start/stop it
	appropriately.

	A good understanding of JNI is highly recommended, since many operations
	in this environment require specific actions from the developers, that are
	not 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 native
	APIs and libraries supported by the Android platform. While a typical
	Android system image includes many native shared libraries, these should
	be considered an implementation detail that might change drastically between
	updates and releases of the platform.

	If an Android system library is not explicitly supported by the NDK
	headers, then applications should not depend on it being available, or
	they risk breaking after the next over-the-air system update on various
	devices.

	Selected system libraries will gradually be added to the set of stable NDK
	APIs.


	III. NDK development in practice:
	---------------------------------

	Here's a very rough overview of how you can develop native code with the
	Android 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 libraries
	your application needs to your application's root project directory. You
	will 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 completely
	in 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 application
	project.

	You are pretty free to organize the content of 'jni' as you want,
	the directory names and structure here will not influence the final
	generated application packages, so you don't have to use pseudo-unique
	names like com.<mycompany>.<myproject> as is the case for application
	package names.

	Note that C and C++ sources are supported. The default C++ file extensions
	supported 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 adjusting
	your 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 your
	sources to the NDK build system. Its syntax is described in details in
	the file docs/ANDROID-MK.html.

	In a nutshell, the NDK groups your sources into "modules", where each module
	can be one of the following:

	- a static library
	- a shared library

	You can define several modules in a single Android.mk, or you can write
	several Android.mk files, each one defining a single module.

	Note that a single Android.mk might be parsed several times by the build
	system 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 should
	include them explicitly in your top-level Android.mk. There is even
	a 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 current
	build file's path.


	III.4/ Writing an Application.mk build file (optional):
	- - - - - - - - - - - - - - - - - - - - - - - - - - - -

	While an Android.mk file describes your modules to the build system, the
	Application.mk file describes your application itself. See the
	docs/APPLICATION-MK.html document to understand what this file allows you
	to 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 simply
	builds all the modules listed from your Android.mk (and all the makefiles
	it 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 its
	content.


	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 use
	a 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's
	project path (Note that unstripped versions are kept for debugging
	purposes, 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.


	III.6/ Specifying custom output directories:
	- - - - - - - - - - - - - - - - - - - - - - -

	By default, ndk-build places all intermediate generated files under
	$PROJECT/obj. You can however select a different location by defining
	the NDK_OUT environment variable to point to a different directory.

	When defined, this variable has two effects:

	1/ It ensures that all files that normally go under $PROJECT/obj are
	stored in $NDK_OUT instead

	2/ It tells ndk-gdb to look into $NDK_OUT, instead of $PROJECT/obj for
	any symbolized (i.e. unstripped) versions of the generated binaries.


	IV. Rebuild your application package:
	- - - - - - - - - - - - - - - - - - -

	After generating the binaries with the NDK, you need to rebuild your
	Android 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 will
	embed your shared libraries, and they will be extracted automatically
	at installation time by the system when you install the package on a
	target device.


	V. Debugging support:
	- - - - - - - - - - -

	The NDK provides a helper script, named 'ndk-gdb' to very easily launch
	a native debugging session of your applications.

	Native debugging can ONLY be performed on production devices running
	Android 2.2 or higher, and does not require root or privileged access, as
	long as your application is debuggable.

	For more information, read docs/NDK-GDB.html. In a nutshell, native debugging
	follows 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 useful
	commands.
	</pre></body></html>