NDK使用二進制庫方法(翻譯)

NDK Prebuilt library support:

NDK使用二進制庫方法：

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.

Android NDK r5版本引入了引用二進制庫（提早編譯好的共享和靜態庫）的能力，能夠在應用中包含和使用提早編譯好的庫。這個功能使如下兩件事有幫助：

1/ 你須要分發你本身的庫給第三方的NDK開發者，可是不想分發源碼。

2/你須要引入本身的庫文件，加快構建速度。

這篇文檔將解釋上面的兩種狀況是如何實現的。

I. Declaring a prebuilt library module:

I. 定義一個預先編譯好的二進制庫模塊：

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:

每一個預先編譯好的庫在構建系統中必須定義成一個單獨的獨立模塊。這裏是一個簡單的例子，咱們假定文件「libfoo.so」和Android.mk位於同一個路徑，Android.mk內容以下：

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>.

注意，爲了定義這樣一個模塊，你只須要作如下事情：

1. 給這個模塊命名（這裏是「foo-prebuild」），這不須要和庫文件名字自己同樣

2.給 LOCAL_SRC_FILES 賦值，指向你的預編譯好的庫所在的路徑。像其餘同樣，提供和LOCAL_PATH對於的相對路徑。注意，必須保證預編譯庫的版本和目標ABI移植。

3.若是包含的是共享庫，應寫成PREBUILT_SHARED_LIBRARY，而不是BUILD_SHARED_LIBRARY。若是是靜態庫，就是PREBUILT_STATIC_LIBRARY。預編譯的模塊不會出發構建過程。只是把預編譯的共享庫拷貝到$PROJECT/obj/local路徑，靜態庫則是拷貝到$PROJECT/libs/<abi>並進行裁剪。

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 in the 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_LIBRARIES := foo-prebuilt include $(BUILD_SHARED_LIBRARY)

II.在其餘模塊中引用預編譯的庫：

簡單的在依賴庫的應用的Android.mk裏LOCAL_STATIC_LIBRARIES或LOCAL_SHARED_LIBRARIES中列出預編譯的模塊的名字就能夠了。例如，一個簡單的應用使用libfoo.so應該是：

include $(CLEAR_VARS)

LOCAL_MODULE := foo-user

LOCAL_SRC_FILES := foo-user.c

LOCAL_SHARED_LIBRARIES := foo-prebuilt

include $(BUILD_SHARED_LIBRARY)

III. Exporting headers for prebuilt libraries:

III. 導出預編譯庫的頭文件：

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.

上面的例子之因此說「簡單」，是由於實際上foo-user.c裏面的代碼會依賴預編譯庫配套的頭文件（如「foo.h」）中的定義。換句話說，foo-user.c須要有一行相似於下面的代碼 #include <foo.h> ，並且你在構建時應該提供他引用的頭文件並放置在編譯器能訪問的路徑下。一個簡單的處理方法是在預編譯的庫的定義中中使用exports。例如，假設"foo.h"是在include目錄下，咱們能夠這樣寫：

 include $(CLEAR_VARS)

LOCAL_MODULE := foo-prebuilt

LOCAL_SRC_FILES := libfoo.so

LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/include

include $(PREBUILT_SHARED_LIBRARY)

IV. Debugging prebuilt binaries:

IV. 調試預編譯的庫

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.

咱們建議你提供包含調試信息的共享庫，拷貝到$PROJECT/libs/<abi>/目錄的版本會被NDK構建系統剝離調試信息，而調試版本能夠供ndk-gdb用於調試目的。

V. ABI Selection of prebuilt binaries:

V. 預編譯庫的ABI適配

As said previously, it is crucial to provide a prebuilt shared library that is compatible with the targeted ABI during the build. To do that, check for the value of TARGET_ARCH_ABI, its value will be: armeabi => when targeting ARMv5TE or higher CPUs armeabi-v7a => when targeting ARMv7 or higher CPUs x86 => when targeting x86 CPUs mips => when targeting MIPS 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.

就像前面說的，提供一個與目標ABI版本匹配的預編譯的庫很是重要。檢查TARGET_ARCH_ABI的值，若是目標版本爲ARMv5TE或更高，值應該是 armeabi 。目標是ARMv7或更高，值應該是armeabi-v7a。目標是x86時值爲x86；目標是MIPS系列CPU時，值爲mips。注意armeabi-v7a系統能運行armeabi的文件。下面是一個例子，咱們提供兩個版本的預編譯庫，並根據ABI選擇對應版本進行拷貝:

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)




咱們假設須要拷貝的預編譯庫在如下目錄中：

Android.mk --> 上面的make文件

armeabi/libfoo.so--> armeabi 版本庫

armeabi-v7a/libfoo.so --> armeabi-v7a 版本庫

include/foo.h --> 導出頭文件

注意：記住你不是必需要提供一個armeabi-v7a版本的預編譯庫，由於armeabi版本就能夠在相關設備商跑的很好。