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February 2, 2012 13:21
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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 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) | |
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 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 | |
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. |
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