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Using Prebuilt Libraries

The NDK supports the use of prebuilt libraries, both static and shared. There are two principal use cases for this functionality:

  • Distributing your own libraries to third-party NDK developers without distributing your sources.
  • Using a prebuilt version of your own libraries to speed up your build.

This page explains how to use prebuilt libraries.

Declaring a Prebuilt Library

You must declare each prebuilt library you use as a single independent module. To do so, perform the following steps:

  1. Give the module a name. This name does not need to be the same as that of the prebuilt library, itself.
  2. In the module's Android.mk file, assign to LOCAL_SRC_FILES the path to the prebuilt library you are providing. Specify the path relative to the value of your LOCAL_PATH variable.

    Note: You must make sure to select the version of your prebuilt library appropriate to your target ABI. For more information on ensuring library support for ABIs, see Selecting ABIs for Prebuilt Libraries.

  3. Include PREBUILT_SHARED_LIBRARY or PREBUILT_STATIC_LIBRARY, depending on whether you are using a shared (.so) or static (.a) library.

Here is a trivial example that assumes the prebuilt library libfoo.so resides in the same directory as the Android.mk file that describes it.

LOCAL_PATH := $(call my-dir)

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

In this example, the name of the module is the same as that of the prebuilt library.

The build system places a copy of your prebuilt shared library into $PROJECT/obj/local, and another copy, stripped of debug information, into $PROJECT/libs/<abi>. Here, $PROJECT is the root directory of your project.

Referencing the Prebuilt Library from Other Modules

To reference a prebuilt library from other modules, specify its name as the value of the LOCAL_STATIC_LIBRARIES or LOCAL_SHARED_LIBRARIES variable in the Android.mk files associated with those other modules.

For example, the description of a module using libfoo.so might be as follows:

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

Here, LOCAL_MODULE is the name of the module referring to the prebuilt; LOCAL_SHARED_LIBRARIES is the name of the prebuilt, itself.

Exporting Headers for Prebuilt Libraries

The code in foo-user.c depends on specific declarations that normally reside in a header file, such as foo.h, distributed with the prebuilt library. For example, foo-user.c might have a line like the following:

#include <foo.h>

In such a case, you need to provide the header and its include path to the compiler when you build the foo-user module. A simple way to accomplish this task is to use exports in the prebuilt module definition. For example, as long as header foo.h is located under the include directory associated with the prebuilt module, you can declare it as follows:

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 the build system exports the path to the prebuilt library's include directory, prepending that path onto the value of the LOCAL_C_INCLUDES for the module dependent on it.

This operation allows the build system to find the necessary headers.

Debugging Prebuilt Libraries

We recommend that you provide prebuilt shared libraries containing debug symbols. The NDK build system always strips the symbols from the version of the library that it installs into $PROJECT/libs/<abi>/, but you can use the debug version for debugging with ndk-gdb.

Selecting ABIs for Prebuilt Libraries

You must make sure to select the right version of your prebuilt shared library for your targeted ABI. The TARGET_ARCH_ABI variable in the Android.mk file can point the build system at the appropriate version of the library.

For example, assume that your project contains two versions of library libfoo.so:

armeabi/libfoo.so
x86/libfoo.so

The following snippet shows how to use TARGET_ARCH_ABI so that the build system selects the appropriate version of the library:

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)

If you have specified armeabi as the value of TARGET_ARCH_ABI, the build system uses the version of libfoo.so located in the armeabi directory. If you have specified x86 as the value TARGET_ARCH_ABI, the build system uses the version in the x86 directory.

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