How to link a prebuilt shared Library to an Android NDK project?

Question

Here I used this Android.mk file in jni/ folder.

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

# Here we give our module name and source file(s)
LOCAL_MODULE    := offlineDownload
LOCAL_SRC_FILES := offline_download.c

LOCAL_SHARED_LIBRARIES :=../lib/libpackext.so.1.0
LOCAL_LDLIBS := -L$(SYSROOT)/usr/lib -llog

include $(BUILD_SHARED_LIBRARY)

And make one lib folder in project directory and put my prebuilt .so library and make one Android.mk file which contains following

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)

LOCAL_MODULE    := packext
LOCAL_SRC_FILES := libpackext.so.1.0
LOCAL_C_INCLUDES := $(LOCAL_PATH)/../include

include $(PREBUILT_SHARED_LIBRARY)

And when i use ndk-build -B command than i got undefined reference to packageExtraction . Here I use my prebuilt library functions means I can't link my prebuilt shared library to my offlinedownload library.

So any body please help me to solved out this issue.

Answer 1

Here is a complete Android.mk file for using a 3rd party shared library. The library (libffmpeg.so) is placed in the jni folder. Its "LOCAL_EXPORT_C_INCLUDES" specifies where the header files are kept for the library.

LOCAL_PATH := $(call my-dir)

include $(CLEAR_VARS)
LOCAL_MODULE := ffmpeg
LOCAL_SRC_FILES := libffmpeg.so
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/../../ffmpeg/libs/arm-linux-androideabi4.7_1/include
include $(PREBUILT_SHARED_LIBRARY)


include $(CLEAR_VARS)
LOCAL_MODULE    := ffmpegandroid
LOCAL_SRC_FILES := ffmpegandroid.c
LOCAL_SHARED_LIBRARIES := ffmpeg
include $(BUILD_SHARED_LIBRARY)

If you wanted to support multiple architectures then you could specify:

APP_ABI := armeabi armeabi-v7a x86 mips

in your jni/Application.mk and change the LOCAL_SRC_FILES to something like:

LOCAL_SRC_FILES := $(TARGET_ARCH_ABI)/libffmpeg.so

and place a libffmpeg.so at jni/armeabi/libffmpeg.so, jni/armeabi-v7a/libffmpeg.so etc ..

Answer 2

Android NDK official hello-libs CMake example

https://github.com/googlesamples/android-ndk/tree/840858984e1bb8a7fab37c1b7c571efbe7d6eb75/hello-libs

Just worked for me on Ubuntu 17.10 host, Android Studio 3, Android SDK 26, NDK 15.2. so I strongly recommend that you base your project on it.

The shared library is called libgperf , the key code parts are:

• hello-libs/app/src/main/cpp/CMakeLists.txt :

   // -L add_library(lib_gperf SHARED IMPORTED) set_target_properties(lib_gperf PROPERTIES IMPORTED_LOCATION ${distribution_DIR}/gperf/lib/${ANDROID_ABI}/libgperf.so) // -I target_include_directories(hello-libs PRIVATE ${distribution_DIR}/gperf/include) // -lgperf target_link_libraries(hello-libs lib_gperf) 

• on C++ code, use: #include <gperf.h>

• header location: hello-libs/distribution/gperf/include/gperf.h

• lib location: distribution/gperf/lib/arm64-v8a/libgperf.so

• app/build.gradle :

   android { sourceSets { main { // let gradle pack the shared library into apk jniLibs.srcDirs = ['../distribution/gperf/lib'] 

  Then, if you look under /data/app on the device, libgperf.so will be there as well.

• If you only support some architectures, see: Gradle Build NDK target only ARM

The example git tracks the prebuilt shared libraries, but it also contains the build system to actually build them as well: https://github.com/googlesamples/android-ndk/tree/840858984e1bb8a7fab37c1b7c571efbe7d6eb75/hello-libs/gen-libs

Answer 3

You have to do either one of the following:

1. Cut and paste everything except LOCAL_PATH := $(call my-dir) from your second Android.mk into your first.

2. Put the following in the end of your first Android.mk:

   $(call import-module,packext)

   Also make sure that you set your NDK_MODULE_PATH environment variable to a path where the Android.mk-file defining the module packext can be found.

You also have to change the LOCAL_SHARED_LIBRARIES in the same way mgiza said in the first answer. I suppose the packageExtraction that you got undefined reference to is in your prebuilt library so unless you have other linking problems this should solve the issue.

Answer 4

Have a look at the ndk documentation for prebuilts:

android-ndk/docs/PREBUILTS.html

You have to change

LOCAL_SHARED_LIBRARIES :=../lib/libpackext.so.1.0

to

LOCAL_SHARED_LIBRARIES := packext

Be sure that your folder containing the Android.mk for the packext module is named packext and can be found in in your NDK_MODULE_PATH.