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