Develop Reference

Android NDK: Linking x86 shared library

I was given a shared library built on Linux x86, let's call it libA.so, and I want to use the function calls provided by this library SDK.
I am having issues building and have a few questions:
1) I will be able to build for x86, but will I be able to build for arm? I believe the answer is no, meaning I cannot run on a Nexus 5 for example.
2) The ndk-build complains of the #include that should be resolved by my LOCAL_SHARED_LIBRARIES. I am not sure why that is. My Android.mk is as follow:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := B
LOCAL_SRC_FILES := B.cpp
LOCAL_SHARED_LIBRARIES := A
LOCAL_LDLIBS := -llog
include $(BUILD_SHARED_LIBRARY)
Can someone please help me resolve my Android.mk? I don't understand why it is complaining about my include statement in B.cpp. Please let me know if I can run B in an arm environment, although the SDK I am relying on was built on x86.

If your shared library libA.so has been compiled for linux-x86, it will certainly not run on android x86 targets (mainly because it needs to be linked to Bionic C library instead of glibc), and absolutely not on android arm devices.
Then, to solve your second issue, if you can get properly compiled android shared libraries for your android targets, you would include your library this way:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := A
LOCAL_SRC_FILES := ../libA/prebuilts/$(TARGET_ARCH_ABI)/libA.so # path to libA .so file, depending on the target ABI.
LOCAL_EXPORT_C_INCLUDES := ../libA/includes # path to libA headers.
include $(PREBUILT_SHARED_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := B
LOCAL_SRC_FILES := B.cpp
LOCAL_SHARED_LIBRARIES := A
LOCAL_LDLIBS := -llog
include $(BUILD_SHARED_LIBRARY)

Android NDK/JNI: Building a shared library that depends on other shared libraries

I am writing an android app that wants to make JNI calls into a shared library built in using the NDK. The trick is this shared library calls functions provided by OTHER shared libraries. The other shared libraries are C libraries that have been compiled elsewhere.
Here's what I've tried:
My Environment:
I'm working in Eclipse. I've added native support and have a jni library. In that library I have my code and a \lib directory where I have copied my other .so files.
Attempt #1 Android.mk: Just telling it where the libs are
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := native_lib
LOCAL_SRC_FILES := native_lib.cpp
LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib1
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib2
include $(BUILD_SHARED_LIBRARY)
This builds just fine, but when I try to run I get errors indicating that dlopen(libnative_lib) failed because it couldn't load libsupport_lib1.
Coming here I found this:
Can shared library call another shared library?
which said that I needed to call load library on all necessary libraries. Great!
Attempt #2 Opening each library first
static {
System.loadLibrary("support_lib1");
System.loadLibrary("support_lib2");
System.loadLibrary("native_lib");
}
Again, this builds just fine, however when I run I get a new error:
couldn't load libsupport_lib1. findLibrary returned null.
Now we're getting somewhere. It must not be loading the libraries over to the target.
Attempt #3 Copying .so files into project/libs/armeabi
Didn't work. When Eclipse builds it deleted the files I dropped in there.
Attempt #4 Creating a new module for each library
So then I found this:
Android NDK: Link using a pre-compiled static library
It's about static libraries, but maybe I am having a similar problem. The gist is that I need to declare a module for each library. So my new Android.mk looks like this:
LOCAL_PATH := $(call my-dir)
#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib1
LOCAL_SRC_FILES := $(LOCAL_PATH)/lib/support_lib1.so
include $(BUILD_SHARED_LIBRARY)
#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib2
LOCAL_SRC_FILES := $(LOCAL_PATH)/lib/support_lib2.so
include $(BUILD_SHARED_LIBRARY)
#build native lib
include $(CLEAR_VARS)
LOCAL_MODULE := native_lib
LOCAL_SRC_FILES := native_lib.cpp
LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib1
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib/support_lib2
include $(BUILD_SHARED_LIBRARY)
This builds! Even better, armeabi has the sos now! Even BETTER I get the following messages when I try to run it (telling me that support_lib1 and 2 were opened by LoadLibrary:
Trying to load lib /data/app-lib/com.example.tst/libsupport_lib1.so
added shared lib /data/app-lib/com.example.tst/libsupport_lib1.so
no JNI_OnLoad found in /data/app-lib/com.example.tst/libsupport_lib1.so, skipping init
but then...
dlopen failed: Could not locate symbol func_that_exists_in_libsupport_lib.so referenced by libnative_lib.so
Edit: Attempt 5: Use PREBUILT_SHARED_LIBRARY
So I found this:
How can i Link prebuilt shared Library to Android NDK project?
which seems to be exactly what I'm asking. Their answer seems to be 'don't use 'build_shared_library' but instead 'use PREBUILT_SHARED_LIBRARY
Okay, let's try.
LOCAL_PATH := $(call my-dir)
#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib1
LOCAL_SRC_FILES := $(LOCAL_PATH)/lib/support_lib1.so
include $(PREBUILT_SHARED_LIBRARY)
#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib2
LOCAL_SRC_FILES := $(LOCAL_PATH)/lib/support_lib2.so
include $(PREBUILT_SHARED_LIBRARY)
#build native lib
include $(CLEAR_VARS)
LOCAL_MODULE := native_lib
LOCAL_SRC_FILES := native_lib.cpp
LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_SHARED_LIBRARIES := support_lib1 support_lib2
include $(BUILD_SHARED_LIBRARY)
Build... fails! The build complains about missing symbols now.
Edit: Attempt 6: Flatten everything
So I went back to the prebuilts documentation in the NDK. It says:
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:
Give the module a name (here 'foo-prebuilt'). This does not need to correspond to the name of the prebuilt library itself.
Assign to LOCAL_SRC_FILES the path to the prebuilt library you are providing. As usual, the path is relative to your LOCAL_PATH.
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/.
So let's try flattening everything out to match the trivial example. I copied my libraries out of their cozy /lib folder and put them in the jni root. I then did this:
LOCAL_PATH := $(call my-dir)
#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib1
LOCAL_SRC_FILES := support_lib1.so
include $(PREBUILT_SHARED_LIBRARY)
#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib2
LOCAL_SRC_FILES := support_lib2.so
include $(PREBUILT_SHARED_LIBRARY)
#build native lib
include $(CLEAR_VARS)
LOCAL_MODULE := native_lib
LOCAL_SRC_FILES := native_lib.cpp
LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_SHARED_LIBRARIES := support_lib1 support_lib2
include $(BUILD_SHARED_LIBRARY)
and... same error. Moreover I'm most definitely NOT seeing library files getting copied to $PROJECT/obj/local.
sooooo.... now what?

Your problem is with the naming convention. NDK and Android insist on the shared library names to always begin with lib. Otherwise, the libraries will not be linked properly, and not copied to the libs/armeabi folder properly, and not installed on the device (copied to /data/data/package/lib directory properly.
If you rename support_lib1.so to libsupport_1.so and support_lib2.so to libsupport_2.so, and put these two files in jni/lib directory, then your Attempt #5 will work with minor change:
LOCAL_PATH := $(call my-dir)
#get support_lib1
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib1
LOCAL_SRC_FILES := lib/libsupport_1.so
include $(PREBUILT_SHARED_LIBRARY)
#get support_lib2
include $(CLEAR_VARS)
LOCAL_MODULE := support_lib2
LOCAL_SRC_FILES := lib/libsupport_2.so
include $(PREBUILT_SHARED_LIBRARY)
#build native lib
include $(CLEAR_VARS)
LOCAL_MODULE := native_lib
LOCAL_SRC_FILES := native_lib.cpp
LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog
LOCAL_SHARED_LIBRARIES := support_lib1 support_lib2
include $(BUILD_SHARED_LIBRARY)
BTW, I don't think you need this -L$(SYSROOT)/../usr/lib.
PS Don't forget to update the Java side, too:
static {
System.loadLibrary("support_lib1");
System.loadLibrary("support_lib2");
System.loadLibrary("native_lib");
}

Not sure if this is exactly where you are at, but here's what I know about these sorts of things.
Make each prebuilt libary its own separate Makefile. Multiple targets in Android.mk tends to get wonky. Sad.
Include each make file using $(call import-add-path) and $(call import-module)
Export as much as you can from the prebuilt's make files, using the LOCAL_EXPORT_ family of variables.
Prebuilt Shared Library Android.mk
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := my_module_name
MY_LIBRARY_NAME := shared_library_name
### export include path
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/include
### path to library
LOCAL_SRC_FILES := libs/$(TARGET_ARCH_ABI)/lib$(MY_LIBRARY_NAME).so
### export dependency on the library
LOCAL_EXPORT_LDLIBS := -L$(LOCAL_PATH)/libs/$(TARGET_ARCH_ABI)/
LOCAL_EXPORT_LDLIBS += -l$(MY_LIBRARY_NAME)
include $(PREBUILT_SHARED_LIBRARY)
This is assuming that the prebuilt libaries live in a dir structure like this
+ SharedProjectFolderName
+--- Android.mk
+--- include/
+-+- libs/$(TARGET_ARCH_ABI)/
|- libshared_library_name.so
If you are not building for multiple ABI, I guess you can leave that bit out
The Project's Android.mk
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := my_jni_module
## source files here, etc...
### define dependency on the other library
LOCAL_SHARED_LIBRARIES := my_module_name
include $(BUILD_SHARED_LIBRARY)
$(call import-add-path,$(LOCAL_PATH)/path/to/myLibraries/)
$(call import-module,SharedProjectFolderName)
$(call import-module,AnotherSharedProject)
I recommend you put all shared libraries in one folder. When you say $(call import-module,SharedProjectFolderName) it looks for a folder containing an Android.mk along the search path you told it (import-add-path)
By the way, you probably shouldn't specify LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib. It should be finding the proper libs from NDK by itself. Adding more linker paths will probably confuse it. The proper way is to export the linker paths as flags from the sub-modules.
ALSO, you can use ndk-build V=1 to get a ton of info on why it can't find paths, etc

The -L option gives the linker a directory path in which to look for libraries. The -l option gives the linker a library file name to link in. Library file names must begin with "lib". Your libraries should be named libsupport_lib1.so and libsupport_lib2.so. If you do that, then this is probably what you should do (replacing attempt #1):
LOCAL_LDLIBS := -L$(SYSROOT)/../usr/lib -llog -lsupport_lib1 -lsupport_lib2
LOCAL_LDLIBS += -L$(LOCAL_PATH)/lib
The linker will prefix the library name you specify using -l with "lib" and suffix it with ".so". (Why do you have -L$(SYSROOT)/../usr/lib?)
I believe that attempts #1 and #2 failed because you did not link your libraries into your executable - they are not mentioned in a -l option. By the way, you can verify this yourself. Unzip the .apk file and look in the lib directory and subdirectories. Are your .so files in there?
Looking at the error:
but then... dlopen failed: Could not locate symbol func_that_exists_in_libsupport_lib.so referenced by libnative_lib.so
Can you supply the entire message? dlopen() loads and links libraries into the running process.

Android.mk Unable to link with libcrypto

Case:
I am building an app which uses libcrypto and libssl. I am trying to use prebuilt libcrypto.so and libssl.so and compile my application.
But I keep getting undefined reference errors.
My App/Android.mk
LOCAL_PATH := $(call my-dir)
my_LOCAL_PATH := $(LOCAL_PATH)
include $(call all-subdir-makefiles)
LOCAL_PATH := $(my_LOCAL_PATH)
common_SRC_FILES := \
src/foo.c
include $(CLEAR_VARS)
LOCAL_SRC_FILES := $(common_SRC_FILES)
LOCAL_C_INCLUDES += $(LOCAL_PATH)/../openssl/include
LOCAL_SHARED_LIBRARIES += mylibssl mylibcrypto
include $(BUILD_STATIC_LIBRARY)
My App/mylibssl/Android.mk
Building mylibssl [from a prebuilt libssl.so]
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := mylibssl
LOCAL_SRC_FILES := libssl.so
LOCAL_MODULE_TAGS := optional
LOCAL_MODULE_CLASS := SHARED_LIBRARIES
LOCAL_MODULE_SUFFIX := .so
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/../../openssl/include
include $(BUILD_PREBUILT)
My App/mylibcrypto/Android.mk
Building mylibcrypto [from a prebuilt libcrypto.so]
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := mylibcrypto
LOCAL_SRC_FILES := libcrypto.so
LOCAL_MODULE_TAGS := optional
LOCAL_MODULE_CLASS := SHARED_LIBRARIES
LOCAL_MODULE_SUFFIX := .so
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/../../openssl/include
include $(BUILD_PREBUILT)
I keep getting
error: undefined reference to 'BIO_new_mem_buf'
error: undefined reference to 'PEM_read_bio_X509_AUX'
error: undefined reference to 'BIO_free'
...
I have spent several hours trying to figure out and am totally stuck. Please help!

I am building an app which uses libcrypto and libssl... But I keep getting undefined reference errors.
error: undefined reference to 'BIO_new_mem_buf'
error: undefined reference to 'PEM_read_bio_X509_AUX'
error: undefined reference to 'BIO_free'
These are linker errors, and mean you are not linking against the OpenSSL library. Its probably a path problem, assuming you have an Android version of the OpenSSL library available.
If you need an an Android version of the OpenSSL library, then you can build it yourself or find it on Github. To build it yourself, see FIPS Library and Android (just ignore the FIPS stuff). The steps required are (and note the leading dot "." when running the script):
cd openssl-1.0.1f/
wget http://wiki.openssl.org/images/7/70/Setenv-android.sh
chmod a+x *.sh
. ./setenv-android.sh
./config shared no-ssl2 no-hw no-engine --openssldir=/usr/local/ssl/android-18/ <other options>
make depend
make all
For option two, you can search Github with https://www.google.com/q=openssl+android+site:github.com.
Building mylibcrypto [from a prebuilt libcrypto.so]
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := mylibcrypto
LOCAL_SRC_FILES := libcrypto.so
This is not going to produce expected results, and will likely result in a crash if you get it to work. The problem is Android uses 0.9.8, and you are probably building against 1.0.1.
What happens is Zygote loads OpenSSL 0.9.8 at startup (its the parent of all Android Java programs). When your app is launched, Zygote is forked so OpenSSL is already mapped into the address space. That means the version of OpenSSL you are carrying around is not loaded. Later, you crash because 0.9.8 and 1.0.1 are not binary compatible (i.e., they are ABI incompatible).
You are correct in building a wrapper shared object (mylibcrypto.so). However, mylibcrypto.so will need to statically link against OpenSSL to avoid the above problem. That is, mylibcrypto.so will need to link against libcrypto.a and libssl.a.

Compiling shared and static libraries into a shared library using the Android NDK

I have come across a problem with trying to compile cURL with SSL support into an app.
So far, I have successfully compiled the openSSL package into libcrypt.so and libssl.so.
I believe I have successfully compiled a version of libcurl.a with SSL support using the configure script and running it through the cross-chain compiler found in the NDK (under a linux environment).
Now, I am attempting to write a .so library under Eclipse that can be called by the Java code of an Android App.
Here is the file structure so far:
Project Folder ---> jni ---> include ---> curl ---> curl headers
| |
| -> openssl ---> ssl and crypto headers
|
-> libcrypto.so
-> libssl.so
-> libcurl.a
-> jniProcessRequest.c
-> Android.mk
Android.mk reads:
LOCAL_PATH := $(call my-dir)
MY_PATH := $(LOCAL_PATH)
include $(CLEAR_VARS)
LOCAL_PATH := $(MY_PATH)
LOCAL_MODULE := crypto
LOCAL_SRC_FILES := libcrypto.so
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/openssl/
include $(PREBUILT_SHARED_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := ssl
LOCAL_SRC_FILES := libssl.so
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/openssl/
include $(PREBUILT_SHARED_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := curl
LOCAL_SRC_FILES := libcurl.a
LOCAL_EXPORT_C_INCLUDES := $(LOCAL_PATH)/curl/
include $(PREBUILT_STATIC_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := JNIProcessRequest
LOCAL_SRC_FILES := JNIProcessRequest.c
LOCAL_SHARED_LIBRARIES := crypto ssl
LOCAL_STATIC_LIBRARIES := curl
LOCAL_LDLIBS := -llog
include $(BUILD_SHARED_LIBRARY)
jniProcessRequest.c:
#include <jni.h>
#include <stdlib.h>
#include "include/curl/curl.h"
#include <android/log.h>
JNIEXPORT void JNICALL Java_com_example_jniprocessrequest_MainActivity_jniProcessRequest(JNIEnv * env, jobject obje){
CURL *conn;
conn = curl_easy_init();
}
Every time I attempt to compile the above, I have undefined reference errors in Eclipse:
make: *** [obj/local/armeabi/libJNIProcessRequest.so] Error 1
undefined reference to 'curl_easy_init'
I am thinking this is some sort of linkage error but am struggling to find where the error is occurring. I have spent nearly two days trying all different methods of placing the shared libraries in differing places, switching the static libcurl with a shared libcurl, altering the Android.mk file and following tutorials on how to get cURL working in Android.
Any help would be greatly appreciated!

I believe that the problem was stemming from an incorrectly built libcurl.a static library. I replaced my library with one that was compiled by a user on GitHub and without changing any code, the linkage errors disappeared.

You use curl as a prebuilt module.
As stated by the NDK documentation you should take care to wrap it under include $(CLEAR_VARS) and (most importantly) include $(PREBUILT_STATIC_LIBRARY) directives as follow:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := foo-prebuilt
LOCAL_SRC_FILES := libfoo.a
include $(PREBUILT_STATIC_LIBRARY)
Right after take care to wrap your JNIProcessRequest module with include $(CLEAR_VARS) and include $(BUILD_SHARED_LIBRARY).
For more details please refer to the docs/PREBUILTS.html section that you can find within your Android NDK folder (it looks like this).

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

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.

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 ..

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

You have to do either one of the following:
Cut and paste everything except LOCAL_PATH := $(call my-dir) from your second Android.mk into your first.
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.

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.