Develop Reference

Use a shared library with Android NDK

I have a C++ project compiled in a shared library. I need to use that library (.so file) in Android with the NDK.
The idea is that someone else would compile the C++ project and give me the .so file, so that I don't have to compile it on my computer.
I need to reference the .so file in my Android.mk file, and all the solutions I tried did not work. That's where I'm stuck.
So:
is it possible to do what I described without creating one shared library per platform?
how can I reference my .so library in my Android.mk? All the solutions I tried did not work.
To reproduce everything from scratch, I did these steps:
1°) Create a HelloWorld C++ project:
Test.h:
#include <iostream>
class Test {
public:
Test();
~Test();
static std::string getTest();
};
Test.cpp:
#include "Test.h"
std::string Test::getTest()
{
return "Hey!!\n";
}
2°) Create the shared library:
g++ -Wall -shared -fPIC -o libTest.so Test.cpp
This gaves me a libTest.so file, which I can use on a C++ project. Now I want to use it on an Android project.
3°) Create a basic empty Android project:
package: com.so.test
name: CppTest
4°) Create a "Test.java" file next to the MainActivity.java file containing:
package com.so.test;
public class Test {
static {
System.loadLibrary("hellondk");
}
public native String getMessage();
}
5°) Open a terminal and:
cd /path/to/my/android/project/app/src/main/java
javac com/so/test/Test.java
javah com.so.test.Test
This creates the "com_so_test_Test.h" file.
cd /path/to/my/android/project
mkdir jni
mv app/src/main/java/com_so_test_Test.h jni
touch jni/com_so_test_Test.cpp
6°) Add the following code in "com_so_test_Test.cpp" file:
#include <jni.h>
#include <stdio.h>
#include "com_so_test_Test.h"
#include "Test.hpp"
JNIEXPORT jstring JNICALL Java_com_so_test_Test_getMessage(JNIEnv* env, jobject ob)
{
return env->NewStringUTF(Test::getTest().c_str());
}
7°) Copy "Test.h" and "libTest.so" files created on steps 1 & 2 into "jni" folder
8°) Create an "Android.mk" file in the "jni" folder containing:
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := hellondk
LOCAL_SRC_FILES := com_so_test_Test.cpp
include $(BUILD_SHARED_LIBRARY)
9°) Create an "Application.mk" file in the "jni" folder containing:
APP_STL := stlport_static
10°) Back in the terminal, run:
/path/to/your/ndk/ndk-build
And that's the step where I'm stuck. I need to reference the libTest.so file in the Android.mk, but all the solutions I tried did not work. Any idea?
EDIT: here is the error I get:
[arm64-v8a] Compile++ : hellondk <= com_so_test_Test.cpp
[arm64-v8a] SharedLibrary : libhellondk.so
./obj/local/arm64-v8a/objs/hellondk/com_so_test_Test.o: In function `Java_com_so_test_Test_getMessage':
/Users/timsandra/Desktop/CppTest/jni/com_so_test_Test.cpp:8: undefined reference to `Test::getTest()'
clang++: error: linker command failed with exit code 1 (use -v to see invocation)
make: *** [obj/local/arm64-v8a/libhellondk.so] Error 1

Android Studio JNI with C++ UnsatisfiedLinkError

I am using Android Studio 2.1.2 I am not using Experimental Plugin
Please Check the following files and Check the error I am getting.
I solved the issue. Edited file is as follows. They way I fixed it may not be the correct way as I am setting property to use deprecated way, but it works. Experimental plugin can still be unstable. I will try with experimental plugin soon.
build.gradle from Module
sourceSets.main {
jniLibs.srcDir 'src/main/libs'
/*jni.srcDirs = [] not using this, I commented this. Please check SO links which explained when to use this and when not to use this*/
}
following 4 files are in jni folder of main
Android.mk
LOCAL_PATH := $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := mylib
LOCAL_SRC_FILES := HelloJni.cpp
include $(BUILD_SHARED_LIBRARY)
Application.mk
APP_ABI := all
HelloJni.cpp
#include <jni.h>
#include <Header.h>
JNIEXPORT jstring JNICALL Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni(JNIEnv *env,jobject thiz)
{
env-> NewStringUTF ( "Hellofrom JNI!");
}
Header.h
#include <jni.h>;
using namespace std;
#ifndef HEADER
#define HEADER
extern "C" {
JNIEXPORT jstring JNICALL Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni(JNIEnv *env, jobject obj);
}
#endif //NDKSAMPLEAPP_HEADER_H
MainActicity.java
static
{
System.loadLibrary("mylib");
}
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
TextView tvHello=(TextView)findViewById(R.id.tvHello);
tvHello.setText(getStringFromJni());
}
public native String getStringFromJni();
Here when I hover on method getStringFromJni it shows Can't resolve corresponding JNI function
I have set NDK path in Project Structure and in Path environment variable too.
I am getting following error
Process: com.example.m1035325.ndksampleapp, PID: 12831
java.lang.UnsatisfiedLinkError: No implementation found for java.lang.String com.example.m1035325.ndksampleapp.MainActivity.getStringFromJni() (tried Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni and Java_com_example_m1035325_ndksampleapp_MainActivity_getStringFromJni__)
at com.example.m1035325.ndksampleapp.MainActivity.getStringFromJni(Native Method)
I searched a lot on SO also but I am not getting what I missed?
No error now , above error is fixed. Please check my answer to this question.

I think the problem is in your Android.mk file:
LOCAL_SOURCE_FILE := HelloJni.cpp
AFAIK the Android build system doesn't use variable of that name. It should be:
LOCAL_SRC_FILES := HelloJni.cpp

One important change I made is in file gradle.properties, is as follows
android.useDeprecatedNdk=true
so its related to Android Studio version 2.1.2 , for this version experimental plugin is the default option to use and the approach I am using is deprecated.
I will be trying with experimental plugin soon.

about mangled name of functions in c++

ı have closed source shared library that links against libmedia.so from android
but unforunatly in >android 5.0 ABI changes that made by google broke my closed source lib (thanx google)
now ı have the following error --> dlopen failed cannot locate symbol "_ZN7android11MediaPlayer13setDataSourceEPKcPKNS_11KeyedVectorINS_7String8ES4_EE" referenced by "mylib.so"
then ı hexedited my closed source library(mylib.so) to load myhack.so (that ı built myself) instead of libmedia.so (so ı simply made a trick)
here is the Android.mk of myhack.so library:
include $(CLEAR_VARS)
LOCAL_SRC_FILES := hack.cpp
LOCAL_SHARED_LIBRARIES := libmedia
LOCAL_MODULE := myhack
LOCAL_MODULE_TAGS := optional
LOCAL_MODULE_CLASS := SHARED_LIBRARIES
include $(BUILD_SHARED_LIBRARY)
and here is the hack.cpp:
extern "C" void _ZN7android11MediaPlayer13setDataSourceERKNS_2spINS_17IMediaHTTPServiceEEEPKcPKNS_11KeyedVectorINS_7String8ES9_EE();
extern "C" void _ZN7android11MediaPlayer13setDataSourceEPKcPKNS_11KeyedVectorINS_7String8ES4_EE() {
return _ZN7android11MediaPlayer13setDataSourceERKNS_2spINS_17IMediaHTTPServiceEEEPKcPKNS_11KeyedVectorINS_7String8ES9_EE();
}
let me explain a bit:
1-as I said I hexedited my closed source lib to load myhack.so instead of libmedia.so (source file and android.mk file of myhack.so are in above)
2-I linked myhack.so to libmedia.so(as you can see) to provide other libmedia functions via myhack.so
LOCAL_SHARED_LIBRARIES := libmedia
3-
lost symbol: _ZN7android11MediaPlayer13setDataSourceEPKcPKNS_11KeyedVectorINS_7String8ES4_EE
current symbol in libmedia.so: _ZN7android11MediaPlayer13setDataSourceERKNS_2spINS_17IMediaHTTPServiceEEEPKcPKNS_11KeyedVectorINS_7String8ES9_EE
so if lost symbol is called, it will return original function
my question is should I use extern "C" void or extern "C" int..... int or void? which one? setDataSource is a function and I dont think it is returning integer value so it should be void ı think but ım not %100 sure
could anyone help me please? thanx

Sooner or later you will get in troble with it. I remember problems with skia on project I work on. Some vendors were doing small changes with interfaces and that was causing various crashes.
To find return type you can investigate sources for android, steps are below:
You can use https://demangler.com/, to see signature of your function, it should be:
android::MediaPlayer::setDataSource(android::sp<android::IMediaHTTPService> const&, char const*, android::KeyedVector<android::String8, android::String8> const*)
now lets search android sources for such signature, you can find it here:
http://androidxref.com/6.0.1_r10/xref/frameworks/av/media/libmedia/mediaplayer.cpp#148
so its return type is status_t, which is typedef int status_t;, so as you have assumed its int.

Linker trouble when using LuaJIT prebuilt static library with android under cygwin

I think I spent most of yesterday unsuccessfully wrestling with this, any help would greatly appreciated and make me extremely happy! Even a next step to try to find the root of the issue is something I'm stuck on at the moment!
I have an Android 2.2 project that's trying to reference a prebuilt LuaJIT static library but ndk-build gives me this error:
test_android.cpp:25: undefined reference to `luaL_newstate'
I built LuaJIT as liblua.a, I've placed that in the root of my JNI directory with the relevant headers. I've have one Android.mk as shown below:
LOCAL_PATH:= $(call my-dir)
include $(CLEAR_VARS)
LOCAL_MODULE := lua
LOCAL_SRC_FILES := liblua.a
include $(PREBUILT_STATIC_LIBRARY)
include $(CLEAR_VARS)
LOCAL_MODULE := test
LOCAL_CFLAGS := -Werror
LOCAL_SRC_FILES := test_android.cpp
LOCAL_LDLIBS := -llog -lGLESv2
LOCAL_STATIC_LIBRARIES := lua
include $(BUILD_SHARED_LIBRARY)
In test_andrdoid.cpp I've got this code:
extern "C"
{
#include "lua.h"
#include "lauxlib.h"
#include "lualib.h"
}
void test()
{
lua_State* lua = lua_open();
}
This seems like a linker error, for some reason the static library file is not being correctly referenced. But, to me, the makefile seems correct.
Any help would be greatly appreciated!
To start with: is there anyway to see how everything is being linked
together and if my shared library module is really get access
to the static library?
Additional Information
Here's extra information that I think could be relevant!
Building the library
Maybe it's the static lib file that's not correct? (Is there anywhere I could download a prebuilt one to check?). I made it with this script (from the LuaJIT website). I'm using the latest stable LuaJIT, 1.1.8
NDK=/cygdrive/c/android-ndk-r8b
NDKABI=8
NDKVER=$NDK/toolchains/arm-linux-androideabi-4.4.3
NDKP=$NDKVER/prebuilt/linux-x86/bin/arm-linux-androideabi-
NDKF="--sysroot $NDK/platforms/android-$NDKABI/arch-arm"
make linux HOST_CC="gcc -m32" CROSS=$NDKP TARGET_FLAGS="$NDKF"
This builds fine and creates a liblua.a in the /src/ directory. (I ran nm on it and it lists out all the function prototypes I'd expect). I don't know if there's anything else I can do to ensure it's really a build for ARM?
NDKABI=8 means I'm targeting Android 2.2
Setting up the test Android Project
I create a brand new 2.2 android project using this command:
android create project --target 3 --name test --path . --activity TestActivity --package com.test
Target 3 maps to Android 2.2 on my system (using android list devices).
I create the jni folder and have a test_android.h and test_android.cpp. Then I use ndk-build to build them - which works fine when I'm not trying to reference LuaJIT. When I do try and use Lua I get the following error:
Full Error Message
Cygwin : Generating dependency file converter script
Compile++ thumb : test <= test_android.cpp
In file included from jni/test_android.h:3:0, from jni/test_android.cpp:2:
C:/android-ndk-r8b/platforms/android-8/arch-arm/usr/include/jni.h:592:13: note:
the mangling of 'va_list' has changed in GCC 4.4
Prebuilt : liblua.a <= jni/
StaticLibrary : libstdc++.a
SharedLibrary : libtest.so
obj/local/armeabi/objs/test/test_android.o: In function `test()':
C:\Users\Grrr\Documents\mycode\static_lib_test/jni/test_android.cpp:25: undefined reference to `luaL_newstate'
collect2: ld returned 1 exit status
/cygdrive/c/android-ndk-r8b/build/core/build-binary.mk:378: recipe for target `obj/local/armeabi/libtest.so' failed make: *** [obj/local/armeabi/libtest.so] Error 1
Most of the issues I've seen searching around are due to the local library include order, as I only have one library this shouldn't be an issue and suggests I've managed to get something more fundamental wrong :)
Update
I've since built normal Lua and added that as prebuilt static library and it works fine. I suspect its how I've built LuaJIT but I'm not sure how to correctly build it, or find a working prebuilt version.

The linking for this was fine, or at least the makefile was.
The problem was the LuaJIT library wasn't built for ARM (I used objdump -a lualib.a, to check this). I downloaded the latest LuaJIT, ran the script under linux and got an ARM library. Everything links nicely now!

Build Android-openssl library for platform 2.1

I am using open-ssl source given at https://github.com/eighthave/openssl-android
to build a library which can be used in android project.
As per instructions given at README.txt, I am able to compile it for the the Android platform version 2.2 (level -8)
But my app requires it to be 2.1 (level -7) compatible.
I tried following options with the default.properties file ( https://github.com/eighthave/openssl-android/blob/master/default.properties )
1) set target=android-7
2) set target=android-5
But when I compile it using command ndk-build, it gives following error
Compile thumb : crypto <= dsa_vrf.c
Compile thumb : crypto <= dso_dl.c
Compile thumb : crypto <= dso_dlfcn.c
/Crypto/openssl-android/crypto/dso/dso_dlfcn.c: In function 'dlfcn_pathbyaddr':
/Crypto/openssl-android/crypto/dso/dso_dlfcn.c:445: error: 'Dl_info' undeclared (first use in this function)
/Crypto/openssl-android/crypto/dso/dso_dlfcn.c:445: error: (Each undeclared identifier is reported only once
/Crypto/openssl-android/crypto/dso/dso_dlfcn.c:445: error: for each function it appears in.)
/Crypto/openssl-android/crypto/dso/dso_dlfcn.c:445: error: expected ';' before 'dli'
/Crypto/openssl-android/crypto/dso/dso_dlfcn.c:455: error: 'dli' undeclared (first use in this function)
make: *** [obj/local/armeabi/objs/crypto/dso/dso_dlfcn.o] Error 1
As per error message- Dl_info is not defined. but if we go to file dso_dlfcn.c , the definition for the structure is already provided. (https://github.com/eighthave/openssl-android/blob/master/crypto/dso/dso_dlfcn.c)
And this code compiled for target=android-8 in default properties file, but not for android-7 or android-5.
Request you to help me to resolve this error. and let me know what all changes needs to be done in order to compile it for android 2.1 platform.
Thanks in advance.

Try to include the following piece of code into dso_dlfcn.c:
typedef struct {
const char *dli_fname; /* Pathname of shared object that
contains address */
void *dli_fbase; /* Address at which shared object
is loaded */
const char *dli_sname; /* Name of nearest symbol with address
lower than addr */
void *dli_saddr; /* Exact address of symbol named
in dli_sname */
} Dl_info;
int dladdr(const void *addr, Dl_info *info) { return 0; }
Before:
#ifdef __linux
# ifndef _GNU_SOURCE
# define _GNU_SOURCE /* make sure dladdr is declared */
# endif
#endif
After that in my case the library is built.

Try to install with latest NDK version and update Application.mk file appropriately.
LOCAL_PATH := $(call my-dir)
APP_PLATFORM := android-19
NDK_TOOLCHAIN_VERSION := clang
APP_ABI := armeabi-v7a
APP_STL := gnustl_static
APP_CPPFLAGS += -frtti
APP_CPPFLAGS += -fexceptions
APP_CPPFLAGS += -DANDROID
APP_PROJECT_PATH := $(shell pwd)
APP_BUILD_SCRIPT := $(LOCAL_PATH)/../Android.mk
The above 2 problems will be solved.

I had one issue with #Yuri's solution and had to improve it alittle. My APP_ABI is set to all in Application.mk. In my case it meant that among armeabi and armeabi-v7a I'm building also for x86 and mips. I have also android-9 target installed in android sdk to use in other projects. x86 and mips are supported by ndk starting from android-9. As written in the docs, when ndk-build will start building these targets, it will switch to android-9 target automatically. And what? - Yes, it will fail to compile :-). Here is my solution:
In crypto/Android.mk find the line local_c_flags :=
-DNO_WINDOWS_BRAINDEATH. After the line write this http://pastebin.com/7euUVD7A.
Yuri's code has to be inserted into if defined: http://pastebin.com/V58gTSBU
By the way, I've inserted Yuri's block after #include <openssl/dso.h> but not before #ifdef __linux