Undefined reference to android log write

OpenCV in Android Studio’s NDK fails with «undefined reference» error #17519

Comments

orangesomethingorange commented Jun 10, 2020 •

System information (version)

• OpenCV => ❔ 4.3 / 3.4
• Operating System / Platform => ❔ Linux Ubuntu 16.04
• Compiler => ❔ Android Studio (Gradle / Ninja / CMake / clang++)

Detailed description

I’m trying to use OpenCV on Android’s NDK.
I have two similar environments — one integrated with OpenCV 4.3 and the other with OpenCV 3.4.
I created an Android C++ app.
For the 4.3 integration I followed this tutorial.
For the 3.4 integration I followed this tutorial.
I have a very basic native-lib.cpp file (see below), which is trying to use cv::imread .

In my code I provide the last trial — which includes the OpenCVLoader.initAsync instance, but I also tried the OpenCVLoader.initDebug way and System.loadLibrary(«opencv_java4») , but the error remains.

I’m aware of the following issues:

and noticed the recent update regarding link issues (which I assumed was my problem):

I tried to build for my SDK version or against libc++, as described in the following:

and noticed some other issues, which do not use exactly the same setup (for example, using ndk-build instead of cmake):

but failed to find anything new.

When trying to run the app I get the following errors:

Isn’t undefined reference a linker issue?
Did I forget a compiler flag or something?

Steps to reproduce

Follow one of the tutorials (4.3 tutorial or 3.4 tutorial) to integrate OpenCV library into Android Studio.

Create the following simple native-lib.cpp:

and call it from this simple MainActivity.java:

NOTES:

• When calling OpenCVLoader.initAsync , both the 4.3 and the 3.4 version use the OPENCV_VERSION_3_4_0 flag, since OPENCV_VERSION on 4.3 results in an error. I also tried the OpenCVLoader.initDebug and it produces the same error.
• When commenting out the image = imread line in native-lib.cpp , the app runs OK, and returns the string (all cases).

the app’s build.gradle:

CMakeLists.txt:

Error

ERROR:
undefined reference to «cv::imread»

CLANG++:
/path/to/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ —target=aarch64-none-linux-android29 —gcc-toolchain=/path/to/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64 —sysroot=/path/to/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/sysroot -fPIC -g -DANDROID -fdata-sections -ffunction-sections -funwind-tables -fstack-protector-strong -no-canonical-prefixes -D_FORTIFY_SOURCE=2 -Wformat -Werror=format-security -frtti -fexceptions -O0 -fno-limit-debug-info -Wl,—exclude-libs,libgcc.a -Wl,—exclude-libs,libgcc_real.a -Wl,—exclude-libs,libatomic.a -static-libstdc++ -Wl,—build-id -Wl,—fatal-warnings -Wl,—no-undefined -Qunused-arguments -shared -Wl,-soname,libnative-lib.so -o /path/to/app/build/intermediates/cmake/debug/obj/arm64-v8a/libnative-lib.so CMakeFiles/native-lib.dir/native-lib.cpp.o /path/to/opencv-4.3.0-android-sdk/OpenCV-android-sdk/sdk/native/libs/arm64-v8a/libopencv_java4.so /path/to/Sdk/ndk/21.1.6352462/toolchains/llvm/prebuilt/linux-x86_64/sysroot/usr/lib/aarch64-linux-android/29/liblog.so -ljnigraphics -llog -ldl -lz -latomic -lm

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alalek commented Jun 10, 2020

What is about OpenCV Android samples from SDK package?

orangesomethingorange commented Jun 14, 2020

What is about OpenCV Android samples from SDK package?

I assume you’re referring to this example
These are somewhat old and refer to Android app development on Eclipse, using ndk-build .
My (and maybe every other recent) Android app project is built on Android Studio, with CMake.
That’s exactly the reason I tried the above tutorials — working with ndk-build on Eclipse seemed obsolete.

Also, the Hello World sample uses the Camera, which I have no intention of using.
All I need is just some basic functionality 🙂

orangesomethingorange commented Jun 14, 2020

I can see that Mat is well defined, for example, but imread not.
Could this be a linking issue with CMake?

alalek commented Jun 14, 2020 •

Unpack OpenCV SDK and load «samples» into Android Studio.

You have found a link on EOL version of OpenCV. «gradle» support is available on 4.x versions.

Usage questions should go to Users OpenCV Q/A forum: http://answers.opencv.org

orangesomethingorange commented Jun 15, 2020

Unpack OpenCV SDK and load «samples» into Android Studio.

You have found a link on EOL version of OpenCV. «gradle» support is available on 4.x versions.

Usage questions should go to Users OpenCV Q/A forum: http://answers.opencv.org

Not sure what you mean that:
Following the docs link at docs.opencv.org, from this repository, I get to this tutorial, and following opencv.org/ or a Google search I get to the link I sent before.

Both cases describe the integration of OpenCV (I’ve tried versions 4.3 or 3.4) into Eclipse.
I currently have Android Studio integrated with both OpenCV versions (in separate projects), but trying to use it via Gradle results in the above described undefined reference error.
Do you have a link describing the Gradle support you’ve mentioned?

alalek commented Jun 15, 2020

Almost all Android tutorials are outdated — added the «documentation» label.

OpenCV 4.3.0 binaries were built using NDK r18b.
Related symbols are:

Note about the std::__ndk1:: prefix.

• use NDK16 in your application which uses __ndk1 for std
• or rebuild OpenCV binaries from sources with NDK21.1

orangesomethingorange commented Jun 16, 2020 •

Almost all Android tutorials are outdated — added the «documentation» label.

OpenCV 4.3.0 binaries were built using NDK r18b.
Related symbols are:

Note about the std::__ndk1:: prefix.

Downgrading 4.3 project to NDK16 keeps the undefined reference to cv::imread exactly the same.

Trying to build 4.3 project with NDK18 (for symbols), posed an Expected NDK STL shared object file libgnustl_shared.so issue.

Downgrading 3.4 project to NDK16 enabled it to build, but it fails to run when including imread , and crash on app startup. I can’t see anything on the debug log, and the Logcat seems to run the same regardless. I’m not even sure what exactly crashes and where to look.

(*) BTW, for anyone else’s future reference, using NDK16 poses a different issue — it only supports up to API level 26/27, and I need API 28 (above preferred).

1. A CMake build with NDK21 was successful, produced some static libraries ( libopencv_calib3d.a , libopencv_core.a , libopencv_dnn.a , libopencv_features2d.a , etc. ) and shared library libopencv_java4.so .
   Though linking to the shared library reports successful, I can’t find it using the NDK.
   Something is still missing.

orangesomethingorange commented Jun 17, 2020

Here’s an example for a build output (after a successful previous build):

Linking the shared library didn’t pose any errors, but still — there’s no OpenCV code which may be included in the NDK.
Any «#include [some_opencv_header]» results in fatal error: ‘[some_opencv_header]’ file not found

orangesomethingorange commented Jun 17, 2020

The same build, with adding (in CMakeLists.txt ) the following:

results in undefined reference error from protobuf :

orangesomethingorange commented Jun 18, 2020

I still can’t figure out what’s preventing the link / packaging.
Here’s a CMakeLists.txt file I’m using to configure build and link.

alalek commented Jun 18, 2020

set(CMAKE_ANDROID_STL_TYPE gnustl_static ) # GNU libstdc++ Static

By default OpenCV uses:

I believe different symbols come from this difference. We need the same compiler and the same compiler flags.

orangesomethingorange commented Jun 23, 2020 •

Thanks for the comment @alalek .
I’m using clang++ and changed CMAKE_ANDROID_STL_TYPE gnustl_static to CMAKE_ANDROID_STL_TYPE c++_shared .
I still get the same error, though.

Build log shows that build and link went OK, so my guess is that now it’s a packaging issue. I think that Gradle fails to pack the library into the APK for some reason.
From here I can assume it all should be packed correctly (though I also tried creating jniLibs directory and copying the shared library into it).

Still, I can’t access the OpenCV library from native code 😞

orangesomethingorange commented Jun 23, 2020

ndk-build

Another attempt to build OpenCV, using ndk-build instead of cmake (rest is the same — using Gradle in Android Studio, with clang++).

Android.mk:

Application.mk:

App’s build.gradle:

I manually build the library by cd -ing into the app/src/main/jni (where the .mk files are) and running /path/to/ndk-build , before running build with Android Studio (as suggested in step 6 of the Native/C++ section of this OpenCV doc).

Shared library is being created and build and link seem to be successful, when no calls are done from native code.
If and when I try to include some OpenCV code in native code ( #include or #include «opencv2/core.hpp» for example) I get file not found error.
Including function calls in code, again, results in undefined reference error.

alalek commented Jun 23, 2020

Another attempt to build OpenCV

I see that you trying to use OpenCV, instead of rebuilding it from the sources.
Again, existed OpenCV binaries are not suitable for your configuration with your Android NDK/SDK versions.
You should use «platforms/android/build_sdk.py» script first and get new libopencv_java4.so.

Android package build logs are here.

orangesomethingorange commented Jun 24, 2020

Another attempt to build OpenCV

I see that you trying to use OpenCV, instead of rebuilding it from the sources.
Again, existed OpenCV binaries are not suitable for your configuration with your Android NDK/SDK versions.
You should use «platforms/android/build_sdk.py» script first and get new libopencv_java4.so.

Android package build logs are here.

You’re correct — I was, again, trying to use the packaged SDK.

Regarding build_sdk.py :
This script was used by Gradle. In previous attempts to build it locally the script was used and created a new libopencv_java4.so shared library. When I tried to use it — it got me the undefined reference error (the first time).

After your comment I thought — maybe you meant that I should run the script standalone?
Invoking the script outside the environment (not setting the —use_android_buildtools flag) makes the script call cmake v.3.6 via the NDK, and makes it fail on cmake_minimum_required error, when called by cmake v.3.5.1.
Also, exporting all the CMAKE variables and debugging is just like creating the whole environment from scratch.

So I ran build_sdk.py with the —use_android_buildtools flag, and no libopencv_java4.so was created (all the build and output folders were created, but not the library).

So, I’m guessing you meant to use it via Gradle, correct?

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‘undefined reference to’ функция, использующая Android Studio NDK

У меня есть 3 класса, native-lib.cpp, tracker.cpp и tracker.hpp. Однако во время сборки я получаю сообщение об ошибке, если я использую функцию isTracking из tracker.cpp в файле navtive-lib.cpp.

Что пошло не так: выполнение выполнено для задачи ‘: app: externalNativeBuildDebug’.

Сбой команды сборки. Ошибка при выполнении процесса /home/user/Android/Sdk/cmake/3.6.4111459/bin/cmake с аргументами <--build/home/user/AndroidStudioProjects/project/app/.externalNativeBuild/cmake/debug/x86 --target tracker>[1/2] Создание объекта CXX CMakeFiles/tracker.dir/src/main/cpp/native-lib.cpp.o [2/2] Связывание общей библиотеки CXX../../../../build/intermediates/cmake/debug/obj/x86/libtracker.so FAILED:: &&/home/user/Android/Sdk/ndk-bundle/toolchains/llvm/prebuilt/linux-x86_64/bin/clang++ —target = i686-none-linux-android — -g cc-toolchain =/home/user/Android/Sdk/ndk-bundle/toolchains/x86-4.9/prebuilt/linux-x86_64 —sysroot =/home/user/Android/Sdk/ndk-bundle/sysroot -fPIC -система /home/user/Android/Sdk/ndk-bundle/sysroot/usr/include/i686-linux-android -D__ANDROID_API__ = 23 -g -DANDROID -ffunction- разделы -funwind-tables -fstack-protector-strong -no-canonical-prefixes -mstackrealign -Wa, —noexecstack -Wformat -Werror = format-security -O0 -fno-limit-debug-info -Wl, —exclude-libs, libgcc.a —sysroot/home/user/Android/Sdk/ndk-bun dle/platform/android-23/arch-x86 -Wl, —build -id -Wl, — warn -shared -textrel -Wl, — фатальные предупреждения -Wl, — no-undefined -Wl, — z, noexecstack -Qunused-arguments -Wl, — z, relro -Wl, — z, теперь -shared -Wl, -soname, libtracker.so -o../../../../build/intermediates/cmake/debug/obj/x86/libtracker.so CMakeFiles/tracker.dir/src/main/cpp/tracker.cpp.o CMakeFiles/tracker.dir/src/main/cpp/native-lib.cpp.o/home/user/Android/Sdk/ndk-bundle/platforms/android-23/arch-x86/usr/lib/liblog.so../../../../src/main/jniLibs/x86/libopencv_java3.so -lm «/home/user/Android/Sdk/ndk-bundle/sources/cxx-stl/gnu-libstdc++/4.9/libs/x86/libgnustl_static.a» &&: /home/user/AndroidStudioProjects/project/app/src/main/cpp/native-lib.cpp:28: ошибка: неопределенная ссылка на «Tracker :: isTracking()» clang++:

ошибка: команда компоновщика завершилась с кодом выхода 1 (используйте -v, чтобы увидеть вызов) ninja: build stopped: подкоманда не удалась.

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undefined reference to `__android_log_print’

Posted by: admin November 28, 2017 Leave a comment

What is wrong with my make file?

Try the following in your Android.mk file:

You need to add

If you use Android Studio and gradle, it ignores Android.mk. Add this to your build.gradle file:

If you upgrade to Android Studio 2.1, above answers do not work, need use ldLibs.add() to load the lib as below:

For Android Studio 2.2 and tools.build:gradle:2.2.0 using CMake add or edit row in CMakeLists.txt:

Thats connecting log library to yours.

We can link a shared library in Android in 3 ways.
In below 3 cases, the lines mentioned should be added in Android.mk

So here are the three ways.

For some reason if 1 doesnt work(it did not work for me), You can try below 2 ways

Of course you also need to include #include in your C/H file.

If using the new Gradle NDK integration in Android Studio 1.3, you need to add ldLibs = [“android”, “log”] to your android.ndk options – Stephen Kaiser Sep 24 at 4:20

use ldLibs.addAll([«android», «log»]) for the experimental plugin

Yes, you do need to add: LOCAL_LDLIBS := -llog as the other answers/comments have specified, however the original question did not specify if he use the jni library as: LOCAL_JNI_SHARED_LIBRARIES or as LOCAL_REQUIRED_MODULES .

I can pretty much say for sure that he has it used it as: LOCAL_REQUIRED_MODULES because of the LOCAL_EXPORT_LDLIBS := -llog in the question… unless that was added after an edit.

If you use LOCAL_REQUIRED_MODULES the shared library is installed in /system/lib instead of into the apk, because it is a required module. Therefore you will need to add LOCAL_LDLIBS := -L$(SYSROOT)/usr/lib -llog instead of just LOCAL_LDLIBS := -llog so that when the build system is building & linking the jni shared library, it will have the -llog definitions in the correct place, available to be built under $OUT/root/system/lib . Otherwise you will continue to get the same answer, even if you only add LOCAL_LDLIBS := -llog .

So, those who commented that the -L is not needed, and the other answer was correct, they were actually incorrect in this situation.

This helped for me:

In the android studio version 2.2 and higher, there is inbuilt support for CPP when you create a new project. Also, the liblog.so is included by default. Nothing to be done apart from including the header file (android/log.h).

Checkout app/CMakeLists.txt that is created by the studio when we create new android studio project. We can see that the find_library() block and target_link_libraries() block for loglib are already present.

Also, pay attention towards the function syntax. It should be:

__android_log_print (int priority, const char *tag, const char *fmt,…);

In my case, I had left out tag parameter and ended up spending good 3 days in figuring it out.

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