I'm working on an Android app that uses some 3rd party native libraries. We need to add some functionality to these libraries. So I wrote a set of C++ classes with the code that I need and also cross-compile openssl, as we are using openssl crypto functions.
I made a demo app to test my code and everything is working fine.
Now I need to send this code to the people that are developing the 3rd party libraries, and they will add our code and wrap all the native code in a JAR for us to use it in our app.
I used this example NDK hello libs as a base. As I want to send my code as a shared library (.so file), I did this in the 'CMakeList.txt':
cmake_minimum_required(VERSION 3.4.1)
set(distribution_DIR ${CMAKE_SOURCE_DIR}/../distribution)
add_library(lib_crypto STATIC IMPORTED)
set_target_properties(lib_crypto PROPERTIES IMPORTED_LOCATION
${distribution_DIR}/openssl/lib/${ANDROID_ABI}/libcrypto.a)
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++11")
### This code just calls my my-lib ###
add_library(native-lib SHARED
src/main/cpp/native-lib.cpp)
### This is the lib I want to build ###
add_library(my-lib SHARED
src/main/cpp/*****.cpp
# more files...
)
target_include_directories(my-lib PRIVATE ${distribution_DIR}/openssl/include)
target_link_libraries(
native-lib
my-lib
)
target_link_libraries(
neclic
lib_crypto
)
I built the APK and took 'my-lib.so' from it. Then I made a new project using this lib (and including the .h headers) and everything is working as expected.
My questions is:
Is there a better way of doing this? May I expect any problem when I send this library to be integrated with the original code? For example, do they need to use the same NDK version? Or 'my-lib.so' can be used in any android app if the arch is supported?
Thanks!
Generally speaking, you don't need to send your libneclic.so to be packed in a JAR. You can use it in your APK as is, unless the 3rd party libraries that your collegues develop outside, need your library as dependency.
Actually, the supported way to disribute compiled Java libraries with native components are not JAR, but rather AAR files. But this is a technicality that is not important for this discussion.
If you use your libneclic.so directly in your APK, that is, you have Java class(es) that uses JNI to call functions in your library, you can keep the same CMakeList.txt in your project, or you can put the the built library in JniLibs folder, and Android Studio will pick it from there when it packages the APK.
On the other hand, if the library you built has exported functions that will be called from some 3rd party library, make sure that both use same shared STL (luckily, with the latest NDK release 18, there is only one shared STL implementation available, c++_shared.
It is highly recommended that the libraries are built with same NDK version, even though, most likely, moderate difference between these versions will not cause negative results. Each NDK release brings significant improvements, not only in performance, but also fixes bugs, including some security ones.
Still, it is safer to agree on a common older release of NDK, than to keep track of all possible inconsistencies.
Note that by default, CMake uses c++_static STL to build your libneclic.so, and you must override that, by setting ANDROID_STL.
Related
We have a project that uses a library that is built on top of Google's Mediapipe, which is built using the Bazel build system.
The project itself is an Android Native Library, built using Gradle with CMake
externalNativeBuild {
cmake {
cppFlags "-std=c++17 -fopenmp -static-openmp -fexceptions -frtti -Wl,-s -Wno-unused-command-line-argument"
arguments "-DANDROID_STL=c++_shared", "-DOpenCV_DIR=${opencvDir}", "-DANDROID_ARM_NEON=TRUE"
}
}
So we end up with 2 (or more later, also dependent on OpenCV for example) shared object libraries - the actual SDK & the Mediapipe project.
We're seeing issues that are similar to this, which lead me to look into the runtime part of our project.
E/libc++abi: terminating with uncaught exception of type std::bad_cast: std::bad_cast
I saw this comment on that issue thread, and adding
System.loadLibrary("c++_shared");
Solved the crash.
However, this is not a practical solution as the project we're building would provide a native SDK in the form of multiple .so files and I wouldn't want to force our clients to have to explicitly load the shared runtime library before using our library.
The gradle library has "-DANDROID_STL=c++_shared" flag, so this is using the shared one, but I couldn't find any way to compile Mediapipe (with Bazel) using c++_shared. I couldn't find any reference to using shared runtime when compiling Bazel projects (except for this, which isn't exactly relevant and the solution didn't help me)
We might be able to work around this by setting -DANDROID_STL=c++_static, but this has other issues, mainly, it violates Android's guidelines for using multiple shared libraries, though it might be possible for for middleware vendors
So the question is,
Is it possible to build Mediapipe (or any other Bazel based) using c++_shared Android STL
If not, are there any other options to solve the runtime conflicts
Is it even a runtime conflict or something else?
I managed to get it working as suggested by using c++_static on all of our shared objects (SDK, Mediapipe, OpenCV and others)
To make this a little more complicated I'm loading a native library I've compiled with ndk and cmake into Unity which will be deployed on Android. This was all working fine until I wanted to link another shared library to the library I was building. Now my library fails to load and I believe it is because it can't find the new shared library. I can say that the new shared library works on its own because I'm using it in another NDK project.
I see that Unity copies the new .so library over to the apps install directory and puts it along with my original library in a /data/appname/lib/arm64 directory. I have also tried installing the library in /system/lib64 but no luck. I have compiled my library without the new shared library and it works, and then only modified cmake to add it back in and my library will no longer load. I have also objdump'd it to make sure it is x64 as well as exported the symbols in case it was corrupted. So I'm wondering, is there a way to find out where my library file is looking?
This is how I include the new shared library in my CmakeList.txt
add_library(libusb SHARED IMPORTED)
set_target_properties(libusb PROPERTIES IMPORTED_LOCATION D:/projects/arm64-v8a/libusb1.0.so)
where D:\projects\arm64-v8a is the location of the library file on my build machine. I wonder if this is screwing things up. But I use the same syntax in my Android studio Cmake project and I can access the library but I'm sure that's different somehow. Any help is always appreciated.
Thank you
The imported library may have its own dependencies, which must also be copied to your APK (or AAR). To check this, run objdump -p libusb1.so | findstr "NEEDED".
It will look in the lib/[platform-abi] directory of your app. But there is a list of steps for using prebuilt libraries:
https://developer.android.com/ndk/guides/prebuilts
Have you checked those?
I'm compiling a 3rd-party Java library for Android that uses JNI. I read the relevant pages on adding C++ support on developer.android but I'm still confused about a couple of issues regarding C++ STL runtime that I was hoping I could clear up here:
1- My library has no control over the app it will be embedded in, so I don't know if there will be other libraries that might use a static/shared STLs. If I use a static C++ runtime with ANDROID_STL=c++_static, is it safe, or should I have to worry about another library that could be using something like gnustl_static which might conflict with mine?
2- If I use a shared C++ runtime with ANDROID_STL=c++_shared, is it a guarantee that a specific element in the STL will use the libc++ runtime or could it be possible to use gnustl if it doesn't exist? For example, If I was using std::string with a shared c++ runtime (c++_shared) in an app that has another library of gnustl_static, will my std::string implementation be taken from libc++ or gnustl?
Ideally, I'd like to have a very stripped down version of a static c++ runtime with (c++_static) that only includes std::vector, std::string and std::map. I was actually planning to use something like -ffunction-sections as described here and #768.
Please advise and thank you.
Environment Details
Pkg.Desc = Android NDK
Pkg.Revision = r15c
Android Studio = 3.1.2
system: cmake Host OS: Arch Linux ($ uname -r % 4.18.5-arch1-1-ARCH)
Compiler: Clang++
STL: c++_static/c++_shared
Your concern is a very real one. But if handled properly, you can find a robust way out.
The warnings about using single C++ runtime across all libraries in the app (and the whole idea to define C++ support in NDK as APP_STL vs. most other flags such as LOCAL_CFLAGS or LOCAL_SHARED_LIBRARIES, are relevant for the native libraries that are connected. JNI libraries that never communicate directly (except through their corresponding Java layers) can use different C++ runtimes. Another point is that normal build will only package one C++ runtime shared lib into the APK. Note that versioning is also a potential hazard: if a developer who adds your library uses a different NDK release, there might be collisions or unexpected side effects when his version of STL runtime works with your code.
Therefore, to achieve maximum flexibility, your library should use a static C++ runtime. This may effect the size of the binary, but if, as you say, you use only a limited subset of STL, this extra will be rather small.
The bottom line, you will have minimum to worry about if build your shared library with libc++_static.
I'd like to compile a library (static but also could be shared) written in C++ so that is statically contains all the C++ runtime functionality that I use in the library, i.e. another project can simply include my .a or .so file without having to worry about further dependencies. How can I do this using the NDK/Android build system?
So far, I have
APP_STL := c++_static
in my Application.mk and build my library using
include $(BUILD_STATIC_LIBRARY)
in my Android.mk after setting up the project. However, if I then pull in the resulting library into a second project (say, a dynamic library) that does not specify an STL (e.g. pure C) I get a load of symbol not found linker errors. Only when I specify to use the c++_static STL in the second project do these errors go away indicating that c++_static was not pulled into the first library. (I know this isn't surprising but it's not the behaviour I want).
Using good ol' make files and specifying build flags myself allows me to combine static libraries. My problem is coaxing the NDK build system to do it :)
if you compile a C++ runtime statically to your lib, unfortunately another project have to worry about this dependency:
If several .so files are using static C++ runtimes, this can lead to corruption and crashes because of the sharing of global variables used by the runtimes.
If you compile everything (your lib and the final project) into a single .so file, you can use one static C++ runtime, and it's better then if it's declared inside the end project. Because forcing the static inclusion of the C++ runtime in your lib would prevent the end project of using a C++ runtime.
I am developing a library for Android which has both Java and Native components. The Native library depends on the GNU STL. I was linking against gnustl_static, but have ran into a few strange problems that were resolved by linking gnustl_shared. (for example the question Filesystem and locale). For the time being, I have settled on gnustl_shared.
Here's where the question is -- the consumer of my library is expected to link against the Native .so to add code of their own. I understand from reading that all the C-runtimes must match.
Demanding my clients use gnustl_shared seems like it might not work (as far as diplomacy and ease of integration goes)
If I build with gnustl_shared, the JAR file generated contains libgnustl_shared.so, which causes a problem building an APK which also relies on this library. -- I suppose I can just not include it in the JAR file?
If I build against gnustl_static do I avoid all these problems?
If my API contains refernces to STL objects (mostly std::string and std::vector), will I face problems if my clients do not use GNU STL?
thanks