How to properly link OpenCV in a C++ cross-platform library for Android and iOS?

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I am developing a C++ library, including OpenCV, which will be used in a cross-platform Xamarin solution through a wrapper and the NuGet packaging system (see this guide). I configured a CMakeLists.txt file but I simply cannot get OpenCV to be correctly linked for both static (iOS) and dynamic (Android) libraries.

I tried to change the OpenCV_DIR variable, install and build OpenCV from sources and manually include the content of the OpenCV_INCLUDE_DIRS variable but nothing worked. I also noticed that the linking works when only using cv::Point. But the linking does not work when using cv::Mat, which I do not understand the reason.

The following is the CMakeLists.txt that I am using :

cmake_minimum_required (VERSION 3.2)
project (MyLib C CXX)
enable_testing()

set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=c++11")
MESSAGE(STATUS "CMAKE_CXX_FLAGS: " ${CMAKE_CXX_FLAGS})

# Source and headers files
set(SOURCES File1.cpp File2.cpp)
set(HEADERS File1.h File2.h)

# Library
if(BUILD_SHARED_LIBS)
  add_library (MyLib SHARED ${SOURCES} ${HEADERS})
  target_compile_definitions(MyLib PUBLIC IS_BUILDING_SHARED)
else()
  add_library (MyLib STATIC ${SOURCES} ${HEADERS})
endif()

# Dependencies
set(OpenCV_DIR /usr/local/Cellar/opencv/4.5.0_1/lib/cmake/opencv4)
find_package(OpenCV REQUIRED)
message(STATUS "OpenCV_INCLUDE_DIRS = ${OpenCV_INCLUDE_DIRS}")
message(STATUS "OpenCV_LIBS = ${OpenCV_LIBS}")
message(STATUS "OpenCV_DIR = ${OpenCV_DIR}")
include_directories(${OpenCV_INCLUDE_DIRS})
target_link_libraries(MyLib ${OpenCV_LIBS})

The following shows the location of OpenCV's files that are used during the build process. Everything seems alright.

-- OpenCV_INCLUDE_DIRS = /usr/local/Cellar/opencv/4.5.0_1/include/opencv4
-- OpenCV_LIBS = opencv_calib3d;opencv_core;opencv_dnn;opencv_features2d;opencv_flann;opencv_gapi;opencv_highgui;opencv_imgcodecs;opencv_imgproc;opencv_ml;opencv_objdetect;opencv_photo;opencv_stitching;opencv_video;opencv_videoio;opencv_alphamat;opencv_aruco;opencv_bgsegm;opencv_bioinspired;opencv_ccalib;opencv_datasets;opencv_dnn_objdetect;opencv_dnn_superres;opencv_dpm;opencv_face;opencv_freetype;opencv_fuzzy;opencv_hfs;opencv_img_hash;opencv_intensity_transform;opencv_line_descriptor;opencv_mcc;opencv_optflow;opencv_phase_unwrapping;opencv_plot;opencv_quality;opencv_rapid;opencv_reg;opencv_rgbd;opencv_saliency;opencv_sfm;opencv_shape;opencv_stereo;opencv_structured_light;opencv_superres;opencv_surface_matching;opencv_text;opencv_tracking;opencv_videostab;opencv_viz;opencv_xfeatures2d;opencv_ximgproc;opencv_xobjdetect;opencv_xphoto
-- OpenCV_DIR = /usr/local/Cellar/opencv/4.5.0_1/lib/cmake/opencv4

Android

The following is the commands that I am using to build the Android dynamic library (.so). I have installed the NDK and am building for each ABI (x86, x86_64, armeabi-v7a, arm64-v8a).

cmake ../.. -DCMAKE_TOOLCHAIN_FILE=/Users/$USER/Library/Android/sdk/ndk-bundle/build/cmake/android.toolchain.cmake -DANDROID_NATIVE_API_LEVEL=21 -DANDROID_ABI=$abi_name -DBUILD_SHARED_LIBS=ON
cmake --build . --config Release

I directly get an error when building the library which is the following.

ld: error: /usr/local/Cellar/opencv/4.5.0_1/lib/libopencv_gapi.4.5.0.dylib: unknown file type
ld: error: /usr/local/Cellar/opencv/4.5.0_1/lib/libopencv_stitching.4.5.0.dylib: unknown file type
[...]
ld: error: /usr/local/Cellar/opencv/4.5.0_1/lib/libopencv_rapid.4.5.0.dylib: unknown file type
ld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
clang++: error: linker command failed with exit code 1 (use -v to see invocation)

iOS

The following is the commands that I am using to build the iOS static library (.a). I am using leetal's ​cmake toolchain file from this repository.

cmake ../.. -G Xcode -DCMAKE_TOOLCHAIN_FILE=../../ios.toolchain.cmake -DPLATFORM=OS64COMBINED -DBUILD_SHARED_LIBS=OFF
cmake --build . --config Release

The compilation of the static library seems to work because no error message is printed. However, when the library is used in the final Xamarin solution, the linked library cannot be found and the following error is shown.

Native linking failed, undefined symbol: cv::Mat::deallocate(). Please verify that all the necessary frameworks have been referenced and native libraries are properly linked in. (MT5210)

Question

What am I missing in order to properly compile and link OpenCV into my C++ library ?

I am working on macOS Big Sur and uses the following tools versions:

  • cmake : 3.20.0-rc5
  • ndk : 23.0.7196353
  • apple clang : 12.0.0

I hope that the description of my problem is clear enough and I thank you in advance for any help.

android-ndk opencv cmake c++ ios
1 Answers

We had same problems, including Xamarin's DllNotFoundException with message from last comment, which led me to this topic. What fixed the exception for us in the end was linking statically to OpenCV *.a libs instead of linking to the shared libopencv_java4.so file. So we now have a huge 30MB nativelib.so file for each android ABI in build output, instead of a pair of small nativelib.so and libopencv_java4.so per ABI. CMakeLists looks like this:

set( OpenCV_DIR "~/opencv/build/OpenCV-android-sdk/sdk/native/jni" )
find_package( OpenCV REQUIRED )
    
target_link_libraries( # Specifies the target library.
                           nativelib
                           ${OpenCV_LIBS})

Another thing in our project is we use OpenCV optional modules and had to create a custom OpenCV build, which I guess ensures our native library and OpenCV are compiled against same NDK version. I suppose using the prebuilt OpenCV distribution and compiling against a different NDK version could lead to problems too otherwise.

user_1053950
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