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I am trying to build a project with libtorch and opencv as dependencies. I am using cmake as my build system due to the fact that it is recommended for both these libraries. I am currently stuck, I am trying to get a minimal program to compile, using both libtorch and opencv.

My program looks like this

#include <opencv2/opencv.hpp> #include <torch/torch.h> void showImage(cv::Mat); at::Tensor imgToTensor(std::string img_path); using namespace cv; using std::cout; using std::endl; int main() { std::string img_path = "./images/01 HEAVENLY STAR080.png"; auto tensor = imgToTensor(img_path); cout << tensor << endl; } at::Tensor imgToTensor(std::string img_path){ Mat origImage; Mat normalizedImage; Mat sizedImage(500, 200, CV_32FC3); origImage = imread(img_path, 1); origImage.convertTo(normalizedImage, CV_32FC3); resize(normalizedImage, sizedImage, sizedImage.size(), 0, 0, INTER_LINEAR); auto input = torch::from_blob(sizedImage.data, {sizedImage.rows, sizedImage.cols, 3}); return input; } void showImage(Mat image){ namedWindow("Display window", WINDOW_AUTOSIZE); imshow("Display window", image); waitKey(0); }

This is my CMakeLists.txt:

cmake_minimum_required(VERSION 2.8 FATAL_ERROR) project(ConvNet) set(Torch_DIR /usr/local/libtorch/share/cmake/Torch) find_package(OpenCV REQUIRED) find_package(Torch REQUIRED) include_directories( ${OpenCV_INCLUDE_DIRS} ) set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${TORCH_CXX_FLAGS}") add_executable(main main.cpp) target_link_libraries(main "${OpenCV_LIBS}" "${TORCH_LIBRARIES}")

This is the output of cmake, so i know that the libraries are found:

-- The C compiler identification is GNU 9.3.0 -- The CXX compiler identification is GNU 9.3.0 -- Check for working C compiler: /usr/bin/cc -- Check for working C compiler: /usr/bin/cc - works -- Detecting C compiler ABI info -- Detecting C compiler ABI info - done -- Detecting C compile features -- Detecting C compile features - done -- Check for working CXX compiler: /usr/bin/c++ -- Check for working CXX compiler: /usr/bin/c++ - works -- Detecting CXX compiler ABI info -- Detecting CXX compiler ABI info - done -- Detecting CXX compile features -- Detecting CXX compile features - done -- Found OpenCV: /usr/local (found version "4.3.0") -- Looking for pthread.h -- Looking for pthread.h - found -- Performing Test CMAKE_HAVE_LIBC_PTHREAD -- Performing Test CMAKE_HAVE_LIBC_PTHREAD - Failed -- Looking for pthread_create in pthreads -- Looking for pthread_create in pthreads - not found -- Looking for pthread_create in pthread -- Looking for pthread_create in pthread - found -- Found Threads: TRUE -- Found Torch: /usr/local/libtorch/lib/libtorch.so -- Configuring done -- Generating done -- Build files have been written to: /home/jacob/Documents/KTH/KEX/codeEnvironment/ML_Classification_Toolkit/ML_tool/ConvNet/build

and this is the error i get:

/usr/bin/ld: CMakeFiles/main.dir/main.cpp.o: in function `imgToTensor(std::string)': main.cpp:(.text+0x8d9): undefined reference to `cv::imread(std::string const&, int)' /usr/bin/ld: CMakeFiles/main.dir/main.cpp.o: in function `showImage(cv::Mat)': main.cpp:(.text+0xbac): undefined reference to `cv::namedWindow(std::string const&, int)' /usr/bin/ld: main.cpp:(.text+0xc0d): undefined reference to `cv::imshow(std::string const&, cv::_InputArray const&)' collect2: error: ld returned 1 exit status make[2]: *** [CMakeFiles/main.dir/build.make:122: main] Error 1 make[1]: *** [CMakeFiles/Makefile2:96: CMakeFiles/main.dir/all] Error 2 make: *** [Makefile:104: all] Error 2

Any help would be greatly appreciated!

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  • 2
    This is my first time using cmake, I have no idea what I'm doing. Thanks a ton for the constructive criticism! Commented Apr 27, 2020 at 11:02
  • Are you sure that OpenCV_LIBS variable actually contains OpenCV library? See that question (and comments to it) about checking this fact. Commented Apr 27, 2020 at 11:06
  • Yes, it does. The same syntax works when im compiling only opencv. Commented Apr 27, 2020 at 11:19
  • Putting message(STATUS "${Open_CV_LIBS}") prints a list of libraries. Commented Apr 27, 2020 at 11:26
  • So it is OpenCV_LIBS or Open_CV_LIBS variable which contains a list of libraries? Your last comment seems to contradict to the code in the question post. Anywhere, you may build the project with make VERBOSE=1 and check exact command line used for linking. Commented Apr 27, 2020 at 11:39

2 Answers 2

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The version of libtorch I downloaded did not support cxx11 abi, and was therefore not compatible with opencv. Fixed by changing version of libtorch used.

The version I was using was the pre-cxx11 abi from here: https://pytorch.org/get-started/locally/

I switched to the cxx11 abi.

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2

Don't use that legacy cmake stuff

CMake simplifies a lot the process of building and linking libraries together.

Instead of manually tell the library path and linker options you can just create a dependency of your target with the library.

Following a short snippet on how it should it look like:

cmake_minimum_required(VERSION 3.10 FATAL_ERROR) project(ConvNet) find_package(OpenCV REQUIRED COMPONENTS opencv_highgui) find_package(Torch REQUIRED) add_executable(main main.cpp) target_link_libraries(main PRIVATE opencv_highgui torch)

Observations

  1. There is no include directory! This is because the include directory of the library is something that the library itself knows!
  2. target_link_libraries even if it seems the same there's a lot of difference here! Because now we're not telling the compiler what to do but linking the target main to the targets opencv_highgui and torch.

opencv_highgui and torch are targets like main. Targets are created using add_library and add_executable.

A target has a public and a private interface. To set the target properties we call the function target_* (for instance target_compile_features(mytarget PUBLIC cxx_std_20) says that the target mytarget will use C++20 features and if someone links to us he will use the same compiler option automatically`

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4 Comments

Alright, I updated my cmake file to your suggestion. I still defined Torch_DIR as it wouldnt work otherwise. However, Im at the same error. Are you saying that the problem could be due to compatibility issues between opencv and libtorch?
I will try to compile on my laptop... maybe I made some typo, let me download opencv and torch :)
It might be due to the 2 libraries using different ABI's? Is there anyway for me to check this?
I think the problem is that i downloaded a pre-compiled libtorch library which is compiled with _GLIBCXX_USE_CXX11_ABI=0 flag, this breaks compatibility with opencv. I am going to download it from source and build libtorch myself and see if it works.

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