Linking OpenCV libraries (undefined reference to)

[Petros626]

Hello,

I know it's not a bug request yet I don't know how to create a template like other users without specific problem. I already used the reference to stackoverflow but did not find anything.

VSCode: 1.76.2 OS: Ubuntu 20.04 (VM)

The problem is that last week everything was linked correctly (libraries were found) and now there is an undefined reference to a function. In my opinion, tasks.json as well as c_cpp_properties.json should be configured correctly.

The error, when I try to build:

Many thanks in advance

[bobbrow]

The error is coming from the linker. It doesn't know where to find the implementation for the functions you included via the header files so you need to include the libraries you want to link to as part of the build command.

This SO post sheds some light on the issue and references a tool that can help you identify the paths to the libraries that need to be linked as part of the build: https://stackoverflow.com/questions/23162399/linking-opencv-libraries-with-g

I'm not sure if the VS Code tasks will do inline shell commands or not, but it's worth a try. You can experiment with something like in this in your tasks.json file:

"tasks": [
  {
    "type": "cppbuild",
    "label": "build with opencv",
    "command": "/usr/bin/g++",
    "args": [
      "-fdiagnostics-color=always",
      "-g",
      "`pkg-config --libs --cflags opencv`",
      "${file}",
      "-o",
      "${fileDirname}/${fileBasenameNoExtension}"
    ]
  }
]

[Petros626]

Hey @bobbrow,

I found out, that the order of the compiler commands in tasks.json was wrong, so the linke couldn't find the required libraries. The corrected order looks like your example. Transfered to mine:

"tasks": [
  {
    "type": "cppbuild",
    "label": "C/C++: g++ build active file",
    "command": "/usr/bin/g++",
    "args": [
      "-fdiagnostics-color=always",
      "-g",
      "${file}",
      "-L/usr/local/lib",
      "-I/usr/local/include/opencv4",
      "-lopencv_gapi",
      "-lopencv_highgui",
      "-lopencv_ml",
      "-lopencv_objdetect",
      "-lopencv_photo",
      "-lopencv_stitching",
      "-lopencv_video",
      "-lopencv_videoio",
      "-lopencv_imgcodecs",
      "-lopencv_dnn",
      "-lopencv_calib3d",
      "-lopencv_features2d",
      "-lopencv_flann",
      "-lopencv_imgproc",
      "-lopencv_core",
      "-o",
      "${fileDirname}/${fileBasenameNoExtension}"
    ]
  }
]