Import Non Standard C++ Lib Using Import "C"

Getting Help Code Review
1

Guys I am trying to call a C++ code using import “C” in VS Code (not vs studio). That worked for a simple “hello world” which only have standard libraries required, nevertheless when I tried to call an opencv code which requires its own libraries the compiler could not find them:

# command-line-arguments
.\cam.go:4:10: fatal error: opencv2/opencv.hpp: No such file or directory
    4 | //#include <opencv2/opencv.hpp>
      |          ^~~~~~~~~~~~~~~~~~~~
compilation terminated.

Below is my code. It is a code to open an jpg image from a specified directory.

package main

//#include <stdio.h>
//#include <opencv2/opencv.hpp>
//
//using namespace cv;
//int main(int argc, char** argv )
//{
//
//Mat image;
//image = imread("C:\\Users\\Desktop\\profile-picture.jpg");
//if ( !image.data )
//{
//printf("No image data \n");
//return -1;
//}
//namedWindow("Display Image", WINDOW_AUTOSIZE );
//imshow("Display Image", image);
//waitKey(0);
//return 0;
//}
import "C"

func main() {

}

How can I show to the compiler where is the library? I think that is the problem.

2

You need to tell cgo where to look. Here’s a snippet extracted from some of my code to give you an idea:

/*
#cgo windows CFLAGS: -DTAG_WINDOWS
#cgo linux freebsd CFLAGS: -DTAG_POSIX
#cgo linux freebsd LDFLAGS: -ldl
#cgo CFLAGS: -I ${SRCDIR}/sdk/samples/common/inc

CFLAGS, CPPFLAGS, CXXFLAGS, FFLAGS and LDFLAGS may be defined with pseudo #cgo directives within these comments to tweak the behavior of the C, C++ or Fortran compiler.

1 Like
3

Unfortunately I did not understand this documentation. This do not look to be very friendly as go used to. First, I do not know which directory I should referrer. I write below a short version of the opencv folder structure:

C:Opencv/
├─ build/
│  ├─ bin/
│  ├─ etc/
│  ├─ x64/
│      ├─ vc14/
│      ├─ vc15/
│          ├─ bin/
│          ├─ lib/
│  ├─ python/
│  ├─ java/
│  ├─ include/
│          ├─ opencv2/
│              ├─ opencv.hpp
├─ sources/
│  ├─ 3rdparty/
│  ├─ apps/
│  ├─ cmake/
│  ├─ data/
│  ├─ doc/
│  ├─ include/
│      ├─ opencv2/
│          ├─ opencv.hpp
│  ├─ modules/
│  ├─ platforms/
│  ├─ samples/

In the flag // #cgo LDFLAGS: -L${SRCDIR}/libs -lfoo , what would be the directory that should I use to call the #include <opencv2/opencv.hpp>, the source or the build? How write it inside the -L${SRCDIR}? As it is possible see on the structure above, the libs directory is not the same directory of the includes. Please help me, this even looks like my loved go.

4

This has more to do with C than it does with Go. cgo lets Go call C code. -I, -L, and -l are flags to the C compiler and linker. You really need to understand how those work to be able to specify anything, but in short, -I points to directories that contain files to be #included. -L points to directories containing libraries, and -l indicates a library that should be linked.

You might do well to extract your C code to a .c file and see if you can get the C compiler to build it. Then you’ll know what you need to specify for cgo.

1 Like