How to include compiler flags in Visual Studio Code?

Question

I have a program that I am trying to run in the Visual Studio Code debugger that uses fftw functions. It compiles with the command

g++ dimer.cpp -std=c++11 -lfftw3 

on the terminal on my computer without complaining about undefined references. However, after generating a launch.json file, my program complains about fftw library functions and about the -std=c++14 compiler flag.

I believe that it needs just the extra flags of -std=c++11 and -lfftw3 for the debugger in Visual Studio Code to work. I am using Microsoft's C/C++ extension and the Code Runner extension.

I am trying to transform a Mathematica document of code into c++.

Below are the errors I get from the output.

Executing task: /usr/bin/g++ -g /home/msammartino/Documents/twochain/dimer.cpp -o /home/msammartino/Documents/twochain/dimer <

In file included from /usr/include/armadillo:54:0,
             from /home/msammartino/Documents/twochain/dimer.cpp:6:
/usr/include/armadillo_bits/compiler_setup.hpp:530:108: note: #pragma message: NOTE: suggest to enable C++14 mode for faster code; add -std=c++14 to compiler flags
 #pragma message ("NOTE: suggest to enable C++14 mode for faster code; add -std=c++14 to compiler flags")
                                                                                                        ^
/tmp/ccgb7Xsv.o: In function `r2r_dsine_fftw_forward_dimer(int, double*, double*, Eigen::Matrix<double, 2, 2, 0, 2, 2> (&) [2048], Eigen::Matrix<double, 2, 2, 0, 2, 2> (&) [2048])':
/home/msammartino/Documents/twochain/dimer.cpp:99: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:100: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:101: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:102: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:103: undefined reference to `fftw_execute'
/home/msammartino/Documents/twochain/dimer.cpp:104: undefined reference to `fftw_execute'
/home/msammartino/Documents/twochain/dimer.cpp:105: undefined reference to `fftw_execute'
/home/msammartino/Documents/twochain/dimer.cpp:106: undefined reference to `fftw_execute'
/tmp/ccgb7Xsv.o: In function `r2r_dsine_fftw_backward_dimer(int, double*, double*, Eigen::Matrix<double, 2, 2, 0, 2, 2> (&) [2048], Eigen::Matrix<double, 2, 2, 0, 2, 2> (&) [2048])':
/home/msammartino/Documents/twochain/dimer.cpp:166: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:167: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:168: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:169: undefined reference to `fftw_plan_r2r_1d'
/home/msammartino/Documents/twochain/dimer.cpp:170: undefined reference to `fftw_execute'
/home/msammartino/Documents/twochain/dimer.cpp:171: undefined reference to `fftw_execute'
/home/msammartino/Documents/twochain/dimer.cpp:172: undefined reference to `fftw_execute'
/home/msammartino/Documents/twochain/dimer.cpp:173: undefined reference to `fftw_execute'
collect2: error: ld returned 1 exit status
The terminal process terminated with exit code: 1

Terminal will be reused by tasks, press any key to close it.

Please let me know about any problems with the way I have asked this question.

Solution

The easy option is to pass them as args in your tasks.json configuration:

{
  "version": "2.0.0",
  "tasks": [
    {
      "label": "build-all",
      "type": "shell",
      "args": [
          "-std=c++11",
          "-lfftw3",
          "-L",
          "/path/to/libs",
          "/path/to/file.cpp"
      ],
      "command": "g++",
    }
  ]
}

The more maintainable, shareable option is to create a Makefile and set them all there:

# Specify compiler to be used
CXX = g++
CXXFLAGS += -g -std=c++11 -fPIC -march=x86-64

# Specify paths to headers
INCLUDES += -I include

# Specify paths to the libraries
LDFLAGS  += -L /path/to/libs

# Specify the link libraries
LLIBS    += -lfftw3

# ... add other configs ...

$(OBJ_DIR)/%.o: $(SRC_DIR)/%.cpp $(OBJ_DIR)
    $(CXX) -c $(CXXFLAGS) $(INCLUDES) $< -o $@

$(OBJ_DIR)/$(PROGRAM): $(OBJS)
    $(CXX) $(LDFLAGS) $^ $(LLIBS) -o $@

Then in your task configuration, just call make:

{
  "version": "2.0.0",
  "tasks": [
    {
      "label": "build-all",
      "type": "shell",
      "options": {
          "cwd": "${workspaceFolder}",
          "env": {
            ...
          }
      },
      "command": "make -f Makefile.x86_64",
    }
  ]
}

If you have env-dependent paths, you can specify a variable in your Makefile (ex. MY_LIBS) and then set them in the env block of the task configuration (ex. "MY_LIBS": "/path/to/libs").

The advantage of the Makefile option is that:

• People who don't use VS Code can still compile your code (from console or another IDE).
• If you are using a CI/CD pipeline, you don't need a separate configuration. You can use the same Makefile to build locally with VS Code and to build with CI/CD.
• You can commit the Makefile to a repository, and then just use environment variables in your local tasks.json configuration to specify env-specific settings.

Comments

[MattSamm]

I found this link useful in understanding what Make files are. cs.colby.edu/maxwell/courses/tutorials/maketutor

[MattSamm]

Im not sure how I opened my tasks.json file but its located in a hidden ".vscode" file in the directory of my VSC file. I get a permission error when I try and run the debugger. Do I need to include sudo and if so, do you know how to do that? How would I attach my tasks.json file in this question?

[Gino Mempin]

That's the expected location of the tasks.json file, in a .vscode folder in your current VSCode workspace. I find it a bit odd that the debugger requires sudo... It seems some files, binaries, or maybe VSCode itself was installed as root and you don't have permissions to access those files. Have you tried compiling your code outside of VS Code? From the console?

[MattSamm]

My code compiles fine in terminal without sudo. I used "g++ dimer.cpp -std=c++14 -lfftw3". It seems to infinitely run/ doesn't finish running when I call ./a.out. Would this cause problems with the debugger?

[Gino Mempin]

You shouldn't be requiring sudo anyway to compile and run, so that should be fine. As for the "infinitely run", that's a problem now of your application. The debugger will just behave the same way, if it infinitely runs on the terminal, it will also just infinitely run with the VS Code debugger.