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C and Cpp Debugging in VS Code

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Brian "Beej Jorgensen" Hall edited this page Dec 14, 2017 · 9 revisions

Prerequisites

First, go to the extensions panel in VS Code and search for C/C++. Choose the extension by Microsoft (ms-vscode.cpptools).

Install that extension and restart VSC.

Here is an example project.

Building the Project in VSC

What we want to be able to do is build the project with CTRL-SHIFT-B (or COMMAND-SHIFT-B).

The tasks.json file controls which tasks VSC can run. The one we're interested in is the Default Build Task.

  1. With VSC open to your root project directory, hit CTRL-SHIFT-P (or COMMAND-SHIFT-P).
  2. Type tasks and choose "Tasks: Configure Default Build Task".
  3. Choose "Create tasks.json file from template".
  4. Choose "Others".

At this point you should be editing tasks.json.

  1. Change the label to something representative, like Build
  2. Change the command to the command you want to run, like gcc -Wall -o foo foo.c or make
  3. Hit CTRL-SHIFT-P (or COMMAND-SHIFT-P) again.
  4. Type default and choose "Tasks: Configure Default Build Task" again.
  5. Select the value you put in for label in step 1.
  6. Save tasks.json.

At this point, your tasks.json should look something like this:

{
  // See https://go.microsoft.com/fwlink/?LinkId=733558
  // for the documentation about the tasks.json format
  "version": "2.0.0",
  "tasks": [
    {
      "label": "Build",
      "type": "shell",
      "command": "make",
      "group": {
        "kind": "build",
        "isDefault": true
      }
    }
  ]
}

Now you should be able to hit CTRL-SHIFT-B (or COMMAND-SHIFT-B) and see a build.

Adding PROBLEMS Panel Support (OPTIONAL)

Adding a Problem Matcher will automatically extract error information from the build and put it in the PROBLEMS panel to make it easier to jump to those line numbers.

Here is a tasks.json file with a problemMatcher for C/C++ added in:

{
  // See https://go.microsoft.com/fwlink/?LinkId=733558
  // for the documentation about the tasks.json format
  "version": "2.0.0",
  "tasks": [
    {
      "label": "Build",
      "type": "shell",
      "command": "make",
      "group": {
        "kind": "build",
        "isDefault": true
      },
      "problemMatcher": {
        "owner": "cpp",
        "fileLocation": ["relative", "${workspaceFolder}"],
        "pattern": {
          "regexp": "^(.*):(\\d+):(\\d+):\\s+(warning|error):\\s+(.*)$",
          "file": 1,
          "line": 2,
          "column": 3,
          "severity": 4,
          "message": 5
        }
      }
    }
  ]
}

tasks.json will be found in the .vscode/ directory in the project workspace.

Debugging the Project in VSC

IMPORTANT: You must build your C code with the -g switch to gcc or clang. This causes debugging information to be included in the binary.

Setting up the Debugger

To set up debugging, perform the following steps:

  1. Open the debugging panel (with the "anti-bug" symbol on the left).

  2. Where it says "No Configurations on the top, pull down that menu.

  3. Choose "Add Configuration".

  4. Choose C++

  5. It'll pop up an editor for launch.json. Edit the name to be something you like, e.g. "My Server"

  6. Change program to be the name of your binary from the build step, e.g. "${workspaceFolder}/myserver"

  7. If you are using fork(), it might be helpful to modify GDB's detach-on-fork setting to off in setupCommands.text, for example:

    "text": "-enable-pretty-printing -gdb-set detach-on-fork off",

  8. Save launch.json.

Example launch.json:

{
  // Use IntelliSense to learn about possible attributes.
  // Hover to view descriptions of existing attributes.
  // For more information, visit: https://go.microsoft.com/fwlink/?linkid=830387
  "version": "0.2.0",
  "configurations": [

    {
      "name": "Hello world",
      "type": "cppdbg",
      "request": "launch",
      "program": "${workspaceFolder}/hello",
      "args": [],
      "stopAtEntry": false,
      "cwd": "${workspaceFolder}",
      "environment": [],
      "externalConsole": true,
      "MIMode": "gdb",
      "setupCommands": [
        {
          "description": "Enable pretty-printing for gdb and add better child debugging",
          "text": "-enable-pretty-printing -gdb-set detach-on-fork off",
          "ignoreFailures": true
        }
      ]
    }
  ]
}

launch.json will be found in the .vscode/ directory in the project workspace.

Running the Debugger

  1. Perform a build.
  2. In the editor for the file you wish to debug, click just left of the line number where you want a breakpoint. A red dot will appear.
  3. In the debugger panel, click the green run arrow at the top, or select Debug→Start Debugging, or hit F5.
  4. The run should break on the line with the breakpoint.

If the run doesn't break on the line you specified you probably didn't compile with the -g switch for gcc or clang. Make sure to rebuild.

Variables

The debugging panel will show local variables in the VARIABLES pane. You can add expressions to watch in the WATCH pane, e.g.

node->next->val

If you have a char a[1000] that you want to view as a string instead of an array, add this to the WATCH section:

(char *)a

Console I/O

Console input and output will be in another window that pops up. It might hide behind the VSCode window, so look for it there.

Controls

Once you've hit the breakpoint, you can do one of these main things:

  • Continue: F5 or green arrow. Keep running to the next breakpoint or the end.
  • Step Over: F10 or "jump over" icon. Execute this line and go to the next without entering any functions.
  • Step Into: F11 or "down arrow" icon. If this line is a function call, step into that function.
  • Step Out: SHIFT-F11 or "up arrow" icon. Continue running until you get out of this function.

See the "Debug" pulldown menu for all options.

Happy hacking!

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