memo: C++ | Debug with VS Code

Debug Config

  1. Open “launch.json” ➡ Clik blue button “Add Configuration…” at right-bottom corner ➡ Select “C/C++: (gdb) Launch”.

    Then, a snippest of configuration with name "(gdb) Launch" is inserted into the “configurations” field in the config file.

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    {
    "version": "0.2.0",
    "configurations": [ 

        {
            "name": "(gdb) Launch",
            "type": "cppdbg",
            "request": "launch",
            "program": "${workspaceFolder}/test",
            "args": [],
            "stopAtEntry": false,
            "cwd": "${fileDirname}",
            "environment": [],
            "externalConsole": false,
            "MIMode": "gdb",
            "setupCommands": [
                {
                    "description": "Enable pretty-printing for gdb",
                    "text": "-enable-pretty-printing",
                    "ignoreFailures": true
                },
                {
                    "description": "Set Disassembly Flavor to Intel",
                    "text": "-gdb-set disassembly-flavor intel",
                    "ignoreFailures": true
                }
            ],
            "preLaunchTask": "g++ compile"
        },
    ]
}

    This newly created configuration can be selected in the drop-down box beside the gear.

  2. Add a task in tasks.json.

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    {
    "version": "2.0.0",
    "tasks": [
      {
        "label": "g++ compile",
        "type": "shell",
        "command": "g++",
        "args": [
          "${workspaceFolder}/test.cpp",
          "-g",
          "-o", "test"
        ],
        "options": {
          "cwd": "${workspaceFolder}"  
        },
        "group": {
          "kind": "build",
          "isDefault": true
        }
      },
    ]
}

The code “test.cpp” to be debugged is as follows:

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int main(){
    int x = 1; 
    int exampleArray[5];
    for (int i=0; i<5; i++)
        exampleArray[i] = i;

    int* exampleArray_heap = new int[5];
    for (int i=0; i<5; i++)
        exampleArray_heap[i] = i;
    delete[] exampleArray_heap
}

View Memory

After debugging started,

  1. Find the address of a variable x:

    Add expression &x into “Watch” panel. SO

  2. Use VS Code’s generic memory view

    • Install extention Hex Editor;
    • Clik the binary data icon $[^{01}_{10}]$ following the address (not variables).

Another ways to view memory

  1. Using extension Memmory View

    • Press F1, MemoryView: Add new memory view (for debugger)

    • Type the address of x: 0x7fffffffdbf0. Then, “MEMORY” section appers in the bottom panel.

  2. Use GDB command -exec x/64xb 0x7fffffffdc00

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    -exec x/64xb 0x555558854e20
0x555558854e20:	0x03	0x00	0x00	0x00	0x00	0x00	0x00	0x00
0x555558854e28:	0x00	0x00	0x00	0x00	0x00	0x00	0x00	0x00
    • The byte at 0x555558854e20 is 3.

Inspect array

  1. Array on stack is the pointer to the array. The pointer is stored in: &exampleArray= 0x7fffffffdc00

  2. After initializing, memory at exampleArray is 0,1,2,3,4:

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    00007fffffffdc00  00 00 00 00 01 00 00 00 02 00 00 00 03 00 00 00
00007fffffffdc10  04 00 00 00 ff 7f 00 00 00 50 58 2b fa da fc 53
    • This array takes 16 bytes.

  3. Array on heap is the address of pointer, which is stored in: &exampleArray_heap= 0x7fffffffdbf8

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    00007fffffffdbf0  01 00 00 00 04 00 00 00 c0 b2 56 55 55 55 00 00

  4. That’s an address: 00 00 55 55 55 56 b2 c0 (0x000055555556b2c0), because the endianness of computer is in reverse order.

    After initializing by the for loop, heap memory of the array is:

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    000055555556b2c0  00 00 00 00 01 00 00 00 02 00 00 00 03 00 00 00
000055555556b2d0  04 00 00 00

Read Address

Pointer’s value is an address.

  1. Use for loop to read 8 bytes one-by-one:

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    // Read the "address" (0x55555556b2c0) from memory (0x7fffffffdbf0) in bytes
unsigned char* bytePointer = (unsigned char*)&exampleArray_heap;
const int numBytesToRead = 8;
unsigned char buffer[numBytesToRead];
for (int i=0; i<numBytesToRead; i++)
    buffer[i] = *(bytePointer + i);
for (int i=0; i<numBytesToRead; i++)
    std::cout << std::hex << static_cast<int>(buffer[i]) << " ";

    Output:

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    c0 b2 56 55 55 55 0 0

  2. Another way is converting the type of pointer to (long long*)

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    std::cout << std::hex << *(long long*)&exampleArray_heap << std::endl;

    And it will automatically reverse the number:

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    55555556b2c0

Python Calls Cpp

(2024-01-29)

Attach the GDB used for C++ application to the running Python debugger, following steps in Example debugging mixed Python C++ in VS Code - Nadiah Pardede Kristensen

  • Preparation: Compile the C++ code to a python package:

    1. Make a folder for compiling the cpp package:

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      (AIkui) yi@yi-Alien:~/Downloads/Cpp_Study$ tree
.
├── cpp_ext_myadd
|   ├── debug_w_cpp.py
│   ├── myAdd.cpp
│   ├── setup.cfg
│   └── setup.py
└── .vscode
    ├── c_cpp_properties.json
    ├── launch.json
    └── settings.json

      where the myAdd.cpp file refers to code

      The "setup.py" looks like: 
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      from distutils.core import setup, Extension

def main():
    setup(name="myadd",
          ext_modules=[Extension("myadd", ["myAdd.cpp"])],
          )
if __name__ == "__main__":
    main()

    2. Install package:

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      (AIkui) yi@yi-Alie:~/Downloads/Cpp_Study$ pip install ./cpp_ext_myadd/

      The customize package myadd will be installed in the current environment. So, VS Code requires “Python:Select Interpreter” by pressing F1 to match the envrionment.

    3. Pyhon code: 
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      import myadd
print("going to ADD SOME NUMBERS")
x = myadd.myadd(5,6)
print(x)

      Add a breakpoint at line 2 before getting into package functions. And add a breakpoint (at z = x + y;) in the cpp file. Otherwise, the debugger won’t pause.

  1. The method that needs to manually find the process ID:

    1. Add 2 configurations for “Python” and “C/C++: gdb (Attach)” separately in the launch.json.

      Default contents generated after clicking the blue button "Add Configuration..." 
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      {
"version": "0.2.0",
"configurations": [
{
    "name": "(gdb) Attach",
    "type": "cppdbg",
    "request": "attach",
    "program": "/home/yi/anaconda3/envs/AIkui/bin/python3.10",
    "processId": "${command:pickProcess}",
    "MIMode": "gdb",
    "setupCommands": [
        {
            "description": "Enable pretty-printing for gdb",
            "text": "-enable-pretty-printing",
            "ignoreFailures": true
        },
        {
            "description": "Set Disassembly Flavor to Intel",
            "text": "-gdb-set disassembly-flavor intel",
            "ignoreFailures": true
        }
    ]
},
{
    "name": "Python: Current File",
    "type": "python",
    "request": "launch",
    "program": "${file}",
    "console": "integratedTerminal",
    "justMyCode": true
},
] }

    2. With focusing on debug_w_cpp.py, select the dubugger “Python: Current File”, and then click the green start button to kick off the debugging.

      When the debugger hits the breakpoint in debug_w_cpp.py, find its process ID:

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      (base) yi@yi-Alien:~$ ps aux | grep python
root         905  0.0  0.0  42744  1680 ?        Ss    2023   0:00 /usr/bin/python3 /usr/bin/networkd-dispatcher --run-startup-triggers
root        1278  0.0  0.0 121200  1436 ?        Ssl   2023   0:00 /usr/bin/python3 /usr/share/unattended-upgrades/unattended-upgrade-shutdown --wait-for-signal
yi       1605315  0.0  0.1  38840 26336 pts/5    S+   16:17   0:04 /usr/bin/python3.8 -O /usr/bin/ranger
yi       1698762  2.9  0.9 1179385796 150624 ?   Sl   18:42   0:04 /usr/share/code/code --ms-enable-electron-run-as-node /home/yi/.vscode/extensions/ms-python.vscode-pylance-2023.12.1/dist/server.bundle.js --cancellationReceive=file:bad7d71dedf58b5bb22a36398d8eb2bcf4447338c7 --node-ipc --clientProcessId=1698586
yi       1700442  0.4  0.1 840012 18472 ?        Sl   18:44   0:00 /home/yi/anaconda3/envs/AIkui/bin/python /home/yi/.vscode/extensions/ms-python.python-2023.22.1/pythonFiles/lib/python/debugpy/adapter
yi       1700450  0.3  0.1 250260 18376 pts/6    Sl   18:44   0:00 /home/yi/anaconda3/envs/AIkui/bin/python /home/yi/.vscode/extensions/ms-python.python-2023.22.1/pythonFiles/lib/python/debugpy/adapter/../../debugpy/launcher 45763 -- /home/yi/Downloads/Cpp_Study/cross_lang_debug_host.py
yi       1700456  3.1  0.1 414788 30468 pts/6    Sl+  18:44   0:00 /home/yi/anaconda3/envs/AIkui/bin/python /home/yi/.vscode/extensions/ms-python.python-2023.22.1/pythonFiles/lib/python/debugpy/adapter/../../debugpy/launcher/../../debugpy
--connect 127.0.0.1:55977 --configure-qt none --adapter-access-token 2bb19c19a6a1592229fd27c7c8e3e0a37c002b320568e3b3c8fe450a3d343404 /home/yi/Downloads/Cpp_Study/cross_lang_debug_host.py
yi       1700778  0.0  0.0  12116  2776 pts/7    S+   18:45   0:00 grep --color=auto python

      The desired process is the one attached with a token: 1700456

    3. Switch the debugger to “gdb (Attach)”, and then click the green start button again, it will ask the process ID just found.

      Then, a prompt pops:

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      (base) yi@yi-Alienware:~/Downloads/Cpp_Study$  /usr/bin/env /bin/sh /tmp/Microsoft-MIEngine-Cmd-00cvveuw.5cg 
Superuser access is required to attach to a process. Attaching as superuser can potentially harm your computer. Do you want to continue? [y/N]

      However, after I input “y”, the terminal said executing GDB requires elevated permission, but it didn’t prompt me to enter password.

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      ==== AUTHENTICATING FOR org.freedesktop.policykit.exec ===
Authentication is needed to run `/usr/bin/gdb' as the super user
Authenticating as: Yi Cao,,, (yi)
Password: [1] + Stopped (tty output)       /usr/bin/pkexec "/usr/bin/gdb" --interpreter=mi --tty=${DbgTerm} 0<"/tmp/Microsoft-MIEngine-In-tkwd0bog.wdj" 1>"/tmp/Microsoft-MIEngine-Out-n1p4gbjg.vvb"
You have stopped jobs.

  1. Extension: Python C++ Debugger eliminates the need of manually entering process ID.

    Its default configuration in launch.json is:

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    {
  "version": "0.2.0",
  "configurations": [
    {
      "name": "Python C++ Debug",
      "type": "pythoncpp",
      "request": "launch",
      "pythonConfig": "default",
      "cppConfig": "default (gdb) Attach",
    }
  ]
}

    However, the authentication error persists.

  1. Bypass the authentication:

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    echo 0 | sudo tee /proc/sys/kernel/yama/ptrace_scope

    Refer to: Debugging mixed Python C++ in VS Code. Can’t enter sudo password - SO

    • “This solution doesn’t require reboot, but it’s not permanent.” , as explained in Attaching gdb (C++ debugger) to remote python process in VSCode - SO (OP is Mark Harris) (Found by Perplexity)

      Original answer 
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      $ gdb --pid=30428
...
Attaching to process 30428
Could not attach to process.  If your uid matches the uid of the target
process, check the setting of /proc/sys/kernel/yama/ptrace_scope, or try
again as the root user.  For more details, see /etc/sysctl.d/10-ptrace.conf
ptrace: Operation not permitted.

In this example, I found that the debugger cannot step downward after entering the cpp file, instead, it gets stuck at the line following the breakpoint.

However, the method works in the project of AIkui’s CUDA extension tutorial, where the debugger moves line-by-line after jumping into “interpolation_kernel.cu” files (function: trilinear_fw_cu()), whereas it can’t step into kernel functions, which may require CUDA-GDB, rather than GDB.

  • It seems that after GDB concludes the execution of C++ programm, it will stay at the last line, and won’t automatically return to the Python debugger. So, I have to click the “disconnect” button on the control bar to free the GDB.

Other problems:

  1. If the file currently open in the editor is not the Python file “debug_w_cpp.py”, but rather “myAdd.cpp”, an error pops upon clicking the start button:

  2. The following command for installing cpp packege, referring How to pass –debug to build_ext when invoking setup.py install?, requires the path to source file relative path to workspaceFolder in the “setup.py”: ext_modules=[Extension("myadd", ["./cpp_ext_myadd/myAdd.cpp"])]

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    (AIkui) yi@yi-Alienware:~/Downloads/Cpp_Study$ python3.10 ./cpp_ext_myadd/setup.py build_ext --debug install

(2024-04-18)