CSCI 315

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Debugging in C++ with GDB ​

Introduction ​

Common Types of Bugs ​

  • Syntax Errors: Mistakes in the code that prevent it from compiling.
  • Logical (Semantic) Errors: The program runs but produces incorrect results.

What is Debugging? ​

  • The process of finding and fixing errors (bugs) in a program.
  • Can involve various techniques, including print statements, code reviews, and using debugging tools.

Why Debug with Debuggers? ​

  • Even the best programmers make mistakes.
  • Debugging is an essential skill for every programmer.
  • Learning to use a debugger effectively can save you hours of frustration.

Debugging Approaches ​

  • Print statements (std::cout).
  • Code reviews and pair programming.
  • Debugging tools like GDB.

Debugging with GDB ​

What is GDB? ​

  • GDB stands for GNU Project Debugger and was first written in 1986.

  • A debugging tool for C++ (and other languages).

Logo for GDB, designed by Jamie Guinan, who wrote, “For a fish, the archer fish is known to shoot down bugs from low hanging plants by spitting water at them.”
Logo for GDB, designed by Jamie Guinan, who wrote, “For a fish, the archer fish is known to shoot down bugs from low hanging plants by spitting water at them.”

  • Lets you step through the code one instruction at a time to see what is happening.

  • Loads executable files that were compiled with debug information (-g).

Compiling a Program for Debugging ​

g++ -g <source> -o <executable>

Launching and Exiting GDB ​

  • gdb Start GDB.
  • gdb <program> Start GDB.
  • gdb --args Start GDB and pass arguments to executable.
  • quit Exit GDB.

Run a Program to be Debugged ​

  • run Run the executable that was previously set.
  • file Set the executable to run.

Breakpoints ​

  • break <where> Set a new breakpoint.
  • enable <breakpoint#> Enable a disabled breakpoint
  • disable <breakpoint#> Disable a breakpoint.
  • delete <breakpoint#> Remove a breakpoint.
  • clear Delete all breakpoints.

Stepping through Lines of Code ​

  • step Go to next instruction (source line), diving into function calls.
  • next Go to next instruction (source line) in this function.
  • continue Continue execution until the next breakpoint.
  • finish Continue until the current function returns.

Variables and Memory ​

  • print Print content of variable/memory location.
  • display Like print but shows the information with each step.
  • undisplay Remove the display with the given number.
  • info locals Print the local variables or arguments in the current stack frame.

GDB in VSCodium or VS Code ​

Prerequisites ​

  1. Install the C/C++ Debug (gdb) extension.

  2. If running in WSL, install the Open Remote - WSL extension.

  3. Open the lab/project folder (not just a source file or the whole repository).

    • If using WSL, make sure to connect to WSL (remote connection).

  4. Open a C++ file.

Project Setup ​

  1. Open a C++ File in the Folder.

  2. Click on the “Run” menu at the top.

  3. Click on the “Add Configuration” menu item and select “G++ (GDB/LLDB)”.

  4. Click “Add Configuration” on the bottom right.

  5. Select “C/C++: (gdb) Launch”.

  6. Modify the “name”, “program”, “args”, and “cwd” fields as desired.

https://code.visualstudio.com/docs/cpp/launch-json-reference

Use .gitignore files ​

There are often files you do not want to share (executable, debug configuration).

Git will not track files and folders specified in a .gitignore file.

.vscode/*
obj/*
main
test-runner