Debugging, Doubly-Linked Lists, Project 1

Chapter 16

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

• 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 VS Code

Prerequisites to GDB in VS Code

• If running in WSL, install the WSL extension.

• Install the C++ extension.

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

• Open a C++ file.

Setup Project for GDB in VS Code

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

Doubly-Linked List

What if we want to traverse the list in reverse?

Three Options:

1. We can use recursion to iterate to the end and then perform the operation when we “unwind.”

2. We can use a helper data structure (a stack).

3. We can implement a doubly-linked list type, where every node has a next and previous pointer.

   

   Example of a doubly-linked list.

   

   Example of a doubly-linked list.

Prepending a Doubly-Linked List

Step 0:

Initial State before prepending to a doubly-linked list.

Initial State before prepending to a doubly-linked list.

Step 1:

Create the new node.

Create the new node.

Step 2:

Update pNewNode->pNext to mpHead.

Update pNewNode->pNext to mpHead.

Step 3:

Update mpHead->pPrev to pNewNode.

Update mpHead->pPrev to pNewNode.

Step 4:

Update mpHead to pNewNode.

Update mpHead to pNewNode.

Appending a Doubly-Linked List

Step 0:

Initial State before appending to a doubly-linked list.

Initial State before appending to a doubly-linked list.

Step 1:

Create the new node.

Create the new node.

Step 2:

Update pNewNode->pNext to mpTail.

Update pNewNode->pNext to mpTail.

Step 3:

Update mpTail->pNext to pNewNode.

Update mpTail->pNext to pNewNode.

Step 4:

Update mpTail to pNewNode.

Update mpTail to pNewNode.

Intro. to the Standard Template Library (SLT)

Go here for lecture notes on the STL.

Homework

• Lab 9: Doubly-Linked Lists

• Project 1: Large Map