Unit 8

Agenda¶

Why debugging matters
Build types in CMake
Using gdb (GNU Debugger)
Example: debugging a simple C++ bug

Why Debugging?¶

Every developer writes bugs – debugging is how we learn.
Tools like gdb help find logic or memory errors efficiently.
Understanding how to inspect variables and execution flow = essential engineering skill.

CMake Build Types¶

CMake supports different build configurations:

cmake -DCMAKE_BUILD_TYPE=Debug ..
cmake -DCMAKE_BUILD_TYPE=Release ..

Debug: includes symbols for gdb (-g flag)
Release: optimized build without debug info

Simple Example Program with a Bug¶

src/main.cpp

#include <iostream>
#include <vector>
using namespace std;

int main() {
    vector<int> data = {1, 2, 3, 4};
    for (int i = 0; i <= data.size(); i++) // <-- Bug: <= causes out-of-bounds
        cout << data[i] << endl;
}

Step 1: Compile with Debug Info¶

mkdir build && cd build
cmake -DCMAKE_BUILD_TYPE=Debug ..
make

Step 2: Start gdb¶

gdb ./main

At the gdb prompt:

(gdb) break main
(gdb) run
(gdb) next
(gdb) print i
(gdb) backtrace
(gdb) quit

Common gdb Commands¶

Command	Description
`run`	start the program
`break <line/function>`	set a breakpoint
`next` / `step`	execute next line (step over/into)
`continue`	resume program
`print <var>`	show variable value
`backtrace`	show call stack
`quit`	exit gdb

Example Debugging Session¶

gdb ./main
(gdb) break 5
(gdb) run
(gdb) print i
(gdb) next
(gdb) print data[i]
(gdb) continue

gdb output:

Program received signal SIGSEGV, Segmentation fault.

Debugging Tips¶

Always compile with -g
Use small, reproducible test cases
Don't just fix symptoms – understand why the bug occurred
Document fixes clearly in Git commits

Survival Package (Debugging)¶

cmake -DCMAKE_BUILD_TYPE=Debug
gdb ./program
Commands: break, run, next, step, print, backtrace