CIS241

System-Level Programming and Utilities

C - Debugging

Erik Fredericks, frederer@gvsu.edu
Fall 2025

Based on material provided by Erin Carrier, Austin Ferguson, and Katherine Bowers

Debugging

What is debugging?

• Trying to fix a broken code

What is a debugger?

• A program that lets you see what’s going on inside a program as it runs

Why use a debugger?

• Helps fix segfaults and logic errors
• Provides more insights than print statements

Debugging

We’ll be using gdb (GNU Project Debugger)

Steps to run gdb:

1. Compile your code in debug mode

• gcc -g my_code.c

2. Launch gdb with your executable

• gdb ./a.out

Mac users...

You will likely not have gdb

• You can use lldb instead.

Use will be very similar, but some commands will have different names.

Reference: https://lldb.llvm.org/use/map.html

Using gdb

Using gdb

You can run commands within gdb, basics include:

• run - Start executing your program from the beginning
• run arg1 arg2 for command line args
• run < input_file > output_file (redirect)
• quit - exit gdb
• kill - stop program execution

Debugging a segfault

♪~ ᕕ(ᐛ)ᕗ

1. Start gdb to run program

2. run

3. Examine the callstack with backtrace or bt

4. Load a frame (f) using its number (e.g., f 1)

Debugging a segfault

5. You can use list to get more context

6. Check the state of your variables

• Print a variable with print (or p) (e.g., p size)

• Print n items of an array with *array@n

7. Try to deduce the problem! You can always examine other frames

Breakpoints

You can also use breakpoints to pause your code

To set a breakpoint:

• break function_name
• break line_number
• break line_number if condition

To list all breakpoints: info break

Breakpoints

continue will resume until next breakpoint

step to execute one line of code

• step n to execute n lines of code

enable/disable breakpoint_number

delete breakpoint_number (delete for all)

Other tools

Valgrind: "a suite of tools for debugging and profiling programs"

• Valgrind is very powerful, but very complex.

Recommendation: memcheck

• valgrind --tool=memcheck ./a.out

This will check for memory leaks and other hard-to-spot memory issues!