Exercise 4: Using a Debugger

This is a video-focused exercise where I show you how to use the debugger that comes with your computer to debug your programs, detect errors, and even debug processes that are currently running. Please watch the accompanying video to learn more about this topic.

GDB Tricks

Here's a list of simple tricks you can do with GNU Debugger (GDB):

• gdb --args [PROGRAM]: Normally, gdb takes arguments you give it and assumes they are for itself. Using --args passes them to the program.
• thread apply all bt: Dump a backtrace for all threads. It's very useful.
• gdb --batch --ex run --ex bt --ex q --args [PROGRAM] [ARGS]: Run the program so that if it bombs, you get a backtrace.

GDB Quick Reference

The video is good for learning how to use a debugger, but you'll need to refer back to the commands as you work. Here is a quick reference to the GDB commands that I used in the video so you can use them later in the book:

• run [args]: Start your program with [args].
• break [file:]function: Set a break point at [[file:]function](file:]function). You can also use b.
• backtrace: Dump a backtrace of the current calling stack. Shorthand is bt.
• print expr: Print the value of expr. Shorthand is p.
• continue: Continue running the program. Shorthand is c.
• next: Next line, but step over function calls. Shorthand is n.
• step: Next line, but step into function calls. Shorthand is s.
• quit: Exit GDB.
• help: List the types of commands. You can then get help on the class of command as well as the command.
• cd, pwd, make: just like running these commands in your shell.
• shell: Quickly start a shell so you can do other things.
• clear: Clear a breakpoint.
• info break, info watch: Show information about breakpoints and watchpoints.
• attach pid: Attach to a running process so you can debug it.
• detach: Detach from the process.
• list: List out the next ten source lines. Add a - to list the previous ten lines.

LLDB Quick Reference

In OS X, you no longer have GDB and instead must use a similar program called LLDB Debugger (LLDB). The commands are almost the same, but here's a quick reference for LLDB:

• run [args]: Start your program with [args].
• breakpoint set --name [file:]function: Set a break point at [[file:]function](file:]function). You can also use b, which is way easier.
• thread backtrace: Dump a backtrace of the current calling stack. Shorthand is bt.
• print expr: Print the value of expr. Shorthand is p.
• continue: Continue running the program. Shorthand is c.
• next: Next line, but step over function calls. Shorthand is n.
• step: Next line, but step into function calls. Shorthand is s.
• quit: Exit LLDB.
• help: List the types of commands. You can then get help on the class of command as well as the command itself.
• cd, pwd, make: just like running these commands in your shell.
• shell: Quickly start a shell so you can do other things.
• clear: Clear a breakpoint.
• info break, info watch: Show information about breakpoints and watchpoints.
• attach -p pid: Attach to a running process so you can debug it.
• detach: Detach from the process.
• list: List out the next ten source lines. Add a - to list the previous ten sources.

You can also search online for quick reference cards and tutorials for both GDB and LLDB.

Valgrind

In the videos I will frequently use a tool called Valgrind, or a similar tool to perform memory checks of my code. I don't explicitly use this tool in the book and instead say "the debugger" to refer to GDB, LLDB, Valgrind, anything you can use to analyze your code. Watch the videos to see me use Valgrind and many other tools to check my C source code, and especially the video for this exercise.