Debugging with GDB: The GNU Source-Level Debugger, Tenth Edition, for GDB version 8.1.50.20180116-git. This book is available for free at gnu.org. This book is printed in grayscale.
The purpose of a debugger such as gdb is to allow you to see what is going on "inside" another program while it executes - or what another program was doing at the moment it crashed. gdb can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:

- Start your program, specifying anything that might affect its behavior.

- Make your program stop on specified conditions.

- Examine what has happened, when your program has stopped.

- Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another.