Debugging with GDB

Basic Commands

1. b <location> // breakpoint (ocation an be memory addresses or names e.g. monitor.c:71, main, *0x7c00)
- b <location> if <condition>

2. c // continue running till breakpoint or interrupt
3. s // will step into functions
4. n // step over functions
5. si // same as s but for assembly instructions
6. ni // same as n but for assembly instructions
7. finish // runs code until the current function returns
8. advance <location> // runs code until the instructionpointer gets to the specified location.

Watchpoints

Like breakpoints, but with more complicated conditions.

• watch <expression> will stop execution whenever the expression’s value changes.\
• watch -l <address> will stop execution whenever the contents of the specified memory address change.

Examining

• x prints the raw contents of memory in whatever format you specify (x/x for hexadecimal, x/i for assembly, x/d for decimals etc).
• print evaluates a C expression and prints the result as its proper type. It is often more useful than x. e.g. p *((struct elfhdr *) 0x10000)
• info registers prints the value of every register.
• i frame or info frame prints the current stack frame.
• i args followed by p *argv@argc: prints out the arguments to the function
• i locals: prints info about local variables
• bt or backtrace: get the full backtrace of all stack frames. Followed by frame <no> and i frame to inspect in more details.

TUI (Text User Interface)

TUI is a terminal interface which uses the curses library to show the source file, the assembly output, the program registers and GDB commands in separate text windows.

1. tui enable
2. layout asm // view assembly code
3. layout reg // view registers
4. focus reg // can scroll reg window 
5. layout split // C source code and asm 

Written by Melodies Sim who lives and works in the Bay Area, California turning novel ideas into reality. Check out her projects on Github.