CS351 - Computer Organization

GDB - The GNU Debugger

GDB TUI mode (Text User Interface)

The TUI mode represents a nice curses interface to gdb.

Compile your programs with:

Language	Options
GNU C	`gcc -ggdb` ...
clang	`clang -gdwarf` ...
nasm	`nasm -g -F dwarf -felf64 prog.s` ... `ld -o prog prog.o`

Then issue the command: gdb prog

Then inside of GDB issue the following commands:

tui enable
tui reg general
break _start
run [<params>]

Then use 'step', 'next', 'si', 'ni', etc as normal, but enjoy the curses window. The source window should be the selected one, which will respond to arrow keys and scroll wheel events to move around. The enter key will repeat the last command run over and over.

To direct the output of the program to a different terminal window (because you will not see the output because of the curses nature of the tui window,) use "tty" in the terminal you want to direct the output to, then in the gdb window use "tty ", such as "tty /dev/pts/4" for example, then "run" the program (re-run it if using debug.)

gdb program

-p pid - Debug the already running program with PID 'pid'
-c core - Use the core file as the processes memory.

Use:

unlimit coredump (tcsh)
ulimit -c unlimited (bash)

to enable core-dumps.

Common commands:

Command	What it does
`run` args...	Runs the program
`c`	Continue the program
`next`	Step the program (skips function calls)
`step`	Step one source line (descends into function calls)

`list` [file`:`]func	List the source where the program is stopped at
`break` location	Set a break-point at location
`catch` event	Catch an event such as "fork", "signal", etc.
`watch` expr	Break whenever a data location changes.
`awatch` expr	Break whenever a data location is read/written.

`print` expr	Print the value of expr
`where/bt`	Print a stack-trace of where the program is currently
`up`	Go up the previous stack frame
`down`	Go down a stack frame

`help`	GDB help
`quit`	Exit gdb

Assembly debugging:

Command	What it does
`info registers` `i r`	Dumps all the registers and their values
`info frame` `i f`	Dump information about the current stack frame
`print $reg` `p $reg`	Print a specific register and its value
`print (char *)($rsp+8)`	Can apply C type to a dereferenced address.
`display /3i $pc`	Display 3 instructions relative to the program counter when stepping. *You probably always want to do this before stepping*
`ni`	Go to next instruction (stepping over calls)
`si`	Step to next instruction (stepping into calls)
`x /8g $rsp`	Examine 8 (giant (8-byte) words) starting at $rsp

TUI keybindings

Key	What it does
Ctrl-X 1	Use TUI mode w/ only one window
Ctrl-X 2	Use TUI mode w/ at least two windows
Ctrl-X o	Change the active output window
Ctrl-X a	Exit TUI mode
Ctrl-X s	Switch in/out of TUI mode
PgUp / PgDown	Scroll active window one page up/down
Up/Down/Left/Right	Scroll active window one line/column
Ctrl-L	Refresh the display