gdb Cheat Sheet

Note on Supported Commands

Most valid gdb commands can be used in mdb (as long as they run in remote execution mode – but that’s nearly all the useful ones I can think of). The main exceptions are start and run and these wouldn’t work in MPI mode anyway because you cannot (as far as I know) restart and MPI application from within the process.

mdb launches the binary to stop at main by default, which can optionally overridden to any other valid breakpoint if this is not suitable (see Quick Start for more info). Therefore instead of using start or run commands, as you can non-MPI applications, mdb will already be waiting at the breakpoint specified by -b/--breakpt.

For MPI debugging gdb’s reverse commands (e.g., reverse-next, reverse-step etc.) are not supported. Each gdb instance is not MPI-aware and you cannot step backwards through MPI_init(), for example. If you want to “go backwards” you should restart the mdb debug session.

Useful Commands

The following commands may be helpful when using mdb. All of these can be passed to gdb using the command command [gdb command] (or optionally specify which ranks to run the command on with command [ranks] [gdb command]). A full list of gdb commands can be found in gdb’s manual.

Moving through the program

Command	Action	Example	Aliases
next	run to next line	command n	n
step	step into function	command s	s
backtrace	print backtrace	command bt	bt and where
continue	continue running program	command c	c
until	run until {expression}	command u {expression}	u
up	move up one frame	command up	up
down	move down one frame	command down	do

Breakpoints and Watchpoints

Command	Action	Example	Aliases
breakpoint	set a breakpoint	command b {expression}	b
		command b [filename]:[line_no]
	(conditional)	command b [func_name] if myrank < 2
watchpoint	set a watchpoint	command watch [variable_name]	watch
	(on write)	command watch foo
	(on read)	command rwatch foo
	(on read-write)	command awatch foo
delete [num]	delete breakpoint #2	command del 2	del

Displaying Information

Command	Action	Example	Aliases
print	print variable {var}	command p {var}	p
display	show variable {var} after every command	command disp {var}	disp
info {option}	print information for:
	(breakpoints)	command info break	info b
	(locals)	command info locals	info loc

Changing Execution

Command	Action	Example	Aliases
set variable	change value of variable {var}	command set var {var}={value}	set var

Single Key Mode (TUI)

This can be used inside an interactive gdb session when the tui is activated. To enter Single Key Mode press CTRL-x s. To exit press q. This allows you to use a single key press to issue the following commands:

Single Key Mode

Key	Command
c	continue
d	down
f	finish
n	next
o	nexti (the letter “o” stands for “step [o]ver”)
r	run
s	step
i	stepi (the letter “i” stands for “step [i]nto”)
u	up
v	info locals
w	where
q	(exit Single Key Mode)