GDB

Running

# gdb <program> [core dump]

Start GDB (with optional core dump)

# gdb --args <program> <args...>

Start GDB and pass arguments

# gdb --pid <pid>

Start GDB and attach to process

set args <args...>

Set arguments to pass to program to be debugged

run

Run the program to be debugged

kill

Kill the running program

Breakpoints

break <where>

Set a new breakpoint

delete <breakpoint#>

Remove a breakpoint

clear

Delete all breakpoints

enable <breakpoint#>

Enable a disabled breakpoint

disable <breakpoint#>

Disable a breakpoint

Watchpoints

watch <where>

Set a new watchpoint

delete <watchpoint#>

Delete a watchpoint

enable <watchpoint#>

Enable a disabled watchpoint

disable <watchpoint#>

Disable a watchpoint

Where

function_name

Break/watch the named function

line_number

Break/watch the line number in the current source file

file:line_number

Break/watch the line number in the named source file

Conditions

break/watch <where> if <condition>

Break/watch at the given location if the condition is met. Condition may be almost any C expression that evaluates to true or false

condition <breakpoint#> <condition>

Set/change the condition of an existing break or watchpoint

Examining the Stack

backtrace

where

Show call stack

backtrace full

where full

Show call stack, also print the local variables in each frame

frame <frame#>

Select the stack frame to operate on

Stepping

step

Go to next instruction (source line), diving into function

next

Go to next instruction (source line) but do not dive into function

finish

Continue until the current function returns

continue

Continue normal execution

Variables and Memory

print/format <what>

Print content of variable/memory location/register

display/format <what>

Like print, but print the information after each stepping instruction

undisplay <display#>

Remove the display with the given number

enable display <display#>

disable display <display#>

Enable or disable the display with the given number

x/nfu <address>

Print memory

n: How many units to print (default 1)
f: Format character (like printf)
u: Unit

Unit is one of: b (Byte), h (Half-word/two bytes), w (Word/four bytes), g (Giant-word/eight bytes)

Format

a

Pointer

c

Read as integer, print as character

d

Integer, signed decimal

f

Floating point number

o

Integer, print as octal

s

Try to treat as C string

t

Integer, print as binary (t = 'two')

u

Integer, unsigned decimal

x

Integer, print as hexadecimal

What

expression

Almost any C expression, including function calls (must be prefixed with a cast to tell GDB the return value type)

file_name::variable_name

Content of the variable defined in the name file (static variables)

function_name::variable_name

Content of the variable defined in the name function (if on the stack)

{type}address

Content at address, interpreted as being of the C type type

$register

Content of named register. Interesting registers are $esp (stack pointer), $ebp (frame pointer) and $eip (instruction pointer)

Threads

thread <thread#>

Choose thread to operate on

Manipulate the Program

set var <variable_name>=<value>

Change the content of a variable to the given value

return <expression>

Force the current function to return immediately, passing the given value

Sources

directory <directory>

Add directory to the list of directories that is searched for sources

list

list <filename>:<function>

list <filename>:<line_number>

list <first>, <last>

Shows the current or given source context. The filename may be omitted. If last is omitted the context starting at start is printed instead of centered around it

set listsize <count>

Set how many lines to show in list

Signals

handle <signal> <options>

Set how to handle signals

Options are:
(no)print: (Do not) print a message when signals occur
(no)stop: (Do not) stop the program when signals occur
(no)pass: (Do not) pass the signal to the program

Information

disassemble

disassemble <where>

Disassemble the current function or given location

info args

Print the arguments to the function of the current stack frame

info breakpoints

Print information about the breakpoints and watchpoints

info display

Print information about the display

info locals

Print the local variables in the currently selected stack frame

info sharedlibrary

List loaded shared libraries

info signals

List all signals and how they are currently handled

info threads

List all threads

show directories

Print all directories in which GDB searches for source files

show listsize

Print how many are shown in the list command

whatis variable_name

Print type of named variable

Notes