Quick Command References: GDB

Contents

Start GDB
Configurations
- Print and Display
Debugging Operations

A quick reference to most commonly used GDB commands you may need for debugging.

Start GDB

Compile and Enable Debugging

To allow GDB to examine functions and variables with using their names appeared in the source code/files, it is necessary to add the flag -g when compiling your program (with gcc/g++).
Quoting a good explanation:

“In order to debug a program effectively, you need to generate debugging information when you compile it. This debugging information is stored in the object file; it describes the data type of each variable or function and the correspondence between source line numbers and addresses in the executable code.” — source

Select Program

# specify the executable to trace
file <executable_name>

# set arguments to append when running the executable
set args <arguments>

[Bonus] Debug a Linux Kernel

It’s not uncommon to debug a Linux kernel with using QEMU and GDB, so I’m giving a little summary here. A full article for building and running the kernel for QEMU can be found here.

The following configurations should be set in defconfig to enable the GDB debugging:

CONFIG_DEBUG_INFO=y
CONFIG_DEBUG_KERNEL=y
CONFIG_GDB_SCRIPTS=y

What’s CONFIG_GDB_SCRIPTS for?

“Gdb comes with a powerful scripting interface for python. The kernel provides a collection of helper scripts that can simplify typical kernel debugging steps.” — source

When starting your QEMU, you will want to add the arguments -s -S to make your VM stop and wait for GDB to connect. Then, in another terminal, you can start gdb and connect to your QEMU’ed kernel by:

GDB

# enable Python scripts for kernel debugging with lx- commands
python gdb.COMPLETE_EXPRESSION = gdb.COMPLETE_SYMBOL
add-auto-load-safe-path scripts/gdb/vmlinux-gdb.py

# read symbols
file vmlinux

# connect to the GDB server at the default address
target remote :1234

# check the Linux version
lx-version

# run
continue

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Configurations

Print and Display

To beautify printing variables (ref):

# enable printing with indents and new lines
set print pretty on

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Debugging Operations

Run

There are several ways to advance execution in GDB (ref):

# run the program
run

# continue after a pause
continue

# execute one line at a time
step

# run out of the current function
finish

# step over the current line if it is a function
nexti

Show Variables

# show all (global) variable symbols
info variables

# show variables in current stack (of a function)
info locals

# show arguments of current function
info args

# show the value of the specified variable <variable_name>
print <variable_name>

Set/Delete Breakpoints

To add breakpoints:

# by function name <function_name>
break <function_name>

# by file name and line number
break <filename>:<line_number>

# show existing breakpoints
info breakpoints

To delete breakpoints (ref):

# delete all breakpoints
clear

# delete a breakpoint by ID (displayed in info breakpoints)
delete 1

To temporarily disable or enable one or all break points

# disable all breakpoints
disable b

# enable all breakpoints
enable b

# disable a breakpoint (e.g., the first breakpoint shown in `info b`)
disable 1

# enable a breakpoint
enable 1

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