GDB Tutorials

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What is GDB?

GDB, the GNU Project debugger, allows you to see what is going on `inside' another program while it executes -- or what another program was doing at the moment it crashed.

GDB can do four main kinds of things (plus other things in support of these) to help you catch bugs in the act:

• Start your program, specifying anything that might affect its behavior.
• Make your program stop on specified conditions.
• Examine what has happened, when your program has stopped.
• Change things in your program, so you can experiment with correcting the effects of one bug and go on to learn about another

gdb is in the gnu package on CEC machines. If you don't have this package loaded then type pkgadd gnu at a shell prompt. If you can run g++, then you will be able to run gdb.

gdb can only use debugging symbols that are generated by g++. For Sun CC users, there is the dbx debugger which is very similar to gdb.
gdb is most effective when it is debugging a program that has debugging symbols linked in to it. With g++, this is accomplished using the -g command line argument. For even more information, the -ggdb switch can be used which includes debugging symbols which are specific to gdb. The makefile for this tutorial uses the -ggdb switch


Summary of the info Command

info ...	Information displayed
address sym	Information about where symbol sym is stored. This is either a memory address or a register name.
all-registers	Information about all registers, including floating-point registers.
args	Information about the arguments to the current function (stack frame).
break [bpnum]	Information about breakpoint bpnum if given, or about all breakpoints if not.
breakpoints[bpnum]	Same information as the info break command.
catch	Information on exception handlers active in the current frame.
classes[regexp]	Information about Objective-C classes that matchregexp, or about all classes if regexp is not given.
display	Information about items in the automatic display list.
extensions	Information about the correspondence of filename extensions to source code programming languages.
f [address]	Same information as the info frame command.
files	Information about the current debugging target, including the current executable, core, and symbol files.
float	Information about the floating-point hardware.
frame[address]	With no argument, print information about the current frame. With an address, print information about the frame containing address, but do not make it the current frame.
functions[regexp]	With no argument, print the names and types of all functions. Otherwise, print information about functions whose names match regexp.
handle	The list of all signals and how GDB currently treats them.

Alias	Short for ...	Alias	Short for ...
bt	backtrace	i	info
c	continue	l	list
cont	continue	n	next
d	delete	ni	nexti
dir	directory	p	print
dis	disable	po	print-object
do	down	r	run
e	edit	s	step
f	frame	share	sharedlibrary
fo	forward-search	si	stepi
gcore	generate-core-file	u	until
h	help	where	backtrace

BreakPoints

awatch	Set an expression watchpoint.
break	Set a breakpoint at a line or function.
catch	Set a catchpoint to catch an event.
clear	Clear a given breakpoint.
commands	Specify commands to run when a breakpoint is reached.
condition	Supply a condition to a particular breakpoint.
delete	Delete one or more breakpoints or auto-display expressions.
disable	Disable one or more breakpoints.
enable	Enable one or more breakpoints.
hbreak	Set a hardware-assisted breakpoint.
ignore	Set the ignore-count of a particular breakpoint.
rbreak	Set a breakpoint for all functions matching a regular expression.
rwatch	Set a read watchpoint for an expression.
tbreak	Set a temporary breakpoint.
tcatch	Set a temporary catchpoint.
thbreak	Set a temporary hardware-assisted breakpoint.
watch	Set an expression watchpoint.

Data Analysis

call	Call a function in the program.
delete display	Cancel one or more expressions that have been set to display when the program stops.
delete mem	Delete a memory region.
disable display	Disable one or more expressions that have been set to display when the program stops.
disable mem	Disable a memory region.
disassemble	Disassemble a section of memory.
display	Print the value of an expression each time the program stops.
enable display	Enable one or more expressions that have been set to display when the program stops.
enable mem	Enable a memory region.
inspect	Same as print.
mem	Define attributes for a memory region.
output	Similar to print, but doesn't save the value in history and doesn't print a newline. For scripting.
print	Print the value of an expression.
print-object	Cause an Objective C object to print information about itself.
printf	Print values such as the printf command.
ptype	Print the definition of a given type.
set	Evaluate an expression and save the result in a program variable.

GDB Tutorials # 1

So you now have an executable file (in this case main) and you want to debug it. First you must launch the debugger. The debugger is called gdb and you can tell it which file to debug at the shell prompt. So to debug main we want to type gdb main. Here is what it looks like when I run it:

GDB Tutorials # 2

Inspecting crashes

So already we can see the that the program was at line 28 of main.cc, that this points to 0, and we can see the line of code that was executed. But we also want to know who called this method and we would like to be able to examine values in the calling methods. So at the gdb prompt, we type backtrace which gives me the following output: