Debugging C and C++ programs with gdb (and ddd)

Debugging C and C++ programs with gdb (and ddd)

About gdb and ddd
Getting Started with gdb
Common Comands
gdb info commands for getting application and debugger state
using gdb to debug assembly code and examine memory and register values

Sample gdb sessions
Keyboard shortcuts in gdb
Setting conditional breakpoints and breakpoints in C++
invoking make in gdb
Advanced features attaching to an already running process, signal handling
Links to more gdb information

Some settings (and bug fixes) for ddd

Introduction to gdb and ddd

The purpose of a debugger is to allow you to see what is going on inside yourC program while it runs. In addition, you can use gdb to see what yourprogram was doing at the moment it crashed.

Here are some of the usful actions that gdb can perform:

• Start your program and step through it line by line
• Make your program stop on specified conditions
• Show the values of variables used by your program
• Examine the contents of any frame on the call stack
• Set breakpoints that will stop your program when it reaches a certain point. Then you can step through part of the execution using step and next, and type continue to resume regular execution.

For C and C++ programs, gdb and ddd are debuggers that you can use. dddis a easy-to-use GUI wrapper around an inferior debugger (gdbfor GNU compiled C or C++ code).ddd allows you to interactwith the debugger by using either GUI menu options or the under-lying debugger's command line interface. In addition,ddd automatically displays source code when breakpoints are reached.

There are some example programs and some documentation on using gdbto debug them that you can copy from here:/home/newhall/public/gdb_examples/

Getting started with gdb

C and C++ programs compiled with the GNU compiler and the -goption can be debugged using GNU's debugger gdb (actually, youcan use gdb on code that is not compiled with -g, but unless you like trying tofigure out how assembly code sequences map to your source code I wouldn'trecommend doing so). Also, do not compile with an optimization flag (i.e.don't use -O2), or gdb will have a hard time mapping optimized machine codeto your source code. For example:

% gcc -g myprog.c

To start gdb, invoke gdb on the executable file. For example:

% gdb a.out

If your program terminates with an error, then the operating system willoften dump a core file that contains information about the state of theprogram when it crashed. gdb can be used to examine the contents of a corefile:

% gdb core a.out

One good way to get started when you are trying to track down a bug, is to set breakpoints at the start of every function. In this way, you will quickly be able to determine which function has the problem. Then you can restart the program and step through the offending function line-by-line until you locate the problem exactly.

ddd is invoked in a similar way:

% ddd a.out

Common gdb Commands

(printable version here)

Commonly used gdb commands
--------------------------
gdb also understands abreviations of commands, so you can just type up to 
the unique part of a command name ("cont" for "continue", or "p" for "print")

help                  List classes of all gdb commands
help <topic>          Shows help available for topic or command

where     	      Shows stack: sequence of function calls executed so far
(or backtrace)        (good for pinpointing location of a program crash)
(or bt)

frame                 Shows all stack frames
 frame <frame-num>     Sets current stack frame to <frame-num>
info frame            Show state about current stack frame

run                   Starts program at the beginning
 run command line args
continue              Continues execution from breakpoint

break
 break <line>          Sets breakpoint at line number <line>
 break <func-name>     Sets breakpoint at beginning of function <func-name>
  break main            Sets breakpoint at beginning of program

 tbreak 
Set a temporary breakpoint.
Like "break" except the breakpoint is only temporary,
so it will be deleted when hit.  

continue              Continues execution from breakpoint

condition <bp-num> <exp>   Sets breakpoint number <bp-num> to break only if
                           conditional expression <exp> is true
info break            Shows current breakpoints
disable [breakpoints] [bnums ...]  Disable one or more breakpoints
enable [breakpoints] [bnums ...]   Enable one or more breakpoints 
clear <line>          Clears breakpoint at line number <line>
clear <func-name>     Clears breakpoint at beginning of function <func-name>
delete <bp-num>       Deletes breakpoint number <bp-num>
delete                Deletes all breakpoints

step (or s)           Executes next line of program (steping into functions)
  step <count>          Executes next <count> lines of program
next (or n)           Like step, but treats a function call as a single
  next <count>            instruction
until <line>          Executes program until line number <line>

list                  Lists next few lines of program
  list <line>           Lists lines around line number <line> of program
  list <start> <end>    Lists line numbers <start> through <end>
  list <func-name>      Lists lines at beginning of function <func-name>

print <exp> (or inspect <exp>  Displays the value of expression <exp>

 To print in different formats:
   print/x  <exp> print the value of the expression in hexidecimal  (e.g. print/x 123   displays 0x7b)
   print/t  <exp> print the value of the expression in binary (e.g. print/t 123   displays  1111011)
   print/d  <exp> print the value of the expression as unsigned int format (e.g. print/d 0x1c displays 28)
   print/c  <exp> print the ascii value of the expression (e.g. print/c 99  displays 'c')
   print   (int)<exp> print the value of the expression as signed int format (e.g. print (int)'c' displays 99)

 To represent different formats in the expression (the default is int):
   0x suffix for hex: 0x1c   
   0b suffix for binary: 0b101      (e.g. print 0b101   displays 5,   print 0b101 + 3   displays 8) 
   you can also re-cast expressions using C-style syntax  (int)'c'

 You can also use register values and values stored in memory locations in expressions
   print $eax    # print the value stored in the eax register
   print *(int *)0x8ff4bc10  # print the int value stored at memory address 0x8ff4bc10

x <var, memory address> displays the contents of the memory location given a variable name or a memory address.
  Can display in diffent formats (as an int, a char, a string, ...)

  (ex) assume s1 = "Hello There" is at memory address 0x40062d
  x/s s1      # examine the memory location associated with var s1 as a string
    0x40062d   "Hello There"
	x/4c s1     # examine the first 4 chars in s1
    0x40062d   72 'H'	101 'e'	108 'l'	108 'l'
  x/d s1      # examine the memory location assoc with var s1 as an int 
    0x40062d   72  
  x/8d s1     # the ascii values of the first 8 chars of s1
    0x40062d:	72	101	108	108	111	32	84	104
			
  # can also use the address of a varible as the argument (say temp is an int)
  x  &temp

  # NOTE: format in examine is sticky, for example if you use the command x/c
  #       subsequent executions of x will use /c format. you therefore need to
  #       explicitly change the format to /d /c /s etc. for interpreting memory
  #       contents as differnt type from the previous call to x


display <exp>         Automatic display of <exp> each time a breakpoint reached
   display i+1

whatis <exp>          Shows data type of expression <exp>
info locals           Shows local variables in current stack frame
set variable <variable> = <exp>   Sets variable <variable> to expression <exp>
  set x = 123*y                 # set var x's value to 123*y 

quit                  Quits gdb

generate-core-file: create a core dump file 

info commands for examining runtime and debugger state:

gdb has a large set of info X commands for displaying information aboutdifferent types of runtime state and about debugger state. Here is how to list all the info commands in help, and a description of what a few of the info commands do:

(gdb) help status           # lists a bunch of info X commands

(gdb) info frame            # list information about the current stack frame
(gdb) info locals           # list local variable values of current stack frame
(gdb) info args             # list argument values of current stack frame
(gdb) info registers        # list register values
(gdb) info breakpoints      # list status of all breakpoints

using gdb to debug assembly code and examine memory and register values

ddd is probably easier to use when steping through assembly code than gdbbecause you can have separate windows that show the disassembled code, the register values, and the gdb prompt.

Here are some gdb commands that are useful for debugging at the assembly code level:

disass   list the assembly code for a function or range of addresses
  disass <func_name>           lists assembly code for function
  disass <start>  <end>  lists assembly instructions between start and end address 

break   Set a breakpoint at an instruction 
  break *0x80dbef10   Sets breakpoint at the machine code instruction at address 0x80dbef10

stepi   Executes the next machine code instruction
nexti   Executes the next machine code instruction treats function call as single instr

info
   info registers        # list register values

info  symbol 0xAABBCCD  
                        Describe what symbol is at location ADDR
info line *0xAABBCCDD 
                        Describe which line in source code it is at the address
info proc mappings
                       List of mapped memory regions
 print
   print $eax    # print the value stored in the eax register
   print *(int *)0x8ff4bc10  # print the int value stored at memory address 0x8ff4bc10

x      Display the contents of the memory location given an address.  
                NOTE: the format is sticky (need to explictly change it)

   x/s 0x40062d      # examine the memory location 0x40062d as a string
     0x40062d   "Hello There"
   x/4c 0x40062d     # examine the first 4 char memory locations starting at address 0x40062d
     0x40062d   72 'H'	101 'e'	108 'l'	108 'l'
   x/d s1            # examine the memory location assoc with var s1 as an int 
     0x40062d   72  

set set the contents of memory locations and registers
   set $eax = 10                 # set the value of register eax to 10
   set $esp = $esp + 4           # pop a 4-byte value off the stack 
   set *(int *)0x8ff4bc10 = 44   # at memory address 0x8ff4bc10 store int value 44 

display  at every breakpoint display the given expression

   display $eax

directory         Add directory DIR to beginning of search path for source files
directory  /home/source

set follow-fork-mode child
the new process is debugged after a fork

stepi

Step one instruction exactly.
Argument N means do this N times (or till program stops for another reason).
can be used to debug assembly instructions

nexti
Step one instruction, but proceed through subroutine calls.
Argument N means do this N times (or till program stops for another reason).

watch: Set a watchpoint for an expression.
Usage: watch [-l|-location] EXPRESSION
A watchpoint stops execution of your program whenever the value of
an expression changes.
If -l or -location is given, this evaluates EXPRESSION and watches
the memory to which it refers.
awatch:
Set a watchpoint for an expression.
Usage: awatch [-l|-location] EXPRESSION
A watchpoint stops execution of your program whenever the value of
an expression is either read or written.
If -l or -location is given, this evaluates EXPRESSION and watches
the memory to which it refers.
 rwatch:
Set a read watchpoint for an expression.
Usage: rwatch [-l|-location] EXPRESSION
A watchpoint stops execution of your program whenever the value of
an expression is read.
If -l or -location is given, this evaluates EXPRESSION and watches
the memory to which it refers.

(gdb) watch  -l *0x804a01c
Hardware watchpoint 1: -location *0x804a01c
(gdb) r
Starting program: /home/charles/tmp/test 
Hardware watchpoint 1: -location *0x804a01c

Old value = 100
New value = 10
fun () at test.c:5
5    }

Sample gdb sessions

Below is output from two runs of gdb on programs from ~newhall/public/gdb_examples/.

1. Run 1 is a gdb run of badprog.c. It demonstrates some common gdb commands, and it finds one of the bugs in this program...there are others.

2. Run 2 is a gdb run of segfaulter.c. It demonstrates how to find out where your program is segfaulting (andperhaps why...although valgrind will help more with this type of error).

Run 1: badprog.c

% gcc -g badprog.c           #-- compile program with -g flag


% gdb a.out                  #-- invoke gdb with the executable

GNU gdb 6.4.90-debian
Copyright (C) 2006 Free Software Foundation, Inc.
GDB is free software, covered by the GNU General Public License, and you are
welcome to change it and/or distribute copies of it under certain conditions.
Type "show copying" to see the conditions.
There is absolutely no warranty for GDB.  Type "show warranty" for details.
This GDB was configured as "i486-linux-gnu"...Using host libthread_db library "/lib/tls/libthread_db.so.1".

(gdb) break main              #-- set a breakpoint at the begining of the program's execution

Breakpoint 1 at 0x8048436: file badprog.c, line 36.


(gdb) run                     #-- run the program

Starting program: /home/newhall/public/gdb_examples/a.out 

Breakpoint 1, main () at badprog.c:36      #-- gdb stops at breakpoint	

36	  int arr[5] = { 17, 21, 44, 2, 60 };

(gdb) list                                 #-- list the source code near the break point
31	  return 0;
32	}
33	
34	int main(int argc, char *argv[]) {
35	
36	  int arr[5] = { 17, 21, 44, 2, 60 };
37	
38	  int max = arr[0];
39	
40	  if ( findAndReturnMax(arr, 5, max) != 0 ) {


(gdb) list 11                            #-- list source code around line 11

11	// this function should find the largest element in the array and
12	// "return" it through max
13	// 	array: array of integer values
14	// 	len: size of the array
15	// 	max: set to the largest value in the array 
16	//  	reuturns: 0 on success and non-zero on an error
17	//
18	int findAndReturnMax(int *array1, int len, int max) {
19	
20	  int i;

(gdb) list                              #-- list the next few lines of code
21	
22	  if(!array1 || (len <=0) ) { 
23	    return -1; 
24	  }
25	  max = array1[0];
26	  for(i=1; i <= len; i++) {
27	    if(max < array1[i]) { 
28	      max = array1[i];
29	    }
30	  }

(gdb) next                             #-- execute the next instruction
38	  int max = arr[0];

(gdb)                                 #-- hitting Enter executes the previous command (next in this case)
40	  if ( findAndReturnMax(arr, 5, max) != 0 ) {

 #-- also you can use the up and down arrows to scroll through previous commands


(gdb) print max                        #-- print out the value of max
$1 = 17

(gdb) p arr                            #-- p is short for the print command
$2 = {17, 21, 44, 2, 60}

(gdb) step        #-- step into the function call
                  #-- if we had entered 'next' the entire function call would have been executed

findAndReturnMax (array1=0xbfc5cb3c, len=5, max=17) at badprog.c:22
22	  if(!array1 || (len <=0) ) {             #-- 'step' takes us to the entry point of findAndReturnMax

(gdb) print array1[0]                   #-- lets see what the param values are
$3 = 17
(gdb) p max 
$4 = 17

(gdb) list
17	//
18	int findAndReturnMax(int *array1, int len, int max) {
19	
20	  int i;
21	
22	  if(!array1 || (len <=0) ) { 
23	    return -1; 
24	  }
25	  max = array1[0];
26	  for(i=1; i <= len; i++) {

(gdb) break 26                           #-- set a breakpoint at line 26 (inside findAndReturnMax)

Breakpoint 2 at 0x80483e7: file badprog.c, line 26.

(gdb) cont                               #-- continue the execution
Continuing.

Breakpoint 2, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=17)    #-- gdb hits the next breakpoint 
    at badprog.c:26
26	  for(i=1; i <= len; i++) {

(gdb) p i
$5 = 0

(gdb) n                                   #-- n is short for next
27	    if(max < array1[i]) { 

(gdb) display max                         #-- display will print out the value everytime we hit a breakpoint
1: max = 17                                      

(gdb) display array1[i]
2: array1[i] = 21

(gdb) break 27                            #-- set a breakpoint inside the loop
Breakpoint 3 at 0x80483f0: file badprog.c, line 27.

(gdb) cont                                #-- continue execution
Continuing.

Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=21)
    at badprog.c:27
27	    if(max < array1[i]) { 
#-- display prints these out:
2: array1[i] = 44                        
1: max = 21

(gdb) cont
Continuing.

Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=44)
    at badprog.c:27
27	    if(max < array1[i]) { 
2: array1[i] = 2
1: max = 44

(gdb) cont
Continuing.

Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=44)
    at badprog.c:27
27	    if(max < array1[i]) { 
2: array1[i] = 60
1: max = 44

(gdb) cont
Continuing.

Breakpoint 3, findAndReturnMax (array1=0xbfc5cb3c, len=5, max=60)
    at badprog.c:27
27	    if(max < array1[i]) { 
2: array1[i] = 17
1: max = 60                     #-- so max is 60 here

(gdb) where                 #-- show the stack frames 

#-- findAndReturnMax is the active function at line 27, it was called by main at line 40:
#0  findAndReturnMax (array1=0xbfd043ec, len=5, max=60) at badprog.c:27
#1  0x08048479 in main () at badprog.c:40

 frame 1                         #-- move into main's calling context (stack frame 1) to examine main's state
#1  0x08048479 in main () at badprog.c:40
40        if ( findAndReturnMax(arr, 5, max) != 0 ) {

(gdb) print max                  #-- in main's stack frame max is 17
$1 = 17


(gdb) cont                       #-- continue execution
Continuing.
max value in the array is 17            #-- main prints out value of max after function call 

#-- This looks like a bug:" 
#-- findAndReturnMax set max to 60, but 60 isn't getting "passed back" to main after the call 
#-- to fix this we need either have findAndReturnMax return the value of max or pass max by reference


(gdb) quit                      #-- quit gdb

The program is running.  Exit anyway? (y or n) y

Run 2: segfaulter.c

  % gdb segfaulter

 GNU gdb 6.4.90-debian
 Copyright (C) 2006 Free Software Foundation, Inc.
 GDB is free software, covered by the GNU General Public License, and you are
 welcome to change it and/or distribute copies of it under certain conditions.
 Type "show copying" to see the conditions.
 There is absolutely no warranty for GDB.  Type "show warranty" for details.
 This GDB was configured as "i486-linux-gnu"...Using host libthread_db library "/lib/tls/libthread_db.so.1".

  (gdb) run              #-- just run segfaulter and let it seg fault

 Starting program: /home/newhall/public/gdb_examples/segfaulter 
 Failed to read a valid object file image from memory.

 Program received signal SIGSEGV, Segmentation fault.
 0x080483e1 in initfunc (array=0x0, len=100) at segfaulter.c:15
 15          array[i] = i;

  (gdb) where           #---  let's see where it segfaulted

 #0  0x080483e1 in initfunc (array=0x0, len=100) at segfaulter.c:15
 #1  0x0804846e in main () at segfaulter.c:38

 (gdb) list             #--- let's see code around segfaulting instruction 

 10      int initfunc(int *array, int len) {
 11
 12        int i;
 13
 14        for(i=1; i <= len; i++) {
 15          array[i] = i;
 16        }
 17        return 0;
 18      }
 19

  (gdb) p array[0]          #--- let's print out some values and see what's going on
 Cannot access memory at address 0x0

 #-- it looks like array is a bad address (0x0 is NULL)

  (gdb) p array
 $1 = (int *) 0x0

  (gdb) frame 1             #--- let's see what main is passing to this funtion

 #1  0x0804846e in main () at segfaulter.c:38
 38        if(initfunc(arr, 100) != 0 ) {

  (gdb) print arr           #--- print out arr's value (what we pass to initfunc) 
 $2 = (int *) 0x0
 #--- oops, we are passing NULL to initfunc...we forgot to initialize arr to point to valid memory

Keyboard shortcuts in gdb

gdb supports command line completion; by typing in a prefix you canhit TAB and gdb will try to complete the command line for you.

Also, you can give just the unique prefix of a command as the command andgdb will execute it. For example, rather than entering the commandprint x, you can just enterp x to print out the valueof x.

The up and down arrow keys can be used to scroll through previous command lines, so you do not need to re-type them each time.

If you just hit RETURN at the gdb prompt, gdb will execute themost recent previous command again. This is particularly useful if you aresteping through the execution, then you don't have to typenexteach time you want to execute the next instruction, you can just typeit one time and then hit RETURN.

Setting conditional breakpoints and some issues with setting breakpoints in C++ code

conditional breakpoints

A conditional breakpoint is one that only transfers control to gdb when a certain condition is true. This can be very useful when you only wantgdb control after iteration 1000 of a loop, for example.

To set a condition on a breakpoint, use the condition command with thenumber of the breakpoint followed by the condition on which to triggerthe breakpoint. Here is an example where I'm setting a conditionalbreakpoint that will only be triggered when the condition (i >= 1000) is true:

(gdb) break 28                                  # set breakpoint at line 28
(gdb) info break                                # list breakpoint information
  Num Type           Disp Enb Address    What
   1   breakpoint     keep y   0x080483a3 in foo at loops.c:28

(gdb) condition 1 (i >= 1000)                # set condition on breakpoint 1
(gdb) run   (or continue if already running) 

breakpoints in C++ programs

One complication with gdb and C++ programs, is that you need to specify methods and data members using the "classname::" prefix. In addition, you often need to use a leading ' before a name for gdb to find the symbol, and if methods are overloaded, you need to specify which method it is by listing its full prototype (actually, if you hit TAB gdb will list all possible matches for you and you can pick one of those).

For example, to set a break point in funciton pinPage of the BufMgr class,I'd do the following:

	(gdb) break 'BufMgr::pinPage(int, Page *&, int)'
	

This looks pretty icky, but really I just type break 'BufMgr::p then hit TAB for automatic completion.

	(gdb) break 'BufMgr:: <tab> 
	

will list all methods of the BufMgr class, then you can just pick from the list the method you want to put the breakpoint in.

gdb and make

Within gdb you can invoke "make" to rebuid your executable (assumingthat you have a makefile to build your program). This is a nice feature in the case when you have many breakpoints set and do not want to exit gdb, recompile, re-start gdb with the new a.out, and reset all the breakpoints. However, keep in mind that modifying and recompiling your source code from within gdb may result in your breakpoints not being where you think they should be (adding/removing lines of source code can result in your in your breakpoints no longer being where you want them to be in terms ofthe new version of your source code). You can use the disable or deletecommands to disable or delete old breakpoints.

Some Advanced Features

attaching gdb to a running process

1. get the process's pid

      # ps to get process's pid
      $ ps                     # lists all processes started in current shell					
      $ ps -A | grep a.out     # list all processes pipe through grep for just those named a.out
        PID TTY          TIME CMD
      12345 pts/3     00:00:00 a.out
   

2. attach gdb to the running process

      # gdb <executable> <pid>
      $ gdb a.out 12345
   
   # OR alternative syntax:
   
      # gdb attach <pid>  <executable>
      $ gdb attach 12345 a.out
   

   At this point the process is stopped by gdb; you have the gdb prompt that you can use issue gdb commands like setting breakpoints, or printing out program state before continuing execution.

signal control

Sometimes your process receives signals and you would like to havegdb perform some action when certain signals are delived to the debugged process. For example, if your program issuesa bad adress, it will receive a SIGBUS signal and usually exit. The default behavior of gdb on a SIGBUS it to let the process exit. If, however, you want to examine program state when it recieves a SIGBUS, you can specify that gdb handle this singal differently:

  (gdb) handle SIGBUS stop		# if program gets a SIGBUS, gdb gets control 

You can list how gdb is handling signals using info:

  (gdb) info signal    # list info on all signals

  (gdb) info SIGALRM   # list info just for the SIGALRM signal

ddd settings and bug fixes

Running ddd creates a .ddd directory in your home directory andwill save settings to files here, so that you don't need to resetall your preferences from scratch. You can click and drag to change thesizes of subwindows and choose Menu options to display (or not) certainmenus, register values, machine code, etc.

• To view assembly code: under Source menu choose "Display Machine Code" or Alt+4
• If ddd hangs with "Waiting until gdb ready" message, then one way to fix thisis to wipe out your .ddd directory (you will lose all your saved settings):

  	rm -rf ~/.ddd
  	

gdb Links

common gdb commands (from above)
example gdb sessions (from above)
GDB quick reference card
A very complete GDB reference

Using GDB within Emacs by Ali Erkan

(from: http://www.cs.swarthmore.edu/~newhall/unixhelp/howto_gdb.html)

 

Command Example	Description
gdb executable_file_name	Running the gdb.
(gdb) break main (gdb) b main	Set a breakpoint at the main()
(gdb) clear main	Clear the breakpoint at main()
(gdb) breakADDRESS	Set a breakpoint at the specifiedADDRESS
(gdb) break [function_name]	Set a breakpoint at the [function_name]
(gdb) clear [function_name]	Clear a break point at the [function_name]
(gdb) break [line_number]	Set a breakpoint at the [line_number]
(gdb) clear [line_number]	Clear a breakpoint at the [line_number]
(gdb) break 0x800050b	e.g: Set a breakpoint at 0x800050b
(gdb) run (gdb) r	Run the program in gdb
(gdb) run [argv[1] ] (gdb) run 12345	Run the program in gdb with command line argument. Run the program in gdb with 1234 as a command line argument.
(gdb) help info	Help for info command.
(gdb) info stack	Backtrace of the stack
(gdb) info frame	Display the information about the current stack frame
(gdb) info registers (gdb) i r	To view register contents.
(gdb) info reg ebp	Info for ebp register.
(gdb) info break	To see what breakpoints are currently defined and their conditions.
(gdb) disass main (gdb) disass [function_name] (gdb) disassCopyData	Disassemble the main(). Disassemble the [function_name] Disassemble the CopyData function.
(gdb) step (gdb) stepi	Execute one machine code instruction
(gdb) stepi <n>	Execute n instructions
(gdb) next (gdb) nexti	Like stepi, but will proceed through subroutine calls
(gdb) cont (gdb) continue (gdb) c	Resume execution
(gdb) x/3bcx	Examine in range (3), binary format char format, hex format.
(gdb) x/5xw 0xbffffc04	Examine in hex with word size with the range of 5 starting at 0xbffffc04.
(gdb) x/bx 0xbffffc04	Examine by byte size in hex format at address 0xbffffc04
(gdb) x/s 0xbffffc01	Examine the string at address 0xbffffc01
(gdb) x/i setuid	Examine in assembly format thesetuid() function
(gdb) x/i system	Examine in assembly format thesystem() function
(gdb) x/4w 0xbffcec54	Examine in range of 4 with word size at 0xbffcec54
(gdb) x/10i $eip	Examine the next 10 address in assembly code format instead of ASCII or hex, starting from the address the register eip is pointing to.
(gdb) x ADDR	Prints the contents of the address ADDR in memory
(gdb) x /20b address	Then, the next 20 bytes
(gdb) x /20c address	Then, the next 20 characters
(gdb) p/x $eax	Prints the value in the eax register in hex.
(gdb) print $esp	Prints the value in the esp register in hex.
(gdb) print $esp + 4	Prints the value in the esp+4 register in hex.
(gdb) print TheData	Prints the value in the TheData variable.
(gdb) printf "%s\n", 0x8048552	Prints the string value at address 0x8048552.
(gdb) l	List the program source code.
(gdb) frame (gdb) frame 0	Display the current frame (stack frame). Display the frame 0 (stack frame 0).
(gdb) backtrace (gdb) b	Print the current address and the stack frame.
(gdb) q (gdb) quit	Quit the gdb