Utilizing the
GDB debugger
to analyze
programs
Background and
application
Topics to be discussed:
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Background Information
 The
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Basic commands
Useful practices
 The
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shell
stack
GDB debugger
 Accessing
the shell
 Basic commands
 Homework 2
The Shell
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Bourne Again Shell (BASH)
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Visual Text Editor (vi)
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Command mode (:)
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! (bang) – run shell command
w – save
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With an argument acts like save as
q - quit
Insert mode(i-insert,a-append,o–insert above)
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x – delete character
dd – delete line
#, Shift+g – jump to line
The Shell continued
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Pipe (|) use output from one command as input for another –
command | command
script – used to write all command line activity to a file
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To read (cat scriptname | more)
chmod – change file permissions
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chmod [a,g,u,o] [+,-] [r/w/x] filename
Octal numbers can be used instead, one each for user, group, others,
and all where the binary representation shows permissions (r,w,x)
ls – list files
-a show hidden files –l list attributes
cp – copy file (source destination)
mv – move file (source destination)
rm – remove file –f (force, don’t show warnings)
rmdir – remove directory
man – manual pages for command if installed
Useful Practices
 (RCS)
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Revision Control Systems
Tracks changes in source files
Allows for multiple versions of the same file
When files are checked out, no other user
can edit those files (prevents over writing of
files when multiple people are working on
the same project)
Useful Practices
 Make
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files
Defines compilation (useful when using
multiple files) – limits human error
Uses dependency rules and logic to
compile necessary files
Only compiles files that have been recently
modified (saves compile time)
Sample Make File
Useful Practices
 Extend



your path
Located in .bash_profile (shell specific)
Append a colon and the path you wish to
add
When a command is issued in the shell, the
shell will check your default path, then the
appended path – run programs that exist in
the path from anywhere in the system
The Stack
The stack is used to preserve data, hold local
variables, store return addresses when
subroutines are called, and to pass and return
information between functions.
 When information is saved on the stack, it is
referred to as a push.
 When information is read from the stack, it is
referred to as a pull or pop.
 The stack pointer is automatically
manipulated as data is moved to/from the
stack.
The Stack
A
pointer must be initialized and set to the
bottom of the stack (highest location in
stack). When information is pushed to the
stack, the stack pointer is first adjusted,
then the information is stored, when
information is pulled, it is first pulled, and
then the stack pointer is adjusted.
The Stack Frame
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Every time a function is called, a stack frame
is created.
In addition to local variables, the frame
contains:
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passed parameters from the calling code,
space for results returned from subroutine,
the return address of the subroutine, and
saved register contents.
**Within the subroutine, addressing is done
relative to the frame pointer, which points to
the return address. (stack pointer value at time
of function call)
The Stack Frame

The stack frame should be allocated
immediately upon entering a function and
deallocated prior to exiting.
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The stack pointer is transferred to another
register which is referred to as the frame pointer.
The stack pointer is adjusted to reserve space
for local variables.
This is done to support reentrancy – further
functions can be called, and interrupts can
occur without affecting information in the stack
frame.
Illustration of Stack Frame
Enter Function
Transfer Stack Pointer
Allocate Stack Frame
Call external Function
Return Address
Since all local variables, and the return
address are stored on the stack, no
information will be lost when additional
functions are called. Registers may also be
pushed before calling a new function, and
pulled upon returning.
*Values can be passed/returned by loading
them into registers, or pushing them onto the
stack prior to calling/returning from another
function.
GDB Debugger
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The GDB debugger can be used to debug a number of
different programming languages and/or their associated
binary output.
Starting the debugger:
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The shell can be accessed by typing shell command

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Type gdb from the command line, or gdb filename to start the
debugger and load a file to run.
ex. shell ls
history will display the history of the gdb session
Cheat Sheet
Extended Info
Overview - from Stanford
DDD – GUI based debugger which uses GDB
Commands
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file – file filename will load the symbols from the
target file, and prepare the file for debugging.
run – will begin execution of the target file
display – used to display information when a
breakpoint is reached. display/i $pc will show the
next instruction.
set logging on - starts a log file of all commands
and output during the gdb session.
break

break function name/ line number will set a
breakpoint at the specified function or line
number.
Breakpoints

commands breaknum
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Specify commands to be executed when
breaknum is reached. The commands can be
any list of C statements or gdb commands. This
can be useful to fix code on-the-fly in the
debugger without recompiling
info – can be used to obtain information
about most program and gdb attributes

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registers – show register contents
locals- show local variables/values
Breakpoints
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next – step program until a new source line is
reached
nexti – step one instruction
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Next instructions treat subroutine calls as an
instruction, (subroutine executed, but not
stepped into)
step – step program until a new source line is
reached
stepi – step one instruction

Step instructions will treat subroutines as a part
of the source. Subroutine will be entered and
debugged.
Further Examination
 disassemble
– shows the assembly for a
program from a given memory location

/r option shows machine code
x
(examine) allows you to examine
memory locations with a given format

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/s –string
/#c – character
/#x – byte
/#t – word
Using the source
 set
list # - sets the number of lines of
source to show when list is called
 list – lists the source from the start of the
program or a specified line number
 edit – starts an edit session of the source
file(read only) in the default text editor
using MIME types.
Homework 2

Dell PowerEdge R510 Server- Intel Xeon X5560
processor

Intel® 64 and IA-32 Architectures Software Developer's
Manual Volume 1: Basic Architecture [pdf]
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Register Table
List of Registers
Specific Register Duties
Registers Outlined
Register Overlay
Intel® 64 and IA-32 Architectures Software Developer's
Manual Volume 2A: Instruction Set Reference, A-M [pdf]
Intel® 64 and IA-32 Architectures Software Developer's
Manual Volume 2B: Instruction Set Reference, N-Z [pdf]
ASCII table