Note that gdb may refuse towatch registers before the program running

  gdb <file>

  gdb -h   (lists command line options)

General commands

  r   (start your program)

  b main         (set a breakpoint at the entry to function main)

  b *0x08048b26  (set a breakpoint at the specified address)

  delete 2       (deletes breakpoint 2)

  delete         (deletes all breakpoints)

  ni                 (like si, but skip over subroutine calls)

  ni 2               (like si, but skip over subroutine calls)

  c 2                (continue, ignoring this breakpoint 2 times)

  finish             (run until the current function returns)

  p/a $sp  (print the stack pointer)

  p/a $esp (print the stack pointer)

  disas 0x080489b3 0x080489c3  (display the code between the addresses)

  layout asm (display the assembly code)

  p $eax   (print the contents of %eax)

  p/x $eax (print the contents of %eax as hex)

  p/c $eax (print the contents of %eax as a character)

  x/NFU ADDR (print the contents at ADDR in memory:

     N = number of units to display

  x/a $ebp+8    // print first argument to function as address

  x/f $ebp+12   // print second argument to function as float

  x/d $eax        (contents of *(%eax) as int)

  x/f $eax        (contents of *(%eax) as float)

  x/d *(int*)($ebp)+12       (second arg of prior function as int)       

  x/d *(*(int*)($ebp))+8     (first arg of second prior function as int)

display $eax (print contents of %eax every time the program stops)

  display  (print the auto-displayed items)

  info program (current status of the program)

  info functions (functions in program)

  info registers  (registers and their contents)

  info breakpoints (status of user-settable breakpoints)

------------------------------------------------------------------------

http://dirac.org/linux/gdb/04-Breakpoints_And_Watchpoints.php

loop, first to watch every change to the loop variable, and then to

check a particular value.

0x08048385 <main+17>:   movl   $0x0,-0x8(%ebp)

0x0804838c <main+24>:   jmp    0x8048392 <main+30>

(gdb) b *0x08048385

(gdb) r

(gdb) c

Continuing.

(gdb) c

Continuing.

(gdb) c

Continuing.

(gdb) disable 2

(gdb) watch *((int*)($ebp-0x8)) == 42

Old value = 0

New value = 1

When gdb is watching registers, it will watch those registers in all

code, which may not be what you want.

call (in this code, if you simply try to watch $ebx, then the code for

printf will also trigger the breakpoint):

0x080483c7 <main+71>:   movl   $0x80484b0,(%esp)

0x080483ce <main+78>:   call   0x80482d8 <printf@plt>

Breakpoint 3 at 0x80483d3

(gdb) condition 3 $ebx==42

$1 = 42

Occasionally, your gdb session will look kind of crazy (random text will be everywhere, your will have the same registers displayed multiple times, etc.) -- Fix this by CTRL + L