objdump

Kitt3n · 12-28-2010, 02:01 PM

I just started learning C from a book and it told me to use objdump to look at the code in assembly. However I run into the problem that the output objdump gives me is not the same as is in the book. That other addresses are used I understand. But why is the program not executed the exact same way even know I use the same processor (x86) as is used in the book.

Sins I have not found out yet how I can copy text from the command line in my virtual box in windows I can only give you the example from the book. What I am reffering to is the 3th and 4th column the left.

Example:

Code:

reader:~/booksrc $ objdump -M intel -D a.out | grep -A20 main.:
08048374 <main>:
 8048374:       55                      push   ebp
 8048375:       89 e5                   mov    ebp,esp
 8048377:       83 ec 08                sub    esp,0x8
 804837a:       83 e4 f0                and    esp,0xfffffff0
 804837d:       b8 00 00 00 00          mov    eax,0x0
 8048382:       29 c4                   sub    esp,eax
 8048384:       c7 45 fc 00 00 00 00    mov    DWORD PTR [ebp-4],0x0
 804838b:       83 7d fc 09             cmp    DWORD PTR [ebp-4],0x9
 804838f:       7e 02                   jle    8048393 <main+0x1f>
 8048391:       eb 13                   jmp    80483a6 <main+0x32>
 8048393:       c7 04 24 84 84 04 08    mov    DWORD PTR [esp],0x8048484
 804839a:       e8 01 ff ff ff          call   80482a0 <printf@plt>
 804839f:       8d 45 fc                lea    eax,[ebp-4]
 80483a2:       ff 00                   inc    DWORD PTR [eax]
 80483a4:       eb e5                   jmp    804838b <main+0x17>
 80483a6:       c9                      leave
 80483a7:       c3                      ret
 80483a8:       90                      nop
 80483a9:       90                      nop
 80483aa:       90                      nop
reader:~/booksrc $

ForzaItalia2006 · 12-28-2010, 06:18 PM

Quote:

Originally Posted by Kitt3n

I just started learning C from a book and it told me to use objdump to look at the code in assembly.

Yes, you could use objdump(1) or a debugger (which should be capable to disassemble).

Quote:

Originally Posted by Kitt3n

However I run into the problem that the output objdump gives me is not the same as is in the book. But why is the program not executed the exact same way even know I use the same processor (x86) as is used in the book.

That's hard to tell from just the original assembly code. If my internal de-compiler worked correctly ;-), the original C code seems to be a for (i = 0; i<10; i++) while each iteration prints the same string. Though, what do you mean by "not executed the exact same way"? Do you get a different number of iterations? Don't you get ANY output? Does it run forever? Does your program crash?

Quote:

Originally Posted by Kitt3n

Sins I have not found out yet how I can copy text from the command line in my virtual box in windows I can only give you the example from the book. What I am reffering to is the 3th and 4th column the left.

Your disassembly (and possibly our C-code) would be really helpful ...

Andi