Question

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Using gdb to check register’s values

I know that to read from the memory location below:

 mov %esi, (%eax)

when in GDB I can use

 (gdb) display *(int *)$eax

If a want to read from memory location 0x8(%eax), which command can I use in GDB? I tried to use some variants of the display command above, but I was not successful.

Answer 1

You can use display if you want to see the expression after any new step. This is useful if you want to watch this expression. if you simply want to show the state of an exepression just use print.

For example:

print $eax
print *(int *)$esp

if you want to take a look for example at an int stored at location 0x8(%eax) you can use

print *(int *)($eax+8)

Sometimes it seems you need to omit the $ in front of the registers to make gdb work correctly.

Here a simple dump of a debugging session on a 32 Bit x86 executable:

d:\temp\C++11>gdb test.exe
GNU gdb (GDB) 7.5
Copyright (C) 2012 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.  Type "show copying"
and "show warranty" for details.
This GDB was configured as "i686-pc-mingw32".
For bug reporting instructions, please see:
<http://www.gnu.org/software/gdb/bugs/>...
Reading symbols from d:\temp\C++11\test.exe...(no debugging symbols found)...don
e.
(gdb) start
Temporary breakpoint 1 at 0x4013c1
Starting program: d:\temp\C++11\test.exe
[New Thread 340.0x1bc0]

Temporary breakpoint 1, 0x004013c1 in main ()
(gdb) print $eax
$1 = 1
(gdb) info register $eax
eax            0x1      1
(gdb) info register
eax            0x1      1
ecx            0x28ff30 2686768
edx            0x8e3c8  582600
ebx            0x7efde000       2130567168
esp            0x28ff08 0x28ff08
ebp            0x28ff18 0x28ff18
esi            0x0      0
edi            0x0      0
eip            0x4013c1 0x4013c1 <main+17>
eflags         0x202    [ IF ]
cs             0x23     35
ss             0x2b     43
ds             0x2b     43
es             0x2b     43
fs             0x53     83
gs             0x2b     43
(gdb) print *(int *)$esp
$2 = 2686768
(gdb) print *(int *)($esp+8)
$3 = 0
(gdb)