Using gdb to check register's values

Question

How can I see what values the registers hold? I have the following line of assembly:

 mov 0x8(%rax), %rax
 cpm %ebx, (%rax)

Using the command:

 (gdb) p/x $ebx
 (gdb) p/x $rbx
 $3 = 0xb

I get the value that is stored in this register. However, when I try to see what it is stored int the memory location (%rax) I have the following problem:

(gdb) display *(int *)$rax
Disabling display 10 to avoid infinite recursion.
10: *(int *)$rax = Cannot access memory at address 0x17

I cannot not understand why this happens and moreover how to find out what (%rax) has in.

Answer 1

To see the values of the registers just use

print $rax
info register $rax
info register

In your case it seems rax contains the number 0x17 and the debugger cannot access this location. Please make sure that rax contains a valid address pointing to an integer.

You should use print instead of display if you want to take a quick view. Dont abuse the display command. You can use it if you need a kind of watchdog for an expression.

Here you find a quick reference

http://www.yolinux.com/TUTORIALS/GDB-Commands.html

hope it helps.