How to do relative jumps in assembly?

Question

I'm following Aleph One's awesome introduction to smashing the stack, but I've gotten stuck running one of the example programs he has. The issue is that when I compile the line jmp 0x2a, and disassemble it, that exact line is produced, whereas in aleph's code, it is translated to jmp 0x800015f <main+47>. How do I force gcc to make this a relative jump and not an absolute one (which is what I think is going on).

Answer 1

The problem is the 0x2a. That looks like an absolute address, but what you want is a relative address.

That can be accomplished by labeling the destination and jumping to that: assemblers prefer generating a relative jump most of the time. Or use a relative address. Most assemblers accept something similar to .+0x2a where . is a symbol meaning the current instruction pointer. Some use $ instead.