Jmp x86 explanation

Question

I'm dealing with some x86 code and I know for the most part what the jmp command is used for and what it does, but this one line absolutely perplexes me.

jmp    *0x804a0e0(,%eax,4)

eax is dependent on earlier input and should be an int. 0x804a0e0 contains 60 (base 10). Can someone please explain what this is saying?

Answer 1

jmp *0x804a0e0(,%eax,4)

This instruction is quite simple: given an array of function pointers (the array starts at address 0x804a0e0), and an index i (stored in %eax), call the function, a pointer to which is stored in array[i].

You are most likely to encounter such instruction in C++ (virtual function calls are implemented this way).

Using (gdb) info symbol 0x804a0e0 will tell you what global the array actually corresponds to.