I keep getting segmentation faults. How is a stack used in IA-32 using AT&T?

Question

For a homework assignment, we have been asked to scan a number, increment it by one and print the answer. However, code tends to give a segmentation error.

.global main

mystring: .asciz "Assignment 3: inout\n"

string: .asciz "%d"

main:   pushl $mystring
        call printf
        jmp inout

end:    pushl $0
        call exit

This is basically what we're doing. Running a code like this without the jump works. The problem is in the inout method

inout:  subl $0, %esp             ;what is happening here
        leal -4(%ebp), %eax       ;what am i doing here
        pushl %ebp                ;why do i need to push the base pointer
        movl %esp, %ebp
        pushl %eax
        pushl $string
        call scanf
        incl %eax
        pushl %eax
        pushl $string
        call printf

Which doesn't seem to work. I also have no idea what the beginning (substract, address thing) means, see the commentary for further things. Thanks in advance!

Answer 1

Partial answer, but maybe this will help you track down the problem:

subl $0, %esp would grow the stack if the constant wasn't zero. The top of the stack is usually at the lowest memory address in x86, that is, stacks grow downwards.

leal -4(%ebp), %eax loads the absolute address %ebp - 4 into %eax. This is the pointer to the memory location where the scanfd integer should be stored.

Why you're pushing the base pointer is indeed a relevant question. You usually do that because you modify it afterwards, do stuff, and then pop it when you're done - which it seems you don't. It is usually used with functions that you call though. This might well be the problem here, you call inout a method, but you're using it as a label for jmp only. The code as you have posted it now has no chance of returning to where it was jmped to from.

Wikibooks has a fairly concise introduction to x86 stack frames.