Question

Given the following code :

.section .rodata

input_format1:  .string  "%d%d"
output_format1: .string  "Yes. %d is a power of %d\n"
output_format2: .string  "No. %d is not a power of %d\n"

.section .text
.globl  main
    .type main, @function
main:
        pushl   %ebp
        movl    %esp,   %ebp

        addl    $-8 ,%esp       # moving down the stack
        pushl   %esp
        pushl   4(%esp)
        pushl   $input_format1
        call    scanf           # call scanf to get a number from the user
        addl    $12,%esp
        movl    (%esp),%ebx     # store the actual number
        movl    4(%esp),%ecx


.loop:


        #return from printf:
        movl    %ebp,%esp
        popl    %ebp
        ret

After the program reaches scanf and I press the 1st number ,it crashes.What am I doing wrong ?

Thanks
Ron

Answer 1

The problem is in:

pushl   4(%esp)

That pushes the value in the stack, not the address of such variable.
Compare that with the previous, correct, instruction pushl %esp. This one does push an address.

What you need is pushl %esp+4, but that cannot be done in just one instruction, AFAIK. Instead do something like:

lea 4(%esp), %eax
push %eax

UPDATE:

Your other problem is because each time you do a push %esp gets decremented, so calculating the right address for your local variables is messy. That’s one reason to have a stack frame, and you have one! So use %ebp to refer to your local variables, but with a negative offset:

    pushl   %ebp
    movl    %esp,   %ebp
    addl    $-8 ,%esp       # moving down the stack
    lea     -4(%ebp), %eax
    pushl   %eax
    lea     -8(%ebp), %eax
    pushl   %eax
    pushl   $input_format1
    call    scanf           # call scanf to get a number from the user
    addl    $12,%esp
    movl    -8(%ebp),%ebx     # store the actual number
    movl    -4(%ebp),%ecx