Question

So I’m trying to create a factorial function in assembler

In c:

 #include<stdio.h>



int fat (int n) 
{
      if (n==0) return 1;
      else return n*fat(n-1);
}

    int main (void){

    printf("%d\n", fat(4));
       return 0;

    }

In Assembly:

.text

.global    fat
fat:push    %ebp   
    mov    %esp, %ebp
    movl    $1,%eax
    movl    4(%ebp),%edx

LOOP:cmp    $0,%edx
    je FIM
    sub    $1,%edx
    push    %edx
    call    fat
    imul    %edx,%eax

FIM:mov    %ebp, %esp
    pop    %ebp
    ret

I keep getting the segmentation fault error and I don’t know why…can someone help me?

Answer 1

The offset is probably wrong in this line:

movl    4(%ebp),%edx

The stack has the previous value of %ebp and the return address already, so your offset is going to have to be more than 4.

I recommend stepping through the assembly code with the debugger, and make sure that all the register values are exactly what you expect them to be. You will also have problems with the %edx register across calls unless you save and restore its value, too.