16.4.2 
[first part, with stuff as a global var]

stuff[i].str[j] = '\0';


# stuff[i]
# need to compute the displacement from the base
# of stuff to stuff[i]

# the displacement/offset is (i * sizeof(struct X))

   movl $X_size,%eax
   movl i(%ebp),%ebx
   mull %ebx

# eax now has the offset to stuff[i] from stuff
   addl $stuff,%eax

# eax is the address of stuff[i]
   addl $X_str,%eax

# eax is the address of stuff[i].str
   addl j(%ebp),%eax

# eax is the address of stuff[i].str[j]
   movb $0,(%eax)

# stuff[i].str[j] = 0;

======================================================
for the homework assignment:

int isctrl(char ch)
{
  return (ch >= 0) && (ch <= 31) ? 1 : 0;
}

int isctrl(char ch)
{
  if (ch >= 0)
  {
    if (ch <= 31)
    {
      return 1;
    }
  }
  return 0;
}

void update(char *ptr)
{
  *ptr = *ptr + 17;
}

int isctrl(char ch)
{
  if ((ch >= 0) and (ch <= 31))
    return 1;
  else
    return 0;
}

void update(char *ptr)
{
  *ptr += 17;
}

int main(void)
{
  char state, ch, newch;
  int numread;

  state = 0;
  do
  {
    numread = read(0, &ch, 1);
    if (numread == 1)
    {
      newch = crypt(ch, &state);
      write(1,&newch,1);
    }
  } while (numread <> 0);
}