问无用的已分配堆栈空间？

EN

echo:
  pushl  %ebp
  movl   %esp, %ebp
  pushl  %ebx
  leal   -8(%ebp), %ebx
  subl   $20,  %esp       <-- Why so much space?
  movl   %ebx, (%esp)
  call   gets
  ...

void echo()
{
  char buf[4];
  gets(buf);
  puts(buf);
}