function fixFooter() {
  var clientHeight = window.innerHeight == true ? window.innerHeight : document.documentElement.clientHeight;
  var bodyHeight   = document.getElementById('body').offsetHeight;
  if (clientHeight > bodyHeight) {
    newHeight = clientHeight - 100 + 'px';
    document.getElementById('main').style.height = newHeight;
  }
}
window.onresize = fixFooter;
window.onload   = fixFooter