function setFooter(){

        var h=getHeight();
        var t=document.getElementById('header');
        var m=document.getElementById('main');
        var f=document.getElementById('footer');

        m.style.height='';//reset content height

        var realHeight=h-f.offsetHeight;
        if(realHeight>m.offsetHeight){
            f.style.position='absolute';
            f.style.left=m.offsetLeft+'px';
            f.style.top=realHeight+'px';//-border
            if(realHeight-t.offsetHeight>0)
                m.style.height=realHeight-t.offsetHeight+'px';
         }else{
            f.style.position='relative';
            f.style.left='0px';
            f.style.top='0px';
         }
        
        
    }
    function getHeight(){
        if(typeof(window.innerHeight)=='number'){
            return window.innerHeight;
        }else if(document.documentElement && document.documentElement.clientHeight){
            return document.documentElement.clientHeight;
        }else if(document.body && document.body.clientHeight){
            return document.body.clientHeight;
        }else return 0;
    }