Using body onload with ASP.net MasterPages

we want to use body onload tag in master pages but it will have effect other content pages.The onload function is for only one content page.for this purpose we have used below code in master page.
Master Page :
<script language="javascript">
function masterpage_onload()
{
if(window.content_onload != null)
window.content_onload();
//this condition check whether the rendered page has function are not.

}
</script>

Content Page :

<script language="javascript">
function content_onload()
{
//do something
}
</script>