Access function defined in parent from inside an Iframe

this is the function defined in a.aspx
this page has an iframe which will call this method

<script type="text/javascript">
var myIframeUrl;
function modifyVar(val) {
myIframeUrl = val;
}
</script>
-----------
in the iframe use this statement:
<script type='text/javascript'>parent.modifyVar('" + requestURL + "');</script>"