$.post() problem

$.post() problem

i have this code

var ret = null;
$.post("my_url.php", { my_data : "hello"},
      function(data){
         ret =  data;
         alert("server returned " + data);
      }
);

alert("server response = " + ret);

my_url.php will returned a random value of integer. when i call   alert("server returned " + data); within the callback function of $.pos() its completely work...
but when i call alert("server response = " + ret);  its doesn't work...

i think its because ajax setting "async" is true. but i dont know how to set async:false when i use $.post(). i have tried to use $.ajaxSetup() and set async: false. But it doesn't work..

please help me...
thanks...