How to pass multiple checkboxes using jQuery ajax post

How to pass multiple checkboxes using jQuery ajax post

this is the ajax function

 function submit_form(){
 $.post("ajax.php", {
 selectedcheckboxes:user_ids,
 confirm:"true"
 },
 function(data){
  $("#lightbox").html(data);
  });
 }

and this is my form

<form>
<input type='checkbox' name='user_ids[]' value='1'id='checkbox_1' />
<input type='checkbox' name='user_ids[]' value='2'id='checkbox_2' />
<input type='checkbox' name='user_ids[]' value='3'id='checkbox_3' />
<input name="confirm" type="button" value="confirm" onclick="submit_form();" />
</form>