function checkbox_infome(int checked)
{
//Show a message with the check status
error(str_printf(NULL,"%i",checked));
}
...
//Declare the checkbox
CHECKBOX* checkbox;
//create the checkbox
checkbox = checkbox_create(0,0,128,16,"Checkbox 1",vector(128,128,128),CHECKED,3);
//Register the event
checkbox.CheckedChanged = checkbox_infome;
...