How do I log the value of a string parameter?

ap_StringValue is a macro which logs everything from the address provided up to the first null character:

probe thread
{
  probe <subprogram>
  {
    on_entry
    {
      log("NameParam = ",
          ap_StringValue($1));
      // $1 refers to the string which is the 
      // first parameter of the <subprogram>
    }
  }
}