kdu_message::start_message

This function is invoked on the kdu_message-derived objects passed in to kdu_customize_errors and kdu_customize_warnings, whenever a kdu_error or kdu_warning object (as appropriate) is constructed. In many cases, there may be no need to override this function when deriving new classes from kdu_message. An important exception, however, arises when a single messaging object is to be shared by multiple threads of execution. To avoid message mangling or nasty race conditions, multi-threaded applications should lock a mutex resource when start_message is called, releasing it when flush is called with end_of_message true.