typedef struct my_work {…} my_work_t;
dispatch_source_set_event_handler(reader, ^{
   size_t estimate = dispatch_source_get_data(reader);
   my_work_t *work = produce_work_from_input(fd, estimate);
   if (NULL == work)
   	dispatch_source_cancel(reader);
   else
   	dispatch_async(q_default, ^{ consume_work(work); free(work); } );
});
dispatch_resume(reader);