code: func (f *FileLogger) Printf(format string, v ...interface{}) {
  	_, file, line, _ := runtime.Caller(1) //calldepth=2
  	f.logChan <- fmt.Sprintf("[%v:%v]", shortFileName(file), line) + fmt.Sprintf(format, v...)
  }
docs: // Printf throw logstr to channel to print to the logger.
  // Arguments are handled in the manner of fmt.Printf.