Fflush(stdout) seems to be the same whether it is added or not! ! Why? What exactly does fflush(stdout) do here?

When multiple output functions are used for continuous multiple outputs, output errors can be found. Because the next data is still in the output buffer before the last data is output, the next printf will add another data in the output buffer, and the original data will be washed away, resulting in an output error. in prinf()； Add fflush (stdout) after it; Force immediate output to avoid errors.