if (-1 == r) {
if (EINTR == errno)
continue;
printf ("select err\n");
}
if (0 == r) {
fprintf (stderr, "select timeout\n");
exit (EXIT_FAILURE);
}

if (read_frame ())//如果可读，执行read_frame ()函数，并跳出循环
break;
}

unmap:
for (i = 0; i < n_buffers; ++i)
if (-1 == munmap (buffers[i].start, buffers[i].length))
printf ("munmap error");
close (fd);
fclose (file_fd);
exit (EXIT_SUCCESS);
return 0;
}