if (MAP_FAILED == buffers[n_buffers].start)
printf ("mmap failed\n");
}

for (i = 0; i < n_buffers; ++i)
{
struct v4l2_buffer buf;
CLEAR (buf);

buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = i;

if (-1 == ioctl (fd, VIDIOC_QBUF, &buf))//申请到的缓冲进入列队
printf ("VIDIOC_QBUF failed\n");
}

type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

if (-1 == ioctl (fd, VIDIOC_STREAMON, &type)) //开始捕捉图像数据
printf ("VIDIOC_STREAMON failed\n");

for (;;) //这一段涉及到异步IO
{
fd_set fds;
struct timeva l tv;
int r;

FD_ZERO (&fds);//将指定的文件描述符集清空
FD_SET (fd, &fds);//在文件描述符集合中增加一个新的文件描述符

/* Timeout. */
tv.tv_sec = 2;
tv.tv_usec = 0;

r = select (fd + 1, &fds, NULL, NULL, &tv);//判断是否可读（即摄像头是否准备好），tv是定时

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");
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
if (-1 == ioctl(fd, VIDIOC_STREAMOFF, &type))
printf("VIDIOC_STREAMOFF");
close (fd);
fclose (file_fd);
return 0;
}