前言
v4l2的用户侧使用方法的简单demo，参考：media/v4l/v4l2grab.c
实际上，v4l2的用户侧有libv4l开源库。用户可以使用此开源库快速搭建其用户侧代码。
流程
int main()
{// openfd = v4l2_open("/dev/video0", O_RDWR | O_NONBLOCK, 0);// set formatCLEAR(fmt);fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;fmt.fmt.pix.width = 640;fmt.fmt.pix.height = 480;fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_RGB24;fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;xioctl(fd, VIDIOC_S_FMT, &fmt);// request buffer// 申请的buffer留在内核CLEAR(req);req.count = 2;req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;req.memory = V4L2_MEMORY_MMAP;xioctl(fd, VIDIOC_REQBUFS, &req);buffers = calloc(req.count, sizeof(*buffers));// query buffer// 根据index获取buffer信息（length和offset），然后mmap获取虚拟地址并保存for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {CLEAR(buf);buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;buf.memory = V4L2_MEMORY_MMAP;buf.index = n_buffers;xioctl(fd, VIDIOC_QUERYBUF, &buf);buffers[n_buffers].length = buf.length;buffers[n_buffers].start = v4l2_mmap(NULL, buf.length,PROT_READ | PROT_WRITE, MAP_SHARED,fd, buf.m.offset);}// queue buffer// buffer保存在内核，只需要告知index即可完成queuefor (i = 0; i < n_buffers; ++i) {CLEAR(buf);buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;buf.memory = V4L2_MEMORY_MMAP;buf.index = i;xioctl(fd, VIDIOC_QBUF, &buf);}// stream ontype = V4L2_BUF_TYPE_VIDEO_CAPTURE;xioctl(fd, VIDIOC_STREAMON, &type);// dequeue buffer + queue bufferfor (i = 0; i < 20; i++) {// 使用select或poll或epoll监控fd是否可读，如果可读则往下走CLEAR(buf);buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;buf.memory = V4L2_MEMORY_MMAP;xioctl(fd, VIDIOC_DQBUF, &buf);// 处理buffer，比如写到文件中xioctl(fd, VIDIOC_QBUF, &buf);}// stream offtype = V4L2_BUF_TYPE_VIDEO_CAPTURE;xioctl(fd, VIDIOC_STREAMOFF, &type);// munmapfor (i = 0; i < n_buffers; ++i)v4l2_munmap(buffers[i].start, buffers[i].length);// closev4l2_close(fd);
}