V4L2应用程序开发-01数据采集流程

1 数据采集流程

可以参考这些文件：

• mjpg-streamer\mjpg-streamer-experimental\plugins\input_control\input_uvc.c

• video2lcd\video\v4l2.c

Video for Linux two(Video4Linux2)简称V4L2，是V4L的改进版。V4L2支持三种方式来采集图像：内存映射方式(mmap)、直接读取方式(read)和用户指针。内存映射的方式采集速度较快，一般用于连续视频数据的采集，实际工作中的应用概率更高；直接读取的方式相对速度慢一些，所以常用于静态图片数据的采集；用户指针使用较少，如有兴趣可自行研究。

1.1 buffer的管理

使用摄像头时，核心是"获得数据"。所以先讲如何获取数据，即如何得到buffer。

摄像头采集数据时，是一帧又一帧地连续采集。所以需要申请若干个buffer，驱动程序把数据放入buffer，APP从buffer得到数据。这些buffer可以使用链表来管理。

驱动程序周而复始地做如下事情：

• 从硬件采集到数据

• 从"空闲链表"取出buffer，把数据存入buffer

• 把含有数据的buffer放入"完成链表"

APP也会周而复始地做如下事情：

• 监测"完成链表"，等待它含有buffer

• 从"完成链表"中取出buffer

• 处理数据

• 把buffer放入"空闲链表"

链表操作示意图如下：

1.2 完整的使用流程

参考mjpg-streamer和video2lcd，总结了摄像头的使用流程，如下：

主要是：video2lcd\video\v4l2.c

（1）open：打开设备节点/dev/videoX

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{int i;int iFd;int iError;struct v4l2_capability tV4l2Cap;struct v4l2_fmtdesc tFmtDesc;struct v4l2_format  tV4l2Fmt;struct v4l2_requestbuffers tV4l2ReqBuffs;struct v4l2_buffer tV4l2Buf;int iLcdWidth;int iLcdHeigt;int iLcdBpp;// 打开设备节点/dev/videoXiFd = open(strDevName, O_RDWR);if (iFd < 0){DBG_PRINTF("can not open %s\n", strDevName);return -1;}// ......
}

（2）ioctl VIDIOC_QUERYCAP：Query Capbility，查询能力

• 确认它是否是"捕获设备"，因为有些节点是输出设备

• 确认它是否支持mmap操作，还是仅支持read/write操作

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{int i;int iFd;int iError;struct v4l2_capability tV4l2Cap;struct v4l2_fmtdesc tFmtDesc;struct v4l2_format  tV4l2Fmt;struct v4l2_requestbuffers tV4l2ReqBuffs;struct v4l2_buffer tV4l2Buf;int iLcdWidth;int iLcdHeigt;int iLcdBpp;// 1打开设备节点/dev/videoXiFd = open(strDevName, O_RDWR);if (iFd < 0){DBG_PRINTF("can not open %s\n", strDevName);return -1;}ptVideoDevice->iFd = iFd;iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));// 2Query Capbility，查询能力iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);if (iError) {DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);goto err_exit;}// 2.1捕获设备if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)){DBG_PRINTF("%s is not a video capture device\n", strDevName);goto err_exit;}// 2.2流if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {DBG_PRINTF("%s supports streaming i/o\n", strDevName);}// 2.3读能力if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {DBG_PRINTF("%s supports read i/o\n", strDevName);}// ......
}

（3）ioctl VIDIOC_ENUM_FMT：枚举它支持的格式

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{int i;int iFd;int iError;struct v4l2_capability tV4l2Cap;struct v4l2_fmtdesc tFmtDesc;struct v4l2_format  tV4l2Fmt;struct v4l2_requestbuffers tV4l2ReqBuffs;struct v4l2_buffer tV4l2Buf;int iLcdWidth;int iLcdHeigt;int iLcdBpp;// 1打开设备节点/dev/videoXiFd = open(strDevName, O_RDWR);if (iFd < 0){DBG_PRINTF("can not open %s\n", strDevName);return -1;}ptVideoDevice->iFd = iFd;iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));// 2Query Capbility，查询能力iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);if (iError) {DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);goto err_exit;}// 2.1捕获设备if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)){DBG_PRINTF("%s is not a video capture device\n", strDevName);goto err_exit;}// 2.2流if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {DBG_PRINTF("%s supports streaming i/o\n", strDevName);}// 2.3读能力if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {DBG_PRINTF("%s supports read i/o\n", strDevName);}memset(&tFmtDesc, 0, sizeof(tFmtDesc));tFmtDesc.index = 0;tFmtDesc.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;// 3ioctl VIDIOC_ENUM_FMT：枚举它支持的格式while ((iError = ioctl(iFd, VIDIOC_ENUM_FMT, &tFmtDesc)) == 0) {if (isSupportThisFormat(tFmtDesc.pixelformat)){ptVideoDevice->iPixelFormat = tFmtDesc.pixelformat;break;}tFmtDesc.index++;}// ......
}

（4）ioctl VIDIOC_S_FMT：在上面枚举出来的格式里，选择一个来设置格式

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{int i;int iFd;int iError;struct v4l2_capability tV4l2Cap;struct v4l2_fmtdesc tFmtDesc;struct v4l2_format  tV4l2Fmt;struct v4l2_requestbuffers tV4l2ReqBuffs;struct v4l2_buffer tV4l2Buf;int iLcdWidth;int iLcdHeigt;int iLcdBpp;// 1打开设备节点/dev/videoXiFd = open(strDevName, O_RDWR);if (iFd < 0){DBG_PRINTF("can not open %s\n", strDevName);return -1;}ptVideoDevice->iFd = iFd;iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));// 2Query Capbility，查询能力iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);if (iError) {DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);goto err_exit;}// 2.1捕获设备if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)){DBG_PRINTF("%s is not a video capture device\n", strDevName);goto err_exit;}// 2.2流if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {DBG_PRINTF("%s supports streaming i/o\n", strDevName);}// 2.3读能力if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {DBG_PRINTF("%s supports read i/o\n", strDevName);}memset(&tFmtDesc, 0, sizeof(tFmtDesc));tFmtDesc.index = 0;tFmtDesc.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;// 3ioctl VIDIOC_ENUM_FMT：枚举它支持的格式while ((iError = ioctl(iFd, VIDIOC_ENUM_FMT, &tFmtDesc)) == 0) {if (isSupportThisFormat(tFmtDesc.pixelformat)){ptVideoDevice->iPixelFormat = tFmtDesc.pixelformat;break;}tFmtDesc.index++;}if (!ptVideoDevice->iPixelFormat){DBG_PRINTF("can not support the format of this device\n");goto err_exit;        }/* set format in */GetDispResolution(&iLcdWidth, &iLcdHeigt, &iLcdBpp);memset(&tV4l2Fmt, 0, sizeof(struct v4l2_format));tV4l2Fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Fmt.fmt.pix.pixelformat = ptVideoDevice->iPixelFormat;tV4l2Fmt.fmt.pix.width       = iLcdWidth;tV4l2Fmt.fmt.pix.height      = iLcdHeigt;tV4l2Fmt.fmt.pix.field       = V4L2_FIELD_ANY;/* 如果驱动程序发现无法某些参数(比如分辨率),* 它会调整这些参数, 并且返回给应用程序*/// 4在上面枚举出来的格式里，选择一个来设置格式iError = ioctl(iFd, VIDIOC_S_FMT, &tV4l2Fmt); if (iError) {DBG_PRINTF("Unable to set format\n");goto err_exit;        }ptVideoDevice->iWidth  = tV4l2Fmt.fmt.pix.width;ptVideoDevice->iHeight = tV4l2Fmt.fmt.pix.height;// ......
}

（5）ioctl VIDIOC_REQBUFS：申请buffer，APP可以申请很多个buffer，但是驱动程序不一定能申请到

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{int i;int iFd;int iError;struct v4l2_capability tV4l2Cap;struct v4l2_fmtdesc tFmtDesc;struct v4l2_format  tV4l2Fmt;struct v4l2_requestbuffers tV4l2ReqBuffs;struct v4l2_buffer tV4l2Buf;int iLcdWidth;int iLcdHeigt;int iLcdBpp;// 1打开设备节点/dev/videoXiFd = open(strDevName, O_RDWR);if (iFd < 0){DBG_PRINTF("can not open %s\n", strDevName);return -1;}ptVideoDevice->iFd = iFd;iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));// 2Query Capbility，查询能力iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);if (iError) {DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);goto err_exit;}// 2.1捕获设备if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)){DBG_PRINTF("%s is not a video capture device\n", strDevName);goto err_exit;}// 2.2流if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {DBG_PRINTF("%s supports streaming i/o\n", strDevName);}// 2.3读能力if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {DBG_PRINTF("%s supports read i/o\n", strDevName);}memset(&tFmtDesc, 0, sizeof(tFmtDesc));tFmtDesc.index = 0;tFmtDesc.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;// 3ioctl VIDIOC_ENUM_FMT：枚举它支持的格式while ((iError = ioctl(iFd, VIDIOC_ENUM_FMT, &tFmtDesc)) == 0) {if (isSupportThisFormat(tFmtDesc.pixelformat)){ptVideoDevice->iPixelFormat = tFmtDesc.pixelformat;break;}tFmtDesc.index++;}if (!ptVideoDevice->iPixelFormat){DBG_PRINTF("can not support the format of this device\n");goto err_exit;        }/* set format in */GetDispResolution(&iLcdWidth, &iLcdHeigt, &iLcdBpp);memset(&tV4l2Fmt, 0, sizeof(struct v4l2_format));tV4l2Fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Fmt.fmt.pix.pixelformat = ptVideoDevice->iPixelFormat;tV4l2Fmt.fmt.pix.width       = iLcdWidth;tV4l2Fmt.fmt.pix.height      = iLcdHeigt;tV4l2Fmt.fmt.pix.field       = V4L2_FIELD_ANY;/* 如果驱动程序发现无法某些参数(比如分辨率),* 它会调整这些参数, 并且返回给应用程序*/// 4在上面枚举出来的格式里，选择一个来设置格式iError = ioctl(iFd, VIDIOC_S_FMT, &tV4l2Fmt); if (iError) {DBG_PRINTF("Unable to set format\n");goto err_exit;        }ptVideoDevice->iWidth  = tV4l2Fmt.fmt.pix.width;ptVideoDevice->iHeight = tV4l2Fmt.fmt.pix.height;/* request buffers */memset(&tV4l2ReqBuffs, 0, sizeof(struct v4l2_requestbuffers));tV4l2ReqBuffs.count = NB_BUFFER;tV4l2ReqBuffs.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2ReqBuffs.memory = V4L2_MEMORY_MMAP;// 5申请buffer，APP可以申请很多个buffer，但是驱动程序不一定能申请到iError = ioctl(iFd, VIDIOC_REQBUFS, &tV4l2ReqBuffs);if (iError) {DBG_PRINTF("Unable to allocate buffers.\n");goto err_exit;        }// ......
}

（6）ioctl VIDIOC_QUERYBUF和mmap：查询buffer信息、映射

• 如果申请到了N个buffer，这个ioctl就应该执行N次

• 执行mmap后，APP就可以直接读写这些buffer

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{int i;int iFd;int iError;struct v4l2_capability tV4l2Cap;struct v4l2_fmtdesc tFmtDesc;struct v4l2_format  tV4l2Fmt;struct v4l2_requestbuffers tV4l2ReqBuffs;struct v4l2_buffer tV4l2Buf;int iLcdWidth;int iLcdHeigt;int iLcdBpp;// 1打开设备节点/dev/videoXiFd = open(strDevName, O_RDWR);if (iFd < 0){DBG_PRINTF("can not open %s\n", strDevName);return -1;}ptVideoDevice->iFd = iFd;iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));// 2Query Capbility，查询能力iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);if (iError) {DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);goto err_exit;}// 2.1捕获设备if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)){DBG_PRINTF("%s is not a video capture device\n", strDevName);goto err_exit;}// 2.2流,mmapif (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {DBG_PRINTF("%s supports streaming i/o\n", strDevName);}// 2.3读能力if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {DBG_PRINTF("%s supports read i/o\n", strDevName);}memset(&tFmtDesc, 0, sizeof(tFmtDesc));tFmtDesc.index = 0;tFmtDesc.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;// 3ioctl VIDIOC_ENUM_FMT：枚举它支持的格式while ((iError = ioctl(iFd, VIDIOC_ENUM_FMT, &tFmtDesc)) == 0) {if (isSupportThisFormat(tFmtDesc.pixelformat)){ptVideoDevice->iPixelFormat = tFmtDesc.pixelformat;break;}tFmtDesc.index++;}if (!ptVideoDevice->iPixelFormat){DBG_PRINTF("can not support the format of this device\n");goto err_exit;        }/* set format in */GetDispResolution(&iLcdWidth, &iLcdHeigt, &iLcdBpp);memset(&tV4l2Fmt, 0, sizeof(struct v4l2_format));tV4l2Fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Fmt.fmt.pix.pixelformat = ptVideoDevice->iPixelFormat;tV4l2Fmt.fmt.pix.width       = iLcdWidth;tV4l2Fmt.fmt.pix.height      = iLcdHeigt;tV4l2Fmt.fmt.pix.field       = V4L2_FIELD_ANY;/* 如果驱动程序发现无法某些参数(比如分辨率),* 它会调整这些参数, 并且返回给应用程序*/// 4在上面枚举出来的格式里，选择一个来设置格式iError = ioctl(iFd, VIDIOC_S_FMT, &tV4l2Fmt); if (iError) {DBG_PRINTF("Unable to set format\n");goto err_exit;        }ptVideoDevice->iWidth  = tV4l2Fmt.fmt.pix.width;ptVideoDevice->iHeight = tV4l2Fmt.fmt.pix.height;/* request buffers */memset(&tV4l2ReqBuffs, 0, sizeof(struct v4l2_requestbuffers));tV4l2ReqBuffs.count = NB_BUFFER;tV4l2ReqBuffs.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2ReqBuffs.memory = V4L2_MEMORY_MMAP;// 5申请buffer，APP可以申请很多个buffer，但是驱动程序不一定能申请到iError = ioctl(iFd, VIDIOC_REQBUFS, &tV4l2ReqBuffs);if (iError) {DBG_PRINTF("Unable to allocate buffers.\n");goto err_exit;        }// 6确定最终分配buffer数量ptVideoDevice->iVideoBufCnt = tV4l2ReqBuffs.count;if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING){/* map the buffers */for (i = 0; i < ptVideoDevice->iVideoBufCnt; i++) {memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));tV4l2Buf.index = i;tV4l2Buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Buf.memory = V4L2_MEMORY_MMAP;// 6.1查询iError = ioctl(iFd, VIDIOC_QUERYBUF, &tV4l2Buf);if (iError) {DBG_PRINTF("Unable to query buffer.\n");goto err_exit;}ptVideoDevice->iVideoBufMaxLen = tV4l2Buf.length;// 6.2把内核中分配的buffer的地址映射给应用程序ptVideoDevice->pucVideBuf[i] = mmap(0 /* start anywhere */ ,tV4l2Buf.length, PROT_READ, MAP_SHARED, iFd,tV4l2Buf.m.offset);if (ptVideoDevice->pucVideBuf[i] == MAP_FAILED) {DBG_PRINTF("Unable to map buffer\n");goto err_exit;}}// ......
}

（7）

ioctl VIDIOC_QBUF：把buffer放入"空闲链表"

如果申请到了N个buffer，这个ioctl就应该执行N次

static int V4l2InitDevice(char *strDevName, PT_VideoDevice ptVideoDevice)
{int i;int iFd;int iError;struct v4l2_capability tV4l2Cap;struct v4l2_fmtdesc tFmtDesc;struct v4l2_format  tV4l2Fmt;struct v4l2_requestbuffers tV4l2ReqBuffs;struct v4l2_buffer tV4l2Buf;int iLcdWidth;int iLcdHeigt;int iLcdBpp;// 1打开设备节点/dev/videoXiFd = open(strDevName, O_RDWR);if (iFd < 0){DBG_PRINTF("can not open %s\n", strDevName);return -1;}ptVideoDevice->iFd = iFd;iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);memset(&tV4l2Cap, 0, sizeof(struct v4l2_capability));// 2Query Capbility，查询能力iError = ioctl(iFd, VIDIOC_QUERYCAP, &tV4l2Cap);if (iError) {DBG_PRINTF("Error opening device %s: unable to query device.\n", strDevName);goto err_exit;}// 2.1捕获设备if (!(tV4l2Cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)){DBG_PRINTF("%s is not a video capture device\n", strDevName);goto err_exit;}// 2.2流,mmapif (tV4l2Cap.capabilities & V4L2_CAP_STREAMING) {DBG_PRINTF("%s supports streaming i/o\n", strDevName);}// 2.3读能力if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE) {DBG_PRINTF("%s supports read i/o\n", strDevName);}memset(&tFmtDesc, 0, sizeof(tFmtDesc));tFmtDesc.index = 0;tFmtDesc.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;// 3ioctl VIDIOC_ENUM_FMT：枚举它支持的格式while ((iError = ioctl(iFd, VIDIOC_ENUM_FMT, &tFmtDesc)) == 0) {if (isSupportThisFormat(tFmtDesc.pixelformat)){ptVideoDevice->iPixelFormat = tFmtDesc.pixelformat;break;}tFmtDesc.index++;}if (!ptVideoDevice->iPixelFormat){DBG_PRINTF("can not support the format of this device\n");goto err_exit;        }/* set format in */GetDispResolution(&iLcdWidth, &iLcdHeigt, &iLcdBpp);memset(&tV4l2Fmt, 0, sizeof(struct v4l2_format));tV4l2Fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Fmt.fmt.pix.pixelformat = ptVideoDevice->iPixelFormat;tV4l2Fmt.fmt.pix.width       = iLcdWidth;tV4l2Fmt.fmt.pix.height      = iLcdHeigt;tV4l2Fmt.fmt.pix.field       = V4L2_FIELD_ANY;/* 如果驱动程序发现无法某些参数(比如分辨率),* 它会调整这些参数, 并且返回给应用程序*/// 4在上面枚举出来的格式里，选择一个来设置格式iError = ioctl(iFd, VIDIOC_S_FMT, &tV4l2Fmt); if (iError) {DBG_PRINTF("Unable to set format\n");goto err_exit;        }ptVideoDevice->iWidth  = tV4l2Fmt.fmt.pix.width;ptVideoDevice->iHeight = tV4l2Fmt.fmt.pix.height;/* request buffers */memset(&tV4l2ReqBuffs, 0, sizeof(struct v4l2_requestbuffers));tV4l2ReqBuffs.count = NB_BUFFER;tV4l2ReqBuffs.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2ReqBuffs.memory = V4L2_MEMORY_MMAP;// 5申请buffer，APP可以申请很多个buffer，但是驱动程序不一定能申请到iError = ioctl(iFd, VIDIOC_REQBUFS, &tV4l2ReqBuffs);if (iError) {DBG_PRINTF("Unable to allocate buffers.\n");goto err_exit;        }// 6确定最终分配buffer数量ptVideoDevice->iVideoBufCnt = tV4l2ReqBuffs.count;if (tV4l2Cap.capabilities & V4L2_CAP_STREAMING){/* map the buffers */for (i = 0; i < ptVideoDevice->iVideoBufCnt; i++) {memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));tV4l2Buf.index = i;tV4l2Buf.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Buf.memory = V4L2_MEMORY_MMAP;// 6.1查询iError = ioctl(iFd, VIDIOC_QUERYBUF, &tV4l2Buf);if (iError) {DBG_PRINTF("Unable to query buffer.\n");goto err_exit;}ptVideoDevice->iVideoBufMaxLen = tV4l2Buf.length;// 6.2把内核中分配的buffer的地址映射给应用程序ptVideoDevice->pucVideBuf[i] = mmap(0 /* start anywhere */ ,tV4l2Buf.length, PROT_READ, MAP_SHARED, iFd,tV4l2Buf.m.offset);if (ptVideoDevice->pucVideBuf[i] == MAP_FAILED) {DBG_PRINTF("Unable to map buffer\n");goto err_exit;}}        /* Queue the buffers. */// 7把buffer放入"空闲链表"for (i = 0; i < ptVideoDevice->iVideoBufCnt; i++) {memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));tV4l2Buf.index = i;tV4l2Buf.type  = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Buf.memory = V4L2_MEMORY_MMAP;iError = ioctl(iFd, VIDIOC_QBUF, &tV4l2Buf);if (iError){DBG_PRINTF("Unable to queue buffer.\n");goto err_exit;}}}else if (tV4l2Cap.capabilities & V4L2_CAP_READWRITE){g_tV4l2VideoOpr.GetFrame = V4l2GetFrameForReadWrite;g_tV4l2VideoOpr.PutFrame = V4l2PutFrameForReadWrite;/* read(fd, buf, size) */ptVideoDevice->iVideoBufCnt  = 1;/* 在这个程序所能支持的格式里, 一个象素最多只需要4字节 */ptVideoDevice->iVideoBufMaxLen = ptVideoDevice->iWidth * ptVideoDevice->iHeight * 4;ptVideoDevice->pucVideBuf[0] = malloc(ptVideoDevice->iVideoBufMaxLen);}ptVideoDevice->ptOPr = &g_tV4l2VideoOpr;return 0;err_exit:    close(iFd);return -1;    
}

（8）ioctl VIDIOC_STREAMON：启动摄像头

static int V4l2StartDevice(PT_VideoDevice ptVideoDevice)
{int iType = V4L2_BUF_TYPE_VIDEO_CAPTURE;int iError;// 启动摄像头iError = ioctl(ptVideoDevice->iFd, VIDIOC_STREAMON, &iType);if (iError) {DBG_PRINTF("Unable to start capture.\n");return -1;}return 0;
}

（9）

这里是一个循环：使用poll/select监测buffer，然后从"完成链表"中取出buffer，处理后再放入"空闲链表"

poll/select

static int V4l2GetFrameForStreaming(PT_VideoDevice ptVideoDevice, PT_VideoBuf ptVideoBuf)
{struct pollfd tFds[1];int iRet;struct v4l2_buffer tV4l2Buf;/* poll */tFds[0].fd     = ptVideoDevice->iFd;tFds[0].events = POLLIN;iRet = poll(tFds, 1, -1);if (iRet <= 0){DBG_PRINTF("poll error!\n");return -1;}// ......
}

ioctl VIDIOC_DQBUF：从"完成链表"中取出buffer；处理：前面使用mmap映射了每个buffer的地址，处理时就可以直接使用地址来访问buffer

static int V4l2GetFrameForStreaming(PT_VideoDevice ptVideoDevice, PT_VideoBuf ptVideoBuf)
{struct pollfd tFds[1];int iRet;struct v4l2_buffer tV4l2Buf;/* poll */tFds[0].fd     = ptVideoDevice->iFd;tFds[0].events = POLLIN;iRet = poll(tFds, 1, -1);if (iRet <= 0){DBG_PRINTF("poll error!\n");return -1;}/* VIDIOC_DQBUF */memset(&tV4l2Buf, 0, sizeof(struct v4l2_buffer));tV4l2Buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;tV4l2Buf.memory = V4L2_MEMORY_MMAP;iRet = ioctl(ptVideoDevice->iFd, VIDIOC_DQBUF, &tV4l2Buf);if (iRet < 0) {DBG_PRINTF("Unable to dequeue buffer.\n");return -1;}ptVideoDevice->iVideoBufCurIndex = tV4l2Buf.index;ptVideoBuf->iPixelFormat        = ptVideoDevice->iPixelFormat;ptVideoBuf->tPixelDatas.iWidth  = ptVideoDevice->iWidth;ptVideoBuf->tPixelDatas.iHeight = ptVideoDevice->iHeight;ptVideoBuf->tPixelDatas.iBpp    = (ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_YUYV) ? 16 : \(ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_MJPEG) ? 0 :  \(ptVideoDevice->iPixelFormat == V4L2_PIX_FMT_RGB565) ? 16 :  \0;ptVideoBuf->tPixelDatas.iLineBytes    = ptVideoDevice->iWidth * ptVideoBuf->tPixelDatas.iBpp / 8;ptVideoBuf->tPixelDatas.iTotalBytes   = tV4l2Buf.bytesused;ptVideoBuf->tPixelDatas.aucPixelDatas = ptVideoDevice->pucVideBuf[tV4l2Buf.index];    return 0;
}

ioclt VIDIOC_QBUF：把buffer放入"空闲链表"

（10）停止摄像头

static int V4l2StopDevice(PT_VideoDevice ptVideoDevice)
{int iType = V4L2_BUF_TYPE_VIDEO_CAPTURE;int iError;iError = ioctl(ptVideoDevice->iFd, VIDIOC_STREAMOFF, &iType);if (iError) {DBG_PRINTF("Unable to stop capture.\n");return -1;}return 0;
}

参考资料：

[1] 01_V4L2应用程序开发_数据采集流程_哔哩哔哩_bilibili

[2] mjpg-streamer：https://github.com/jacksonliam/mjpg-streamer

[3] video2lcd：这是百问网编写的APP，它可以在LCD上直接显示摄像头的图像

[4] 这2个源码都放在GIT仓库里：

[5] UVC文档

[6] 参考文章：https://www.xjx100.cn/news/700965.html?action=onClick