V4L2获取usb视频流测试代码

Video4Linux2(Video for Linux Two, 简称V4L2)是Linux中关于视频设备的驱动框架，为上层访问底层的视频设备提供统一接口。V4L2主要支持三类设备：视频输入输出设备、VBI设备和Radio设备，分别会在/dev目录下产生videoX、vbiX和radioX设备节点，其中X是0,1,2等的数字。如USB摄像头是我们常见的视频输入设备。FFmpeg和OpenCV对V4L2均支持。

在使用USB摄像头获取视频时，外部调用V4L2 API的包含的主要头文件为videodev2.h，在/usr/include/linux目录下。

V4L2获取USB视频流执行流程如下：代码参考：https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/capture.c.html

1. 打开设备调用open_device函数：

(1). 调用stat函数，通过设备名字获取设备信息，并将结果保存在结构体stat中，结构体stat内容如下：

struct stat {
    dev_t     st_dev;     /* ID of device containing file */
    ino_t     st_ino;     /* inode number */
    mode_t    st_mode;    /* protection */
    nlink_t   st_nlink;   /* number of hard links */
    uid_t     st_uid;     /* user ID of owner */
    gid_t     st_gid;     /* group ID of owner */
    dev_t     st_rdev;    /* device ID (if special file) */
    off_t     st_size;    /* total size, in bytes */
    blksize_t st_blksize; /* blocksize for file system I/O */
    blkcnt_t  st_blocks;  /* number of 512B blocks allocated */
    time_t    st_atime;   /* time of last access */
    time_t    st_mtime;   /* time of last modification */
    time_t    st_ctime;   /* time of last status change */
};

(2). 调用S_ISCHR函数判断是否是字符设备(character device)；

(3). 调用open函数打开设备，以可读可写方式(O_RDWR)和无阻塞方式(O_NONBLOCK)打开，open函数返回一个文件(或设备)描述符。

 2. 初始化设备调用init_device函数：

(1). 调用ioctl函数，对设备的I/O通道进行管理，查询设备能力，并将结果保存在结构体v4l2_capability中，结构体v4l2_capability内容如下：

struct v4l2_capability {
	__u8	driver[16]; // name of the driver module (e.g. "bttv")
	__u8	card[32]; // name of the card (e.g. "Hauppauge WinTV")
	__u8	bus_info[32]; // name of the bus (e.g. "PCI:" + pci_name(pci_dev) )
	__u32   version; // KERNEL_VERSION
	__u32	capabilities; // capabilities of the physical device as a whole
	__u32	device_caps; // capabilities accessed via this particular device (node)
	__u32	reserved[3]; // reserved fields for future extensions
};

(2). 判断是否是视频捕获设备：V4L2_CAP_VIDEO_CAPTURE；

(3). 判断是否支持流I/O(streaming I/O)：V4L2_CAP_STREAMING；

(4). 调用ioctl函数，查询设备视频裁剪和缩放功能信息，并将结果保存在结构体v4l2_cropcap中，结构体v4l2_cropcap内容如下：

enum v4l2_buf_type {
	V4L2_BUF_TYPE_VIDEO_CAPTURE        = 1,
	V4L2_BUF_TYPE_VIDEO_OUTPUT         = 2,
	V4L2_BUF_TYPE_VIDEO_OVERLAY        = 3,
	V4L2_BUF_TYPE_VBI_CAPTURE          = 4,
	V4L2_BUF_TYPE_VBI_OUTPUT           = 5,
	V4L2_BUF_TYPE_SLICED_VBI_CAPTURE   = 6,
	V4L2_BUF_TYPE_SLICED_VBI_OUTPUT    = 7,
#if 1
	/* Experimental */
	V4L2_BUF_TYPE_VIDEO_OUTPUT_OVERLAY = 8,
#endif
	V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE = 9,
	V4L2_BUF_TYPE_VIDEO_OUTPUT_MPLANE  = 10,
	/* Deprecated, do not use */
	V4L2_BUF_TYPE_PRIVATE              = 0x80,
};

struct v4l2_cropcap {
	__u32 type;	/* enum v4l2_buf_type */
	struct v4l2_rect        bounds;
	struct v4l2_rect        defrect;
	struct v4l2_fract       pixelaspect;
};

(5). 调用ioctl函数，设置当前裁剪矩形，并将结果保存在结构体v4l2_crop中，结构体v4l2_crop内容如下：

struct v4l2_crop {
	__u32 type;	/* enum v4l2_buf_type */
	struct v4l2_rect        c;
};

(6). 调用ioctl函数，设置流数据格式，包括宽、高、像素格式，并将结果保存在结构体v4l2_format中，结构体v4l2_format内容如下：

struct v4l2_pix_format {
	__u32         	width;
	__u32			height;
	__u32			pixelformat;
	__u32			field;		/* enum v4l2_field */
	__u32           bytesperline;	/* for padding, zero if unused */
	__u32          	sizeimage;
	__u32			colorspace;	/* enum v4l2_colorspace */
	__u32			priv;		/* private data, depends on pixelformat */
};

struct v4l2_format { // stream data format
	__u32	 type; // enum v4l2_buf_type; type of the data stream
	union {
		struct v4l2_pix_format		pix;     // V4L2_BUF_TYPE_VIDEO_CAPTURE, definition of an image format
		struct v4l2_pix_format_mplane	pix_mp;  // V4L2_BUF_TYPE_VIDEO_CAPTURE_MPLANE, definition of a multiplanar image format
		struct v4l2_window		win;     // V4L2_BUF_TYPE_VIDEO_OVERLAY, definition of an overlaid image
		struct v4l2_vbi_format		vbi;     // V4L2_BUF_TYPE_VBI_CAPTURE, raw VBI capture or output parameters
		struct v4l2_sliced_vbi_format	sliced;  // V4L2_BUF_TYPE_SLICED_VBI_CAPTURE, sliced VBI capture or output parameters
		__u8	raw_data[200];                   // user-defined, placeholder for future extensions and custom formats
	} fmt;
};

3. 因为io采用的是MMAP即内存映射方式，因此调用init_mmap函数：

(1). 调用ioctl函数，设置内存映射I/O，并将结果保存在结构体v4l2_requestbuffers中，结构体v4l2_requestbuffers内容如下：

enum v4l2_memory {
	V4L2_MEMORY_MMAP             = 1,
	V4L2_MEMORY_USERPTR          = 2,
	V4L2_MEMORY_OVERLAY          = 3,
	V4L2_MEMORY_DMABUF           = 4,
};

struct v4l2_requestbuffers {
	__u32			count;
	__u32			type;		/* enum v4l2_buf_type */
	__u32			memory;		/* enum v4l2_memory */
	__u32			reserved[2];
};

(2). 调用ioctl函数，查询缓冲区状态，并将结果保存在结构体v4l2_buffer中，结构体v4l2_buffer内容如下：

/**
 * struct v4l2_buffer - video buffer info
 * @index:	id number of the buffer
 * @type:	enum v4l2_buf_type; buffer type (type == *_MPLANE for
 *		multiplanar buffers);
 * @bytesused:	number of bytes occupied by data in the buffer (payload);
 *		unused (set to 0) for multiplanar buffers
 * @flags:	buffer informational flags
 * @field:	enum v4l2_field; field order of the image in the buffer
 * @timestamp:	frame timestamp
 * @timecode:	frame timecode
 * @sequence:	sequence count of this frame
 * @memory:	enum v4l2_memory; the method, in which the actual video data is
 *		passed
 * @offset:	for non-multiplanar buffers with memory == V4L2_MEMORY_MMAP;
 *		offset from the start of the device memory for this plane,
 *		(or a "cookie" that should be passed to mmap() as offset)
 * @userptr:	for non-multiplanar buffers with memory == V4L2_MEMORY_USERPTR;
 *		a userspace pointer pointing to this buffer
 * @fd:		for non-multiplanar buffers with memory == V4L2_MEMORY_DMABUF;
 *		a userspace file descriptor associated with this buffer
 * @planes:	for multiplanar buffers; userspace pointer to the array of plane
 *		info structs for this buffer
 * @length:	size in bytes of the buffer (NOT its payload) for single-plane
 *		buffers (when type != *_MPLANE); number of elements in the
 *		planes array for multi-plane buffers
 * @input:	input number from which the video data has has been captured
 *
 * Contains data exchanged by application and driver using one of the Streaming
 * I/O methods.
 */
struct v4l2_buffer {
	__u32			index;
	__u32			type;
	__u32			bytesused;
	__u32			flags;
	__u32			field;
	struct timeval		timestamp;
	struct v4l2_timecode	timecode;
	__u32			sequence;

	/* memory location */
	__u32			memory;
	union {
		__u32           offset;
		unsigned long   userptr;
		struct v4l2_plane *planes;
		__s32		fd;
	} m;
	__u32			length;
	__u32			reserved2;
	__u32			reserved;
};

(3). 调用mmap函数，应用程序通过内存映射将帧缓冲区地址映射到用户空间；通常在需要对文件进行频繁读写时使用，这样用内存读写取代I/O读写，以获得较高的性能。

4. 开始捕捉，调用start_capturing函数：

(1). 调用ioctl函数，VIDIOC_QBUF，并将结果保存在结构体v4l2_buffer中；

(2). 调用ioctl函数，启动流I/O，并将结果保存在结构体v4l2_buf_type中；

5. 主循环，调用mainloop函数：

(1). 清空集合调用FD_ZERO函数，将一个给定的文件描述符加入到集合中调用FD_SET函数；

(2). 调用select函数；

(3). 调用ioctl函数，VIDIOC_DQBUF，并将结果保存在结构体v4l2_buffer中；

(4). 将帧数据写入指定的文件；

(5). 调用ioctl函数，VIDIOC_QBUF，并将结果保存在结构体v4l2_buffer中。

6. 停止捕捉，调用stop_capturing函数：

 (1). 调用ioctl函数，停止流I/O，并将结果保存在结构体v4l2_buf_type中。

7. 调用uninit_device函数：

(1). 调用munmap函数，取消映射设备内存；

(2). 调用free函数，删除calloc申请的内存。

8. 关闭设备，调用close_device函数：

(1). 调用close函数关闭设备。

这里通过ffmpeg查看usb摄像头支持的编码类型，执行结果如下图所示，编码类型支持Raw和Mjpeg两种，对应的像素格式为V4L2_PIX_FMT_YUYV和V4L2_PIX_FMT_MJPEG：

测试代码如下：

#include "funset.hpp"
#include <string.h>
#include <assert.h>
#include <iostream>

#ifndef _MSC_VER

#include <fcntl.h>
#include <unistd.h>
#include <errno.h>
#include <sys/stat.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/mman.h>
#include <sys/ioctl.h>

#include <linux/videodev2.h>

namespace {

#define CLEAR(x) memset(&(x), 0, sizeof(x))

enum io_method {
	IO_METHOD_READ,
	IO_METHOD_MMAP,
	IO_METHOD_USERPTR,
};

struct buffer {
	void *start;
	size_t length;
};

char* dev_name;
enum io_method io = IO_METHOD_MMAP;
int fd = -1;
struct buffer* buffers;
unsigned int n_buffers;
int out_buf;
int force_format = 1;
int frame_count = 10;
int width = 640;
int height = 480;
FILE* f;

void errno_exit(const char *s)
{
	fprintf(stderr, "%s error %d, %s\n", s, errno, strerror(errno));
	exit(EXIT_FAILURE);
}

int xioctl(int fh, int request, void *arg)
{
	int r;

	do {
		r = ioctl(fh, request, arg);
	} while (-1 == r && EINTR == errno);

	return r;
}

void process_image(const void *p, int size)
{
	/*if (out_buf)
		fwrite(p, size, 1, stdout);

	fflush(stderr);
	fprintf(stderr, ".");
	fflush(stdout);*/
	fwrite(p, size, 1, f);
}

int read_frame(void)
{
	struct v4l2_buffer buf;
	unsigned int i;

	switch (io) {
	case IO_METHOD_READ:
		if (-1 == read(fd, buffers[0].start, buffers[0].length)) {
			switch (errno) {
			case EAGAIN:
				return 0;

			case EIO:
				// Could ignore EIO, see spec. fall through

			default:
				errno_exit("read");
			}
		}

		process_image(buffers[0].start, buffers[0].length);
		break;

	case IO_METHOD_MMAP:
		CLEAR(buf);
		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		buf.memory = V4L2_MEMORY_MMAP;
		if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) {
			switch (errno) {
			case EAGAIN:
				return 0;

			case EIO:
				// Could ignore EIO, see spec. fall through

			default:
				errno_exit("VIDIOC_DQBUF");
			}
		}

		assert(buf.index < n_buffers);
		process_image(buffers[buf.index].start, buf.bytesused);
		if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))
			errno_exit("VIDIOC_QBUF");
		break;

	case IO_METHOD_USERPTR:
		CLEAR(buf);
		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		buf.memory = V4L2_MEMORY_USERPTR;

		if (-1 == xioctl(fd, VIDIOC_DQBUF, &buf)) {
			switch (errno) {
			case EAGAIN:
				return 0;

			case EIO:
				// Could ignore EIO, see spec. fall through

			default:
				errno_exit("VIDIOC_DQBUF");
			}
		}

		for (i = 0; i < n_buffers; ++i)
		if (buf.m.userptr == (unsigned long)buffers[i].start && buf.length == buffers[i].length)
			break;

		assert(i < n_buffers);
		process_image((void *)buf.m.userptr, buf.bytesused);
		if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))
			errno_exit("VIDIOC_QBUF");
		break;
	}

	return 1;
}

void mainloop(void)
{
	unsigned int count = frame_count;
	while (count-- > 0) {
		for (;;) {
			fd_set fds;
			struct timeval tv;
			int r;

			FD_ZERO(&fds);
			FD_SET(fd, &fds);

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

			r = select(fd + 1, &fds, NULL, NULL, &tv);
			if (-1 == r) {
				if (EINTR == errno)
					continue;
				errno_exit("select");
			}

			if (0 == r) {
				fprintf(stderr, "select timeout\n");
				exit(EXIT_FAILURE);
			}

			if (read_frame())
				break;
			// EAGAIN - continue select loop
		}
	}
}

void stop_capturing(void)
{
	enum v4l2_buf_type type;

	switch (io) {
	case IO_METHOD_READ:
		// Nothing to do
		break;

	case IO_METHOD_MMAP:
	case IO_METHOD_USERPTR:
		type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		if (-1 == xioctl(fd, VIDIOC_STREAMOFF, &type))
			errno_exit("VIDIOC_STREAMOFF");
		break;
	}
}

void start_capturing(void)
{
	unsigned int i;
	enum v4l2_buf_type type;

	switch (io) {
	case IO_METHOD_READ:
		// Nothing to do
		break;

	case IO_METHOD_MMAP:
		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 == xioctl(fd, VIDIOC_QBUF, &buf))
				errno_exit("VIDIOC_QBUF");
		}
		type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		if (-1 == xioctl(fd, VIDIOC_STREAMON, &type))
			errno_exit("VIDIOC_STREAMON");
		break;

	case IO_METHOD_USERPTR:
		for (i = 0; i < n_buffers; ++i) {
			struct v4l2_buffer buf;
			CLEAR(buf);
			buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
			buf.memory = V4L2_MEMORY_USERPTR;
			buf.index = i;
			buf.m.userptr = (unsigned long)buffers[i].start;
			buf.length = buffers[i].length;

			if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))
				errno_exit("VIDIOC_QBUF");
		}
		type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		if (-1 == xioctl(fd, VIDIOC_STREAMON, &type))
			errno_exit("VIDIOC_STREAMON");
		break;
	}

	f = fopen("usb.yuv", "w");
	if (!f) {
		errno_exit("fail to open file");
	}
}

void uninit_device(void)
{
	unsigned int i;

	switch (io) {
	case IO_METHOD_READ:
		free(buffers[0].start);
		break;

	case IO_METHOD_MMAP:
		for (i = 0; i < n_buffers; ++i)
		if (-1 == munmap(buffers[i].start, buffers[i].length))
			errno_exit("munmap");
		break;

	case IO_METHOD_USERPTR:
		for (i = 0; i < n_buffers; ++i)
			free(buffers[i].start);
		break;
	}

	free(buffers);
	fclose(f);
}

void init_read(unsigned int buffer_size)
{
	buffers = (struct buffer*)calloc(1, sizeof(*buffers));
	if (!buffers) {
		fprintf(stderr, "Out of memory\n");
		exit(EXIT_FAILURE);
	}

	buffers[0].length = buffer_size;
	buffers[0].start = malloc(buffer_size);

	if (!buffers[0].start) {
		fprintf(stderr, "Out of memory\n");
		exit(EXIT_FAILURE);
	}
}

void init_mmap(void)
{
	struct v4l2_requestbuffers req;
	CLEAR(req);
	req.count = 4;
	req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
	req.memory = V4L2_MEMORY_MMAP;
	if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) {
		if (EINVAL == errno) {
			fprintf(stderr, "%s does not support memory mappingn\n", dev_name);
			exit(EXIT_FAILURE);
		} else {
			errno_exit("VIDIOC_REQBUFS");
		}
	}

	if (req.count < 2) {
		fprintf(stderr, "Insufficient buffer memory on %s\n", dev_name);
		exit(EXIT_FAILURE);
	}

	buffers = (struct buffer*)calloc(req.count, sizeof(*buffers));
	if (!buffers) {
		fprintf(stderr, "Out of memory\n");
		exit(EXIT_FAILURE);
	}

	for (n_buffers = 0; n_buffers < req.count; ++n_buffers) {
		struct v4l2_buffer buf;
		CLEAR(buf);
		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		buf.memory = V4L2_MEMORY_MMAP;
		buf.index = n_buffers;
		if (-1 == xioctl(fd, VIDIOC_QUERYBUF, &buf))
			errno_exit("VIDIOC_QUERYBUF");

		buffers[n_buffers].length = buf.length;
		buffers[n_buffers].start =
			mmap(NULL, // start anywhere
			buf.length,
			PROT_READ | PROT_WRITE, // required
			MAP_SHARED, // recommended
			fd, buf.m.offset);

		if (MAP_FAILED == buffers[n_buffers].start)
			errno_exit("mmap");
	}
}

void init_userp(unsigned int buffer_size)
{
	struct v4l2_requestbuffers req;
	CLEAR(req);

	req.count = 4;
	req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
	req.memory = V4L2_MEMORY_USERPTR;

	if (-1 == xioctl(fd, VIDIOC_REQBUFS, &req)) {
		if (EINVAL == errno) {
			fprintf(stderr, "%s does not support user pointer i/on", dev_name);
			exit(EXIT_FAILURE);
		} else {
			errno_exit("VIDIOC_REQBUFS");
		}
	}

	buffers = (struct buffer*)calloc(4, sizeof(*buffers));
	if (!buffers) {
		fprintf(stderr, "Out of memory\n");
		exit(EXIT_FAILURE);
	}

	for (n_buffers = 0; n_buffers < 4; ++n_buffers) {
		buffers[n_buffers].length = buffer_size;
		buffers[n_buffers].start = malloc(buffer_size);

		if (!buffers[n_buffers].start) {
			fprintf(stderr, "Out of memory\n");
			exit(EXIT_FAILURE);
		}
	}
}

void init_device(void)
{
	struct v4l2_capability cap;
	struct v4l2_cropcap cropcap;
	struct v4l2_crop crop;
	struct v4l2_format fmt;
	unsigned int min;

	if (-1 == xioctl(fd, VIDIOC_QUERYCAP, &cap)) {
		if (EINVAL == errno) {
			fprintf(stderr, "%s is no V4L2 device\n", dev_name);
			exit(EXIT_FAILURE);
		} else {
			errno_exit("VIDIOC_QUERYCAP");
		}
	}

	if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
		fprintf(stderr, "%s is no video capture device\n", dev_name);
		exit(EXIT_FAILURE);
	}

	switch (io) {
	case IO_METHOD_READ:
		if (!(cap.capabilities & V4L2_CAP_READWRITE)) {
			fprintf(stderr, "%s does not support read i/o\n", dev_name);
			exit(EXIT_FAILURE);
		}
		break;

	case IO_METHOD_MMAP:
	case IO_METHOD_USERPTR:
		if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
			fprintf(stderr, "%s does not support streaming i/o\n", dev_name);
			exit(EXIT_FAILURE);
		}
		break;
	}

	// Select video input, video standard and tune here
	CLEAR(cropcap);
	cropcap.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
	if (0 == xioctl(fd, VIDIOC_CROPCAP, &cropcap)) {
		crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
		crop.c = cropcap.defrect; // reset to default
		if (-1 == xioctl(fd, VIDIOC_S_CROP, &crop)) {
			switch (errno) {
			case EINVAL:
				// Cropping not supported
				break;
			default:
				// Errors ignored
				break;
			}
		}
	} else {
		// Errors ignored
	}

	CLEAR(fmt);
	fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
	if (force_format) {
		fmt.fmt.pix.width = width;
		fmt.fmt.pix.height = height;
		fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
		fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;

		// Note VIDIOC_S_FMT may change width and height
		if (-1 == xioctl(fd, VIDIOC_S_FMT, &fmt))
			errno_exit("VIDIOC_S_FMT");
	} else {
		// Preserve original settings as set by v4l2-ctl for example
		if (-1 == xioctl(fd, VIDIOC_G_FMT, &fmt))
			errno_exit("VIDIOC_G_FMT");
	}

	// Buggy driver paranoia
	min = fmt.fmt.pix.width * 2;
	if (fmt.fmt.pix.bytesperline < min)
		fmt.fmt.pix.bytesperline = min;
	min = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height;
	if (fmt.fmt.pix.sizeimage < min)
		fmt.fmt.pix.sizeimage = min;

	switch (io) {
	case IO_METHOD_READ:
		init_read(fmt.fmt.pix.sizeimage);
		break;

	case IO_METHOD_MMAP:
		init_mmap();
		break;

	case IO_METHOD_USERPTR:
		init_userp(fmt.fmt.pix.sizeimage);
		break;
	}
}

void close_device(void)
{
	if (-1 == close(fd))
		errno_exit("close");

	fd = -1;
}

void open_device(void)
{
	struct stat st;
	if (-1 == stat(dev_name, &st)) {
		fprintf(stderr, "Cannot identify '%s': %d, %s\n", dev_name, errno, strerror(errno));
		exit(EXIT_FAILURE);
	}

	if (!S_ISCHR(st.st_mode)) {
		fprintf(stderr, "%s is no devicen\n", dev_name);
		exit(EXIT_FAILURE);
	}

	fd = open(dev_name, O_RDWR | O_NONBLOCK, 0); // O_RDWR: required
	if (-1 == fd) {
		fprintf(stderr, "Cannot open '%s': %d, %s\n", dev_name, errno, strerror(errno));
		exit(EXIT_FAILURE);
	}
}

} // namespace

int test_v4l2_usb_stream()
{
	// reference: https://linuxtv.org/downloads/v4l-dvb-apis-new/uapi/v4l/capture.c.html
	dev_name = "/dev/video0";

	open_device();
	init_device();
	start_capturing();
	mainloop();
	stop_capturing();
	uninit_device();
	close_device();

	fprintf(stdout, "test finish\n");
	return 0;
}

#else

int test_v4l2_usb_stream()
{
	fprintf(stderr, "Error: this test code only support linux platform\n");
	return -1;
}

#endif

通过ffplay播放生成的usb.yuv文件，执行结果如下：

GitHub：https://github.com//fengbingchun/OpenCV_Test