Linux v4l2 一 应用层

Android Camera 一 源码路径

Android Camera 二 JNI JAVA和C/CPP图像数据传输流程分析

Android Camera 三 CameraService 和 Client 链接到 HAL

Android Camera 四 Camera HAL 分析

Linux v4l2 一 应用层

Linux v4l2 二 驱动和 usb 摄像头

 

v4l2 ： video for Linux version 2

本文转载： 和菜鸟一起学linux之V4L2摄像头应用流程

                   V4L2官方例程

从 0 开始学 v4l2 。在网上查阅很多资料，写 demo 采集摄像头的图像数据。感谢大牛们分享的技术博客。

这里做个小结，分享知识。

下面给出写 v4l2 应用程序常用的控制命令和结构体

// linux-3.10.y/include/uapi/linux/videodev2.h
// 常用的结构体
struct v4l2_requestbuffers  //申请帧缓冲，对应命令VIDIOC_REQBUFS
struct v4l2_capability      //视频设备的功能，对应命令VIDIOC_QUERYCAP
struct v4l2_input           //视频输入信息，对应命令VIDIOC_ENUMINPUT
struct v4l2_standard        //视频的制式，比如PAL，NTSC，对应命令VIDIOC_ENUMSTD
struct v4l2_format          //帧的格式，对应命令VIDIOC_G_FMT、VIDIOC_S_FMT等
struct v4l2_buffer          //驱动中的一帧图像缓存，对应命令VIDIOC_QUERYBUF
struct v4l2_crop            //视频信号矩形边框

// 常用的控制命令     IOCTL CODES FOR VIDEO DEVICES
VIDIOC_ENUM_FMT     /* 获取当前驱动支持的视频格式 */
VIDIOC_G_FMT        /* 获取设置支持的视频格式 */
VIDIOC_S_FMT        /* 设置捕获视频的格式 */
VIDIOC_TRY_FMT      /* 验证当前驱动的显示格式 */
VIDIOC_CROPCAP      /* 查询驱动的修剪能力 */
VIDIOC_S_CROP       /* 设置视频信号的边框 */ 
VIDIOC_G_CROP       /* 获取视频信号的边框 */
VIDIOC_QUERYSTD     /* 检查当前视频设备支持的标准，例如PAL或NTSC */
VIDIOC_QUERYCAP     /* 获取设备支持的操作 */
VIDIOC_REQBUFS      /* 向驱动提出申请内存的请求 */
VIDIOC_QUERYBUF     /* 向驱动查询申请到的内存 */
VIDIOC_QBUF         /* 将空闲的内存加入可捕获视频的队列 */
VIDIOC_DQBUF        /* 将已经捕获好视频的内存拉出已捕获视频的队列 */
VIDIOC_STREAMON     /* 打开视频流 */
VIDIOC_STREAMOFF    /* 关闭视频流 */
VIDIOC_QUERYCTRL    /* 查询驱动是否支持该命令 */
VIDIOC_G_CTRL       /* 获取当前命令值 */
VIDIOC_S_CTRL       /* 设置新的命令值 */
VIDIOC_G_TUNER      /* 获取调谐器信息 */
VIDIOC_S_TUNER      /* 设置调谐器信息 */
VIDIOC_G_FREQUENCY  /* 获取调谐器频率 */
VIDIOC_S_FREQUENCY  /* 设置调谐器频率 */

应用程序流程：

 

/*
 *  V4L2 video capture example
 *
 *  This program can be used and distributed without restrictions.
 */
 
#include 
#include 
#include 
#include 
 
#include              /* getopt_long() */
 
#include               /* low-level i/o */
#include 
#include 
#include 
#include 
#include 
#include 
#include 
#include 
 
#include           /* for videodev2.h */
 
#include 
 
#define CLEAR(x) memset (&(x), 0, sizeof (x))
 
typedef enum {
	IO_METHOD_READ, IO_METHOD_MMAP, IO_METHOD_USERPTR,
} io_method;
 
struct buffer {
	void * start;
	size_t length;
};
 
static char * dev_name = NULL;
static io_method io = IO_METHOD_MMAP;
static int fd = -1;
struct buffer * buffers = NULL;
static unsigned int n_buffers = 0;
 
FILE *fp;
char *filename = "test.yuv\0";
 
static void errno_exit(const char * s) {
	fprintf(stderr, "%s error %d, %s/n", s, errno, strerror(errno));
 
	exit(EXIT_FAILURE);
}
 
static int xioctl(int fd, int request, void * arg) {
	int r;
 
	do {
		r = ioctl(fd, request, arg);
	} while (-1 == r && EINTR == errno);
 
	return r;
}
 
static void process_image(const void * p, int size) {
	fwrite(p, size, 1, fp);
}
 
static 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.length);
 
		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.length);
 
		if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))
			errno_exit("VIDIOC_QBUF");
 
		break;
	}
 
	return 1;
}
 
static void mainloop(void) {
	unsigned int count;
 
	count = 100;
 
	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. */
		}
	}
}
 
static 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;
	}
}
 
static 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;
	}
}
 
static 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);
}
 
static void init_read(unsigned int buffer_size) {
	buffers = 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);
	}
}
 
static 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 mapping/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 = 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");
	}
}
 
static void init_userp(unsigned int buffer_size) {
	struct v4l2_requestbuffers req;
	unsigned int page_size;
 
	page_size = getpagesize();
	buffer_size = (buffer_size + page_size - 1) & ~(page_size - 1);
 
	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/o/n", dev_name);
			exit(EXIT_FAILURE);
		} else {
			errno_exit("VIDIOC_REQBUFS");
		}
	}
 
	buffers = 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 = memalign(/* boundary */page_size,
				buffer_size);
 
		if (!buffers[n_buffers].start) {
			fprintf(stderr, "Out of memory/n");
			exit(EXIT_FAILURE);
		}
	}
}
 
static 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;
	fmt.fmt.pix.width = 640;
	fmt.fmt.pix.height = 480;
	fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
	fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
 
	if (-1 == xioctl(fd, VIDIOC_S_FMT, &fmt))
		errno_exit("VIDIOC_S_FMT");
 
	/* Note VIDIOC_S_FMT may change width and height. */
 
	/* 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;
	}
}
 
static void close_device(void) {
	if (-1 == close(fd))
		errno_exit("close");
 
	fd = -1;
}
 
static 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 device/n", dev_name);
		exit(EXIT_FAILURE);
	}
 
	fd = open(dev_name, O_RDWR /* required */| O_NONBLOCK, 0);
 
	if (-1 == fd) {
		fprintf(stderr, "Cannot open '%s': %d, %s/n", dev_name, errno,
				strerror(errno));
		exit(EXIT_FAILURE);
	}
}
 
static void usage(FILE * fp, int argc, char ** argv) {
	fprintf(fp, "Usage: %s [options]/n/n"
			"Options:/n"
			"-d | --device name   Video device name [/dev/video]/n"
			"-h | --help          Print this message/n"
			"-m | --mmap          Use memory mapped buffers/n"
			"-r | --read          Use read() calls/n"
			"-u | --userp         Use application allocated buffers/n"
			"", argv[0]);
}
 
static const char short_options[] = "d:hmru";
 
static const struct option long_options[] = { { "device", required_argument,
		NULL, 'd' }, { "help", no_argument, NULL, 'h' }, { "mmap", no_argument,
		NULL, 'm' }, { "read", no_argument, NULL, 'r' }, { "userp", no_argument,
		NULL, 'u' }, { 0, 0, 0, 0 } };
 
int main(int argc, char ** argv) {
	dev_name = "/dev/video0";
 
	for (;;) {
		int index;
		int c;
 
		c = getopt_long(argc, argv, short_options, long_options, &index);
 
		if (-1 == c)
			break;
 
		switch (c) {
		case 0: /* getopt_long() flag */
			break;
 
		case 'd':
			dev_name = optarg;
			break;
 
		case 'h':
			usage(stdout, argc, argv);
			exit(EXIT_SUCCESS);
 
		case 'm':
			io = IO_METHOD_MMAP;
			break;
 
		case 'r':
			io = IO_METHOD_READ;
			break;
 
		case 'u':
			io = IO_METHOD_USERPTR;
			break;
 
		default:
			usage(stderr, argc, argv);
			exit(EXIT_FAILURE);
		}
	}
 
	open_device();
 
	init_device();
 
	start_capturing();
 
	fp = fopen(filename, "wa+");
	mainloop();
	fclose(fp);
 
	stop_capturing();
 
	uninit_device();
 
	close_device();
 
	exit(EXIT_SUCCESS);
 
	return 0;
}