/dev/video设备的操作流程（可作为摄像头设备和IP核的输入输出接口设备）

最近一直都在编写Zynq的双目立体视觉的相关代码，简单总结一下video设备的操作流程，下面代码是从官方历程中摘取下来的，如果需要详细的了解可以去该网站查看：https://linuxtv.org/downloads/v4l-dvb-apis/uapi/v4l/userp.html

以V4L2的操作来完成摄像头的打开（主要分为V4L2_MEMORY_MMAP和V4L2_MEMORY_USERPTR） 可以从以下代码中详细看出两种方式的操作流程。

    #include 
    #include 
    #include 
    #include 

    #include              /* getopt_long() */

    #include               /* low-level i/o */
    #include 
    #include 
    #include 
    #include 
    #include 
    #include 
    #include 

    #include 

    #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;
    };

    static char            *dev_name;
    static enum io_method   io = IO_METHOD_MMAP;
    static int              fd = -1;
    struct buffer          *buffers;
    static unsigned int     n_buffers;
    static int              out_buf;
    static int              force_format;
    static int              frame_count = 70;

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

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

	    return r;
    }

	
	//process_image(数据指针，大小)
    static void process_image(const void *p, int size)
    {
	    if (out_buf)
		    fwrite(p, size, 1, stdout);

	    fflush(stderr);
	    fprintf(stderr, ".");
	    fflush(stdout);
    }

    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.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;
    }

    static void mainloop(void)
    {
	    unsigned int count;

	    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. */
		    }
	    }
    }

	
	//VIDIOC_STREAMOFF
    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;
	    }
    }

	
	//VIDIOC_STREAMON
    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;
	    }
    }

	
	//释放申请的内存
    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 mappingn", 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;

	    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 = 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);
		    }
	    }
    }

    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;
	    if (force_format) {
		    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. */
	    } 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;
	    }
    }

    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 devicen", 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"
		     "Version 1.3\\n"
		     "Options:\\n"
		     "-d | --device name   Video device name [%s]n"
		     "-h | --help          Print this messagen"
		     "-m | --mmap          Use memory mapped buffers [default]n"
		     "-r | --read          Use read() callsn"
		     "-u | --userp         Use application allocated buffersn"
		     "-o | --output        Outputs stream to stdoutn"
		     "-f | --format        Force format to 640x480 YUYVn"
		     "-c | --count         Number of frames to grab [%i]n"
		     "",
		     argv[0], dev_name, frame_count);
    }

    static const char short_options[] = "d:hmruofc:";

    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' },
	    { "output", no_argument,       NULL, 'o' },
	    { "format", no_argument,       NULL, 'f' },
	    { "count",  required_argument, NULL, 'c' },
	    { 0, 0, 0, 0 }
    };

    int main(int argc, char **argv)
    {
	    dev_name = "/dev/video0";

	    for (;;) {
		    int idx;
		    int c;

		    c = getopt_long(argc, argv,
				    short_options, long_options, &idx);

		    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;

		    case 'o':
			    out_buf++;
			    break;

		    case 'f':
			    force_format++;
			    break;

		    case 'c':
			    errno = 0;
			    frame_count = strtol(optarg, NULL, 0);
			    if (errno)
				    errno_exit(optarg);
			    break;

		    default:
			    usage(stderr, argc, argv);
			    exit(EXIT_FAILURE);
		    }
	    }

	    open_device();
	    init_device();
	    start_capturing();
	    mainloop();
	    stop_capturing();
	    uninit_device();
	    close_device();
	    fprintf(stderr, "\\n");
	    return 0;
    }