Video for Linux Two API Specification: Revision 0.24
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# Appendix B. Video Capture Example

    /*
     *  V4L2 video capture example
     *
     *  This program can be used and distributed without restrictions.
     */

    #include <stdio.h>
    #include <stdlib.h>
    #include <string.h>
    #include <assert.h>

    #include <getopt.h>             /* getopt_long() */

    #include <fcntl.h>              /* low-level i/o */
    #include <unistd.h>
    #include <errno.h>
    #include <stdlib.h>
    #include <sys/stat.h>
    #include <sys/types.h>
    #include <sys/time.h>
    #include <sys/mman.h>
    #include <sys/ioctl.h>

    #include <asm/types.h>          /* for videodev2.h */

    #include <linux/videodev2.h>

    #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;

    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)
    {
            fputc ('.', stdout);
            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);

                    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);

                    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);

                    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/video";

            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 ();

            mainloop ();

            stop_capturing ();

            uninit_device ();

            close_device ();

            exit (EXIT_SUCCESS);

            return 0;
    }

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