    /*
     *  V4L2 video capture example
     *
     *  This program can be used and distributed without restrictions.
     *
     *      This program is provided with the V4L2 API
     * see https://linuxtv.org/docs.php for more information
     */

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

    #include <linux/videodev2.h>

    #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;
    }

    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. */
                    }
            }
    }

    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;

            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 = 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 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"
                     "Version 1.3\n"
                     "Options:\n"
                     "-d | --device name   Video device name [%s]\n"
                     "-h | --help          Print this message\n"
                     "-m | --mmap          Use memory mapped buffers [default]\n"
                     "-r | --read          Use read() calls\n"
                     "-u | --userp         Use application allocated buffers\n"
                     "-o | --output        Outputs stream to stdout\n"
                     "-f | --format        Force format to 640x480 YUYV\n"
                     "-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;
    }