.. Permission is granted to copy, distribute and/or modify this
.. document under the terms of the GNU Free Documentation License,
.. Version 1.1 or any later version published by the Free Software
.. Foundation, with no Invariant Sections, no Front-Cover Texts
.. and no Back-Cover Texts. A copy of the license is included at
.. Documentation/media/uapi/fdl-appendix.rst.
..
.. TODO: replace it to GFDL-1.1-or-later WITH no-invariant-sections

file: media/v4l/capture.c
=========================

.. code-block:: c

    /*
     *  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 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;
    }