Documentation/userspace-api/media/v4l/capture.c.rst - linux - Git at Google

 .. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later

 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 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;
     }
	.. SPDX-License-Identifier: GFDL-1.1-no-invariants-or-later

	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 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;
	}