Documentation/DocBook/media/v4l/capture.c.xml - kernel/quantenna - Git at Google

 <programlisting>
 /*
  *  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 &lt;stdio.h&gt;
 #include &lt;stdlib.h&gt;
 #include &lt;string.h&gt;
 #include &lt;assert.h&gt;

 #include &lt;getopt.h&gt;             /* getopt_long() */

 #include &lt;fcntl.h&gt;              /* low-level i/o */
 #include &lt;unistd.h&gt;
 #include &lt;errno.h&gt;
 #include &lt;sys/stat.h&gt;
 #include &lt;sys/types.h&gt;
 #include &lt;sys/time.h&gt;
 #include &lt;sys/mman.h&gt;
 #include &lt;sys/ioctl.h&gt;

 #include &lt;linux/videodev2.h&gt;

 #define CLEAR(x) memset(&amp;(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 &amp;&amp; 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 <link linkend="v4l2-buffer">v4l2_buffer</link> 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, &amp;buf)) {
                         switch (errno) {
                         case EAGAIN:
                                 return 0;

                         case EIO:
                                 /* Could ignore EIO, see spec. */

                                 /* fall through */

                         default:
                                 errno_exit("VIDIOC_DQBUF");
                         }
                 }

                 assert(buf.index &lt; n_buffers);

                 process_image(buffers[buf.index].start, buf.bytesused);

                 if (-1 == xioctl(fd, VIDIOC_QBUF, &amp;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, &amp;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 &lt; n_buffers; ++i)
                         if (buf.m.userptr == (unsigned long)buffers[i].start
                             &amp;&amp; buf.length == buffers[i].length)
                                 break;

                 assert(i &lt; n_buffers);

                 process_image((void *)buf.m.userptr, buf.bytesused);

                 if (-1 == xioctl(fd, VIDIOC_QBUF, &amp;buf))
                         errno_exit("VIDIOC_QBUF");
                 break;
         }

         return 1;
 }

 static void mainloop(void)
 {
         unsigned int count;

         count = frame_count;

         while (count-- &gt; 0) {
                 for (;;) {
                         fd_set fds;
                         struct timeval tv;
                         int r;

                         FD_ZERO(&amp;fds);
                         FD_SET(fd, &amp;fds);

                         /* Timeout. */
                         tv.tv_sec = 2;
                         tv.tv_usec = 0;

                         r = select(fd + 1, &amp;fds, NULL, NULL, &amp;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 <link linkend="v4l2-buf-type">v4l2_buf_type</link> 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, &amp;type))
                         errno_exit("VIDIOC_STREAMOFF");
                 break;
         }
 }

 static void start_capturing(void)
 {
         unsigned int i;
         enum <link linkend="v4l2-buf-type">v4l2_buf_type</link> type;

         switch (io) {
         case IO_METHOD_READ:
                 /* Nothing to do. */
                 break;

         case IO_METHOD_MMAP:
                 for (i = 0; i &lt; n_buffers; ++i) {
                         struct <link linkend="v4l2-buffer">v4l2_buffer</link> buf;

                         CLEAR(buf);
                         buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                         buf.memory = V4L2_MEMORY_MMAP;
                         buf.index = i;

                         if (-1 == xioctl(fd, VIDIOC_QBUF, &amp;buf))
                                 errno_exit("VIDIOC_QBUF");
                 }
                 type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                 if (-1 == xioctl(fd, VIDIOC_STREAMON, &amp;type))
                         errno_exit("VIDIOC_STREAMON");
                 break;

         case IO_METHOD_USERPTR:
                 for (i = 0; i &lt; n_buffers; ++i) {
                         struct <link linkend="v4l2-buffer">v4l2_buffer</link> 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, &amp;buf))
                                 errno_exit("VIDIOC_QBUF");
                 }
                 type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                 if (-1 == xioctl(fd, VIDIOC_STREAMON, &amp;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 &lt; n_buffers; ++i)
                         if (-1 == munmap(buffers[i].start, buffers[i].length))
                                 errno_exit("munmap");
                 break;

         case IO_METHOD_USERPTR:
                 for (i = 0; i &lt; 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 <link linkend="v4l2-requestbuffers">v4l2_requestbuffers</link> req;

         CLEAR(req);

         req.count = 4;
         req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
         req.memory = V4L2_MEMORY_MMAP;

         if (-1 == xioctl(fd, VIDIOC_REQBUFS, &amp;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 &lt; 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 &lt; req.count; ++n_buffers) {
                 struct <link linkend="v4l2-buffer">v4l2_buffer</link> 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, &amp;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 <link linkend="v4l2-requestbuffers">v4l2_requestbuffers</link> req;

         CLEAR(req);

         req.count  = 4;
         req.type   = V4L2_BUF_TYPE_VIDEO_CAPTURE;
         req.memory = V4L2_MEMORY_USERPTR;

         if (-1 == xioctl(fd, VIDIOC_REQBUFS, &amp;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 &lt; 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 <link linkend="v4l2-capability">v4l2_capability</link> cap;
         struct <link linkend="v4l2-cropcap">v4l2_cropcap</link> cropcap;
         struct <link linkend="v4l2-crop">v4l2_crop</link> crop;
         struct <link linkend="v4l2-format">v4l2_format</link> fmt;
         unsigned int min;

         if (-1 == xioctl(fd, VIDIOC_QUERYCAP, &amp;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 &amp; 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 &amp; 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 &amp; 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, &amp;cropcap)) {
                 crop.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
                 crop.c = cropcap.defrect; /* reset to default */

                 if (-1 == xioctl(fd, VIDIOC_S_CROP, &amp;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, &amp;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, &amp;fmt))
                         errno_exit("VIDIOC_G_FMT");
         }

         /* Buggy driver paranoia. */
         min = fmt.fmt.pix.width * 2;
         if (fmt.fmt.pix.bytesperline &lt; min)
                 fmt.fmt.pix.bytesperline = min;
         min = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height;
         if (fmt.fmt.pix.sizeimage &lt; 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, &amp;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, &amp;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;
 }
 </programlisting>
	<programlisting>
	/*
	* 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 <link linkend="v4l2-buffer">v4l2_buffer</link> 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 <link linkend="v4l2-buf-type">v4l2_buf_type</link> 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 <link linkend="v4l2-buf-type">v4l2_buf_type</link> type;

	switch (io) {
	case IO_METHOD_READ:
	/* Nothing to do. */
	break;

	case IO_METHOD_MMAP:
	for (i = 0; i < n_buffers; ++i) {
	struct <link linkend="v4l2-buffer">v4l2_buffer</link> 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 <link linkend="v4l2-buffer">v4l2_buffer</link> 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 <link linkend="v4l2-requestbuffers">v4l2_requestbuffers</link> 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 <link linkend="v4l2-buffer">v4l2_buffer</link> 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 <link linkend="v4l2-requestbuffers">v4l2_requestbuffers</link> 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 <link linkend="v4l2-capability">v4l2_capability</link> cap;
	struct <link linkend="v4l2-cropcap">v4l2_cropcap</link> cropcap;
	struct <link linkend="v4l2-crop">v4l2_crop</link> crop;
	struct <link linkend="v4l2-format">v4l2_format</link> 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;
	}
	</programlisting>