V4L2官方例程

V4l2的官网例程代码：

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

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

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

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

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

#include <linux/videodev2.h>

#define CLEAR(x) memset (&(x), 0, sizeof (x))

typedef enum {
	IO_METHOD_READ, IO_METHOD_MMAP, IO_METHOD_USERPTR,
} io_method;

struct buffer {
	void * start;
	size_t length;
};

static char * dev_name = NULL;
static io_method io = IO_METHOD_MMAP;
static int fd = -1;
struct buffer * buffers = NULL;
static unsigned int n_buffers = 0;

FILE *fp;
char *filename = "test.yuv\0";

static void errno_exit(const char * s) {
	fprintf(stderr, "%s error %d, %s/n", s, errno, strerror(errno));

	exit(EXIT_FAILURE);
}

static int xioctl(int fd, int request, void * arg) {
	int r;

	do {
		r = ioctl(fd, request, arg);
	} while (-1 == r && EINTR == errno);

	return r;
}

static void process_image(const void * p, int size) {
	fwrite(p, size, 1, fp);
}

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.length);

		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 (errn