android/linux usb uvc 摄像头 图像花屏 select函数select timeout错误

#include <stdio.h>

#include <stdlib.h>

#include <string.h>

#include <assert.h> 

#include <getopt.h>             

#include <fcntl.h>             

#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>         

#include <linux/videodev2.h>

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

struct buffer {

        void *                  start;

        size_t                  length;

};



static char *           dev_name        = "/dev/video0";//摄像头设备名

static int              fd              = -1;

struct buffer *         buffers         = NULL;

static unsigned int     n_buffers       = 0;

FILE *file_fd;

static unsigned long file_length;

static unsigned char *file_name;



//////////////////////////////////////////////////////

//获取一帧数据

//////////////////////////////////////////////////////

static int read_frame (void)

{

	struct v4l2_buffer buf;

	unsigned int i;

	CLEAR (buf);

	buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

	buf.memory = V4L2_MEMORY_MMAP;

	ioctl (fd, VIDIOC_DQBUF, &buf); //出列采集的帧缓冲

	assert (buf.index < n_buffers);

	

	printf ("buf.index dq is %d,\n",buf.index);

	fwrite(buffers[buf.index].start, buffers[buf.index].length, 1, file_fd); //将其写入文件中

  

	ioctl (fd, VIDIOC_QBUF, &buf); //再将其入列

	return 1;

}



int main (int argc,char ** argv)

{

	struct v4l2_capability cap; 

	struct v4l2_format fmt;

	unsigned int i;

	enum v4l2_buf_type type;

	

	file_fd = fopen("test-mmap.jpg", "w");//图片文件名

	fd = open (dev_name, O_RDWR /* required */ | O_NONBLOCK, 0);//打开设备

	ioctl (fd, VIDIOC_QUERYCAP, &cap);//获取摄像头参数

	CLEAR (fmt);

	fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

	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;

	ioctl (fd, VIDIOC_S_FMT, &fmt); //设置图像格式

	

	file_length = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height; //计算图片大小

	

	struct v4l2_requestbuffers req;

	CLEAR (req);

	req.count = 4;

	req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

	req.memory = V4L2_MEMORY_MMAP;

	ioctl (fd, VIDIOC_REQBUFS, &req); //申请缓冲，count是申请的数量

	

	if (req.count < 2)

		printf("Insufficient buffer memory\n");

		

	buffers = calloc (req.count, sizeof (*buffers));//内存中建立对应空间

	for (n_buffers = 0; n_buffers < req.count; ++n_buffers) 

	{

		struct v4l2_buffer buf;   //驱动中的一帧

		

		CLEAR (buf);

		buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

		buf.memory = V4L2_MEMORY_MMAP;

		buf.index = n_buffers;

		

		if (-1 == ioctl (fd, VIDIOC_QUERYBUF, &buf)) //映射用户空间

			printf ("VIDIOC_QUERYBUF error\n");



		buffers[n_buffers].length = buf.length;

		buffers[n_buffers].start =

			mmap (NULL /* start anywhere */,    //通过mmap建立映射关系

			buf.length,

			PROT_READ | PROT_WRITE /* required */,

			MAP_SHARED /* recommended */,

			fd, buf.m.offset);

		

		if (MAP_FAILED == buffers[n_buffers].start)

			printf ("mmap failed\n");

	}



	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 == ioctl (fd, VIDIOC_QBUF, &buf))//申请到的缓冲进入列队

			printf ("VIDIOC_QBUF failed\n");

	}

            

	type = V4L2_BUF_TYPE_VIDEO_CAPTURE;

	

	if (-1 == ioctl (fd, VIDIOC_STREAMON, &type)) //开始捕捉图像数据

		printf ("VIDIOC_STREAMON failed\n");

	

	for (;;) //这一段涉及到异步IO

	{

		fd_set fds;

		struct timeval tv;

		int r;

		FD_ZERO (&fds);//将指定的文件描述符集清空

		FD_SET (fd, &fds);//在文件描述符集合中增加一个新的文件描述符

		

		/* Timeout. */

		tv.tv_sec = 10;

		tv.tv_usec = 500;

		r = select (fd + 1, &fds, NULL, NULL, &tv);//判断是否可读（即摄像头是否准备好），tv是定时

		

		if (-1 == r) {

			if (EINTR == errno)

				continue;

				

			printf ("select err\n");

        }

        

		if (0 == r) {

			fprintf (stderr, "select timeout\n");//这里出错			exit (EXIT_FAILURE);

        }



		if (read_frame ())//如果可读，执行read_frame ()函数，并跳出循环

			break;

	}

	

	unmap:

		for (i = 0; i < n_buffers; ++i)

			if (-1 == munmap (buffers[i].start, buffers[i].length))

				printf ("munmap error");



	close (fd);

	fclose (file_fd);

	exit (EXIT_SUCCESS);

	return 0;

}