V4L2 mmap is failed

基于国嵌的H264的那个v4l项目，本人想改为V4L2，但是在板子上显示：
USB_video is OK
VIDIOC_S_FMT is OK
VIDIOC_REQBUFS is OK
buffers is OK
VIDIOC_QUERYBUF is OK
the buf length is 32768
the buf offset is 0
mmap is failed
VIDIOC_QBUF is OK
zc3xx: probe 2wr ov vga 0x0000
VIDIOC_STREAMON is OK
VIDIOC_DQBUF is OK
Segmentation fault

发现原来是mmap没有成功，原代码部分如下：
void init_video() /* bpp == bytes per pixel*/
{
   int ret;
/*设备的打开*/
   cam_fd = open( USB_VIDEO, O_NONBLOCK);
  if( cam_fd<0 )
  printf("Can't open video device\n");
else
printf("USB_video is OK\n");

  CLEAR (fmt);
  fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
  fmt.fmt.pix.width = 100;//帧宽

  fmt.fmt.pix.height = 100;//帧高

  fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;//像素格式

  fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;//隔行扫描
  //设置当前驱动的视频捕获格式
  if (-1 == ioctl (cam_fd, VIDIOC_S_FMT, &fmt))
  errno_exit ("VIDIOC_S_FMT");
else
  printf("VIDIOC_S_FMT is OK\n");
struct v4l2_requestbuffers req;
  CLEAR (req);
req.count = 1;//在缓存队列里保存多少张照片
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;//内存映射
/*向驱动申请帧缓冲，一般不超过5个。*/
if (-1 == ioctl (cam_fd, VIDIOC_REQBUFS, &req))
{
errno_exit ("VIDIOC_REQBUFS");
}
else
printf("VIDIOC_REQBUFS is OK\n");
buffers = calloc (req.count, sizeof (*buffers));//分配连续的内存给刚才申请的缓冲区，并且初始化为0，返回值是首地址（物理地址）
if (!buffers) {
fprintf (stderr, "Out of memory\n");
exit (EXIT_FAILURE);
}
else
printf("buffers is OK\n");

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;//缓冲区序号
if (-1 == ioctl (cam_fd, VIDIOC_QUERYBUF, &buf))//把数据缓存转换成物理地址
errno_exit ("VIDIOC_QUERYBUF");
else
printf("VIDIOC_QUERYBUF is OK\n");
printf("the buf length is %d\n",buf.length);
printf("the buf offset is %d\n",buf.m.offset);
buffers[n_buffers].length = buf.length;
buffers[n_buffers].offset = (size_t) buf.m.offset;
//buffers[n_buffers].start= (char*)mmap (NULL, buf.length,PROT_READ | PROT_WRITE, MAP_SHARED, cam_fd, buf.m.offset);//确定被映射的内存地址
buffers[n_buffers].start = mmap(NULL, buf.length, PROT_READ | PROT_WRITE,
                        MAP_SHARED, cam_fd, buf.m.offset);
if (MAP_FAILED == buffers[n_buffers].start)
printf("mmap is failed\n");
else
printf("mmap is OK\n");
InitLookupTable();
}
}

求高人指教，究竟哪个地方错了！！！！

试下这段代码吧

1. for (n_buffers = 0; n_buffers < req.count; ++n_buffers)
   
2.         {
   
3.                 struct v4l2_buffer buf;
   
4. 
   
5.                 CLEAR(buf);
   
6. 
   
7.                 buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
   
8.                 buf.memory = V4L2_MEMORY_MMAP;
   
9.                 buf.index = n_buffers;
   
10. 
    
11.                 if (-1 == xioctl(m_iFd, VIDIOC_QUERYBUF, &buf))
    
12.                 {
    
13.                         errno_exit("VIDIOC_QUERYBUF");
    
14.                         return false;
    
15.                 }
    
16. 
    
17.                 buffers[n_buffers].length = buf.length;
    
18.                 buffers[n_buffers].start = (unsigned char *) mmap(
    
19.                                 NULL /* start anywhere */, buf.length,
    
20.                                 PROT_READ | PROT_WRITE /* required */,
    
21.                                 MAP_SHARED /* recommended */, m_iFd, buf.m.offset);
    
22. 
    
23.                 if (MAP_FAILED == buffers[n_buffers].start)
    
24.                         return false;
    
25.         }

复制代码

好的我试一下

:'(:'(哎还是如此

the buf length is 32768
这个buf.length值好大

TQ那个usb摄像头测试程序里面的buf.length 才68

已解决结贴

把如何解决的贴上分享一下，大家营造一个良好的学习，交流环境

buffers[n_buffers].start =

(char*)mmap (NULL /* 映射起始地址，NULL内核自动选择 */,

buf.length,//被映射内存块的长度

PROT_EXEC|PROT_READ | PROT_WRITE /*标志映射后能否被读写 */,

MAP_PRIVATE /* 可以被其他进城共享 */,

fd, buf.m.offset);//确定被映射的内存地址

这样就可以了，但是程序还是有问题，有段错误，我是把国嵌的v4l改成v4l2的，搞了好几天了，没成果啊

安仔 发表于 2011-12-26 22:43
buffers[n_buffers].start =

(char*)mmap (NULL /* 映射起始地址，NULL内核自动选择 */,

问下，楼主你这程序可以捕获单幅图像吗？

wyz365889 发表于 2012-3-6 14:35
问下，楼主你这程序可以捕获单幅图像吗？

亲，从摄像头来的数据都是一帧一帧的（也就是单幅的），只不过速度快，就变成了动态的了。