目前，在學習v4l2視頻採集，在網上查找一些簡單的例子，在ubuntu12.04的pc機上能正常使用，使用的攝像頭為中星微攝像頭，在電腦上查詢的參數如下：
wu@wu-desktop:~$ lsusb
Bus 002 Device 002: ID 067b:2303 Prolific Technology, Inc. PL2303 Serial Port
Bus 002 Device 003: ID 1c4f:0002 SiGma Micro Keyboard TRACER Gamma Ivory
Bus 005 Device 002: ID 0ac8:301b Z-Star Microelectronics Corp. ZC0301 Webcam
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 005 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
所測試的網上代碼如下（各位也可以在自己的電腦上測試下）
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#include
#define CLEAR(x) memset (&(x), 0, sizeof (x))
struct buffer {
void * start;
size_t length;
};
static char * dev_name = "/dev/video0";//攝像頭設備名
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;
//////////////////////////////////////////////////////
//獲取一幀數據
//////////////////////////////////////////////////////
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;
/*8.出隊列以取得已採集數據的幀緩衝，取得原始採集數據。VIDIOC_DQBUF*/
int ff = ioctl(fd, VIDIOC_DQBUF, &buf);
if (ff < 0)
printf("failture\n"); //出列採集的幀緩衝
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); //將其寫入文件中
/*9.將緩衝重新入隊列尾,這樣可以循環採集。VIDIOC_QBUF*/
ff = ioctl(fd, VIDIOC_QBUF, &buf); //再將其入列
if (ff < 0)//把數據從緩存中讀取出來
printf("failture VIDIOC_QBUF\n");
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");//圖片文件名
/*1.打開設備文件。 int fd=open(」/dev/video0″,O_RDWR);*********/
fd = open(dev_name, O_RDWR /* required */| O_NONBLOCK, 0);//打開設備
/*2.取得設備的capability，看看設備具有什麼功能，比如是否具有視頻輸入,或者音頻輸入輸出等。VIDIOC_QUERYCAP,struct v4l2_capability*/
int ff = ioctl(fd, VIDIOC_QUERYCAP, &cap);//獲取攝像頭參數
if (ff < 0)
printf("failture VIDIOC_QUERYCAP\n");
/*3.設置視頻的制式和幀格式，制式包括PAL，NTSC，幀的格式個包括寬度和高度等。*/
struct v4l2_fmtdesc fmt1;
int ret;
memset(&fmt1, 0, sizeof(fmt1));
fmt1.index = 0;
fmt1.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
//獲取當前驅動支持的視頻格式
printf("we are here11\n");
while ((ret = ioctl(fd, VIDIOC_ENUM_FMT, &fmt1)) == 0)
{
fmt1.index++;
printf("{ pixelformat = '%c%c%c%c', description = '%s' }\n",
fmt1.pixelformat & 0xFF, (fmt1.pixelformat >> 8) & 0xFF,
(fmt1.pixelformat >> 16) & 0xFF,
(fmt1.pixelformat >> 24) & 0xFF, fmt1.description);
}
//幀的格式，比如寬度，高度等
CLEAR (fmt);
printf("we are here22\n");
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE; //數據流類型，必須永遠是V4L2_BUF_TYPE_VIDEO_CAPTURE
fmt.fmt.pix.width = 320;//640;//寬，必須是16的倍數
fmt.fmt.pix.height = 240;//480;////高，必須是16的倍數
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;//V4L2_PIX_FMT_JPEG;//視頻數據存儲類型//V4L2_PIX_FMT_YUYV;//V4L2_PIX_FMT_YVU420;//V4L2_PIX_FMT_YUYV;
//fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;//視頻數據存儲類型/
fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
//設置當前驅動的頻捕獲格式
ff = ioctl(fd, VIDIOC_S_FMT, &fmt);
if (ff < 0)
printf("failture VIDIOC_S_FMT\n");
//計算圖片大小
printf("we are here33\n");
file_length = fmt.fmt.pix.bytesperline * fmt.fmt.pix.height;
/*4.向驅動申請幀緩衝，一般不超過5個。struct v4l2_requestbuffers*/
struct v4l2_requestbuffers req;
CLEAR (req);
req.count = 4;//緩存數量，也就是說在緩存隊列里保持多少張照片
req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
req.memory = V4L2_MEMORY_MMAP;//或V4L2_MEMORY_USERPTR
ff = ioctl(fd, VIDIOC_REQBUFS, &req); //申請緩衝，count是申請的數量
if (ff < 0)
printf("failture VIDIOC_REQBUFS\n");
if (req.count < 2)
printf("Insufficient buffer memory\n");
buffers = (struct buffer*) calloc(req.count, sizeof(*buffers));//內存中建立對應空間
printf("we are here44\n");
/*5.將申請到的幀緩衝映射到用戶空間，這樣就可以直接操作採集到的幀了，而不必去複製。mmap*/
for (n_buffers = 0; n_buffers < req.count; ++n_buffers)
{
struct v4l2_buffer buf; //驅動中的一幀
CLEAR (buf);
buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
buf.memory = V4L2_MEMORY_MMAP;
buf.index = n_buffers;
//把VIDIOC_REQBUFS中分配的數據緩存轉換成物理地址
if (-1 == ioctl(fd, VIDIOC_QUERYBUF, &buf)) //映射用戶空間
printf("VIDIOC_QUERYBUF error\n");
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);//通過mmap建立映射關係,返回映射區的起始地址
if (MAP_FAILED == buffers[n_buffers].start)
printf("mmap failed\n");
}
printf("we are here55\n");
/*6.將申請到的幀緩衝全部入隊列，以便存放採集到的數據.VIDIOC_QBUF,struct v4l2_buffer*/
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;
//把數據從緩存中讀取出來
if (-1 == ioctl(fd, VIDIOC_QBUF, &buf))//申請到的緩衝進入列隊
printf("VIDIOC_QBUF failed\n");
}
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
/*7.開始視頻的採集。VIDIOC_STREAMON*/
if (-1 == ioctl(fd, VIDIOC_STREAMON, &type)) //開始捕捉圖像數據
printf("VIDIOC_STREAMON failed\n");
for (;;) //這一段涉及到非同步IO
{
fd_set fds;
struct timeval tv;
int r;
FD_ZERO(&fds);//將指定的?件描述符集清空
FD_SET(fd, &fds);//在文件描述符集合中增?????個新的文件描述符
/* Timeout. */
tv.tv_sec = 2;
tv.tv_usec = 0;
r = select(fd + 1, &fds, NULL, NULL, &tv);//判斷是否可讀（即攝像頭是否準備好），tv是定時
if (-1 == r)
{
if (EINTR == errno)
continue;
printf("select err\n");
}
if (0 == r) {
fprintf(stderr, "select timeout\n");
exit(EXIT_FAILURE);
}
if (read_frame())//如果可讀，執行read_frame ()函數，並跳出循環
break;
}
unmap:
for (i = 0; i < n_buffers; ++i)
if (-1 == munmap(buffers->start, buffers->length))
printf("munmap error");
type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
/*10.停止視頻的採集。VIDIOC_STREAMOFF*/
if (-1 == ioctl(fd, VIDIOC_STREAMOFF, &type))
printf("VIDIOC_STREAMOFF");
/*11.關閉視頻設備。close(fd);*/
close(fd);
fclose(file_fd);
exit(EXIT_SUCCESS);
return 0;
}
但是，將其交叉編譯後下載到開發板上，使用同樣的usb攝像頭，則不能正常產生圖片。在以下代碼處產生錯誤：
/*7.開始視頻的採集。VIDIOC_STREAMON*/
if (-1 == ioctl(fd, VIDIOC_STREAMON, &type)) //開始捕捉圖像數據
printf("VIDIOC_STREAMON failed\n");
到底是什麼原因呢，為什麼再pc機能正常識別，在arm9開發板就不能呢？是內核配置出問題啦？那為什麼能執行前面的代碼呢（video0已經產生了啊）麻煩各位指點，不甚感激！！！ 
