V4L2应用程序详解

/** 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> /*getpid()*/#include <error.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;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){FILE* fp;fp = fopen("test","w+"); //w+：打开可读写文件，若文件存在则文件长度清为零，即该文件内容会消失。

若文件不存在则建立该文件。

fputs(p, fp); //成功，返回一个非负整数；出错，返回EOF。

fputs(const char *s,FILE *stream)作用：向指定文件写入一个字符串，缓冲区S中保存的是以‘/0’结尾的字符串，fputs将该字符串写入stream，但不写入结尾的’/0’。

fclose(fp);// fflush(stdout); // fflush(stdout)：清空输出缓冲区，并把缓冲区内容输出}static int read_frame(void)//用来读取一帧数据{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: //当使用不可阻断（O_NONBLOCK）打开文件后，read呼叫无可读取的数据return 0;case EIO: //设备文件读取文件时发生输出输入错误/* Could ignore EIO, see spec. *//* fall through */default:errno_exit("read");}}process_image(buffers[0].start); //保存读到的数据break;/* V4L2有一个数据缓存，存放req.count数量的缓存数据。

数据缓存采用FIFO的方式，当应用程序调用缓存数据时，缓存队列将最先采集到的视频数据缓存送出，并重新采集一张视频数据。

这个过程需要用到两个ioctl命令,VIDIOC_DQBUF和VIDIOC_QBUF*/ case IO_METHOD_MMAP:CLEAR (buf);buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;buf.memory = V4L2_MEMORY_MMAP;/*出队列以取得采集数据的帧缓冲，取得原始的采集数据*/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); //assert的作用是计算括号内的表达式，如果其值为0，就先向stderr打印一条出错信息，然后通过abort来终止程序运行process_image(buffers[buf.index].start);if (-1 == xioctl(fd, VIDIOC_QBUF, &buf)) //重新放入缓存队列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 (erptr == (unsigned long) buffers[i].start&& buf.length == buffers[i].length)break;assert(i < n_buffers);process_image((void *) erptr);if (-1 == xioctl(fd, VIDIOC_QBUF, &buf))errno_exit("VIDIOC_QBUF");break;}return 1;}/*完成图像的连续采集*/4. static void mainloop(void){unsigned int count;count = 100;while (count-- > 0){for (;;){fd_set fds;struct timeval tv;int r;FD_ZERO(&fds); //初始化fd_set结构体变量FD_SET(fd, &fds);//在fd_set结构体上注册文件描述/* Timeout. */_sec = 2; //时间“2S”_usec = 0;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;errno_exit("select");}if (0 == r) { //时间到，返回0fprintf(stderr, "select timeout\n");exit(EXIT_FAILURE);}if (read_frame()) //如果可读，执行read_frame函数，并跳出循环break;/* EAGAIN - continue select loop. */}}}5. static void stop_capturing(void)//停止视频的采集{enum v4l2_buf_type 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;}}3. static void start_capturing(void) //开始视频采集{unsigned int i;enum v4l2_buf_type type;switch (io){case IO_METHOD_READ:/* Nothing to do. */break;case IO_METHOD_MMAP: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 == xioctl(fd, VIDIOC_QBUF, &buf)) //申请到的缓冲进入列队errno_exit("VIDIOC_QBUF");}/*for循环的作用是：将申请到的帧缓冲全部入队列，以便存放采集到的数据.VIDIOC_QBUF,struct v4l2_buffer*/type = V4L2_BUF_TYPE_VIDEO_CAPTURE;if (-1 == xioctl(fd, VIDIOC_STREAMON, &type)) //开始捕捉图像数据errno_exit("VIDIOC_STREAMON");break;case IO_METHOD_USERPTR:for (i = 0; i < n_buffers; ++i){struct v4l2_buffer buf;CLEAR(buf);buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;buf.memory = V4L2_MEMORY_USERPTR;buf.index = i;erptr = (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;}}6. static void uninit_device(void)//注销设备文件{unsigned int i;switch (io) {case IO_METHOD_READ:free(buffers[0].start); //原型void free(void *ptr);参数ptr为指向先前由malloc()、calloc()或realloc()所返回的内存指针。