V4L2摄像头获取单幅图片测试程序（MMAP模式）

#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 = 2;   tv.tv_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;    printf ("select err\n");                        }   if (0 == r) {    fprintf (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;}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;}