V4L2摄像头获取单幅图片测试程序（MMAP模式）

#加了点注释

#Rockie Cheng

#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;
}