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;

}