V4L2简单取图片范例

#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, buf.bytesused, 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 = 320;
    fmt.fmt.pix.height = 240;
    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 (i = 0; i < 1; i++) //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;
}

转载自：http://daimajishu.iteye.com/blog/1085484