帮我写一个v4l2程序采用userptr的方式

好的，这是一个简单的采用userptr方式的v4l2程序示例：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <sys/mman.h>
#include <linux/videodev2.h>

#define WIDTH 640
#define HEIGHT 480
#define BUFFER_COUNT 4

struct buffer {
    void *start;
    size_t length;
};

int main(int argc, char **argv)
{
    int fd;
    struct v4l2_capability cap;
    struct v4l2_format fmt;
    struct v4l2_requestbuffers req;
    struct buffer buffers[BUFFER_COUNT];
    enum v4l2_buf_type type;
    int i, j;

    /* 打开设备 */
    fd = open("/dev/video0", O_RDWR);
    if (fd == -1) {
        perror("open");
        exit(EXIT_FAILURE);
    }

    /* 查询设备信息 */
    if (ioctl(fd, VIDIOC_QUERYCAP, &cap) == -1) {
        perror("ioctl(VIDIOC_QUERYCAP)");
        exit(EXIT_FAILURE);
    }

    /* 查询支持的格式 */
    memset(&fmt, 0, sizeof(fmt));
    fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    if (ioctl(fd, VIDIOC_G_FMT, &fmt) == -1) {
        perror("ioctl(VIDIOC_G_FMT)");
        exit(EXIT_FAILURE);
    }

    /* 设置格式 */
    fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    fmt.fmt.pix.width = WIDTH;
    fmt.fmt.pix.height = HEIGHT;
    fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_YUYV;
    fmt.fmt.pix.field = V4L2_FIELD_INTERLACED;
    if (ioctl(fd, VIDIOC_S_FMT, &fmt) == -1) {
        perror("ioctl(VIDIOC_S_FMT)");
        exit(EXIT_FAILURE);
    }

    /* 请求缓冲区 */
    memset(&req, 0, sizeof(req));
    req.count = BUFFER_COUNT;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_USERPTR;
    if (ioctl(fd, VIDIOC_REQBUFS, &req) == -1) {
        perror("ioctl(VIDIOC_REQBUFS)");
        exit(EXIT_FAILURE);
    }

    /* 映射缓冲区 */
    for (i = 0; i < BUFFER_COUNT; i++) {
        buffers[i].length = WIDTH * HEIGHT * 2;
        buffers[i].start = mmap(NULL, buffers[i].length, PROT_READ | PROT_WRITE, MAP_SHARED | MAP_ANONYMOUS, -1, 0);
        if (buffers[i].start == MAP_FAILED) {
            perror("mmap");
            exit(EXIT_FAILURE);
        }
    }

    /* 将缓冲区加入采集队列 */
    for (i = 0; i < BUFFER_COUNT; i++) {
        struct v4l2_buffer buf;
        memset(&buf, 0, sizeof(buf));
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_USERPTR;
        buf.index = i;
        buf.m.userptr = (unsigned long)buffers[i].start;
        buf.length = buffers[i].length;
        if (ioctl(fd, VIDIOC_QBUF, &buf) == -1) {
            perror("ioctl(VIDIOC_QBUF)");
            exit(EXIT_FAILURE);
        }
    }

    /* 开始采集 */
    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    if (ioctl(fd, VIDIOC_STREAMON, &type) == -1) {
        perror("ioctl(VIDIOC_STREAMON)");
        exit(EXIT_FAILURE);
    }

    /* 采集图像 */
    for (i = 0; i < 10; i++) {
        fd_set fds;
        struct timeval tv;
        int r;
        FD_ZERO(&fds);
        FD_SET(fd, &fds);
        tv.tv_sec = 2;
        tv.tv_usec = 0;
        r = select(fd + 1, &fds, NULL, NULL, &tv);
        if (r == -1) {
            perror("select");
            exit(EXIT_FAILURE);
        }
        if (r == 0) {
            fprintf(stderr, "select timeout\n");
            exit(EXIT_FAILURE);
        }

        /* 取出采集到的图像 */
        for (j = 0; j < BUFFER_COUNT; j++) {
            struct v4l2_buffer buf;
            memset(&buf, 0, sizeof(buf));
            buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
            buf.memory = V4L2_MEMORY_USERPTR;
            buf.index = j;
            if (ioctl(fd, VIDIOC_DQBUF, &buf) == -1) {
                perror("ioctl(VIDIOC_DQBUF)");
                exit(EXIT_FAILURE);
            }
            printf("frame %d, buffer %d\n", i, j);

            /* 处理图像 */
            // TODO: 在这里处理图像数据

            /* 将缓冲区重新加入采集队列 */
            if (ioctl(fd, VIDIOC_QBUF, &buf) == -1) {
                perror("ioctl(VIDIOC_QBUF)");
                exit(EXIT_FAILURE);
            }
        }
    }

    /* 停止采集 */
    if (ioctl(fd, VIDIOC_STREAMOFF, &type) == -1) {
        perror("ioctl(VIDIOC_STREAMOFF)");
        exit(EXIT_FAILURE);
    }

    /* 释放缓冲区 */
    for (i = 0; i < BUFFER_COUNT; i++) {
        if (munmap(buffers[i].start, buffers[i].length) == -1) {
            perror("munmap");
            exit(EXIT_FAILURE);
        }
    }

    /* 关闭设备 */
    if (close(fd) == -1) {
        perror("close");
        exit(EXIT_FAILURE);
    }

    return 0;
}

在该示例程序中，我们使用了`VIDIOC_REQBUFS`命令和`V4L2_MEMORY_USERPTR`内存类型来请求采集缓冲区，并使用`mmap`函数将缓冲区映射到用户空间。在采集图像时，我们使用`VIDIOC_DQBUF`命令将采集到的图像数据从缓冲区中取出，并在处理完数据后重新将缓冲区加入采集队列。