你可以帮我写个v4l2框架的camera取流代码

当然可以，以下是一个简单的v4l2框架的camera取流代码示例：

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

#define CAMERA_DEVICE "/dev/video0"
#define CAPTURE_FILE "frame.raw"
#define IMAGE_WIDTH 640
#define IMAGE_HEIGHT 480
#define BUFFER_COUNT 4

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

struct buffer buffers[BUFFER_COUNT];

int main(int argc, char **argv) {
    int fd;
    struct v4l2_capability cap;
    struct v4l2_format fmt;
    struct v4l2_requestbuffers req;
    enum v4l2_buf_type type;
    fd_set fds;
    struct timeval tv;
    int r, i, j;
    char *p;
    size_t n;
    FILE *fp;

    fd = open(CAMERA_DEVICE, O_RDWR);
    if (fd == -1) {
        perror("open");
        return EXIT_FAILURE;
    }

    if (ioctl(fd, VIDIOC_QUERYCAP, &cap) == -1) {
        perror("VIDIOC_QUERYCAP");
        return EXIT_FAILURE;
    }

    if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
        fprintf(stderr, "The device does not support video capture.\n");
        return EXIT_FAILURE;
    }

    if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
        fprintf(stderr, "The device does not support streaming i/o.\n");
        return EXIT_FAILURE;
    }

    memset(&fmt, 0, sizeof(fmt));
    fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    fmt.fmt.pix.width = IMAGE_WIDTH;
    fmt.fmt.pix.height = IMAGE_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("VIDIOC_S_FMT");
        return EXIT_FAILURE;
    }

    memset(&req, 0, sizeof(req));
    req.count = BUFFER_COUNT;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_MMAP;

    if (ioctl(fd, VIDIOC_REQBUFS, &req) == -1) {
        perror("VIDIOC_REQBUFS");
        return EXIT_FAILURE;
    }

    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_MMAP;
        buf.index = i;

        if (ioctl(fd, VIDIOC_QUERYBUF, &buf) == -1) {
            perror("VIDIOC_QUERYBUF");
            return EXIT_FAILURE;
        }

        buffers[i].length = buf.length;
        buffers[i].start = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd, buf.m.offset);
        if (buffers[i].start == MAP_FAILED) {
            perror("mmap");
            return EXIT_FAILURE;
        }
    }

    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_MMAP;
        buf.index = i;

        if (ioctl(fd, VIDIOC_QBUF, &buf) == -1) {
            perror("VIDIOC_QBUF");
            return EXIT_FAILURE;
        }
    }

    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    if (ioctl(fd, VIDIOC_STREAMON, &type) == -1) {
        perror("VIDIOC_STREAMON");
        return EXIT_FAILURE;
    }

    fp = fopen(CAPTURE_FILE, "wb");
    if (fp == NULL) {
        perror("fopen");
        return EXIT_FAILURE;
    }

    for (i = 0; i < 100; i++) {
        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");
            return EXIT_FAILURE;
        }

        if (r == 0) {
            fprintf(stderr, "select timeout\n");
            return EXIT_FAILURE;
        }

        memset(&buffers[i % BUFFER_COUNT], 0, sizeof(struct buffer));

        struct v4l2_buffer buf;
        memset(&buf, 0, sizeof(buf));
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;

        if (ioctl(fd, VIDIOC_DQBUF, &buf) == -1) {
            perror("VIDIOC_DQBUF");
            return EXIT_FAILURE;
        }

        memcpy(buffers[buf.index].start, (void *)buf.m.offset, buf.bytesused);
        buffers[buf.index].length = buf.bytesused;

        if (fwrite(buffers[buf.index].start, 1, buffers[buf.index].length, fp) != buffers[buf.index].length) {
            perror("fwrite");
            return EXIT_FAILURE;
        }

        if (ioctl(fd, VIDIOC_QBUF, &buf) == -1) {
            perror("VIDIOC_QBUF");
            return EXIT_FAILURE;
        }
    }

    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    if (ioctl(fd, VIDIOC_STREAMOFF, &type) == -1) {
        perror("VIDIOC_STREAMOFF");
        return EXIT_FAILURE;
    }

    for (i = 0; i < BUFFER_COUNT; i++) {
        if (munmap(buffers[i].start, buffers[i].length) == -1) {
            perror("munmap");
            return EXIT_FAILURE;
        }
    }

    if (fclose(fp) == EOF) {
        perror("fclose");
        return EXIT_FAILURE;
    }

    if (close(fd) == -1) {
        perror("close");
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

这段代码使用了v4l2框架来从摄像头中获取视频流，并将每一帧保存到文件中。你可以根据自己的需求进行修改。