使用v4l2获取mjpeg格式图片的C语言代码示例

以下是使用v4l2获取mjpeg格式图片的C语言代码示例：

#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 CAMERA_DEVICE "/dev/video0"
#define CAPTURE_FILE "capture.jpeg"
#define BUFFER_COUNT 4

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

int main()
{
    int fd;
    struct v4l2_capability cap;
    struct v4l2_format fmt;
    struct v4l2_requestbuffers req;
    struct v4l2_buffer buf;
    enum v4l2_buf_type type;
    struct buffer *buffers;
    int i, n_buffers;
    FILE *fp;

    // 打开摄像头设备
    fd = open(CAMERA_DEVICE, O_RDWR);
    if (fd == -1) {
        perror("Failed to open camera device");
        return EXIT_FAILURE;
    }

    // 查询摄像头设备的能力
    if (ioctl(fd, VIDIOC_QUERYCAP, &cap) == -1) {
        perror("Failed to query camera device");
        return EXIT_FAILURE;
    }

    // 检查是否支持视频捕获
    if (!(cap.capabilities & V4L2_CAP_VIDEO_CAPTURE)) {
        fprintf(stderr, "Camera device does not support video capture\n");
        return EXIT_FAILURE;
    }

    // 检查是否支持流形式的视频捕获
    if (!(cap.capabilities & V4L2_CAP_STREAMING)) {
        fprintf(stderr, "Camera device does not support streaming\n");
        return EXIT_FAILURE;
    }

    // 设置摄像头设备的格式
    memset(&fmt, 0, sizeof(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_MJPEG;
    if (ioctl(fd, VIDIOC_S_FMT, &fmt) == -1) {
        perror("Failed to set camera device format");
        return EXIT_FAILURE;
    }

    // 请求摄像头设备的缓冲区
    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("Failed to request camera device buffers");
        return EXIT_FAILURE;
    }

    // 映射摄像头设备的缓冲区
    buffers = calloc(req.count, sizeof(*buffers));
    for (i = 0, n_buffers = 0; i < req.count; i++) {
        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("Failed to query camera device buffer");
            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("Failed to map camera device buffer");
            return EXIT_FAILURE;
        }
        n_buffers++;
    }

    // 启动摄像头设备的视频流
    for (i = 0; i < n_buffers; i++) {
        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("Failed to enqueue camera device buffer");
            return EXIT_FAILURE;
        }
    }
    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    if (ioctl(fd, VIDIOC_STREAMON, &type) == -1) {
        perror("Failed to start camera device stream");
        return EXIT_FAILURE;
    }

    // 从摄像头设备中获取图片
    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("Failed to dequeue camera device buffer");
        return EXIT_FAILURE;
    }

    // 保存图片到文件
    fp = fopen(CAPTURE_FILE, "wb");
    if (fp == NULL) {
        perror("Failed to open capture file");
        return EXIT_FAILURE;
    }
    fwrite(buffers[buf.index].start, buf.bytesused, 1, fp);
    fclose(fp);

    // 停止摄像头设备的视频流
    type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    if (ioctl(fd, VIDIOC_STREAMOFF, &type) == -1) {
        perror("Failed to stop camera device stream");
        return EXIT_FAILURE;
    }

    // 释放摄像头设备的缓冲区
    for (i = 0; i < n_buffers; i++) {
        if (munmap(buffers[i].start, buffers[i].length) == -1) {
            perror("Failed to unmap camera device buffer");
            return EXIT_FAILURE;
        }
    }
    free(buffers);

    // 关闭摄像头设备
    if (close(fd) == -1) {
        perror("Failed to close camera device");
        return EXIT_FAILURE;
    }

    return EXIT_SUCCESS;
}

这段代码使用v4l2库调用摄像头设备的API，请求摄像头设备的缓冲区，映射缓冲区，启动视频流，从视频流中获取一张图片，保存图片到文件，停止视频流，释放缓冲区，最后关闭摄像头设备。请注意，此代码中的图片格式为MJPEG，如果你需要获取其他格式的图片，需要修改代码中的像素格式。