在使用ioctl从相机捕获帧之前，您需要先设置如下格式。

fp_ = open("/dev/video0", O_RDWR);
struct v4l2_format fmt = {0};
fmt.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
fmt.fmt.pix.pixelformat = V4L2_PIX_FMT_H264;
ioctl(fp_, VIDIOC_S_FMT, &fmt);

然后，初始化并Map缓冲区

struct Buffer
{
    void *start;
    unsigned int length;
    unsigned int flags;
};

int buffer_count_ = 4;
Buffer *buffers_;

bool AllocateBuffer()
{
    struct v4l2_requestbuffers req = {0};
    req.count = buffer_count_;
    req.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    req.memory = V4L2_MEMORY_MMAP;

    if (ioctl(fp_, VIDIOC_REQBUFS, &req) < 0)
    {
        perror("ioctl Requesting Buffer");
        return false;
    }

    buffers_ = new Buffer[buffer_count_];

    for (int i = 0; i < buffer_count_; i++)
    {
        struct v4l2_buffer buf = {0};
        buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
        buf.memory = V4L2_MEMORY_MMAP;
        buf.index = i;
        if (ioctl(fp_, VIDIOC_QUERYBUF, &buf) < 0)
        {
            perror("ioctl Querying Buffer");
            return false;
        }

        buffers_[i].start = mmap(NULL, buf.length, PROT_READ | PROT_WRITE, MAP_SHARED, fd_, buf.m.offset);
        buffers_[i].length = buf.length;

        if (MAP_FAILED == buffers_[i].start)
        {
            printf("MAP FAILED: %d\n", i);
            for (int j = 0; j < i; j++)
                munmap(buffers_[j].start, buffers_[j].length);
            return false;
        }

        if (ioctl(fp_, VIDIOC_QBUF, &buf) < 0)
        {
            perror("ioctl Queue Buffer");
            return false;
        }
    }

    return true;
}

STREAMON开始捕获

v4l2_buf_type type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
ioctl(fp_, VIDIOC_STREAMON, &type);

最后从Map的缓冲区中读取一个帧。通常，CaptureImage()将在while循环中。

Buffer CaptureImage()
{
    fd_set fds;
    FD_ZERO(&fds);
    FD_SET(fd_, &fds);
    struct timeval tv = {0};
    tv.tv_sec = 1;
    tv.tv_usec = 0;
    int r = select(fd_ + 1, &fds, NULL, NULL, &tv);
    if (r == 0)
    {
        // timeout
    }

    struct v4l2_buffer buf = {0};
    buf.type = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    buf.memory = V4L2_MEMORY_MMAP;

    while (ioctl(fp_, VIDIOC_DQBUF, &buf) < 0)
    {
        perror("Retrieving Frame");
    }

    struct Buffer buffer = {.start = buffers_[buf.index].start,
                            .length = buf.bytesused,
                            .flags = buf.flags};

    if (ioctl(fp_, VIDIOC_QBUF, &buf) < 0)
    {
        perror("Queue buffer");
    }

    return buffer;
}