Function query_buffer

rustcv-camera/src/backend/linux/sys.rs:275–284

Query a single buffer's offset and length (VIDIOC_QUERYBUF). 查询单个缓冲区的偏移量和大小（VIDIOC_QUERYBUF）。 The returned `m.offset` and `length` are needed for `mmap()`. 返回的 `m.offset` 和 `length` 用于 `mmap()` 映射。

(fd: RawFd, index: u32)

/// The returned `m.offset` and `length` are needed for `mmap()`.
/// 返回的 `m.offset` 和 `length` 用于 `mmap()` 映射。
pub fn query_buffer(fd: RawFd, index: u32) -> io::Result<v4l2_buffer> {
    let mut buf: v4l2_buffer = unsafe { std::mem::zeroed() };
    buf.index = index;
    buf.type_ = V4L2_BUF_TYPE_VIDEO_CAPTURE;
    buf.memory = V4L2_MEMORY_MMAP;
    unsafe {
        v4l2_ioctl(fd, VIDIOC_QUERYBUF, &mut buf as *mut _ as *mut libc::c_void)?;
    }
    Ok(buf)
}

/// Enqueue a buffer — return it to the driver for filling (VIDIOC_QBUF).
/// 将缓冲区入队 —— 归还给驱动以便其写入新数据（VIDIOC_QBUF）。