解引用裸指针

解引用裸指针

创建指针是安全的操作，但解引用指针需要使用 unsafe 方法：

fn main() {
    let mut s = String::from("careful!");
    let r1 = &mut s as *mut String;
    let r2 = r1 as *const String;
    // SAFETY: r1 and r2 were obtained from references and so are guaranteed to
    // be non-null and properly aligned, the objects underlying the references
    // from which they were obtained are live throughout the whole unsafe
    // block, and they are not accessed either through the references or
    // concurrently through any other pointers.
    unsafe {
        println!("r1 is: {}", *r1);
        *r1 = String::from("uhoh");
        println!("r2 is: {}", *r2);
    }
    // NOT SAFE. DO NOT DO THIS.
    /*
    let r3: &String = unsafe { &*r1 };
    drop(s);
    println!("r3 is: {}", *r3);
    */
}

This slide should take about 10 minutes.

我们建议（而且 Android Rust 样式指南要求）为每个 unsafe 代码块编写一条注释，说明该代码块中的代码如何满足其所执行的不安全操作的安全要求。

对于指针解除引用，这意味着指针必须为 valid，即：

指针必须为非 null。
指针必须是 dereferenceable（在单个已分配对象的边界内）。
对象不得已取消分配。
不得并发访问相同位置。
如果通过转换引用类型来获取指针，则底层对象必须处于活跃状态，而且不得使用任何引用来访问内存。

在大多数情况下，指针还必须正确对齐。

The “NOT SAFE” section gives an example of a common kind of UB bug: *r1 has the 'static lifetime, so r3 has type &'static String, and thus outlives s. Creating a reference from a pointer requires great care.

30.2. 解引用裸指针

解引用裸指针