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 x = 10;

    let p1: *mut i32 = &raw mut x;
    let p2 = p1 as *const i32;

    // SAFETY: p1 and p2 were created by taking raw pointers to a local, so they
    // are guaranteed to be non-null, aligned, and point into a single (stack-)
    // allocated object.
    //
    // The object underlying the raw pointers lives for the entire function, so
    // it is not deallocated while the raw pointers still exist. It is not
    // accessed through references while the raw pointers exist, nor is it
    // accessed from other threads concurrently.
    unsafe {
        dbg!(*p1);
        *p1 = 6;
        // Mutation may soundly be observed through a raw pointer, like in C.
        dbg!(*p2);
    }

    // UNSOUND. DO NOT DO THIS.
    /*
    let r: &i32 = unsafe { &*p1 };
    dbg!(r);
    x = 50;
    dbg!(r); // Object underlying the reference has been mutated. This is UB.
    */
}

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 “UNSOUND” section gives an example of a common kind of UB bug: naïvely taking a reference to the dereference of a raw pointer sidesteps the compiler’s knowledge of what object the reference is actually pointing to. As such, the borrow checker does not freeze x and so we are able to modify it despite the existence of a reference to it. Creating a reference from a pointer requires great care.

Comprehensive Rust 🦀

解引用裸指针