铸造任意类型的安全性使用

Question

我写了一些代码.它有效......但它安全吗？

use std::mem;
use std::ptr;
use std::marker::PhantomData;

struct Atomic<T: Copy>(AtomicUsize, PhantomData<T>);

impl<T: Copy> Atomic<T> {
    unsafe fn encode(src: T) -> usize {
        assert!(mem::size_of::<T>() <= mem::size_of::<usize>());

        let mut dst = 0;
        ptr::write(&mut dst as *mut usize as *mut T, src);
        dst
    }

    unsafe fn decode(src: usize) -> T {
        assert!(mem::size_of::<T>() <= mem::size_of::<usize>());
        ptr::read(&src as *const usize as *const T)
    }

    fn new(val: T) -> Atomic<T> {
        unsafe {
            Atomic(AtomicUsize::new(Self::encode(val)), PhantomData)
        }
    }

    fn load(&self, order: Ordering) -> T {
        unsafe { Self::decode(self.0.load(order)) }
    }

    fn store(&self, val: T, order: Ordering) {
        unsafe { self.0.store(Self::encode(val), order) }
    }
}

impl<T: Copy + Default> Default for Atomic<T> {
    fn default() -> Atomic<T> {
        Self::new(T::default())
    }
}

正如你所看到的,我将一个Copy足够小的任意值写入a中usize,并将其运送到一个Atomic.然后我把它作为一个新值读出来.

本质上,我使用usize大小作为内存块size_of::<usize>().

如果这是安全的,下一步是考虑更高级的操作.

unsafe trait PackedInt {}
unsafe impl PackedInt for u8  {}
unsafe impl PackedInt for i8  {}
unsafe impl PackedInt for u32 {}
unsafe impl PackedInt for i32 {}
unsafe impl PackedInt for u64 {}
unsafe impl PackedInt for i64 {}

impl<T: Copy + PackedInt> Atomic<T> {
    fn compare_and_swap(&self, current: T, new: T, order: Ordering) -> T {
        unsafe {
            Self::decode(self.0.compare_and_swap(
                Self::encode(current),
                Self::encode(new),
                order
            ))
        }
    }

    fn fetch_add(&self, val: T, order: Ordering) -> T {
        unsafe {
            Self::decode(self.0.fetch_add(Self::encode(val), order))
        }
    }

    fn fetch_sub(&self, val: T, order: Ordering) -> T {
        unsafe {
            Self::decode(self.0.fetch_sub(Self::encode(val), order))
        }
    }
}

这些当然在溢出时并不总是特别明智(因为两个"相等"的值可能比较由于位之外的位而不相等T),但它们看起来仍然很明确......我想.

那么,这是安全的,为什么？

Answer 1

这几乎是安全的……但也不完全安全。您可能只考虑人们使用Atomic整数和浮点数，但引用也是Copy. 用户使用宽松的负载和存储很容易导致崩溃Atomic<&&u32>。

顺便说一句，您的fetch_add和fetch_sub无法在大端系统上正常工作。