C++ 读取和写入 int 是原子的吗?

问题描述

我有两个线程，一个更新一个 int 一个读取它.这是一个与读写顺序无关的统计值.

I have two threads, one updating an int and one reading it. This is a statistic value where the order of the reads and writes is irrelevant.

我的问题是，我是否需要同步访问这个多字节值?或者，换句话说，可以部分写入完成并被中断，然后读取发生.

My question is, do I need to synchronize access to this multi-byte value anyway? Or, put another way, can part of the write be complete and get interrupted, and then the read happen.

例如，考虑一个值 = 0x0000FFFF，它的增量值为 0x00010000.

For example, think of a value = 0x0000FFFF that gets incremented value of 0x00010000.

有没有我应该担心值看起来像 0x0001FFFF 的时候?当然，类型越大，这种事情发生的可能性就越大.

Is there a time where the value looks like 0x0001FFFF that I should be worried about? Certainly the larger the type, the more possible something like this to happen.

我一直同步这些类型的访问，但很好奇社区的想法.

I've always synchronized these types of accesses, but was curious what the community thinks.

推荐答案

起初人们可能认为对本地机器大小的读取和写入是原子的，但有许多问题需要处理，包括处理器/内核之间的缓存一致性.在 Windows 上使用 Interlocked* 等原子操作，在 Linux 上使用等效操作.C++0x 将有一个原子"模板来将它们包装在一个漂亮的跨平台界面中.现在，如果您使用的是平台抽象层，它可能会提供这些功能.ACE 可以，请参阅类模板 ACE_Atomic_Op.

At first one might think that reads and writes of the native machine size are atomic but there are a number of issues to deal with including cache coherency between processors/cores. Use atomic operations like Interlocked* on Windows and the equivalent on Linux. C++0x will have an "atomic" template to wrap these in a nice and cross-platform interface. For now if you are using a platform abstraction layer it may provide these functions. ACE does, see the class template ACE_Atomic_Op.

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