volatile 如何保证编译器不去自做主张破坏多线程安全。

 volatile

在Andrei Alexandrescu的文章《volatile - Multithreaded Programmer's Best Friend》[2]开头有如下简要描述：

The volatile keyword was devised to prevent compiler optimizations that might render code incorrect in the presence of certain asynchronous events. For example, if you declare a primitive variable as volatile, the compiler is not permitted to cache it in a register -- a common optimization that would be disastrous if that variable were shared among multiple threads. So the general rule is, if you have variables of primitive type that must be shared among multiple threads, declare those variables volatile. But you can actually do a lot more with this keyword: you can use it to catch code that is not thread safe, and you can do so at compile time. This article shows how it is done; the solution involves a simple smart pointer that also makes it easy to serialize critical sections of code.

volatile修饰的变量防止被优化，主要用在多线程程序中。避免编译器优化。编译器进行优化时，它有时会取一些值的时候，直接从寄存器里进行存取，而不是从内存中获取，这种优化在单线程的程序中没有问题，但到了多线程程序中，由于多个线程是并发运行的，就有可能一个线程把某个公共的变量已经改变了，这时其余线程中寄存器的值已经过时，但这个线程本身还不知道，以为没有改变，仍从寄存器里获取，就导致程序运行会出现未定义的行为