Volatile告诉编译器你的变量可以通过其他方式被改变，而不是通过访问它的代码。例如，它可能是一个I/ o映射的内存位置。如果在这种情况下没有指定这一点，一些变量访问可以被优化，例如，它的内容可以保存在寄存器中，并且内存位置不会再次读入。

volatile的边缘用法如下。假设你想计算一个函数f的数值导数:

double der_f(double x)
{
    static const double h = 1e-3;
    return (f(x + h) - f(x)) / h;
}

问题是由于舍入误差，x+h-x通常不等于h。想想看:当你减去非常接近的数字时，你会丢失很多有效的数字，这可能会破坏导数的计算(想想1.00001 - 1)

double der_f2(double x)
{
    static const double h = 1e-3;
    double hh = x + h - x;
    return (f(x + hh) - f(x)) / hh;
}

但是根据您的平台和编译器开关的不同，该函数的第二行可能会被积极优化的编译器删除。所以你可以写

    volatile double hh = x + h;
    hh -= x;

强制编译器读取包含hh的内存位置，从而丧失最终的优化机会。

在我看来，你不应该对volatile期望太高。为了说明这一点，看看尼尔斯·派彭布林克(Nils Pipenbrinck)的高票数回答中的例子。

我想说，他的例子并不适用于volatile。Volatile只用于: 阻止编译器进行有用和理想的优化。这与线程安全、原子访问甚至内存顺序无关。

在这个例子中:

    void SendCommand (volatile MyHardwareGadget * gadget, int command, int data)
    {
      // wait while the gadget is busy:
      while (gadget->isbusy)
      {
        // do nothing here.
      }
      // set data first:
      gadget->data    = data;
      // writing the command starts the action:
      gadget->command = command;
    }

gadget->data = gadget->command = command之前的数据仅由编译器在编译后的代码中保证。在运行时，处理器仍然可能根据处理器架构对数据和命令分配进行重新排序。硬件可能会得到错误的数据(假设gadget映射到硬件I/O)。数据和命令分配之间需要内存屏障。

参见Andrei Alexandrescu的文章，“volatile——多线程程序员最好的朋友”

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.

本文适用于C和c++。

参见Scott Meyers和Andrei Alexandrescu的文章“c++和双重检查锁定的危险”:

So when dealing with some memory locations (e.g. memory mapped ports or memory referenced by ISRs [ Interrupt Service Routines ] ), some optimizations must be suspended. volatile exists for specifying special treatment for such locations, specifically: (1) the content of a volatile variable is "unstable" (can change by means unknown to the compiler), (2) all writes to volatile data are "observable" so they must be executed religiously, and (3) all operations on volatile data are executed in the sequence in which they appear in the source code. The first two rules ensure proper reading and writing. The last one allows implementation of I/O protocols that mix input and output. This is informally what C and C++'s volatile guarantees.

volatile变量可以从编译代码的外部进行更改(例如，程序可以将volatile变量映射到内存映射寄存器)。编译器不会对处理易失性变量的代码应用某些优化——例如，它不会在不将其写入内存的情况下将其加载到寄存器。这在处理硬件寄存器时很重要。

简单来说，它告诉编译器不要对特定变量做任何优化。映射到设备寄存器的变量由设备间接修改。在这种情况下，必须使用volatile。