volatile在C语言中实际上是为了不自动缓存变量的值而存在的。它会告诉编译器不要缓存这个变量的值。因此，每次遇到给定的volatile变量时，它都会生成代码从主存中获取它的值。之所以使用这种机制，是因为该值在任何时候都可以被操作系统或任何中断修改。所以使用volatile可以帮助我们每次都重新访问值。

在Dennis Ritchie设计的语言中，除了地址未被获取的自动对象外，对任何对象的每次访问都表现为计算对象的地址，然后在该地址上读写存储。这使得该语言非常强大，但严重限制了优化机会。

While it might have been possible to add a qualifier that would invite a compiler to assume that a particular object wouldn't be changed in weird ways, such an assumption would be appropriate for the vast majority of objects in C programs, and it would have been impractical to add a qualifier to all the objects for which such assumption would be appropriate. On the other hand, some programs need to use some objects for which such an assumption would not hold. To resolve this issue, the Standard says that compilers may assume that objects which are not declared volatile will not have their value observed or changed in ways that are outside the compiler's control, or would be outside a reasonable compiler's understanding.

Because various platforms may have different ways in which objects could be observed or modified outside a compiler's control, it is appropriate that quality compilers for those platforms should differ in their exact handling of volatile semantics. Unfortunately, because the Standard failed to suggest that quality compilers intended for low-level programming on a platform should handle volatile in a way that will recognize any and all relevant effects of a particular read/write operation on that platform, many compilers fall short of doing so in ways that make it harder to process things like background I/O in a way which is efficient but can't be broken by compiler "optimizations".

正如这里许多人正确地建议的那样，volatile关键字的流行用途是跳过volatile变量的优化。

在阅读了volatile之后，我想到的最好的优点是——在longjmp的情况下防止回滚变量。非本地跳转。

这是什么意思?

它只是意味着在你进行堆栈展开后，最后一个值将被保留，以返回到前一个堆栈帧;通常是在一些错误的情况下。

因为它超出了这个问题的范围，所以我不打算在这里详细讨论setjmp/longjmp，但是值得一读;以及如何使用波动特征来保留最后的价值。

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。

维基上有关于volatile的所有信息:

Volatile(计算机编程)

Linux内核的文档也对volatile做了一个很好的标记:

为什么不应该使用“volatile”类型类

Volatile意味着存储可能在任何时候被改变，而且是在用户程序控制之外被改变。这意味着如果你引用变量，程序应该总是检查物理地址(即映射的输入fifo)，而不是以缓存的方式使用它。