In the early days of C, compilers would interpret all actions that read and write lvalues as memory operations, to be performed in the same sequence as the reads and writes appeared in the code. Efficiency could be greatly improved in many cases if compilers were given a certain amount of freedom to re-order and consolidate operations, but there was a problem with this. Even though operations were often specified in a certain order merely because it was necessary to specify them in some order, and thus the programmer picked one of many equally-good alternatives, that wasn't always the case. Sometimes it would be important that certain operations occur in a particular sequence.

Exactly which details of sequencing are important will vary depending upon the target platform and application field. Rather than provide particularly detailed control, the Standard opted for a simple model: if a sequence of accesses are done with lvalues that are not qualified volatile, a compiler may reorder and consolidate them as it sees fit. If an action is done with a volatile-qualified lvalue, a quality implementation should offer whatever additional ordering guarantees might be required by code targeting its intended platform and application field, without requiring that programmers use non-standard syntax.

不幸的是，许多编译器并没有确定程序员需要什么样的保证，而是选择提供标准规定的最低限度的保证。这使得volatile远没有它应有的用处。例如，在gcc或clang上，一个程序员需要实现一个基本的“移交互斥量”(一个已经获得并释放互斥量的任务直到另一个任务释放互斥量后才会再次释放互斥量)，他必须做以下四件事中的一件:

将互斥量的获取和释放放在编译器不能内联的函数中，并且不能应用整个程序优化。 将互斥锁保护的所有对象限定为volatile——如果所有访问都发生在获得互斥锁之后和释放互斥锁之前，那么就不应该这样做。 使用优化级别0来强制编译器生成代码，就像所有非限定寄存器的对象都是volatile一样。 使用特定于gcc的指令。

相比之下，当使用更适合系统编程的高质量编译器时，例如icc，人们将有另一种选择:

确保在每个需要获取或释放的地方执行volatile-qualified write。

获取一个基本的“传递互斥量”需要一个volatile读取(看看它是否准备好了)，并且不应该需要一个volatile写入(另一方在它被交还之前不会试图重新获取它)，但是必须执行一个毫无意义的volatile写入仍然比gcc或clang下可用的任何选项都要好。

如果你正在从内存中的某个点(比如说，一个完全独立的进程/设备/任何东西)读取数据，则需要使用Volatile。

我曾经在纯c的多处理器系统中使用双端口ram。我们使用硬件管理的16位值作为信号量，以知道另一个家伙什么时候完成。基本上我们是这样做的:

void waitForSemaphore()
{
   volatile uint16_t* semPtr = WELL_KNOWN_SEM_ADDR;/*well known address to my semaphore*/
   while ((*semPtr) != IS_OK_FOR_ME_TO_PROCEED);
}

没有volatile，优化器认为循环是无用的(这家伙从不设置值!他疯了，删掉那代码吧!)，我的代码会在没有获得信号量的情况下继续运行，从而在以后造成问题。

我曾经在20世纪90年代早期开发过一个大型应用程序，其中包含使用setjmp和longjmp进行基于c语言的异常处理。volatile关键字对于那些值需要保存在作为“catch”子句的代码块中的变量是必要的，以免这些变量被存储在寄存器中并被longjmp清除。

In the early days of C, compilers would interpret all actions that read and write lvalues as memory operations, to be performed in the same sequence as the reads and writes appeared in the code. Efficiency could be greatly improved in many cases if compilers were given a certain amount of freedom to re-order and consolidate operations, but there was a problem with this. Even though operations were often specified in a certain order merely because it was necessary to specify them in some order, and thus the programmer picked one of many equally-good alternatives, that wasn't always the case. Sometimes it would be important that certain operations occur in a particular sequence.

Exactly which details of sequencing are important will vary depending upon the target platform and application field. Rather than provide particularly detailed control, the Standard opted for a simple model: if a sequence of accesses are done with lvalues that are not qualified volatile, a compiler may reorder and consolidate them as it sees fit. If an action is done with a volatile-qualified lvalue, a quality implementation should offer whatever additional ordering guarantees might be required by code targeting its intended platform and application field, without requiring that programmers use non-standard syntax.

不幸的是，许多编译器并没有确定程序员需要什么样的保证，而是选择提供标准规定的最低限度的保证。这使得volatile远没有它应有的用处。例如，在gcc或clang上，一个程序员需要实现一个基本的“移交互斥量”(一个已经获得并释放互斥量的任务直到另一个任务释放互斥量后才会再次释放互斥量)，他必须做以下四件事中的一件:

将互斥量的获取和释放放在编译器不能内联的函数中，并且不能应用整个程序优化。 将互斥锁保护的所有对象限定为volatile——如果所有访问都发生在获得互斥锁之后和释放互斥锁之前，那么就不应该这样做。 使用优化级别0来强制编译器生成代码，就像所有非限定寄存器的对象都是volatile一样。 使用特定于gcc的指令。

相比之下，当使用更适合系统编程的高质量编译器时，例如icc，人们将有另一种选择:

确保在每个需要获取或释放的地方执行volatile-qualified write。

获取一个基本的“传递互斥量”需要一个volatile读取(看看它是否准备好了)，并且不应该需要一个volatile写入(另一方在它被交还之前不会试图重新获取它)，但是必须执行一个毫无意义的volatile写入仍然比gcc或clang下可用的任何选项都要好。

您必须使用它来实现自旋锁以及一些(所有?)无锁数据结构 与原子操作/指令一起使用 曾经帮助我克服编译器的错误(在优化过程中错误地生成代码)

Dan Saks在《Volatile as a promise》一文中写道:

易失性对象是其值可能自发变化的对象。也就是说，当你声明一个对象为volatile时，你是在告诉编译器这个对象可能会改变状态，即使程序中没有任何语句显示要改变它。”

以下是他关于volatile关键字的三篇文章的链接:

明智地使用挥发剂 准确放置挥发剂 像承诺一样反复无常