volatile的另一个用途是信号处理程序。如果你有这样的代码:

int quit = 0;
while (!quit)
{
    /* very small loop which is completely visible to the compiler */
}

编译器可以注意到循环体没有触及quit变量，并将循环转换为while (true)循环。即使在信号处理程序上为SIGINT和SIGTERM设置了退出变量;编译器无法知道这一点。

但是，如果quit变量被声明为volatile，编译器将被迫每次加载它，因为它可以在其他地方修改。这正是你在这种情况下想要的。

它有两个用途。这些在嵌入式开发中特别常用。

编译器不会优化使用volatile关键字定义的变量的函数 Volatile用于访问RAM、ROM等中的精确内存位置。这通常用于控制内存映射设备，访问CPU寄存器和定位特定的内存位置。

参见程序集清单示例。 回复:在嵌入式开发中使用C“volatile”关键字

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".

Volatile也很有用，当你想强制编译器不优化特定的代码序列时(例如编写一个微基准测试)。

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。

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。