volatile(读书笔记)

最新推荐文章于 2023-03-20 00:14:02 发布

原创最新推荐文章于 2023-03-20 00:14:02 发布 · 832 阅读

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#读书 #compiler #optimization #byte #service #buffer

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本文详细解释了C++中volatile关键字的作用及使用场景。volatile用于标记可能被外部环境改变的数据，防止编译器对其进行优化。通过一个示例展示了如何声明volatile变量，并在多线程或多任务环境中正确读取这些变量。

The grammar of volatile is the same with that of const, but volatile means "this data may
be changed besides the compiler's eyeshot". For some reason, may be multitask, multithread,
interrupt, this data are changed by the enviroment, so we declare it volatile to tell
compiler not to do any optimization on it.
    If the complier says, "I have read the data into a register, and haven't attained to it
since that", in common, it doesn't need to read it any more. But if it was specified by vo-
latile, the compiler must read it again.

//volatile.cpp
//The volatile keyword

class Comm
{
    const volatile unsigned char byte;
    volatile unsigned char flag;
    enum {bufsize = 100};
    unsigned char buf[bufsize];
    int index;
public:
    Comm();
    void isr() volatile;
    char read(int index) const;
};

Comm::Comm() : index(0), byte(0), flag(0) {}

//Only a demo: won't actually work
//as an interrupt service routine:
void Comm::isr() volatile
{
    flag = 0;
    buf[index++] = byte;
    //Wrap to beginning of buffer:
    if(index >= buffsize)    index = 0;
}

char Comm::read(int index) const
{
    if(index < 0 || index >= bufsize)
        return 0;
    return buf[index];
}

int main()
{
    volatile Comm port;
    port.isr(); //OK
    port.read(0); //Error, read() not volatile
}