how to explain volatile in C ?

roystyle

as title
how to use volatile in C ?
thanks

snowflying

I'm not sure whether the C or C++ compiler will do the same optimization.
As in C++, if you have a code such as

1. #include <stdio.h>
   
2. #include <stdlib.h>
   
3. 
   
4. int main()
   
5. {
   
6.     const int i = 100;
   
7.     int *p = (int*)(&i);
   
8.     *p = 200;
   
9.     print f("%d\n", i);   // delete the space in print f
   
10. 
    
11.     return 0;
    
12. }

複製代碼

you may get the wrong answer 100 , given by the register.
Since some compiler considered it a constant value , the value 100 was loaded
to the register , and give the value from it.
If you declare as "volatile int i" , however , will give the correct answer ,
as it tells the compiler not to do the optimization with the variable i.

(If there is any wrong in the above , please show that. Thanks.)...

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telanx

the key word 'volatile' is to change a variable
A volatile specifier is a hint to a compiler that an object may change its value in ways not specified by the language so that aggressive optimizations must be avoided
for more information,please google

C 語言中要使用 變數 必先宣告 變數的資料型態 datatype
如 char i;
    int j;
    float k;
如果在程式中因演算結果，會改變此資料型態！
此時就要宣告為 volatile i;

caoh

snowflying 發表於 2013-1-1 12:39 AM

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I'm not sure whether the C or C++ compiler will do the same optimization.
As in C++, if you have a c ...

我覺得有點怪但說不出哪怪!
const 在 C 跟 C++ 是不同的, 但無論如何, & 應該會造成變數被分配空間 (l-value)
...

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snowflying

caoh 發表於 2014-1-17 11:37 PM

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我覺得有點怪但說不出哪怪!
const 在 C 跟 C++ 是不同的, 但無論如何, & 應該會造成變數被分配空間 (l-v ...

...

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caoh

c++ const 可以被編成常數, 但取址它就必須分配記憶體空間
我只是想說被取址就有可能會被改變, 應該不能優化, 看來不是這樣
後來想想也合理, const 取址照理說還是 const T* 自己把 const 去掉
不關編譯器的事, 不然就很難分析程式跟優化了

BTW, 樓主可以參考這篇

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...

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