The use of volatile in C language

volatile Keyword means changeable , Remind the compiler to read data from the variable address , Instead of sometimes optimizing to temporarily use the value in the register . If the variable is updated by another program or function , Especially if you modify it in an interrupt , The compiler is likely to optimize variables , There will be inconsistencies .

For example, modify the value of a variable in an interrupt , But when an interrupt is triggered, the compiler doesn't know , Then the compiler will think that the code will not execute , So as to optimize . that volatile This variable tells the compiler that it is mutable , Don't optimize it .

The concept of instruction reordering ：

static int num = 0;
 Threads 1：
    num = 2;
    ready = true;
 Threads 2：
    if (ready) 
      printf("%d",num);
-----------------------------------
 In two threads , There is no data dependency , Maybe threads 1 The order of execution will change to 
    ready = true;
   num = 2;
 It leads to threads 2 Output the default 0  

And add volatile The key word , Will add... After the write operation store Barrier directive , Add... Before reading load barrier , Prevent instruction reordering summary ：

• The variables modified in interrupt service program for other programs to detect need to add volatile;
• In a multitasking environment, the shared flag among tasks should be marked with volatile;
• Memory mapped hardware registers are usually added with volatile explain , Because every time you read and write it, it may have a different meaning ;