Do you know the keyword volatile in C language?

When we learn the 32 keywords of C language, we don't pay much attention to the keyword volatile. Volatile is a type modifier. The Chinese meaning of volatile is "changeable". So when do we use it in the program? We often see the use of this keyword when analyzing the kernel. For example:

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1.struct task_struct {  
2.volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */  
3.void *stack;  
4.atomic_t usage;  
5.unsigned int flags; /* per process flags, defined below */  
6.unsigned int ptrace;  
7........  
struct task_struct { volatile long state; /* -1 unrunnable, 0 runnable, >0 stopped */ void *stack; atomic_t usage; unsigned int flags; /* per process flags, defined below */ unsigned int ptrace; .......

In the application, volatile is mainly designed to modify variables that are accessed and modified by different threads.

The volatile variable means that the variable may be changed unexpectedly, so the compiler should not perform corresponding optimizations. To be precise, the optimizer must carefully re-read the value of this variable each time it is used, rather than using the copy stored in the register.

For example:

extern int a; // Statement 1 Assume that a is a global variable

......

int b=a； //Statement 2

int c=a； //Statement 3

The value of a is used in both statement 2 and statement 3. When the optimizer references the value of a in statement 3, it assumes that the value of a has been read into the register in statement 2, and a has not been modified between statement 2 and statement 3. Therefore, there is no need to read the value of a from the memory, so the value in the register is used directly. However, if an interrupt occurs before statement 3 is executed, and the interrupt happens to modify a, then the value retrieved by statement 3 will be problematic.

If the global variable a above is modified as volatile, the optimizer cannot optimize statement 3 and must retrieve the value from the memory.