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C In language * And & The usage and difference of

keyword static and volatile Meaning and function of

C In language * And & The usage and difference of

c Define a (int a) Variables are ambiguous , In essence, it is to allocate a address space , There is a value in it .
To avoid ambiguity ,c Specifies the a Is the value ,&a Is the address space .

Pointer to an address .
here *a Represents the value stored in the address pointed to ,a It's the address that points to .

void tes(int *i){
	*i=2;
}
int main(int argc, char *argv[]){
	int a=4;
	int* s = &a;
	tes(s);
	printf("%d\n",*s);
	printf("%d\n",s);
}

The above code changes the incoming value through the incoming address , Output *s by 2,s For address .

keyword static and volatile Meaning and function of

static:

c In language static Keywords have two functions , First, file scope , The second is function scope .

File scope keyword static The role of is , With static Declared global variables 、 Functions must not be referenced by other files ;

static Another use is static variables inside functions , It will only be initialized once , And variables are stored in global data segments instead of function stacks , So its lifetime will last until the program exits .
Take the chestnut of callback function ： although counting（） The function calls itself back several times , But static integer variables count Not initialized every time , Instead, it is initialized only once , The purpose of counting is achieved .

volatile:

volatile English means changeable . stay C In language , One definition is volatile Variable means that this variable is likely to be changed unexpectedly , Therefore, we need to be careful . in other words , When the optimizer uses this variable, it must re

Read the value of this variable from virtual memory , Instead of using a backup stored in a register .