C and C++ code

##%zu for sizeof

##return/exit

在main函数中调用exit和return结果是一样的，但在子函数中调用return只是代表子函数终止了，在子函数中调用exit，那么程序终止。

##NULL
#define NULL ((void *)0)

const&pointer

	int a = 1;
	int b = 2;

//指向常量的指针
	//修饰*，指针指向内存区域不能修改，指针指向可以变
	const int * p1 = &a; //等价于int const *p1 = &a;
	//*p1 = 111; //err
	p1 = &b; //ok

//指针常量
	//修饰p1，指针指向不能变，指针指向的内存可以修改
	int * const p2 = &a;
	//p2 = &b; //err
	*p2 = 222; //ok

read from right to left:
const int * p: a pointer p point to const int
p-->a pointer p
*-->point to

int * const p: a const pointer p point to int

pointer - pointer

#include <stdio.h>

int main()
{

    int a[] = {0,1,2,3,4};
    int * p2 = &a[2];
    int * p4 = &a[4];

    int n = p4 - p2;//2
  
    int m = (int)p4 - (int)p2;//8

    
    return 0;
}