c - 我不明白为什么在演员表中使用指针

Question

我明白这个语句是将一个无符号的 volatile char 转换为一个内存地址，但我不明白的是 volatile 之后的指针。

#define PORTC *(unsigned char volatile *)(0x1003)

Answer 1

它说：将数字0x1003视为易失的无符号字符指针；在该地址读取或写入（字节）值，具体取决于它的使用方式：

unsigned char c = PORTC;   // read
PORTC = c + 1;             // write

Answer 2

这不是正在发生的事情。相反，它将值 0x1003 解释为指针，然后取消引用该指针以获取 type 的值volatile unsigned char。本质上，这是一种在固定内存位置访问字节的方法。（“ volatile”强制执行对该内存位置的实际“访问”，这是标准中定义有些模糊的概念。）

Answer 3

(TYPE*) POINTER

是一种被解释为指针类型转换的语法。例如，

int m = 4;
char* p_char = (char*)&m;

因此，p_char是一个指向 char 的指针。如果我们取消引用p_char，即*p_char，指向的位置处的二进制表示p_char将被转换为char(character) 的表示。此值的确切结果未定义。我不知道它会返回什么确切的值，但它会返回一个奇怪的字符č。

为了更深入地理解指针，我想强调指针只是访问以 C++ 语言表示并驻留在计算机内存中的实体的接口之一。

#include <iostream>
using namespace std;

/*
 *  C++ interface to represent entity which resides on computer memory: 
 *      ---------                                             
 *  1) object;
 *  2) pointer;
 *  3) reference;
 *
 *  C++ interpretation of entity through interface: TYPE
 *      --------------                              ----          
 *
 *  What is the function of TYPE?
 *  1) tell compiler the size of an object of this TYPE needed; ( sizeof( int ) -> 4 bytes )
 *  2) when the value of object at which it resides is dereferenced, 
 *     the binary represented value is transformed to some other 
 *     representation value according to the TYPE's interpretation rule; ( if int, interpret it as an int )
 *
 *
 *              +----------------+         
 *              |     0x02105207 |                    
 *              |     0x02105206 |                    
 *              |     0x02105205 |                    
 *       int  n |     0x02105204 |                    
 *              +----------------+                    
 *              |     0x02105203 |                    
 *              |     0x02105202 |                    
 *              |     0x02105201 |                    
 *  ---->int  m |     0x02105200 |                    
 *  |           +----------------+                   
 *  |           |                |                    
 *  |           +----------------+                   
 *  |          ...              ...                    
 *  |           +----------------+                   
 *  ---- int* p |     0x00002298 |                    
 *              +----------------+                   
 *
 *  if the pointer in figure is declared as:
 *
 *       int* p = &m;
 *  
 *  the face of 0x00002298 -> 0x02105200 will not be changed until p is 
 *  assigned to other address value;
 *
 *  
 */

class consecutive_two_int
{
public:
    consecutive_two_int():m(4),n(123){}
    int m;
    int n;
};

int main()
{
    int * p;
    consecutive_two_int obj;
    p = &(obj.m);

    for( int i = 0; i < 8; ++i )
    {
        //  because pointer of char progresses every 1 byte;
        //  increment memory byte by byte through it;
        cout << *(int*)( (char*)p + i ) << "\n";
    }

    return 0;
}

例如，结果是：

4 // the first defined int
2063597568 // undefined
8060928  // undefined
31488  // undefined
123 // the second defined int
268435456  // undefined
1175453696  // undefined
-465170432  // undefined