littleblacklb/CPointer.md

## CPointer.md

      
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C Pointer

https://www.bilibili.com/video/BV1bo4y1Z7xf
https://gist.github.com/LittleBlacklb/ef27515ad9aeae13f956d6c5f302a4d5

There must be some grammatical errors here, please forgive me :(

Initialization

<data type> *<variable name>;: The pointer to the <data type> of <variable name>
& (ampersand)


&<variable>: Return the address of the variable

* (asterisk)


*<variable>: Return the value at variable's address
To access the address of the variable in format

The asterisk meaning between int* p and *p are not the same.
int* is a type that a pointer point to a data of int type.
*p is to dereference the address which stored in variable p.
Pointer arithmetic

int a = 1024;
int *p;
p = &a;  // Assume variable `a` 's address is 2077 
cout << p << endl;  // 2077 in decimal
cout << p + 1 << endl; // 2081 in decimal
int a = 1025;
int* p;
p = &a;
printf("Address=%d; value=%d\n", p, *p);

char* p0;  // 1 char: 1 byte = 8 bit
p0 = (char*)&a;  // Cast int* to char*

// 1025 in memory (small-endian order):
// ... 00000000     00000000    00000100    00000001 ...
// ...(p + 3) ^    (p + 2) ^   (p + 1) ^   (p + 0) ^ ...
// '^' means toward bit
printf("%d\n", (int)*p0);  // 0000100 => 1
printf("%d\n", (int)*(p0 + 1)); // 00000001 => 4
Void Pointer - Generic Pointer

void* <variable name>;
It can't be used to print the value that it points.
It can be used to print itself address.
Char Array

char c[] = "ABC";: It will be stored in the space for array (Stack).
const char* c = "Hello";: It will be stored as compile time constant, so it cannot be modified (Like: c[0] = 'a')
Array & Pointer


The relationship between Array & Pointer.
arr[x] <==> *(arr+x)
(You need to know that arr is a const variable, so it cannot be assigned (Like: arr = var)
It just the same as the relationship between Reference & Pointer
int &a = var <==> int* const a = &var
return a <==> return *a

The array in function parameters

// ...
int get_size(int arr[])
{
    return sizeof(arr) / sizeof(arr[0])       
}

Most of the time, return result is 1 (Depend on the platfrom)
Because the declaration of int arr[] is actually a int pointer which points to the first address of the array.
Compiler translates int arr[] => int* arr
So the variable of arr 's size is the size of pointer.
The purpose is to reduce the time and space cost.(It don't need to copy the array)

mulit-dimensional array

2-dimensional array:

a[i][j] => *(a[i]+j) => *(*(a+i)+j)
a's type: int (*)[]
a[i]'s type: int*

3-dimensional array:

a[i][j][k] => *(a[i][j] + k) => *(*(a[i] + j ) + k ) => *(*(*(a + i) + j ) + k
a's type: int(*)[][]
a[i]'s type: int(*)[]
a[i][j]'s type: int*

Diffence between array type and pointer type


Yes! They are not the same type.
You can't consider them as the same type!

int arr[3][3] = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
int (*p)[3] = arr;
printf("%d\n", p);
printf("%d\n", *p);

This circumstance only happens in static creation!

p and *p's output are both equal
Differences:
p: It's a one-dimensional pointer;
*p: It's a int* pointer.
Explanation

type of array inequals to pointer
In the other word, array is a special pointer.
What is the point of defining this?


It's a fake-pointer (pointer's address = the address it's point to)
Because the static array is continuous.
But dynamic multi-dimension array is not continuous from row to row.


It needs each row's capacity.
For example:
sizeof(int[3]) = 12
As we all know, sizeof(int) * 3 = 12 bytes
So when you do operator like +, C can know plus 1 equal to how many bytes


Much deeper understanding

Static:
int arr[3][3];
for (int y = 0; y < 3; y++)
{
    for (int x = 0; x < 3; x++)
    {
        // These following three are all equivalent
        *(arr[y]+x) = 3 * y + x;
        *(*(arr + y) + x) = 3 * y + x;
        *((*arr + 3 * y) + x) = 3 * y + x;  // Important!
      }
}
int (*p)[3] = arr;
// All equal
printf("%d\n", p);
printf("%d\n", *p);

int b[3][1];
int (*p)[1] = b;
// int **p = b;  // Syntax Error

Dynamic:
int **arr = (int **)calloc(3, sizeof(int *));
for (int y = 0; y < 3; y++)
{
    *(arr + y) = (int *)calloc(3, sizeof(int));
    for (int x = 0; x < 3; x++)
    {
      // ...
    }
}
int** p = arr;
// Not equal
printf("%d\n", p);
printf("%d\n", *p);
It doesn't need [] type for helping.
Because it's structure is different from static array.
It's discontinuous, and is similar to deque in STL:

一段一段的连续空间's beginning address are different.

Data in memory

High Bit & Low Bit


注意高字节在左边，低字节在右边

    1           1           1           1

High Bit                          Low Bit

Byte Order(Endian): Big Endian(大端) & Small Endian(小端)

0b1001111

Big Endian

Low address in mem -------------> High address in mem

1   0   0   1   1   1   1

High Bit <----------------------- Low Bit


Small Endian

Low address in mem -------------> High address in mem
1   1   1   1   0   0   1
Low Bit <------------------------ High Bit


LSB (Least Significant Bit): 最低有效位

0b10010101
The bold underlined bit above called Least Significant Bit
In Big-Endian order, LSB refers to the rightest bit.
Heap & Stack


代码区：存放函数体的二进制代码，由操作系统进行管理的


全局区：存放 全局变量 和 静态变量 以及常量


栈区：由编译器自动分配释放, 存放函数的参数值,局部变量等


堆区：由**程序员分配和释放**,若程序员不释放,程序结束时由操作系统回收


Method of using dynamic memory (heap segment)

C:

Functions:


void *malloc(size_t size)


void *calloc(size_t nitems, size_t size)`


void *realloc(void *ptr, size_t size)


void free(void *ptr)


C++:

Key Words:


new <variable>


delete <variable>


delete[] <variable>


malloc


Signature:
void *malloc(size_t size)

size_t: Positive number. The exact data type depend on the platform (unsigned int, etc.)

Usage:
int* p = (int*)malloc(4 * sizeof(int))


Replacing sizeof(int) with 4 is not recommanded. Because the space of a data type depends on the compiler(platform)


Malloc function return a generic pointer (void*), so we need to do a type casting.


calloc


Signature:
void *calloc(size_t nitems, size_t size)`

size_t nitems: Numbers of elements of specific data type
Difference:


It can be more convenient to allocate a continuous memory.


It will initialize the allocated memory with 0.


realloc


Signature:
void *realloc(void *ptr, size_t size)

Application Scenarios:

The new memory block may bigger than the original one.

The function will mallocate a new memory block & copy the original data to there.
If the old memory block's adjacency is available to use, the function will expand the old memory block.


The new memory block may less than the original one.

The function will free the unnecessary memory block.


//...
int n;
printf("Enter the size of array:\n");
scanf("%d", &n);
int *arr = (int*)calloc(n, sizeof(int));  // Dynamically allocated array
// arr = {0, 0, 0, ...} (n>3)
// ...
free(arr);
Some machine or compiler may allows program to r/w memory block that freed. But sometimes may get crash.
// ...
realloc(NULL, n*sizeof(int)); // equivalent to malloc;
realloc(arr, 0); // equivalent to free;
Dynamic Memory Allocation

Allocate dynamic memory

In C or C++:
int* p;
p = (int*)alloc(sizeof(int));
*p = 0x400;
free(p)
In C++:
int* p;
p = new int;
*p = 0x400;
delete p;
Allocate dynamic array


When you alloc a continuous space, you can use the space as a array.

In C or C++:
// ...
int* p;
const int SIZE = 4;
p = (int*)alloc(SIZE * sizeof(int))  // Allocate a 16 byte continuous space
for (int i=0; i<4; i++, p++)
{
    // p[i] = i;
    *p = i;
}
free(p)
In C++:
int* p;
const int SIZE = 4;
p = new int[4];  // Allocate a 16 byte continuous space
for (int i=0; i<4; i++, p++)
{
    // p[i] = i;
    *p = i;
}
delete[] p;
Memory Leak

Normal:
//...
void func()
{
  printf("void func(): called")
  char chars = "1145141919810";  // 13 bytes
}

int main()
{
  for (int i = 0; i < 100; i++) func();
  return 0;
}

The program's usage of the memory won't be raised up after finish calling the func()

Memory Leak:
//...
void func()
{
  printf("void func(): called")
  char* chars = (char*)calloc(1145, sizeof(char));  // 1145 bytes
  // char* chars = new char[1145];
}

int main()
{
  for (int i = 0; i < 100; i++) func();
  return 0;
}

The program's usage of the memory will be raised up after finish calling the func()
Reason:
The allocated memory block in heap isn't deallocated after use.
Solution:
Add a free(chars) or delete[] chars at the end line of the func()

In Java, C#, etc. The problem can't be happened, because they have their own garbage collection mechanism.
Pointer return from function

//...
int f1(int a) {/* ... */}
int f2(int* a) {/* ... */}
int main() {f1();f2();return 0}

Concept:


int main();: In C/C++ called Calling function


int f1(int a); and int f2(int* a): Call Called function


int f1(var);: Call Call by value


int f1(*var);: Call Call by reference


The correct operation of returning pointer from function:
int* rtnAPointer()
{
  int* p = (int*)malloc(sizeof(int));
  *p = 0x1bf52;
  return p;
}

int main()
{
  int* p = rtnAPointer();
  *p += 1919810;
  printf("%d\n", p);
  free(p);
  return 0;
}
Function Pointer


A pointer which stores a function entry address.

Initialization syntax: <return type> (*<pointer name>)(<type>, ...);
Example:
void (*fp)(int, char);
Counterexample:
int *fp();
This declare a function that return int*, not a function pointer.

Call syntax: *(<pointer name>)(<type>, ...) or <pointer name>(<type>, ...)
Example:
*(f)(0x1bf52, 33);
f(0x1bf52, 33);

Why these two syntax are both correct?
You can compare this with array's syntax:
int arr[] <==> int* arr
*(f)() <==> f()

Application scenario

Callbacks

// ...
void f0() {printf("f0() called");}

void f1(void (*func)()) {func();}

int main() {
  f1(f0);
  return 0;
}
What's the meaning of the callbacks?
In C library function, qsort use it.

void qsort(void *base, size_t nitems, size_t size, int (*compar)(const void *, const void*))


base − This is the pointer to the first element of the array to be sorted.


nitems − This is the number of elements in the array pointed by base.


size − This is the size in bytes of each element in the array.


compar − This is the function that compares two elements.


The reference example from:

https://www.runoob.com/cprogramming/c-function-qsort.html

#include <stdio.h>
#include <stdlib.h>

int values[] = { 88, 56, 100, 2, 25 };

int cmpfunc (const void * a, const void * b)
{
   return ( *(int*)a - *(int*)b );
}

int main()
{
   int n;

   printf("排序之前的列表：\n");
   for( n = 0 ; n < 5; n++ ) {
      printf("%d ", values[n]);
   }

   qsort(values, 5, sizeof(int), cmpfunc);

   printf("\n排序之后的列表：\n");
   for( n = 0 ; n < 5; n++ ) {
      printf("%d ", values[n]);
   }
 
  return 0;
}
Sucked definition of pointer


https://mp.weixin.qq.com/s/g80HxtEfmS4nasxUOS79_w

Extra: Pointer and its application scanerios (Base on ARM Cortex-M)

Memory Address


Memory is byte addressable

Memory is an array of bytes
Each byte has a memory address
Smallest data that can be addressed is a byte

The reason that why boolean data type is 1 byte.


Each address has 32 bits (ARM Cortex-M)


Pointer


The value of a pointer is simply the memory address of some variable

If a variable occupies multiple bytes in memory, its address is the lowest address of all bytes it occupies.


The pink address is this integer's
This is small-endian order

TEST

/*
 * @Date: 2022-07-21 22:39:45
 * @LastEditors: littleblackLB
 * @LastEditTime: 2022-07-22 00:16:55
 * @FilePath: \Cpp\src\Test.cpp
 */

#include <iostream>

using namespace std;

// 一个指针数组，拥有三个元素，每个元素指向一个函数原型为 void func()
typedef void (*(ARR[3]))();
// 一个函数指针返回一个数组，这是一个指针数组，容量为10，
// 每个元素指向一个函数原型为 void func()
typedef ARR *(*FUNC)(); // 等价于 typedef void(*(*(*FUNC)())[3])();

void func2()
{
    printf("%d\n", 888);
}

void func3()
{
    printf("%d\n", 889);
}

void func4()
{
    printf("%d\n", 890);
}

ARR *func1()
{
    ARR *rtn = new ARR[3];
    // 解引用rtn指针到数组头指针，解数组头指针，并赋值为func2
    **rtn = func2;  // 等价于*rtn[0] = func2;
    *rtn[1] = func3;
    *rtn[2] = func4;
    return rtn;
}

int main(int argc, char const *argv[])
{
    system("chcp 65001");
    FUNC f = func1;
    ARR *fs = (*f)();  // 等价于ARR *fs = f();
    for (int i = 0; i < 3; i++)
    {
        (*fs[i])();  // 等价于fs[i]();
    }
    delete[] fs;
    system("pause");
    return 0;
}
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