C language first level pointer

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Learning goals ：

Master the first level pointer knowledge

• Master the first level pointer knowledge

Learning content ：

1. Pointer basis :

1.1 Definition of pointer

Pointer definition ： data type * Variable name
Such as ：int * p

int Is the data type of the pointer
p Is the variable name of the pointer
about * The function of is to reference the variable value pointed to by the pointer , Reference refers to the address of the variable

int a = 10;
int* p =  &a ;

& Here is the address character
int * p = &a; To make p Pointer to a The address of , Pictured above .
after p The address of 0x002 What is stored in is a The address of 0x001.

1.2 The size of the pointer

Pointer size is determined by CPU The addressing bits of the operation mode determine
One byte =8 position
The processor is in 32 Bit operation mode , Logical addressing bits 32, The pointer is 32 position , namely 4 Bytes
The processor is in 64 Bit operation mode , Logical addressing bits 64, The pointer is 64 position , namely 8 Bytes

When running a program in the compiler , The pointer size varies with the running environment

2. Pointer dereference function

Dereference is to return the corresponding object in the memory address
Quoting ： * Variable of pointer type
Such as 1.1 Definition of pointer In the code p The address is stored a The address of ,p Is equal to a The address of ,* p It is equal to returning a The value in the address .
modify a or * p The value of is modified a Address Value inside .

Example ：

void add(int *val)
{
    
	(*val)++;
}


int main() 
{
    
	int a = 1;
	add(&a);
	printf("%d \n", a);
	return 0;
}

Running results ：

The function in the code above add() Take the integer pointer as a formal parameter ,val Is the variable name of the pointer , * val Is the value of the transmitted address .
adopt main Function will a The address of the pass in function add() in , stay add() Function , Shape parameter val The address for a The address of , * val by a Value . Yes a Add one to the data in the address (*val)++, That is to say a++, Final a = 2.

3. Windows data storage

X86(32 position ) Operation in environment ：
stay windows in , Data is stored in small end mode （ The low order data is stored in the low order address ）

The stack area ： When using compilers , The compiler will automatically allocate and free memory , To assign local variables to the running program , Function parameter , Return the data , Return address, etc .
The address size is 32 position ,4 byte .

from 0x0000 0000 To 0xffff ffff

int main() 
{
    
	int a = 1;
	int b = 2;
	return 0;
}

The main function main() Before running , Is already a variable a,b Allocated memory space .
Stack memory partition By high address To The order of low addresses .
1 The space is allocated to a, Deposit a Value 1.
2 Assigned to b, Deposit b Value 2.

The storage of arrays in the stack ：

First, according to the size of the array, open up enough memory space from high address to low address , Then in the opened memory space, follow the small end mode , Store the data in the array from low address to high address .

Array arr[4] ={1,2,3,4}; The process is as follows

4. The operation of the pointer

You can access the addresses of different positions by adding and subtracting pointer variables , To get the effect you want .

4.1 give an example ：

4.1.1 Access array

int main() 
{
    
	int arr[4] = {
    1,2,3,4};
	int* p = &arr[0];
	for(int i = 0;i<4;i++)
	{
    
	  printf("%-3d",*(p++));
	}
	return 0;
}

Running results ：

Through the address of the pointer after ++ To get arr Address in array , It is used to output the value in the address by dereference .

4.1.2 Before the m Put the number behind

function Reverse1： Yes begin To end Data in reverse order With a pointer

void Reverse1(int* arr, int begin, int end)// Yes  begin  To  end  Data in reverse order   With a pointer 
{
    
	assert(arr != NULL && begin <= end);
	int temp = 0;
	int* brr = arr;
	int len = (end - begin + 1) / 2;
	for (int i = 0; i < len; i++, begin++, end--)
	{
    
		temp = *(arr + begin);
		*(arr + begin) = *(brr + end);
		*(brr + end) = temp;
	}
}

function Adjust： Before the m Put the number behind

void Adjust(int* arr, int len, int m) // Before the m Put the number behind 
{
    
	assert(arr != NULL);
	Reverse1(arr, 0, len-1);// Yes arr The whole is in reverse order 
	Reverse1(arr, 0, len - 1 - m);// To the front len-m Elements in reverse order 
	Reverse1(arr, len - m, len - 1);// Right back m Elements in reverse order 
}

int main() 
{
    
    int arr[] = {
     1,2,3,4,5,6,7 };
	int len = sizeof(arr) / sizeof(arr[0]);
	Adjust(arr, len, 4); 

	for (int i = 0; i < len; i++) 
	{
    
		printf("%-2d", arr[i]);
	}
		return 0;
}	

Running results ：