First acquaintance with C language - first acquaintance with pointer

Recognize the pointer , First of all, we need to know what memory is .

1. Memory

Memory is a very important memory on a computer , All programs in a computer run in memory .
So in order to use memory efficiently , Just put Memory is divided into small memory units , The size of each memory unit is 1 Bytes .
In order to effectively access every unit of memory , Just give it to Memory units are numbered , These numbers are called the address of the memory unit .
How did these numbers come into being ？
stay 32/64 A platform , There is 32/64 Root address line , These address lines are physical lines , After the power is on , Generate electrical signals （ The positive charge is 1, Negatively charged 0）, Then the electrical signal is converted into digital information , namely 32/ or 64 Bitwise by 0,1 A binary sequence of components , The binary sequence corresponding to each memory unit is its number .

We need to know , Every time we define a variable , You need memory to allocate a suitable space for this variable , For example, integer. int Distribute 4 Bytes ,char Distribute 1 Bytes ,double Distribute 8 Bytes .
Variables are created in memory （ To allocate space in memory ）, Each memory cell has an address , therefore Variables also have addresses .
Take out the variable address as follows ：

#include <stdio.h>
int main()
{
    
	int num = 10;
	&num;
	printf("%p\n", &num);
	return 0;
}

2. Pointer to the variable

Since the address of the variable can use the address operator （&） Take out , Can you put one The address of the variable is stored ？
Certainly. ！！！
stay C Language , There is a special variable for storing addresses , be called Pointer to the variable .
Definition method of pointer variable ：
type * Pointer variable name ;（* Indicates that the variable is a pointer variable ）
Let's demonstrate ：

int num = 10;
int *p;//p For an integer pointer variable 
p = #

In this way, the address of an integer variable is placed in an integer pointer variable .

Now we know how to save the address of the variable , So can we Use our stored address to find this variable Well ？
Certainly. , It's like you have the address of a friend of yours , You can find his home through this address .
If you want to find this variable through the pointer , Still need to use This operator , Here it is called Dereference operator *.
for instance ：

#include <stdio.h>
int main()
{
    
 int num = 10;
 int *p = &num;
 *p = 20;
 printf("%d\n", num);
 return 0;
}

Look at this code , Print out num What are the results ？


Take the example of a reshaped pointer , It can be extended to other types , Such as ：

#include <stdio.h>
int main()
{
    
 char ch = 'w';
 char* pc = &ch;
 *pc = 'q';
 printf("%c\n", ch);
 return 0; 
}

3. The size of the pointer variable

Think about a problem , The size of the integer variable is 4 Bytes ,char type 1 Bytes ,double8 Bytes , that What is the size of the pointer variable ？ Is the size of different types of pointer variables different ？
Let's test it ：

#include <stdio.h>

int main()
{
    
    printf("%d\n", sizeof(char *));
    printf("%d\n", sizeof(short *));
    printf("%d\n", sizeof(int *));
    printf("%d\n", sizeof(double *));
    return 0; 
}

What is the result of the operation ？

Why are different types of pointer variables the same size ？ And why 4 Bytes? ？
as a result of ：
The pointer is used to store the address , So the size of the pointer variable depends on the size of the address , On the same platform, the size of the address is fixed .
32 The address under the bit platform is 32 individual bit position （ namely 4 Bytes ）
64 The address under the bit platform is 64 individual bit position （ namely 8 Bytes ）
stay 32 A platform , The address of the memory unit is determined by 32 individual 1,0 The number that makes up the binary sequence , That's it 32 A bit , namely 4 Bytes .
Empathy , stay 64 A platform ,64 individual 0,1 The binary sequence of composition constitutes the number , That's it 64 A bit , namely 8 Bytes .
So let's verify that ：
stay 32 A platform ：

4 Bytes
64 A platform ：

8 Bytes

therefore , We come to the conclusion that Conclusion ：
The size of pointer variables is fixed on the same platform :
The size of the pointer is 32 Bit platform is 4 Bytes ,64 Bit platform is 8 Bytes .
The above is a preliminary understanding of pointer .