9. Pointer of C language (5) how many bytes does the pointer variable occupy

  This article is the... Of the pointer series 5 piece , If it feels good , You can follow and collect , It will be updated continuously in the future

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Catalog

1. Preliminary knowledge sizeof() function

2. What is stored in the pointer variable storage ？

3. How do we know how many bytes the pointer variable points to ？

4. Why do pointer variables account for 8 Bytes? ？

（1） If , We want to pass CPU Read a value in memory , So what is the operation ？

（2） Why is 8 Bytes? ？

（3） For example

summary

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A pointer in 32 Bit of computer , Occupy 4 Bytes ;
A pointer in 64 Bit of computer , Occupy 8 Bytes .

The computers we use now are 64 Bit , So it should all be 8 Bytes

But why is that ？ We analyze it layer by layer

1. Preliminary knowledge sizeof() function

sizeof( data type )

         The return value is the number of bytes occupied by the data type

sizeof（ Variable name ）     

         Returns the number of bytes occupied by the variable

              hypothesis p Point to char type （1 Bytes ）

              hypothesis q Point to int type （4 Bytes ）

              hypothesis r Point to double type （8 Bytes ）

p,q,r Whether the number of bytes occupied by itself is the same ？

int main(int argc, char const *argv[])
{
    char ch = 'A';
    int i = 99;
    double x = 66.6;

    char *p = &ch;
    int *q = &i;
    double *r = &x;
    printf("%d %d %d", sizeof(p), sizeof(q), sizeof(q));
    return 0;
}

We passed the code test , See all pointer variables p,q,r Size of bytes occupied All are 8 .

2. What is stored in the pointer variable storage ？

There are one byte and one number in memory

double The type is 8 Bytes , That is to say, it has eight numbers

Pointer to the variable Save only the address of the first byte

3. How do we know how many bytes the pointer variable points to ？

This is due to The data type of the pointer variable itself

If it is int* Explain that the pointed variable accounts for 4 Bytes

 

  If we know it is int Pointer variable of type , Then count down from the address variable it saves 4 A unit （ A unit is a byte ）

4. Why do pointer variables account for 8 Bytes? ？

p,q,r Only the address of the first byte is saved , That is, they only occupy

1 Byte address , But why 8 Bytes?

Specify the address of the first byte （ It's just a bunch of numbers ） adopt 8 Bytes to save

  Here we also need to know some knowledge about the principle of computer composition

First , We need to understand , The pointer is the address , The address is the pointer .  The address is the number of the memory unit . therefore , A pointer takes up a few bytes , How long is the memory unit number equal to one address .

  In the computer ,CPU Cannot exchange data directly with hard disk ,CPU Data can only be exchanged directly with memory . and CPU It's through Address bus 、 data bus 、 Control bus Three lines and memory for data transmission and operation .

The width of the address bus determines CPU The ability to address ;
The control bus determines CPU Control ability and control mode of other controls .
The width of the data bus determines CPU The transmission volume of a single data transmission , That is, the data transmission speed ;

（1） If , We want to pass CPU Read a value in memory , So what is the operation ？

First ,CPU Through the address bus , Find the address of this value in memory ;
then , Know whether the operation is read or write through the control bus ;
Last , Via data bus , Transfer this value to CPU in .

 （2） Why is 8 Bytes? ？

A pointer takes up a few bytes , How long is the memory unit number equal to one address .

We generally need 32 individual 0 or 1 You can find all the addresses in memory , and 32 individual 0 or 1 The combination of , Namely 32 position , That is to say 4 The size of bytes , therefore , We just need 4 You can find all the data in bytes . therefore , stay 32 Bit computer , Pointer 4 Bytes . Empathy , stay 64 Bit computer , Pointer 8 Bytes .

We know 64 The computer address bus of bit consists of 64 root ,64 Bit is 8 Bytes

（3） For example

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Memory address is like a road number , Use a few digits to indicate the number of doorplates you need to see .
For example there are 0-2^64-1 A house number , Then the length of the house number needs 64 Number to form

from 0000……00000（64 individual ）

To 1111……111111（64 individual ）

The address is the number of the memory unit , Each house number consists of 64 Bit composition is 64/8 = 8 Bytes

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summary

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1. A pointer variable , No matter how many bytes it points to , The pointer variable itself accounts for 4 Bytes （32 position ）,8 Bytes （64 position ）

2. The address of a variable is represented by the address of the first byte of the variable

3. The pointer is the address , The address is the pointer .  The address is the number of the memory unit . therefore , A pointer takes up a few bytes , How long is the memory unit number equal to one address .

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