Deep analysis of C language data storage in memory

author ：   In the end —NULL

motto ： Make a little progress every day , Persistence makes a big difference ！

Article content ： C Storage of language data in memory .

 

Preface ：

After a period of study, we have learned that in C Language contains various data , But do we really understand the meaning of type , And how different data types are stored in memory . Next, we will dissect one by one .

Catalog

1. A detailed description of data types ：

1. Plastic surgery Family

2. Floating point family

3. Construction type

4. Pointer types

5. Empty type

2. The meaning of type

3. Storage of different data types in memory  

1. Concept introduction ：

 2. The storage form of data in memory ：

3. Storage order of data in memory ：

1. Large and small end ：

2. Judge the big and small ends ：

 4.char Type of data stored in memory

1. A signed char Store data range in memory ：

2. Unsigned char Store data range in memory ：

5. Storage rules of floating point numbers in memory

1. Floating point storage rules ：

2. Rules for storing floating-point numbers in memory

3. Rules for fetching floating-point numbers from memory

4. give an example ：

6. summary

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1. A detailed description of data types ：

1. Plastic surgery Family

char
 unsigned char
 signed char
short
 unsigned short [int]
 signed short [int]
int 
 unsigned int
 signed int
long
 unsigned long [int]
 signed long [int]

2. Floating point family

float
double

3. Construction type

> An array type
> Type of structure struct
> Enumeration type enum
> Joint type union

4. Pointer types

int *pi;
char *pc;
float* pf;
void* pv;

5. Empty type

void

2. The meaning of type

1. Different data types determine how much memory space we apply for in memory when defining variables .

2. Different data types determine the form in which we store data in the open space

3. Storage of different data types in memory  

1. Concept introduction ：

Data is stored in binary form in the computer ： There are three representations , namely Original code , Inverse code , Complement code .

The three representations have corresponding sign bits ： highest ‘0’ It means a positive number , highest ‘1’ A negative number

Three different representation rules ：

Original code ： Write it directly according to the corresponding binary bit
Inverse code ： The sign bits remain the same , The other bits are reversed bit by bit

Complement code ： Add one to the inverse code to get the complement

 2. The storage form of data in memory ：

int  a= - 10;

summary ： Through debugging, it is found that the data is stored in the form of complement in memory .

3. Storage order of data in memory ：

1. Large and small end ：

The phenomenon ：

Why is the data we define opposite to the data stored in memory ？

Big end storage ： Low byte order （ Low weight bit ） The data of is stored at a high address , High byte order （ High weight bit ） The data of is stored at the low address .

Small end storage ： Low byte order （ Low weight bit ） The data of is stored at the low address , High byte order （ High weight bit ） The data of is stored at a high address .

Whether to use big end storage or small end storage for storing data depends on the compiler . stay VS Small end storage .

2. Judge the big and small ends ：

Title Description ： Write a piece of code , Determine whether the compiler used is small end storage or large end storage .

Ideas ： Shaping data 1 Forced to char type , Then apply the solution

#include<stdio.h>

int  check_sys()
{
	int i = 1;
	return *(char*)&i;
}
int main()
{
	if (check_sys())
	{
		printf(" The small end \n");
	}
	else
	{
		printf(" Big end \n");
	}

	return 0;
}

 4.char Type of data stored in memory

char

signed char ;  // The signed

unsigned char;// Unsigned

1. A signed char Store data range in memory ：

 

summary ： The signed char The data storage range of type is -128~127

2. nothing Symbol char Store data range in memory ：

 

summary ： The storage range of unsigned data in memory is 0~255

5. Storage rules of floating point numbers in memory

1. Floating point storage rules ：

According to international standards IEEE（ Institute of electrical and Electronic Engineering ） 754, Any binary floating point number V It can be expressed in the following form ：
(-1)^S * M * 2^E
(-1)^S The sign bit , When S=0,V Is a positive number ; When S=1,V It's a negative number .
M Represents a significant number , Greater than or equal to 1, Less than 2.
2^E Indicates the index bit .

give an example ：

5.5
Binary form ：101.1
Scientific enumeration ：1.011*2^2
Add sign bits ：（-1）^0*1.011*2^2
                        0==S  M==1.001  E==2

2. Rules for storing floating-point numbers in memory

1.32 Bit single precision floating point number ：

highest 1 Bits are sign bits S, And then there was 8 Bit index E, The rest M yes 23 Significant digits （ Does not include 1, In order to improve the accuracy ）.

 

2.64 Bit double progress floating-point number ：

highest 1 Bits are sign bits S, And then there was 11 Bit index E, The rest M yes 52 Significant digits （ Does not include 1, In order to improve the accuracy ）.

 

Index E Storage rules for ：

E Is an unsigned number , therefore E by 8 position , The scope is 0~255, If E by 11 position , The scope is 0~2047.

But because floating-point numbers are negative , So when storing single precision floating-point numbers, add 127, Double precision floating-point number plus 1023

3. Rules for fetching floating-point numbers from memory

1.E Not all for 0 Or not all of them 1：

Index E The value of minus 127（1023）, Add 1.

2.E All for 0：

Floating point E be equal to 1-127 perhaps （1-1023） That's the true value .

The significant number is not preceded by 1, It's reduced to 0.xxxxx Decimals of , This is to indicate positive and negative 0, And close to 0 A very small number of .

3.E All for 1：

At this time , If the significant number M All for 0, Express ± infinity （ It depends on the sign bit s）;

4. give an example ：

#include<stdio.h>

int main()
{
 int n = 9;
 float *pFloat = (float *)&n;
 printf("n The value of is ：%d\n",n);
 printf("*pFloat The value of is ：%f\n",*pFloat);
 *pFloat = 9.0;
 printf("num The value of is ：%d\n",n);
 printf("*pFloat The value of is ：%f\n",*pFloat);
 return 0;
}

Code interpretation ：

 

 

6. summary

The above is about the storage rules of integer data and floating-point numbers in memory . I hope it helped you , If there are errors and deficiencies in the middle content, please leave a comment .