Deep analysis of data storage in memory - C language

1. Introduction to data types

1. Basic classification of types

Before writing an introduction to data types , Let's first briefly introduce release Version and debug Memory differences between versions ：
Let's put the following code in VS I'm gonna run it in , The results are quite different

	int i = 0;
	int arr[] = {
     1,2,3,4,5,6,7,8,9,10 };
	for (i = 0; i <= 12; i++)
	{
    
		arr[i] = 0;
		printf("hehe\n");
	}

Put this code in debug The result under version is hehe Dead cycle , As shown in the figure below

You can see it here ,hehe It's in an endless cycle

Put this code in Release The result under version is 13 individual hehe
The fundamental reason is that under these two versions , Data is stored in different ways
The following is a diagram ：
The above figure shows the difference between the two , When the compiler goes from low address to high address ,debug In the environment ,arr At the end of the array, if you continue to run down, it will change i Value , send i The value of is initialized to 0.release In the environment arr The end of the array does not change i Value , Therefore, it will not fall into a dead cycle .

next , recall c Basic types of data in language ：

1、 Plastic surgery Family

char
notes ： The essence of character types is ASCII Code value , It's plastic surgery , Therefore, it is divided into plastic surgery family .

unsigned char
signed char

short

unsigned short [int]
signed short [int]

int

unsigned int
signed int

long

unsigned long [int]
signed long [int]

except char type , Other types of data without specific instructions , The default is There are sign types .char The type depends on the compiler .

2、 Floating point family

float     Low precision , The range of stored values is small 
double   High precision , The range of stored values is larger 

3、 Construction type ( Custom type , We can create new types )

 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

1、void  Indicates empty type （ No type ）

2、 Usually applied to the return type of a function 、 The parameters of the function 、 Pointer types  

See the following code example

void test(void)
{
    
	// first  void  Indicates that the function has no return value 
	// the second  void  Indicates that the function does not require any arguments 
	printf("hehe\n");
}
int main()
{
    
	test(1);
	return 0;
}

2、 Shaping storage in memory

1、 Source code 、 Inverse code 、 Complement code

 There are three ways to represent integers in a computer , The original code 、 Inverse and complement 
 The three representations have two parts: sign bit and numeric bit , The sign bits are 
 use 0 Express “ just ”, use 1 Express “ negative ”, And three kinds of numeric bit negative integers 
 The expression methods are different .

Original code ： Directly translate binary into binary in the form of positive and negative numbers Just make it .
Inverse code ： Change the sign bit of the original code , The other bits can be inverted in turn .
Complement code ： Inverse code +1 You get the complement .

The original code of a positive number 、 Inverse code 、 The complement is the same , For plastic surgery ： The data stored in memory is actually Complement code
We won't explain the specific reasons here .

2、 About the concept of size end

What is the big and small end ？ In fact, it is the storage mode of data in memory , Large end storage mode and small end storage mode .

Big end （ Storage ） Pattern ： The low bit of data is stored in the high address of memory , And the high end of the data , Low address saved in memory
in ;
The small end （ Storage ） Pattern ： The low bit of data is stored in the low address of memory , And the high end of the data ,, Stored in a high address in memory .

Now let's look at a topic ： Determine whether the storage mode of the current machine is big end storage or small end storage .

#include <stdio.h>
	int check_sys()
	{
    
		int i = 1;
		return (*(char*)&i);
		// See the following figure for specific reasons 
	}
	int main()
	{
    
		int ret = check_sys();
		if (ret == 1)
		{
    
			printf(" The small end \n");
		}
		else
		{
    
			printf(" Big end \n");
		}
		return 0;
	}

3、 Floating point storage in memory

（ The focus of this blog ）

1、

(-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 .

for example V=5.0 ： Floating point numbers are stored as 101.0

If we want to figure out S、M、E, Then there can only be one decimal place

example ： V=9.5=1001.1=1.0011*2^3
therefore S=0,M=1.0011,E=3
however , There are exceptions to everything , therefore , Not all floating-point numbers can be represented in this way
for example ：V=9.6=1001.10… And 1001.11 Hover between , It cannot be expressed accurately

float —> 4byte —>32bit
double—>8byte—>64bit
although double Type ratio float The accuracy of the type should be large , But they still may not be able to save the decimal memory completely .

2、 It is worth noting that floating point numbers are used in memory S、M、E In the form of

 about 32 Floating point number of bits , The highest 1 Bits are sign bits s,
 And then 8 Bits are exponents E, The rest 23 Bits are significant numbers M.  

 about 64 Floating point number of bits , The highest 1 Bits are sign bits S,
 And then 11 Bits are exponents E, The rest 52 Bits are significant numbers M

3、 Index E Is a complex number
First E Is an unsigned integer, which means , If E by 8 position , Its value range is 0 ~ 255; If E by 11 position , Its value range is 0~2047. however , We know , In scientific counting E You can have negative numbers , therefore , In memory E The true value of must be added with an intermediate number , about 8 Bit E, The middle number is 127; about 11 Bit E, The middle number is 1023. such as ,2^10 Of E yes 10, So save it as 32 When floating-point numbers are in place , Must be saved as 10+127=137, namely 10001001.

If there is an error , I hope you can make comments or private letters ！