bit field in c language

位域的概念

数据在存储的时候,It is not necessary to occupy a complete unit,Only need to occupy one or a few binary bits,Limit the number of bits of data,节约内存资源

 简单来说就是,比如一个int,Usually takes up four bytes,Then four bytes are32个位,But the data we actually use doesn't use that many bits,For example, we want to store the month,Then the month is not1-12呢,Then we can put this at this timeintType is designed to be four bits,Then it can store the largest data is1111,对吗,也就是0-15的范围,存储2^4个数据,Then this range is the reasonable range we want.

在c语言中,Bit fields are generally used in structures

话不多说,直接上代码

#include <stdio.h>
#include <stdlib.h>

struct Data
{
	unsigned int num1 : 1;//1个位 ,二进制为1,也就是只能表示0或者1
	unsigned int num2 : 2;//2个位,二进制为00或者11 0->3 
	unsigned int num3 : 3;//3个位 000 111 0->7
};

void main()
{
	printf("%d\n", sizeof(struct Data));
	system("pause");
}

运行结果：

Let's talk about how to calculate the size of the bit field structure later,再说一点就是,Bit fields are equivalent to limiting the data size,不可越界,比如下面的代码

#include <stdio.h>
#include <stdlib.h>

struct Data
{
	unsigned int num1 : 1;//1个位 ,二进制为1,也就是只能表示0或者1
	unsigned int num2 : 2;//2个位,二进制为00或者11 0->3 
	unsigned int num3 : 3;//3个位 000 111 0->7
};

void main()
{
	struct Data data1;
	data1.num1 = 2;
	printf("%d\n", data1.num1);
	system("pause");
}

 The result will be printed above：

Once out of bounds the result we want will not be printed.

How to determine the size of a struct with bitfields

#include <stdio.h>
#include <stdlib.h>

union MyUnion1
{
	char bj[5];//5
	int num;
};

union MyUnion2
{
	char bj[5];
	int num[2];
};

struct stu
{
	//The size of the union is the size of the largest data type occupied,But must be divisible by the widest primitive type
	union{
		char bj[5];
		int bh[2];//
	}class;//8个字节//0 8
	char xm[8];//8 8
	float cj;//16 4 
};//16 + 4 = 20

//Here, it is considered according to the byte alignment of the structure
struct test
{
	char f1 : 3;//0 1
	short f2 : 4;//2 2
	char f3 : 5;//4 1
};//5->6 才是short的整数倍

struct test1
{
	//The same type can overlap
	char num1 : 6;
	char num2 : 3;
	//上面占2个字节
	//The following overlap occupies four bytes
	int num3 : 5;
	int num5 : 20;//2-》4 + 4 = 8
};//8

struct test2{
	char f1 : 3;//0 1
	char f2;//non-bit field fields1 1//Don't worry about overlapping
	char f3 : 5;//2 1 ->3
};//3

int main() {
	printf("%d\n", sizeof(struct test));
	printf("%d\n", sizeof(struct test1));
	printf("%d\n",sizeof(struct test2));
	getchar();
	return 0;
}

运行结果：

How to read an integer using a bitfield、The binary digits of a floating point number

1.Let's first look at reading an integer

#include <stdio.h>
#include <stdlib.h>


struct bit
{
	unsigned char ch1 : 1;
	unsigned char ch2 : 1;
	unsigned char ch3 : 1;
	unsigned char ch4 : 1;
	unsigned char ch5 : 1;
	unsigned char ch6 : 1;
	unsigned char ch7 : 1;
	unsigned char ch8 : 1;
};//The same data type occupies the overlap1个字节,Exactly eight digits

void main()
{
	int num = 2;
	struct bit *p = &num;//指向intThe first address of the type
	int length = 4;//int占四个字节,也就是4
	//4 3 2 1
	while (length--) {
		//注意,Data is parsed from right to left
		printf("%d%d%d%d %d%d%d%d ",
			(p + length)->ch8,
			(p + length)->ch7,
			(p + length)->ch6,
			(p + length)->ch5,
			(p + length)->ch4,
			(p + length)->ch3,
			(p + length)->ch2,
			(p + length)->ch1);
	}
	system("pause");
}

运行结果

 2.Let's look at the reading of a floating-point number

#include <stdio.h>
#include <stdlib.h>


struct bit
{
	unsigned char ch1 : 1;
	unsigned char ch2 : 1;
	unsigned char ch3 : 1;
	unsigned char ch4 : 1;
	unsigned char ch5 : 1;
	unsigned char ch6 : 1;
	unsigned char ch7 : 1;
	unsigned char ch8 : 1;
};//The same data type occupies the overlap1个字节,Exactly eight digits

void main()
{
	float num = 19.625f;
	struct bit *p = &num;//指向intThe first address of the type

	int length = 4;
	//4 3 2 1
	while (length--) {
		printf("%d%d%d%d %d%d%d%d ",
			(p + length)->ch8,
			(p + length)->ch7,
			(p + length)->ch6,
			(p + length)->ch5,
			(p + length)->ch4,
			(p + length)->ch3,
			(p + length)->ch2,
			(p + length)->ch1);
	}

	printf("%p\n", p);

	system("pause");
}

运行结果：

Take a look at the floating point memory graph