C language & bit field

Bit field （ Binary digit ）

effect : Can name a byte and eight bits separately and use , Significantly improve memory utilization
use ： Single chip microcomputer
Definition ： When defining a structure, add ： Write the position occupied
Regulations ： The width of the bit field cannot exceed the length of the attached data type

struct Text{
    
	unsigned int a:2;
	unsigned int b:1;
};

A structure defined here , Inside a Account for only a 2 Binary digit ,b Account for only a 1 position , The whole structure only accounts for 3 position , Cannot exceed the structure data type length interpretation ：unsigned int Type account 4 Bytes , altogether 32 Bit binary , You can't give a Assignment proportion 33 position .

Nameless bit field

struct Text{
    
	unsigned int a:2;
	unsigned int b:1;
	unsigned int  :1;
};

It's a bit field without a name , The function is mostly to adjust the position of variables , Round up the whole byte

Tips :

Support the type of bit field ： int、signed int and unsigned int,_Bool
Other compilers have extensions , Can support more types .

Logical bit operators （ In order of priority ）

~ According to the not ： Put the... Of the operand 1 Turn into 0,0 Turn into 1;
& Bitwise AND ： The corresponding bit operands on both sides are 1 The result is true, Otherwise false
^ Bitwise XOR ： When the corresponding operands on both sides are different, it is 1, Otherwise 0
| Press bit or ： One of the operands on both sides is 1 The results for 1, Otherwise 0
Besides ~ Cannot be associated with = combination , Everything else can be compared with = combination , Usage is similar. +=

int mask = 0xff;
int a = 0xabcd;
int b = 0xbbbb;
a &= mask;// equivalent a = a & mask;
b ^= mask;//b = b ^ mask;

Shift Operators

1、 Left shift operator : There are two parameters ( Binary number to be shifted )<<( Shift left digit ), Fill the blank after moving , Move a few bits to the left and multiply the source operand by 2 Several power

11001010 << 2 == 00101000

2、 Shift right operator : Shift the source operand a few bits to the right by 2 Several power

11001010 >> 2 == 00110010

Be careful ： The above instructions are limited to unsigned integers on the left , The right shift is not greater than the maximum left width , Other behaviors and undefined behaviors are treated differently

application

1、 Judgment bit ： Judge whether the number is 1, For example, judgment 1100 1010 Whether the first and third places of are 1 We can use a mask 1010 0000 Bitwise and with the source operand , If the result is 1010 0000 Then the judgment is true Otherwise false.
2、 open （ close ） position ： Which digit can be set as 1（ perhaps 0）, Such as the third place , Also use mask 0010 0000（1101 1111） Bitwise OR... With the source operand |（&）, The result is to put the third position 1（0）.

11001010 | 00100000 == 11101010

3、 Transpose ： You can transpose the number , For example, mask the third and fifth bits 0010 1000 XOR with the source operand ^, Just transpose the third and fifth positions

11001010 ^ 00101000 == 11100010