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Manually save data in individual bits

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beginner13

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Hi! I'm a C beginner. I have questions, let say if I have a structure of message containing of Id, Length, and 3 bytes of data, and I want to manually saved in individual bits of data, how can I do that? I confused on how to use typedef struct and union. Below is the sample code:

typedef struct
{
uint8_t Id;
uint8_t Length;
uint8_t DataField[3];
}AA;


struct DataField
{
unsigned int b0:1;
unsigned int b1:1;
unsigned int b2:1;
unsigned int b3:1;
unsigned int b4:1;
unsigned int b5:1;
unsigned int b6:1;
unsigned int b7:1;
unsigned int b8:1;
unsigned int b9:1;
unsigned int b10:1;
unsigned int b11:1;
unsigned int b12:1;
unsigned int b13:1;
unsigned int b14:1;
unsigned int b15:1;
unsigned int b16:1;
unsigned int b17:1;
unsigned int b18:1;
unsigned int b19:1;
unsigned int b20:1;
unsigned int b21:1;
unsigned int b22:1;
unsigned int b23:1;
}u;

By the way, the DataField is declared as above. It's not necessary for a data to be 24 bits. It can be bit 0 - bit 8, it can be bit 10 - bit 15 and so on. So, for each case, I want to ignore the other bits.
 

What a union does is let the compiler know that you want to be able to access the same location in memory in more than one way. So, for a simple example, here's how I do this in my HD44780 LCD code:

Code: 
union
{
	struct
	{
		unsigned bit0: 1;
		unsigned bit1: 1;
		unsigned bit2: 1;
		unsigned bit3: 1;
		unsigned bit4: 1;
		unsigned bit5: 1;
		unsigned bit6: 1;
		unsigned bit7: 1;
	};
	unsigned char LCD_dataByte;
}LCD_dataBits;

Then I can set all eight bits by going:

Code: 
LCD_dataBits.LCD_dataByte = [a byte of data (e.g. a char or unsigned char)];

Or I can set an individual bit by going:

Code: 
LCD_dataBits.bit0 = [1 or 0];
 

If you have a float value which is 4 bytes and int8 (1 byte) then you can use a Union like this.
Code C - [expand]
1
2
3
4
5
6
7
8
9
10

union{
   float myfloat;
   int8  bit[4];
 
}myUnion
 
myUnion.byte[0] = 4;
myUnion.byte[1] = 6;
myUnion.byte[2] = 8;
myUnion.byte[3] = 4;


myUnion.myfloat will be the the bytes 4, 8, 6, 4 converted to float representation.

Instead of float and byte use byte (char) variable and bit (bit / sbit) in union to access individual bits.

This might help https://stackoverflow.com/questions/47981/how-do-you-set-clear-and-toggle-a-single-bit-in-c
 

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