联合的位域使用

Question

I want to create a simple package with bit fields by using union. But when I tried to set "bit1" to 1, then all off my bit fields became "1". How can I solve this problem, I want to do that bit field part by using union not by using struct.

So here my struct;

    struct {
    union{
        uint8_t bit1 :1 ;
        uint8_t bit2 :1 ;
        uint8_t bit3 :1 ;
        uint8_t bit4 :1 ;
        uint8_t bit5 :1 ;
        uint8_t bit6 :1 ;
        uint8_t bit7 :1 ;
        uint8_t bit8 :1 ;


    }bits;
    uint8_t trial;


    }myStruct_t;

int main(int argc, char *argv[]) {
        myStruct_t.bits.bit1 = 1;
        myStruct_t.bits.bit2 = 0;
        printf("%x",myStruct_t.bits);
        printf("%x",myStruct_t.bits.bit1);
        printf("%x",myStruct_t.bits.bit2);

    return 0;
}

And the output is: 000.

Answer 1

Swap your union with your struct .

Ie I believe you want a union of a struct and an int, not a struct of a union and an int. (I kept the now misleading name myStruct_t , which probably should be myUnion_t now.)

#include <stdio.h>
#include <stdint.h>

union
{
    struct
    {
        uint8_t bit1 :1 ;
        uint8_t bit2 :1 ;
        uint8_t bit3 :1 ;
        uint8_t bit4 :1 ;
        uint8_t bit5 :1 ;
        uint8_t bit6 :1 ;
        uint8_t bit7 :1 ;
        uint8_t bit8 :1 ;
    }bits;
    uint8_t trial;
}myStruct_t;

int main(int argc, char *argv[])
{
    myStruct_t.trial=0; // use the encompassing union member for init
    myStruct_t.bits.bit1 = 1; // now use the bitwise view to set bits
    myStruct_t.bits.bit2 = 0;
    printf("%x",myStruct_t.trial);
    printf("%x",myStruct_t.bits.bit1);
    printf("%x",myStruct_t.bits.bit2);

    return 0;
}

The output I get is 110, you might see 12810. Apart from the absent newlines, which would have helped, it means:

• the encompassing 8bit-view has a value of 1, at least with the endianess of my environment - you might see 128
• the bit 1 has a value of 1
• the bit 2 has a value of 0