使用位域来定义位变量，执行位操作，多用于flag变量，YES or NO 的标记

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struct bit_t {
        unsigned char   b0:1;                   /* bit0 */
        unsigned char   b1:1;                   /* bit1 */
        unsigned char   b2:1;                   /* bit2 */
        unsigned char   b3:1;                   /* bit3 */
        unsigned char   b4:1;                   /* bit4 */

        unsigned char   b5:1;                   /* bit5 */
        unsigned char   b6:1;                   /* bit6 */
        unsigned char   b7:1;                   /* bit7 */
};

typedef union byte_u {
struct bit_t bit;
char byte;
};

联合变量定义

union byte_u key2;

位域使用
#define volEncodeCurrentState0 key2.bit.b0
#define volEncodeCurrentState1 key2.bit.b1
#define volEncodeOldState0         key2.bit.b2
#define volEncodeOldState1         key2.bit.b3
#define volEncodeIncFlag               key2.bit.b4
#define volEncodeDecFlag          key2.bit.b5

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