要实现这个功能应该怎么写

1万 · 2012-3-22 22:54

union {
   struct {
      unsigned b0:1;
      unsigned b1:1;
      unsigned b2:1;
      unsigned b3:1;
      unsigned b4:1;
      unsigned b5:1;
      unsigned b6:1;
      unsigned b7:1;
      }oneBit;
      unsigned char allBits;
   } myFlag;
#define fast myFlag.oneBit.b0
#define mid    myFlag.oneBit.b1
#define slow myFlag.oneBit.b2
#define flow myFlag.oneBit.b3
#define voice myFlag.oneBit.b4
#define pause myFlag.oneBit.b5
#define white myFlag.oneBit.b6
#define on_off  myFlag.oneBit.b7
#define mark myFlag.allBits
上面的语句在PICC中编译出来每一个占一个位，但在KEIL C中编译出来怎么占一个字节啊？如果在在KEIL C 中要实现如上功能，应该怎么写呢？

1万 · 2012-3-22 22:54

KEIL C直接支持位定义功能,不用这么麻烦
单个位定义
bit flag;
bit b;
if(flag)....
b = flag;
组合方式
char bdata my_flag;
sbit flag0 = my_flag^0;
sbit flag1 = my_flag^1;
sbit flag2 = my_flag^2;
...

my_flag = 3;
if(flag0){
...
}

1万 · 2012-3-22 22:56

最不喜欢用联合体了因为不会用但是从可移植性考虑

1万 · 2012-3-22 22:57

这个不能怪Keil，是C标准规定了位域是2字节对齐的……也就是说就算你只定义了1bit的位域，它也要占用2字节，其它的15bit就这么空着……不信试一下以下代码：
typedef struct
{
      unsigned b0:1;
      unsigned b1:1;
      unsigned b2:1;
      unsigned b3:1;
      unsigned b4:1;
      unsigned b5:1;
      unsigned b6:1;
      unsigned b7:1;
}oneBitStructs;
typedef union
{
      oneBitStructs oneBit;
//       unsigned char allBits;
}myFlag;

myFlag flag;
void main(void)
{
      int size = sizeof(myFlag);
      size = sizeof(oneBitStructs);
      while(1)
            ;
}
优化级别设为0
单步调试过程可以看到myFlag这个联合体占用了2字节，结构体oneBitStructs也是一样