关于另一个 union 中 union 字段的编译错误

Question

考虑下面的代码，我已经写了:

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

union myAccess {
    uint16_t access16;
    struct {
        uint8_t lo;
        uint8_t hi;
        } access8;
    };

union myByte{
    uint8_t  BYTE;
    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;
        }BIT;
    };

int main()
{

   union myAccess U;
   U.access8.lo=0xF1;
   U.access8.hi=0x55;
   printf("%x\n",U);


   union myByte B;
   B.BYTE=0;
   B.BIT.BIT4=1;
   printf("%x\n",B);

    return 0;
}

输出是:

Gaurav@Gaurav-PC /cygdrive/d
$ ./LSI
2255f1
61279210

现在当我如下修改我的代码时:

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

union myAccess {
    uint16_t access16;
    struct {
        uint8_t lo;
        union myByte hi;//here
        } access8;
    };

union myByte{
    uint8_t  BYTE;
    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;
        }BIT;
    };

int main()
{

    union myAccess U;
    U.access8.lo=0xF1;
    U.access8.hi.BYTE=0x55;
    printf("%x\n",U);
    return 0;
}

这里显示编译错误

Gaurav@Gaurav-PC /cygdrive/d
$ gcc -Wall LSI.c -o LSI
LSI.c:8: error: field `hi' has incomplete type
LSI.c: In function `main':
LSI.c:33: warning: unsigned int format, myAccess arg (arg 2)
LSI.c:33: warning: unsigned int format, myAccess arg (arg 2)

我做错了什么？

Answer 1

在第二个例子中，当定义了union myAccess时，它的字段hi的类型是union myByte，但是那个类型还没有定义.您需要将 union myByte 的定义放在 union myAccess 之前。