Compilation error about a union field in another union

Question

Consider below code, I have written:

#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;
}

The output is:

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

Now when I modify my code as below:

#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;
}

It is showing compilation error at here

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)

What am I doing wrong?

Answer 1

In the second example, when union myAccess is defined, its field hi has type union myByte, but that type is not defined yet. You need to put the definition of union myByte before union myAccess.

Answer 2

You need to declare the union myByte before you reference it within your other myAccess union.

Working example here.

Answer 3

You are not declaring the union myByte before using it in as a member of the struct in myAccess. The compiler needs to know what is the type myByte before it can be used.

Trying to use myByte as type in myAccess is logically equivalent as trying to use a variable before declaring it:

int main(void) {
   int a = 1, b = 1;
   c = a + b;
   int c;
   return c;
}

This will throw a compilation error claiming that c is undeclared.