How to know that which variable from Union is Used?

Question

If I declare a Union as:

union TestUnion
{
    struct 
    {
      unsigned int Num;
      unsigned char Name[5];
    }TestStruct;
    unsigned char Total[7];
};         


        

Answer 1

The member to use is the one you last wrote to; the other(s) are off limits. You know which member you last wrote to, don't you? After all, it was you who wrote the program :-)

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As for you secondary question: the compiler is allowed to insert 'padding bytes' in the structure to avoid unaligned accesses and make it more performant.

example of a possible distribution of bytes inside your structure

Num    |Name     |pad
- - - -|- - - - -|x x x
0 1 2 3|4 5 6 7 8|9 a b