Unions and values stored-CodePudding

I know that a union allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Consider this program:

    #include <stdio.h>

    union integer {

         short s;
         int i;
         long l;
    };

    int main() {

         union integer I;

         scanf("%hi", &I.s);
         scanf("%d", &I.i);
         scanf("%ld", &I.l);

         printf("%hi - %d - %ld ", I.s, I.i, I.l );
    }

Suppose we enter the values 11, 55, 13 the program will give as output

13 - 13 - 13, no problem here. However, if i were to create three different variables of type struct integer

    #include <stdio.h>

    union integer {

         short s;
         int i;
         long l;
    };

    int main() {

         union integer S;
         union integer I;
         union integer L;

         scanf("%hi", &S.s);
         scanf("%d", &I.i);
         scanf("%ld", &L.l);

         printf("%hi - %d - %ld ", S.s, I.i, L.l );
    
    }

than all the values will be preserved. How come? By using three variables am i actually using three unions, each holding just one value?

CodePudding user response：

The union members s, i and l of the same variable share the same memory. Reading a different member than you have written last is undefined behavior.

If you define 3 variables of the same union type it is not much different from defining 3 variables of type int. Every variable has its own memory, and every variable can hold only one of the union members.

CodePudding user response：

What output do you expect from this code? Three different values or the same one?

    #include <stdio.h>

    int main() {

         short S;
         int I;
         long L;

         scanf("%hi", &S);
         scanf("%d", &I);
         scanf("%ld", &L);

         printf("%hi - %d - %ld ", S, I, L );
    
    }

You are declaring three separate variables, even if they are unions, all of them have their storage.