union_type.c

#include <stdio.h>

typedef struct {
  int foo;
  int bar;
} A;
typedef struct {
  char foo;
  char bar;
} B;

typedef union
{
  int type;
  A a;
  B b;
} Returnable;

Returnable func (void)
{
  Returnable r;

  r.a.foo = 200;
  r.a.bar = 300;
  r.type = 100;
  return r;
}

int main (void)
{
  Returnable r = func();

  printf("%i\n", r.a.foo);

  return -1;
}