Why do we need C Unions?

Unions are often used to convert between the binary representations of integers and floats:

union
{
  int i;
  float f;
} u;

// Convert floating-point bits to integer:
u.f = 3.14159f;
printf("As integer: %08x\n", u.i);

Although this is technically undefined behavior according to the C standard (you’re only supposed to read the field which was most recently written), it will act in a well-defined manner in virtually any compiler.

Unions are also sometimes used to implement pseudo-polymorphism in C, by giving a structure some tag indicating what type of object it contains, and then unioning the possible types together:

enum Type { INTS, FLOATS, DOUBLE };
struct S
{
  Type s_type;
  union
  {
    int s_ints[2];
    float s_floats[2];
    double s_double;
  };
};

void do_something(struct S *s)
{
  switch(s->s_type)
  {
    case INTS:  // do something with s->s_ints
      break;

    case FLOATS:  // do something with s->s_floats
      break;

    case DOUBLE:  // do something with s->s_double
      break;
  }
}

This allows the size of struct S to be only 12 bytes, instead of 28.