Unions in C

union
{
  int x;
  float y;
} u;

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

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;
  }
}

typedef union
{
    struct {
        unsigned char byte1;
        unsigned char byte2;
        unsigned char byte3;
        unsigned char byte4;
    } bytes;
    unsigned int dword;
} HW_Register;
HW_Register reg;

reg.dword = 0x12345678;
reg.bytes.byte3 = 4;

typedef union
{
    struct {
        unsigned char b1:1;
        unsigned char b2:1;
        unsigned char b3:1;
        unsigned char b4:1;
        unsigned char reserved:4;
    } bits;
    unsigned char byte;
} HW_RegisterB;
HW_RegisterB reg;

x = reg.bits.b2;

union {
   int a;
   int b;
   int c;
} myUnion;

Connection
     /       |
  Network   USB     VirtualConnection

struct Connection
{
    int type;
    union
    {
        struct Network network;
        struct USB usb;
        struct Virtual virtual;
    }
};

typedef union {
    unsigned char control_byte;
    struct {
        unsigned int nibble  : 4;
        unsigned int nmi     : 1;
        unsigned int enabled : 1;
        unsigned int fired   : 1;
        unsigned int control : 1;
    }
} ControlRegister;

typedef union MyUnion MYUNION;
union MyUnion
{
   int MyInt;
   float MyFloat;
};

MYUNION u;
u.MyInt = 10;

u.MyFloat = 1.0;

set a to b times 7.

#define ELEM_SYM_SET        0
#define ELEM_SYM_TO         1
#define ELEM_SYM_TIMES      2
#define ELEM_SYM_FULLSTOP   3
#define ELEM_VARIABLE     100
#define ELEM_CONSTANT     101

typedef struct {
    int elem_type;
    union {
        char *elem_str;
        int elem_val;
    }
} tElem;

tElem e;
e.elem_type = ELEM_SYM_SET;

tElem e;
e.elem_type = ELEM_VARIABLE;
e.elem_str = strdup ("b");   // make sure you free this later

tElem e;
e.elem_type = ELEM_CONSTANT;
e.elem_val = 7;   // make sure you free this later

+------------------------+
0x1010 |                        |
0x1011 |       elem_type        |
0x1012 |                        |
0x1013 |                        |
       +-------------+----------+
0x1014 |             |          |
0x1015 | elem_string | elem_val |
0x1016 |             |          |
0x1017 |             |          |
       +-------------+----------+

+-------------+
0x1010 |             |
0x1011 |  elem_type  |
0x1012 |             |
0x1013 |             |
       +-------------+
0x1014 |             |
0x1015 | elem_string |
0x1016 |             |
0x1017 |             |
       +-------------+
0x1018 |             |
0x1019 |  elem_val   |
0x101A |             |
0x101B |             |
       +-------------+

struct variant {
    int type;
    double number;
    char *string;
};

struct variant {
    int type;
    double number;
    char *string;
};

struct variant {
    int type;
    union {
        double number;
        char *string;
    } value;
};

struct _mydata {
    int which_one;
    union _data {
            int a;
            float b;
            char c;
    } foo;
} bar;

switch (bar.which_one)
{
   case INTEGER  :  /* access bar.foo.a;*/ break;
   case FLOATING :  /* access bar.foo.a;*/ break;
   case CHARACTER:  /* access bar.foo.a;*/ break;
}

typedef union
{
  unsigned char color[4];
  int       new_color;
}       u_color;

union data {
    int data;
    struct {
        unsigned char higher;
        unsigned char lower;
    } parts;
};

union _Obj {
    union _Obj* _M_free_list_link;
    char _M_client_data[1];    /* The client sees this.        */
};