A union is a variable which may hold (at different times) objects of different sizes and types. C uses the union statement to create unions, for example:
union number
{
short shortnumber;
long longnumber;
double floatnumber
} anumber
defines a union called number and an instance of it called anumber. number is a union tag and acts in the same way as a tag for a structure.
Members can be accessed in the following way:
printf("%ld\n",anumber.longnumber);
This obvliously will displays the value of longnumber.
When the C compiler is allocating memory for unions it will always reserve enough room for th largest member (in the above example this is 8 bytes for the double).
In order that the program can keep track of the type of union variable being used at a given time i is common to have a structure (with union embedded in it) and a variable which flags the union type:
An example is:
typedef struct
{ int maxpassengers;
}jet;
typedef struct
{ int liftcapacity;
}helicopter;
typedef struct
{ int maxpayload;
} cargoplane;
typedef union
{ jet jetu;
helicopter helicopteru;
cargoplane cargoplaneu;
} aircraft;
typedef struct
{ aircrafttype kind;
int speed;
aircraft description;
} an_aircraft;
This example defines a base union aircraft which may either be jet, helicopter, or cargoplane.
In the an_aircraft structure there is a kind member which indicates which structure is being held at the time.