Unions

union's are just like struct's, except that all of the member fields are squezed into the same memory and only one of them can be used consistently.
To be specific, if I have
```
union token
  {
  long i;
  double d; 
  char c;
  } t;
```
then sizeof(union token) is the max of sizeof(long), sizeof(double), and sizeof(c). The following code is valid:
```
t.d = 3.14159;
printf( "%f", t.d );
```
but this code is undefined:
```
t.i = 12345;
printf( "%f", t.d );
```
Of course, the only reason people ever use unions (other than to save space) is to access low level memory structures in strange ways which rely on the particular way unions are implemented on their system. Avoid unions, if you can.

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