No facility is provided in C to handle variable length records directly, though unions do afford a means of handling some variability. A union in C may contain data that can have different types and sizes at different times. Thus unions provide a means of using a single storage area for distinct purposes. Unions are declared in the same manner as structures. The compiler allocates enough memory to hold the largest member that can be stored in the union. Unions are consequently of fixed size but allow variation in the type of data stored. The programmer has to keep track of which type is currently stored in the union. Unions would not be of significant help in Example 2.4 with the list of rentees. In this case there can be more than a few distinct rentees for each car, and unions do not fit this type of variability. Lists, the topic of the next chapter, would be used for this example.
union book_lookup
{
float location;
char title[50];
};
Variables of type book_lookup could contain either the title of a book or its location in a library (based on the Dewey Decimal System). Unions thus give some measure of variability in records. However, the records do not have variable lengths; they are of fixed length.
Unions may occur in structures, and structures may occur in unions.
struct
{
int library_id;
union book_lookup information;
}catalog[100];
Each individual structure of the catalog array can contain an "i.d." number and either a real number or character strings of length 50 in any of its entries. Then, for example, catalog[40].information.title refers to the member title in entry 40 of catalog.