union -- C Keyword

Multiply declare a variable

A union defines an area of storage that can accept any one of several types
of  data.  In  effect, it  is a  multiple declaration  of a  variable.  For
example, a union may be declared to consist of an int, a double, and a char
*. Any one of these three  elements can be held by the union at a time, and
will be handled appropriately by it.  For example, the declaration

union {
    int number;
    double bignumber;
    char *stringptr;
} example;

allows example  to hold either  an int, a  double, or a pointer  to a char,
whichever is needed at the time.   All of these have the same address.  The
elements of  a union are accessed  like those of a  struct: for example, to
access number from the above example, type example.number.

unions are  helpful in dealing  with heterogeneous data,  especially within
structures; however,  you are  responsible for  keeping track of  what data
type the union  is holding at any given time.   Passing a double to a union
and then reading the union as though it held an int will yield results that
are unpredictable, and probably unwelcome.

Example

For an example  of how to use a union in  a program, see the entry for byte
ordering.

See Also

C keywords,
initialization,
struct,
structure
ANSI Standard, §3.1.2.5, §3.5.2.1

© 2012 by Stephen A. Ness