C Unions

Unions are similar to structures. A union is declared and used in the same ways that a structure is. A union differs from a structure in that only one of its members can be used at a time. The reason for this is simple. All the members of a union occupy the same area of memory. They are laid on top of each other.

Defining, Declaring, and Initializing Unions

Unions are defined and declared in the same fashion as structures. The only difference in the declarations is that the keyword union is used instead of struct. To define a simple union of a char variable and an integer variable, you would write the following:

union shared {
 char c;
 int i;

This union, shared, can be used to create instances of a union that can hold either a character value c or an integer value i. This is an OR condition. Unlike a structure that would hold both values, the union can hold only one value at a time. Figure 11.7 illustrates how the shared union would appear in memory.

A union can be initialized on its declaration. Because only one member can be used at a time, only one can be initialized. To avoid confusion, only the first member of the union can be initialized. The following code shows an instance of the shared union being declared and initialized:

union shared generic_variable = {`@'};

Notice that the generic_variable union was initialized just as the first member of a structure would be initialized.

Note : The union can hold only one value at a time.

The union Keyword

union tag {
 /* additional statements may go here */

The union keyword is used for declaring unions. A union is a collection of one or more variables (union_members) that have been grouped under a single name. In addition, each of these union members occupies the same area of memory.

The keyword union identifies the beginning of a union definition. It's followed by a tag that is the name given to the union. Following the tag are the union members enclosed in braces. An instance, the actual declaration of a union, also can be defined. If you define the structure without the instance, it's just a template that can be used later in a program to declare structures. The following is a template's format:

union tag {
 /* additional statements may go here */

To use the template, you would use the following format:

union tag instance;

To use this format, you must have previously declared a union with the given tag.

Accessing Union Members

Individual union members can be used in the same way that structure members can be used--by using the member operator (.). However, there is an important difference in accessing union members. Only one union member should be accessed at a time. Because a union stores its members on top of each other, it's important to access only one member at a time.

Example : An example of the wrong use of unions.

1: /* Example of using more than one union member at a time */
2: #include <stdio.h>
4: main()
5: {
6: union shared_tag {
7: char c;
8: int i;
9: long l;
10: float f;
11: double d;
12: } shared;
14: shared.c = `$';
16: printf("\nchar c = %c", shared.c);
17: printf("\nint i = %d", shared.i);
18: printf("\nlong l = %ld", shared.l);
19: printf("\nfloat f = %f", shared.f);
20: printf("\ndouble d = %f", shared.d);
22: shared.d = 123456789.8765;
24: printf("\n\nchar c = %c", shared.c);
25: printf("\nint i = %d", shared.i);
26: printf("\nlong l = %ld", shared.l);
27: printf("\nfloat f = %f", shared.f);
28: printf("\ndouble d = %f\n", shared.d);
30: return 0;
31: }

char c = $
int i = 4900
long l = 437785380
float f = 0.000000
double d = 0.000000
char c = 7
int i = -30409
long l = 1468107063
float f = 284852666499072.000000
double d = 123456789.876500