Union – C

A union is a special data type available in C that allows to store different data types in the same memory location. You can define a union with many members, but only one member can contain a value at any given time. Unions provide an efficient way of using the same memory location for multiple-purpose.

Defining a Union

To define a union, you must use the union statement in the same way as you did while defining a structure. The union statement defines a new data type with more than one member for your program. The format of the union statement is as follows −

union [union tag] {
   member definition;
   member definition;
   ...
   member definition;
} [one or more union variables];

The union tag is optional and each member definition is a normal variable definition, such as int i; or float f; or any other valid variable definition. At the end of the union’s definition, before the final semicolon, you can specify one or more union variables but it is optional. Here is the way you would define a union type named Data having three members i, f, and str −

union Data {
   int i;
   float f;
   char str[20];
} data;

Now, a variable of Data type can store an integer, a floating-point number, or a string of characters. It means a single variable, i.e., same memory location, can be used to store multiple types of data. You can use any built-in or user defined data types inside a union based on your requirement.

The memory occupied by a union will be large enough to hold the largest member of the union. For example, in the above example, Data type will occupy 20 bytes of memory space because this is the maximum space which can be occupied by a character string. The following example displays the total memory size occupied by the above union −

#include <stdio.h>
 #include <string.h> 
union Data {
 int i; float f; char str[20];
 }; 
int main( ) { 
union Data data;
 printf( "Memory size occupied by data : %d\n", sizeof(data));
 return 0; 
}

When the above code is compiled and executed, it produces the following result −

Memory size occupied by data : 20

Accessing Union Members

To access any member of a union, we use the member access operator (.). The member access operator is coded as a period between the union variable name and the union member that we wish to access. You would use the keyword union to define variables of union type. The following example shows how to use unions in a program −

#include <stdio.h>
 #include <string.h> 
union Data { 
int i; float f; char str[20]; 
}; 
int main( ) {
 union Data data;
 data.i = 10;
 data.f = 220.5; 
strcpy( data.str, "C Programming");
 printf( "data.i : %d\n", data.i);
 printf( "data.f : %f\n", data.f);
 printf( "data.str : %s\n", data.str);
 return 0; 
}

When the above code is compiled and executed, it produces the following result −

data.i : 1917853763
data.f : 4122360580327794860452759994368.000000
data.str : C Programming

The C programming language provides a keyword called typedef, which you can use to give a type a new name. Following is an example to define a term BYTE for one-byte numbers −

typedef unsigned char BYTE;

After this type definition, the identifier BYTE can be used as an abbreviation for the type unsigned char, for example..

BYTE  b1, b2;

By convention, uppercase letters are used for these definitions to remind the user that the type name is really a symbolic abbreviation, but you can use lowercase, as follows −

typedef unsigned char byte;

You can use typedef to give a name to your user defined data types as well. For example, you can use typedef with structure to define a new data type and then use that data type to define structure variables directly as follows −

#include <stdio.h>
 #include <string.h> 
typedef struct Books { 
char title[50]; 
char author[50];
 char subject[100];
 int book_id; 
} 
Book; 
int main( ) { 
Book book;
 strcpy( book.title, "C Programming");
 strcpy( book.author, "Nuha Ali"); 
strcpy( book.subject, "C Programming Tutorial"); 
book.book_id = 6495407; 
printf( "Book title : %s\n", book.title);
 printf( "Book author : %s\n", book.author);
 printf( "Book subject : %s\n", book.subject);
 printf( "Book book_id : %d\n", book.book_id);
 return 0;
 }

When the above code is compiled and executed, it produces the following result −

Book  title : C Programming
Book  author : Nuha Ali
Book  subject : C Programming Tutorial
Book  book_id : 6495407

typedef vs #define

#define is a C-directive which is also used to define the aliases for various data types similar to typedef but with the following differences −

• typedef is limited to giving symbolic names to types only where as #define can be used to define alias for values as well, q., you can define 1 as ONE etc.
• typedef interpretation is performed by the compiler whereas #define statements are processed by the pre-processor.

The following example shows how to use #define in a program −

#include <stdio.h> 
#define TRUE 1 
#define FALSE 0 
int main( ) { 
printf( "Value of TRUE : %d\n", TRUE);
 printf( "Value of FALSE : %d\n", FALSE);
 return 0;
 }

When the above code is compiled and executed, it produces the following result −

Value of TRUE : 1
Value of FALSE : 0