Unions In C Language

Unions:

A union is data type where the data area is shared by two or more members generally of different type at different times.

For Example :-

 union u_tag {
	short ival ;
	float fval ;
	char cval ;
	} uval ;

The size of uval will be the size required to store the largest single member, 4 bytes in this case to accommodate the floating point member.

Union members are accessed in the same way as structure members and union pointers are valid.

uval.ival = 10 ;
uval.cval = 'c' ;

When the union is accessed as a character we are only using the bottom byte of storage, when it is accessed as a short integer the bottom two bytes etc. It is up to the programmer to ensure that the element accessed contains a meaningful value.

A union might be used in conjunction with the bit-field struct status in the previous section to implement binary conversions in C.

For Example :-

union conversion {
unsigned short num ;
	struct status bits ;
} number ;

We can load number with an integer

scanf( "%u", &number.num );

Since the integer and bit--field elements of the union share the same storage if we now access the union as the bit--field variable bits we can interpret the binary representation of num directly.

i.e. if ( uvar.bits.bit15 )
putchar( '1' ) ;
	else
		putchar('0') ;
	...
	if ( uvar.bits.bit0 )
		putchar( '1' ) ;
	else
		putchar('0') ;

Admittedly rather inefficient and inelegant but effective.