C - Structures - Bit Field

C language:  - BIT FIELDS in Structures

·        C permits us to use small bit fields to hold data items and thereby to pack several data items in a word or 2 bytes of memory.

·        A bit field is a set of adjacent bits whose size can be from 1to16 bits in lengths.

·        The name and size of bit fields are declared using a structure.

·        The syntax of bit fields is

Struct tag- name

{

   data-type  name1: bit-lenth;

   data-type  name2: bit-lenth;

    ………………………

    ………………………

   data- type  nameN: bit-lenth;

}

·        The data type is either int or unsigned int or signed int

·        The length is the number of bits(one bit for signed int or int).

The rules for bit fields :

1)    The first field always starts with the first bit of the word.

2)    A bit field cannot overlap integer boundaries.

3)    There can be unnamed fields declared with size.

Example:

Unsigned  :  bit-length

4)    There can be unused bits in a word.

5)    We cannot take the address of a bit field variable.

6)    We cannot use scanf,pointers,arrays to read values into bit fields.

7)    Bit fields should be assigned values that are within the range of their size.

Example:

Struct  personal

{

   Unsigned sex                   :  1

   Unsigned age                   :  7

   Unsigned m_status         :  1

   Unsigned children           :  3

   Unsigned                           :  4

} emp;

This defines a variable name emp with four bit fields. The range of values each field could have as follows:

            Bit field            Bit length        Range  of value

             Sex                       1                   0 or 1

             Age                       7                   0 or 127      

            M_status              1                   0 or 1

           Children                 3                   0 to 7