Bit field in c | example of bit field | My CS Tutorial

BIT FIELD IN C | C - BIT FIELDS | BIT FIELD IN C PROGRAMING | DECLARATION AND IMPLEMENTATION OF BIT FIELDS IN C |Bit field in c | example of bit field | My CS Tutorial

Bit Field

A bit field is a data structure used in computer programming. C bit field consists of a number of adjacent computer memory locations which have been allocated to hold a sequence of bits, stored so that any single bit or group of bits within the set can be addressed.They can only be declared inside a structure or a union, and allow you to specify some very small objects of a given number of bits in length. A bit field is most commonly used to represent integral types of known, fixed bit-width.

Bit field in c | My CS Tutorial

Suppose your C program contains a number of TRUE/FALSE variables grouped in a structure called status, as follows:-

struct
{
 unsigned int widthValidated;
 unsigned int heightValidated;
} status;

This structure requires 8 bytes of memory space but in actual, we are going to store either 0 or 1 in each of the variables.
The C programming language offers a better way to utilize the memory space in such situations.
If you are using such variables inside a structure, then you can define the width of a variable which tells the C compiler that you are going to use only those
number of bytes. For example, the above structure can be rewritten as follows:-

struct
{
 unsigned int widthValidated : 1;
 unsigned int heightValidated : 1;
} status;

The above structure requires 4 bytes of memory space for status variable, but
only 2 bits will be used to store the values.
If you will use up to 32 variables, each one with a width of 1 bit, then also the
status structure will use 4 bytes. However, as soon as you have 33 variables, it will allocate the next slot of the memory and it will start using 8 bytes. Let us
check the following example to understand the concept:-

#include <stdio.h>
#include <string.h>
/* define simple structure */
struct
{
 unsigned int widthValidated;
unsigned int heightValidated;
} status1;
/* define a structure with bit fields */
struct
{
 unsigned int widthValidated : 1;
 unsigned int heightValidated : 1;
} status2;
int main( )
{
 printf( "Memory size occupied by status1 : %d\n", sizeof(status1));
 printf( "Memory size occupied by status2 : %d\n", sizeof(status2));
 return 0;
}

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

Memory size occupied by status1 : 8
Memory size occupied by status2 : 4

Bit Field Declaration

The declaration of a bit-field has the following form inside a structure:-

struct
{
 type [member_name] : width ;
};

The following table describes the variable elements of a bit field:-

Elements and Description

type     :-      An integer type that determines how a bit-field's value is
interpreted. The type may be int, signed int, or unsigned int.

member_name      :-      The name of the bit-field.

width      :-      The number of bits in the bit-field. The width must be less
than or equal to the bit width of the specified type.


The variables defined with a predefined width are called bit fields. A bit field can
hold more than a single bit; for example, if you need a variable to store a value from 0 to 7, then you can define a bit-field with a width of 3 bits as follows:-

struct
{
 unsigned int age : 3;
} Age;

The above structure definition instructs the C compiler that the age variable is
going to use only 3 bits to store the value. If you try to use more than 3 bits,
then it will not allow you to do so. Let us try the following example:-

#include <stdio.h>
#include <string.h>
struct
{
 unsigned int age : 3;
} Age;
int main( )
{
 Age.age = 4;
 printf( "Sizeof( Age ) : %d\n", sizeof(Age) );
 printf( "Age.age : %d\n", Age.age );
 Age.age = 7;
 printf( "Age.age : %d\n", Age.age );
 Age.age = 8;
printf( "Age.age : %d\n", Age.age );
 return 0;
}

When the above code is compiled, it will compile with a warning and when
executed, it produces the following result:-

Sizeof( Age ) : 4
Age.age : 4
Age.age : 7
Age.age : 0


BIT FIELD IN C | C - BIT FIELDS |Bit field in c | example of bit field | BIT FIELD IN C PROGRAMING | DECLARATION AND IMPLEMENTATION OF BIT FIELDS IN C | My CS Tutorial

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