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Programming languages - C Programming

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Lesson Description


Lession - #19 C Bit Field


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 −
Sr.No. Element & Description
1 type
An integer type that determines how a bit-field's value is interpreted. The type may be int, signed int, or unsigned int.
2 member_name
The name of the bit-field.
3 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