C Tutorials
| Bit Field |
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 situation. 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 above structure can be re-written as follows:
struct
{
unsigned int widthValidated : 1;
unsigned int heightValidated : 1;
} status;
Now the above structure will require 4 bytes of memory space for status variable but only 2 bits will be used to store the values. If you will use upto 32 variables each one with a width of 1 bit , then also status structure will use 4 bytes, but as soon as you will have 33 variables then it will allocate 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;
}
| Bit Field Declaration |
The declaration of a bit-field has the form inside a structure:
struct
{
type [member_name] : width ;
};
Below the description of variable elements of a bit field:
| Elements | Description |
|---|---|
| type | An integer type that determines how the bit-field's value is interpreted. The type may be int, signed int, 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 only 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 C compiler that age variable is going to use only 3 bits to store the value, if you will 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;
}
|
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