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Bit Fields in C
In this section we will what is bit fields in C.
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 bits 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 bits. For example, the above structure can be re-written as follows −
struct {
unsigned int widthValidated : 1;
unsigned int heightValidated : 1;
} status;The above structure requires 4 bits 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 bits. However as soon as you have 33 variables, it will allocate the next slot of the memory and it will start using 8 bits. Let us check the following example to understand the concept −
Example Code
#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
", sizeof(status1));
printf( "Memory size occupied by status2 : %d
", sizeof(status2));
return 0;
}Output
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 | 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 –
Example Code
#include <stdio.h>
#include <string.h>
struct {
unsigned int age : 3;
} Age;
int main( ) {
Age.age = 4;
printf( "Sizeof( Age ) : %d
", sizeof(Age) );
printf( "Age.age : %d
", Age.age );
Age.age = 7;
printf( "Age.age : %d
", Age.age );
Age.age = 8;
printf( "Age.age : %d
", Age.age );
return 0;
}Output
Sizeof( Age ) : 4 Age.age : 4 Age.age : 7 Age.age : 0
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