C: Bit Fields

In C, we can specify size (in bits) of structure and union members. The idea is to use memory efficiently when we know that the value of a field or group of fields will never exceed a limit or is withing a small range. For example, consider the following declaration of date without the use of bit fields.

#include <stdio.h>
  
// A simple representation of the date 
struct date { 
    unsigned int d; 
    unsigned int m; 
    unsigned int y; 
}; 
  
int main() 
{ 
    printf("Size of date is %lu bytes\n", 
           sizeof(struct date)); 
    struct date dt = { 31, 12, 2014 }; 
    printf("Date is %d/%d/%d", dt.d, dt.m, dt.y); 
} 

Using a compiler where an unsigned int takes 4 bytes, the result showed that the stucture ‘date’ takes 12 bytes:

Size of date is 12 bytes
Date is 31/12/2014

Since we know that the value of d is always from 1 to 31, the value of m is from 1 to 12, we can optimize the space using bit fields:

#include <stdio.h>
  
// Space optimized representation of the date 
struct date { 
    // d has value between 1 and 31, so 5 bits 
    // are sufficient 
    unsigned int d : 5; 
  
    // m has value between 1 and 12, so 4 bits 
    // are sufficient 
    unsigned int m : 4; 
  
    unsigned int y; 
}; 
  
int main() 
{ 
    printf("Size of date is %lu bytes\n", sizeof(struct date)); 
    struct date dt = { 31, 12, 2014 }; 
    printf("Date is %d/%d/%d", dt.d, dt.m, dt.y); 
    return 0; 
}

the result showed that the stucture ‘date’ takes 8 bytes:

Size of date is 8 bytes
Date is 31/12/2014