Example of C Bit Fields
In this example, we compare the size difference between the structure that does not specify bit fields and the structure that has specified bit fields.
Structure Without Bit Fields
Consider the following declaration of date without the use of bit fields.
C
// C Program to illustrate the structure without bit field#include <stdio.h>// A simple representation of the datestruct date { unsigned int d; unsigned int m; unsigned int y;};int main(){ // printing size of structure 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);} |
Size of date is 12 bytes Date is 31/12/2014
The above representation of ‘date’ takes 12 bytes on a compiler whereas an unsigned int takes 4 bytes. Since we know that the value of d is always from 1 to 31, and the value of m is from 1 to 12, we can optimize the space using bit fields.
Structure with Bit Field
The below code defines a structure named date with a single member month. The month member is declared as a bit field with 4 bits.
struct date
{
// month has value between 0 and 15,
// so 4 bits are sufficient for month variable.
int month : 4;
};
However, if the same code is written using signed int and the value of the fields goes beyond the bits allocated to the variable, something interesting can happen.
Below is the same code but with signed integers:
C
// C program to demonstrate use of Bit-fields#include <stdio.h>// Space optimized representation of the datestruct date { // d has value between 0 and 31, so 5 bits // are sufficient int d : 5; // m has value between 0 and 15, so 4 bits // are sufficient int m : 4; 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;} |
Size of date is 8 bytes Date is -1/-4/2014
Explanation
The output comes out to be negative. What happened behind is that the value 31 was stored in 5 bit signed integer which is equal to 11111. The MSB is a 1, so it’s a negative number and you need to calculate the 2’s complement of the binary number to get its actual value which is what is done internally.
By calculating 2’s complement you will arrive at the value 00001 which is equivalent to the decimal number 1 and since it was a negative number you get a -1. A similar thing happens to 12 in which case you get a 4-bit representation as 1100 and on calculating 2’s complement you get the value of -4.
Bit Fields in C
In C, we can specify the size (in bits) of the structure and union members. The idea of bit-field 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 within a small range. C Bit fields are used when the storage of our program is limited.
Need of Bit Fields in C
- Reduces memory consumption.
- To make our program more efficient and flexible.
- Easy to Implement.