C Bit Fields: Full example code


The following two examples demonstrate the use of bit fields to reduce memory consumption of certain applications.

In the first example we create a compressed ‘bit’ struct and on the second we create a weird struct representation for bytes to show that the size of that struct is significantly less that the original one.

First example: Using bit fields to create a ‘bit’ structure

bits_bit_fields.c (compressed) (67 downloads)
#include <stdio.h>

// Weird structure to represent a 'bit'
typedef struct {
    unsigned char value;
} bit;

// 'bit' structure using bit fields
typedef struct {
    unsigned char value : 1;
} bit_bit_field;

int main( ) {

    printf( "Memory size occupied by 'bit' struct : %zu bytes\n", sizeof(bit));
    printf( "Memory size occupied by 'bit_bit_field' struct : %zu bytes\n", sizeof(bit_bit_field));

    bit bits[8];
    bit_bit_field bits_bit_field[8];

    printf( "Memory size occupied by 'bits' array : %zu bytes\n", sizeof(bits));
    printf( "Memory size occupied by 'bits_bit_field' array : %zu bytes\n", sizeof(bits_bit_field));

    // Setting the value of the first 'bit' and then printing it.
    // We will use various values for this test to show that when you set a value to a bit field
    // that is greater that the allowed size it will fill it using the last bits only.
    // It will not spill data though to neighbouring 'bits'.
    unsigned char value;
    for (value = 0; value < 4; value++)
    {
        printf("Input Value: %d\n", value);
        int bits_i;
        const int bits_length = (sizeof(bits) / sizeof(bit));
        for (bits_i = 0; bits_i < bits_length; bits_i++)
        {
            if (bits_i % 2)
            {
                bits[bits_i].value = 0;
            }
            else
            {
                bits[bits_i].value = value;
            }
            printf("%d", bits[bits_i].value);
        }
        printf("\n");

        int bits_bit_field_i;
        const int bits_bit_field_length = (sizeof(bits_bit_field) / sizeof(bit_bit_field));
        for (bits_bit_field_i = 0; bits_bit_field_i < bits_bit_field_length; bits_bit_field_i++)
        {
            if (bits_bit_field_i % 2)
            {
                bits_bit_field[bits_bit_field_i].value = 0;
            }
            else
            {
                bits_bit_field[bits_bit_field_i].value = value;
            }
            printf("%d", bits_bit_field[bits_bit_field_i].value);
        }
        printf("\n");
    }
    return 0;
}
bits_bit_fields.c (compressed) (67 downloads)

Execution output

Memory size occupied by 'bit' struct : 1 bytes
Memory size occupied by 'bit_bit_field' struct : 1 bytes
Memory size occupied by 'bits' array : 8 bytes
Memory size occupied by 'bits_bit_field' array : 8 bytes
Input Value: 0
00000000
00000000
Input Value: 1
10101010
10101010
Input Value: 2
20202020
00000000
Input Value: 3
30303030
10101010

Second example: Using bit fields to create a ‘byte’ structure where each ‘bit’ is another named member

bytes_bit_fields.c (compressed) (58 downloads)
#include <stdio.h>

// Weird structure to represent a 'byte'
typedef struct {
    unsigned char bit_0;
    unsigned char bit_1;
    unsigned char bit_2;
    unsigned char bit_3;
    unsigned char bit_4;
    unsigned char bit_5;
    unsigned char bit_6;
    unsigned char bit_7;
} byte;

// 'byte' structure using bit fields
// Unfortunately we cannot declare an array where the values are bit fields,
// so we have to declare each member separately.
// We instruct the compiler to use only one bit per element.
typedef struct {
    unsigned char bit_0 : 1;
    unsigned char bit_1 : 1;
    unsigned char bit_2 : 1;
    unsigned char bit_3 : 1;
    unsigned char bit_4 : 1;
    unsigned char bit_5 : 1;
    unsigned char bit_6 : 1;
    unsigned char bit_7 : 1;
} byte_bit_field;

int main( ) {

    printf( "Memory size occupied by 'byte' struct : %zu bytes\n", sizeof(byte));
    printf( "Memory size occupied by 'byte_bit_field' struct : %zu bytes\n", sizeof(byte_bit_field));

    byte bytes[8];
    byte_bit_field bytes_bit_field[8];

    printf( "Memory size occupied by 'bytes' array : %zu bytes\n", sizeof(bytes));
    printf( "Memory size occupied by 'bytes_bit_field' array : %zu bytes\n", sizeof(bytes_bit_field));

    // Setting the value of the first 'bit' and then printing it.
    // We will use various values for this test to show that when you set a value to a bit field
    // that is greater that the allowed size it will fill it using the last bits only.
    // It will not spill data though to neighbouring 'bits'.
    unsigned char value;
    for (value = 0; value < 4; value++)
    {
        printf("Input Value: %d\n", value);
        int bytes_i;
        const int bytes_length = (sizeof(bytes) / sizeof(byte));
        for (bytes_i = 0; bytes_i < bytes_length; bytes_i++)
        {
            if (bytes_i % 2)
            {
                bytes[bytes_i].bit_3 = 0;
            }
            else
            {
                bytes[bytes_i].bit_3 = value;
            }
            printf(" %d  ", bytes[bytes_i].bit_3);
        }
        printf("\n");

        int bytes_bit_field_i;
        const int bytes_bit_field_length = (sizeof(bytes_bit_field) / sizeof(byte_bit_field));
        for (bytes_bit_field_i = 0; bytes_bit_field_i < bytes_bit_field_length; bytes_bit_field_i++)
        {
            if (bytes_bit_field_i % 2)
            {
                bytes_bit_field[bytes_bit_field_i].bit_3 = 0;
            }
            else
            {
                bytes_bit_field[bytes_bit_field_i].bit_3 = value;
            }
            printf("%d%d%d ",
                   bytes_bit_field[bytes_bit_field_i].bit_2,
                   bytes_bit_field[bytes_bit_field_i].bit_3,
                   bytes_bit_field[bytes_bit_field_i].bit_4);
        }
        printf("\n");
    }
    return 0;
}
bytes_bit_fields.c (compressed) (58 downloads)

Execution output

Memory size occupied by 'byte' struct : 8 bytes
Memory size occupied by 'byte_bit_field' struct : 1 bytes
Memory size occupied by 'bytes' array : 64 bytes
Memory size occupied by 'bytes_bit_field' array : 8 bytes
Input Value: 0
 0   0   0   0   0   0   0   0  
000 000 000 000 000 000 000 000 
Input Value: 1
 1   0   1   0   1   0   1   0  
010 000 010 000 010 000 010 000 
Input Value: 2
 2   0   2   0   2   0   2   0  
000 000 000 000 000 000 000 000 
Input Value: 3
 3   0   3   0   3   0   3   0  
010 000 010 000 010 000 010 000

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