apply checksum in c to a char

Question

How do you apply checkksum to a char in c?

Answer 1

A checksum reduces a sequence of bits to a shorter sequence, such that a change to the larger sequence results in a random change to the shorter checksum.

A char is already quite small. To produce a checksum you will need a bitfield. Actually, you will need two, since one bitfield alone will be padded at least to a full byte.

struct twochars_checksum {
    unsigned sum_a : CHAR_BIT / 2;
    unsigned sum_b : CHAR_BIT / 2;
};

void sum_char( char c, struct twochars_checksum *dest, int which ) {
    int sum;
    sum = c ^ c >> CHAR_BIT / 2; // suboptimal, but passable
    if ( which == 0 ) {
        dest->sum_a = sum;
    } else {
        dest->sum_b = sum;
    }
}

Answer 2

Suggest to follow similar approach to transfer a byte of data with checksum.

The algorithm for calculating checksum is quite simple and is as follows.

1.Check if the bit is on then add the corresponding bit value ( ie 2 to the power of bit position ) to the check sum.

2.If the bit is off then detect the sum by 1.

Note : You can use your own checksum algorithm by altering function calculate_checksum().
You can include your own processing logic in set_transfer_data().

#include <stdio.h>
typedef unsigned char   uint8_t;
typedef unsigned short  uint16_t;
typedef unsigned int    uint32_t;
#define NUM_BITS        (8)


uint16_t calculate_checksum(const uint8_t data)
{
    uint16_t checksum = 0;
    uint8_t  bit_index = 0;
    uint8_t  bit_value = 0;
    while( bit_index < NUM_BITS )   
    {
        bit_value = 1 << bit_index++;
        checksum += ( data & bit_value ) ? bit_value : -1;
    }
    return ( checksum );
}

uint8_t set_transfer_data( uint32_t *dest_data , const uint8_t src_data , const uint16_t checksum )
{
    uint8_t return_value = 0;
    *dest_data = checksum << NUM_BITS | src_data ;
    return ( return_value );
}

int main()
{
        uint8_t         return_value = 0;
        uint8_t         source_data = 0xF3;
        uint32_t        transfer_data = 0;
        uint16_t        checksum = 0;

        checksum = calculate_checksum( source_data );

        printf( "\nChecksum calculated = %x",checksum );

        return_value = set_transfer_data( &transfer_data,source_data,checksum );

        if( 0 == return_value )
        {
            printf( "\nChecksum added successfully; transfer_data = %x",
                    transfer_data );
        }
        else
        {
            printf( "\nError adding checksum" );
        }
        return ( 0 );
}