Calculating checksum with example

So, I had worked earlier on a message passing ping application and we wanted to incorporate the checksum into the payload.

We used the following structure:

struct tst_msg_hdr {
        uint_t          tmh_magic;      /* 4 */
        uchar_t         tmh_flags;      /* 5 */
        llt_nid_t       tmh_src;        /* 6 */
        llt_nid_t       tmh_dst;        /* 7 */
        llt_port_t      tmh_port;       /* 8 */
        ushort_t        tmh_pktnum;     /* 10 */
        uint_t          tmh_len;        /* Linux 14 */
        ushort_t        tmh_hdrcksum;   /* Linux 16, 14 otherwise */
        ushort_t        tmh_cksum;      /* Linux 18, 16 otherwise */
};

So we wanted to calculate the tmh_hdrcksum which is the checksum of only the header portion of tst_msg_hdr.

                mp->tmh_magic = HDR_MAGIC;

                mp->tmh_flags = 0;

                mp->tmh_src = mynode;

                mp->tmh_dst = node;

                mp->tmh_port = port;

                mp->tmh_pktnum = long_to_ushort(mp - start_mpool);

                mp->tmh_len = size_to_uint(size);

                /* calculate checksum only if verification is requested */

                if (veriflg) {

                        mp->tmh_hdrcksum = tmh_cksum((uchar_t *) mp,

                                (uchar_t *) &mp->tmh_hdrcksum);

                }

                mp->tmh_cksum   = 0;

                mp++;

So I pass the start and end address of the portion of memory we want to calculate the checksum on.

ushort_t tmh_cksum(unsigned char *ckstart, unsigned char *ckend)

{

        uchar_t sum = 0, rot = 0, *curp;

        for (curp = ckstart; curp < ckend; curp++) {

                sum ^= *curp;

                rot = (rot << 1) ^ *curp;

        }

        return (rot << 8) | sum;

}

This is a simple checksum calculation function: XOR each byte (from start byte to end byte) with previous sum.

Then calculate the rot = ( rot << 1) ^ *curp;

The finally return last 8 bits of rot  XORed with sum.