compute_ip_checksum.c

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#include <stdint.h>
#include <ip_checksum.h>
 
unsigned long compute_ip_checksum(const void *addr, unsigned long length)
{
        const uint8_t *ptr;
        volatile union {
                uint8_t  byte[2];
                uint16_t word;
        } value;
        unsigned long sum;
        unsigned long i;
        /* In the most straight forward way possible,
         * compute an ip style checksum.
         */
        sum = 0;
        ptr = addr;
        for (i = 0; i < length; i++) {
                unsigned long v;
                v = ptr[i];
                if (i & 1)
                        v <<= 8;
                /* Add the new value */
                sum += v;
                /* Wrap around the carry */
                if (sum > 0xFFFF)
                        sum = (sum + (sum >> 16)) & 0xFFFF;
        }
        value.byte[0] = sum & 0xff;
        value.byte[1] = (sum >> 8) & 0xff;
        return (~value.word) & 0xFFFF;
}
 
unsigned long add_ip_checksums(unsigned long offset, unsigned long sum,
        unsigned long new)
{
        unsigned long checksum;
        sum = ~sum & 0xFFFF;
        new = ~new & 0xFFFF;
        if (offset & 1) {
                /* byte swap the sum if it came from an odd offset
                 * since the computation is endian independent this
                 * works.
                 */
                new = ((new >> 8) & 0xff) | ((new << 8) & 0xff00);
        }
        checksum = sum + new;
        if (checksum > 0xFFFF)
                checksum -= 0xFFFF;
        return (~checksum) & 0xFFFF;
}