IP校验和（Just for reminding）

LZHWWY

unsigned short csum(unsigned char *addr, int count)
{
       /* Compute Internet Checksum for "count" bytes
        * beginning at location "addr".
       */
       register long sum = 0;

       while( count > 1 )
       {
           /* This is the inner loop */
           sum += * (unsigned short) addr++;
           count -= 2;
        }

       /* Add left-over byte, if any */
       if( count > 0 )
           sum += * (unsigned char *) addr;

       /* Fold 32-bit sum to 16 bits */
       while (sum>>16)
           sum = (sum & 0xffff) + (sum >> 16);

       return ~sum;
}


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