UDP计算校验和

 static unsigned short checksum(unsigned short *pwbuffer,uint32_t size)
 {
     unsigned long cksum=0;
     while(size > 1){
         cksum+=*pwbuffer++;
         size-=sizeof(uint16_t);
     }
     if(size){
         cksum+=*(uint8_t*)pwbuffer;
     }
     cksum=(cksum>>16)+(cksum&0xffff);
     cksum+=(cksum >> 16);
     return (unsigned short)(~cksum);
 }
 static unsigned int inline
 udp_checksum(unsigned long saddr,
              unsigned long daddr,
              unsigned short *buffer,
              int size)
 {
     unsigned char rawBuf[1024];
     struct pseudo_hdr
     {
         struct in_addr  src;
         struct in_addr  dst;
         uint8_t         mbz;
         uint8_t         proto;
         uint16_t        len;
     } __attribute__((__packed__));
     unsigned long sum = 0;
     struct pseudo_hdr *ph;
     int ph_len = sizeof(struct pseudo_hdr);
     ph = (struct pseudo_hdr *)rawBuf;
     ph->src.s_addr = saddr;
     ph->dst.s_addr = daddr;
     ph->mbz = 0;
     ph->proto = IPPROTO_UDP;
     ph->len = htons(size); //这里的长度为udp header + payload 的总和
     //buffer = udpheader + payload,  size = sizeof(udpheader + payload)
     memcpy(rawBuf + ph_len, buffer, size);
     //ph_len + size 是虚假头长＋UDP长＋payload长。来计算checksum
     sum = checksum((unsigned short*)rawBuf,ph_len+size);
     return(sum);
 }