用C语言写一个IPv6 UDP checksum的计算代码

我可以回答这个问题。以下是一个示例代码：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <netinet/in.h>

unsigned short checksum(unsigned short *ptr, int nbytes);

int main(int argc, char **argv) {
char *data = "Hello, world!";
int datalen = strlen(data);
int udplen = datalen + 8; // UDP header is 8 bytes
char *udp = malloc(udplen);
memset(udp, 0, udplen);
struct sockaddr_in6 src, dst;
memset(&src, 0, sizeof(src));
memset(&dst, 0, sizeof(dst));
src.sin6_family = AF_INET6;
dst.sin6_family = AF_INET6;
inet_pton(AF_INET6, "::1", &src.sin6_addr);
inet_pton(AF_INET6, "::1", &dst.sin6_addr);
src.sin6_port = htons(12345);
dst.sin6_port = htons(54321);
memcpy(udp + 6, &src.sin6_port, 2);
memcpy(udp + 8, &dst.sin6_port, 2);
memcpy(udp + 20, data, datalen);
unsigned short sum = checksum((unsigned short *)udp, udplen);
printf("Checksum: 0x%04x\n", sum);
free(udp);
return 0;
}

unsigned short checksum(unsigned short *ptr, int nbytes) {
unsigned long sum = 0;
while (nbytes > 1) {
sum += *ptr++;
nbytes -= 2;
}
if (nbytes == 1) {
sum += *(unsigned char *)ptr;
}
sum = (sum >> 16) + (sum & 0xffff);
sum += (sum >> 16);
return (unsigned short)(~sum);
}