计算tcp/udp/icmp校验和的c代码

以下是计算TCP/UDP/ICMP校验和的C代码：

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

unsigned short checksum(unsigned short *buf, int nwords)
{
    unsigned long sum;

    for (sum = 0; nwords > 0; nwords--)
        sum += *buf++;

    sum = (sum >> 16) + (sum & 0xffff);
    sum += (sum >> 16);

    return (unsigned short)(~sum);
}

int main()
{
    char data[] = "Hello, world!";
    int datalen = strlen(data);

    // Allocate memory for pseudo-header
    char *pseudoheader = malloc(12 + datalen);
    memset(pseudoheader, 0, 12 + datalen);

    // Fill in pseudo-header
    struct in_addr src, dst;
    inet_aton("192.168.0.1", &src);
    inet_aton("192.168.0.2", &dst);
    memcpy(pseudoheader, &src, 4);
    memcpy(pseudoheader + 4, &dst, 4);
    pseudoheader[9] = IPPROTO_TCP;
    *(unsigned short *)(pseudoheader + 10) = htons(datalen);

    // Allocate memory for TCP header + data
    char *tcpdata = malloc(datalen);
    memset(tcpdata, 0, datalen);
    memcpy(tcpdata, data, datalen);

    // Calculate TCP checksum
    unsigned short tcpchecksum = checksum((unsigned short *)pseudoheader, 6) +
                                  checksum((unsigned short *)tcpdata, datalen / 2);
    printf("TCP checksum: 0x%04x\n", tcpchecksum);

    // Free memory
    free(pseudoheader);
    free(tcpdata);

    return 0;
}

请注意，这只是一个示例代码，实际应用中需要根据具体情况进行修改。