tcp校验和c语言代码

以下是一个计算TCP校验和的C语言代码：

#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);
    char src_ip[] = "192.168.0.1";
    char dst_ip[] = "192.168.0.2";
    unsigned short src_port = 1234;
    unsigned short dst_port = 5678;

    char packet[4096];
    memset(packet, 0, sizeof(packet));

    struct iphdr *iph = (struct iphdr *)packet;
    struct tcphdr *tcph = (struct tcphdr *)(packet + sizeof(struct iphdr));

    iph->ihl = 5;
    iph->version = 4;
    iph->tos = 0;
    iph->tot_len = sizeof(struct iphdr) + sizeof(struct tcphdr) + datalen;
    iph->id = htons(54321);
    iph->frag_off = 0;
    iph->ttl = 255;
    iph->protocol = IPPROTO_TCP;
    iph->check = 0;
    iph->saddr = inet_addr(src_ip);
    iph->daddr = inet_addr(dst_ip);

    tcph->source = htons(src_port);
    tcph->dest = htons(dst_port);
    tcph->seq = htonl(1);
    tcph->ack_seq = 0;
    tcph->doff = 5;
    tcph->fin = 0;
    tcph->syn = 1;
    tcph->rst = 0;
    tcph->psh = 0;
    tcph->ack = 0;
    tcph->urg = 0;
    tcph->window = htons(5840);
    tcph->check = 0;
    tcph->urg_ptr = 0;

    memcpy(packet + sizeof(struct iphdr) + sizeof(struct tcphdr), data, datalen);

    tcph->check = checksum((unsigned short *)(packet + sizeof(struct iphdr)), sizeof(struct tcphdr) + datalen);

    printf("TCP checksum: 0x%x\n", tcph->check);

    return 0;
}

这个代码可以计算TCP报文的校验和，其中包括IP头和TCP头，以及TCP数据。你可以将需要发送的数据放在`data`数组中，将源IP地址和目的IP地址放在`src_ip`和`dst_ip`字符串中，将源端口和目的端口放在`src_port`和`dst_port`变量中。最终计算出的校验和会被存放在`tcph->check`变量中。