C++ Socket庫與IP分片處理

在C++中，處理Socket通信和IP分片主要涉及到以下幾個步驟：

1. 創建Socket
2. 綁定IP地址和端口
3. 監聽連接
4. 接受連接
5. 讀取和發送數據
6. 處理IP分片

以下是一個簡單的示例，展示了如何使用C++的Socket庫進行通信并處理IP分片：

#include <iostream>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <vector>
#include <netinet/ip.h>

int main() {
    int sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        std::cerr << "Error creating socket" << std::endl;
        return 1;
    }

    sockaddr_in server_addr, client_addr;
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    server_addr.sin_port = htons(12345);

    if (bind(sockfd, (struct sockaddr *)&server_addr, sizeof(server_addr)) < 0) {
        std::cerr << "Error binding socket" << std::endl;
        return 1;
    }

    if (listen(sockfd, 5) < 0) {
        std::cerr << "Error listening on socket" << std::endl;
        return 1;
    }

    socklen_t client_addr_len = sizeof(client_addr);
    int client_sockfd = accept(sockfd, (struct sockaddr *)&client_addr, &client_addr_len);
    if (client_sockfd < 0) {
        std::cerr << "Error accepting connection" << std::endl;
        return 1;
    }

    std::vector<char> buffer(1024);
    ssize_t recv_len = recv(client_sockfd, buffer.data(), buffer.size(), 0);
    if (recv_len < 0) {
        std::cerr << "Error receiving data" << std::endl;
        return 1;
    }

    // Process IP fragments
    struct ip *ip_header = reinterpret_cast<struct ip *>(buffer.data());
    int total_length = ntohs(ip_header->ip_len);
    int fragment_offset = ntohs(ip_header->ip_off) & 0x1fff;
    int more_fragments = (ip_header->ip_off & 0x800) != 0;

    while (total_length > 0) {
        if (fragment_offset == 0) {
            // Reassemble the first fragment
            int payload_length = total_length - ip_header->ip_hl * 4;
            if (payload_length > buffer.size()) {
                std::cerr << "Error: payload length exceeds buffer size" << std::endl;
                return 1;
            }
            std::copy(buffer.begin(), buffer.begin() + payload_length, buffer.begin());
        } else {
            // Skip the fragment header
            std::advance(buffer.begin(), fragment_offset * 8);
        }

        // Process the payload
        std::cout << "Received payload: " << std::string(buffer.begin(), buffer.begin() + payload_length) << std::endl;

        total_length -= payload_length;
        fragment_offset = 0;
        more_fragments = false;
    }

    close(client_sockfd);
    close(sockfd);

    return 0;
}

這個示例中，我們創建了一個簡單的TCP服務器，監聽端口12345。當客戶端連接并發送數據時，服務器會處理IP分片并將完整的數據打印到控制臺。

請注意，這個示例僅用于演示目的，實際應用中可能需要處理更多的錯誤情況和邊界條件。在實際項目中，你可能需要使用更高級的庫（如Boost.Asio）來簡化Socket編程和IP分片處理。