recvfrom() 是一個用于接收來自套接字的數據的函數，通常用于無連接的數據報套接字（如 UDP）

1. 簡單的 UDP 服務器：

#include<iostream>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <cstring>

const int BUFFER_SIZE = 1024;

int main() {
    int sockfd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_addr_size;
    char buffer[BUFFER_SIZE];

    // 創建套接字
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        std::cerr << "Error creating socket"<< std::endl;
        return 1;
    }

    // 配置服務器地址
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(8080);

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

    std::cout << "Server listening on port 8080..."<< std::endl;

    while (true) {
        // 接收數據
        client_addr_size = sizeof(client_addr);
        ssize_t received = recvfrom(sockfd, buffer, BUFFER_SIZE - 1, 0, (struct sockaddr *)&client_addr, &client_addr_size);
        if (received < 0) {
            std::cerr << "Error receiving data"<< std::endl;
            continue;
        }

        buffer[received] = '\0';
        std::cout << "Received message: "<< buffer<< std::endl;

        // 發送響應
        sendto(sockfd, "Message received", strlen("Message received"), 0, (struct sockaddr *)&client_addr, client_addr_size);
    }

    close(sockfd);
    return 0;
}

這個示例展示了如何使用 recvfrom() 函數創建一個簡單的 UDP 服務器。服務器監聽端口 8080，接收客戶端發送的數據，并向客戶端發送確認消息。

2. 多線程 UDP 服務器：

#include<iostream>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <unistd.h>
#include <cstring>
#include<thread>
#include <mutex>

const int BUFFER_SIZE = 1024;
std::mutex mtx;

void handle_client(int sockfd, struct sockaddr_in client_addr, socklen_t client_addr_size) {
    char buffer[BUFFER_SIZE];

    while (true) {
        // 接收數據
        ssize_t received = recvfrom(sockfd, buffer, BUFFER_SIZE - 1, 0, (struct sockaddr *)&client_addr, &client_addr_size);
        if (received < 0) {
            std::cerr << "Error receiving data"<< std::endl;
            break;
        }

        buffer[received] = '\0';
        mtx.lock();
        std::cout << "Received message from client: "<< buffer<< std::endl;
        mtx.unlock();

        // 發送響應
        sendto(sockfd, "Message received", strlen("Message received"), 0, (struct sockaddr *)&client_addr, client_addr_size);
    }
}

int main() {
    int sockfd;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_addr_size;

    // 創建套接字
    sockfd = socket(AF_INET, SOCK_DGRAM, 0);
    if (sockfd < 0) {
        std::cerr << "Error creating socket"<< std::endl;
        return 1;
    }

    // 配置服務器地址
    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = INADDR_ANY;
    server_addr.sin_port = htons(8080);

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

    std::cout << "Server listening on port 8080..."<< std::endl;

    while (true) {
        // 等待客戶端連接
        client_addr_size = sizeof(client_addr);
        if (recvfrom(sockfd, nullptr, 0, MSG_PEEK, (struct sockaddr *)&client_addr, &client_addr_size) < 0) {
            std::cerr << "Error waiting for client connection"<< std::endl;
            continue;
        }

        // 為客戶端創建新線程
        std::thread client_thread(handle_client, sockfd, client_addr, client_addr_size);
        client_thread.detach();
    }

    close(sockfd);
    return 0;
}

這個示例展示了如何使用 recvfrom() 函數創建一個多線程 UDP 服務器。服務器監聽端口 8080，為每個連接的客戶端創建一個新線程以處理請求。這樣可以同時處理多個客戶端請求。

這些示例僅用于說明 recvfrom() 函數在實際項目中的應用。在實際開發中，你可能需要根據項目需求進行更多的錯誤處理和功能實現。