能举个c++upd多线程服务器的例子吗？

当然可以，以下是一个简单的 C upd 多线程服务器的例子：

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <sys/socket.h>
#include <arpa/inet.h>
#include <pthread.h>
#include <unistd.h>

#define BUF_SIZE 1024
#define MAX_CLIENT 256

void *handle_client(void *arg);

int main(int argc, char *argv[]) {
    int server_sock, client_sock;
    struct sockaddr_in server_addr, client_addr;
    socklen_t client_addr_size;
    pthread_t t_id;

    if (argc != 2) {
        printf("Usage: %s <port>\n", argv[0]);
        exit(1);
    }

    server_sock = socket(PF_INET, SOCK_DGRAM, 0);
    if (server_sock == -1) {
        perror("socket() error");
        exit(1);
    }

    memset(&server_addr, 0, sizeof(server_addr));
    server_addr.sin_family = AF_INET;
    server_addr.sin_addr.s_addr = htonl(INADDR_ANY);
    server_addr.sin_port = htons(atoi(argv[1]));

    if (bind(server_sock, (struct sockaddr*)&server_addr, sizeof(server_addr)) == -1) {
        perror("bind() error");
        exit(1);
    }

    while (1) {
        client_addr_size = sizeof(client_addr);
        char buf[BUF_SIZE];
        int str_len = recvfrom(server_sock, buf, BUF_SIZE, 0, (struct sockaddr*)&client_addr, &client_addr_size);
        if (str_len == -1) {
            perror("recvfrom() error");
            break;
        }
        buf[str_len] = 0;
        printf("Received from %s:%d: %s\n", inet_ntoa(client_addr.sin_addr), ntohs(client_addr.sin_port), buf);

        pthread_create(&t_id, NULL, handle_client, (void*)&client_sock);
        pthread_detach(t_id);
    }

    close(server_sock);
    return 0;
}

void *handle_client(void *arg) {
    int client_sock = *((int*)arg);
    char buf[BUF_SIZE];
    int str_len;

    while ((str_len = read(client_sock, buf, BUF_SIZE)) != 0) {
        write(client_sock, buf, str_len);
    }

    close(client_sock);
    return NULL;
}

这个服务器使用 UDP 协议，可以同时处理多个客户端的请求。当有客户端连接时，服务器会创建一个新的线程来处理该客户端的请求。