在C++中，可以使用fcntl函數或者ioctl函數來設置recv函數為非阻塞模式。下面是一個簡單的示例代碼：

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

int main() {
    int sockfd;
    struct sockaddr_in serverAddr;
    
    // 創建socket
    sockfd = socket(AF_INET, SOCK_STREAM, 0);
    if (sockfd < 0) {
        std::cerr << "Error creating socket" << std::endl;
        return -1;
    }
    
    // 設置recv函數為非阻塞模式
    int flags = fcntl(sockfd, F_GETFL, 0);
    fcntl(sockfd, F_SETFL, flags | O_NONBLOCK);
    
    // 連接服務器
    serverAddr.sin_family = AF_INET;
    serverAddr.sin_port = htons(8080);
    serverAddr.sin_addr.s_addr = inet_addr("127.0.0.1");
    
    if (connect(sockfd, (struct sockaddr *)&serverAddr, sizeof(serverAddr)) < 0) {
        std::cerr << "Error connecting to server" << std::endl;
        return -1;
    }
    
    // 使用非阻塞recv函數接收數據
    char buffer[1024];
    int bytesReceived = recv(sockfd, buffer, sizeof(buffer), 0);
    if (bytesReceived < 0) {
        std::cerr << "Error receiving data" << std::endl;
    } else {
        std::cout << "Received " << bytesReceived << " bytes: " << buffer << std::endl;
    }
    
    close(sockfd);
    
    return 0;
}

在上面的代碼中，通過fcntl函數將socket設置為非阻塞模式，然后調用recv函數進行接收數據。如果沒有數據可接收，recv函數將立即返回，不會阻塞等待數據到達。