somewrong happen when trying to transfer image files using websocketpp

[MasterL-min]

My startup sequence is to start the server first, and then the client. The client transmits the binary content of an image file to the server, and the server writes it into the file after receiving it. However, I found that the server did not respond when I started the client for the first time. , it does not even show that there is a client connected to the server. Only when I start the client for the second time can the server receive the file correctly. I want to know why this is. The client and server codes are as follows:
client:

#include <iostream>
#include <fstream>
#include <websocketpp/config/asio_no_tls_client.hpp>
#include <websocketpp/client.hpp>

// Define a WebSocket++ client type based on the ASIO network transport
using WebSocketClient = websocketpp::client<websocketpp::config::asio_client>;

int main() {
    // Create a WebSocket++ client
    WebSocketClient client;

    try {
        // Set up the client
        client.init_asio();

        // Specify the WebSocket server URI you want to connect to
        std::string serverUri = "ws://localhost:9002"; // Replace with your WebSocket server's URL

        // Set up the client connection
        websocketpp::lib::error_code ec;
        WebSocketClient::connection_ptr con = client.get_connection(serverUri, ec);

        if (ec) {
            std::cerr << "Error connecting to server: " << ec.message() << std::endl;
            return 1;
        }

        // Connect to the WebSocket server
        client.connect(con);

        // Read an image file (e.g., image.jpg) for transmission
        std::ifstream inputFile("file1.jpg", std::ios::binary);
        std::string imageData((std::istreambuf_iterator<char>(inputFile)), std::istreambuf_iterator<char>());

        // Register a callback for successful connection
        con->set_open_handler([&client, &con, &imageData](websocketpp::connection_hdl) {
            std::cout << "Connected to server" << std::endl;

            // Send the image data to the server as a binary message
            client.send(con, imageData, websocketpp::frame::opcode::binary);
        });

        // Run the WebSocket++ event loop
        client.run();

    } catch (websocketpp::exception const &e) {
        std::cerr << "WebSocket++ exception: " << e.what() << std::endl;
        return 1;
    }

    return 0;
}

server:

#include <iostream>
#include <fstream>
#include <websocketpp/config/asio_no_tls.hpp>
#include <websocketpp/server.hpp>

// Define a WebSocket++ server type based on the ASIO network transport
using WebSocketServer = websocketpp::server<websocketpp::config::asio>;

int main() {
    // Create a WebSocket++ server
    WebSocketServer server;

    try {
        // Initialize the server
        server.init_asio();

        // Bind the server to a specific port (e.g., 9002)
        server.set_reuse_addr(true);
        server.listen(9002);

        // Start the server accept loop
        server.start_accept();

        server.set_open_handler([&server](websocketpp::connection_hdl hdl) {
            std::cout << "New connection accepted." << std::endl;
        });

        // Set up the message handler
        server.set_message_handler([&server](websocketpp::connection_hdl hdl, WebSocketServer::message_ptr msg) {
            try {
                // Extract the received binary data (image) from the message
                const std::string& receivedImageData = msg->get_payload();
                printf("begin to receive\n");
                // Save the received image to a file (e.g., received_image.jpg)
                std::ofstream outputFile("received_image.jpg", std::ios::binary);
                outputFile.write(receivedImageData.c_str(), receivedImageData.size());
                outputFile.close();

                std::cout << "Received and saved an image." << std::endl;
            } catch (const std::exception& e) {
                std::cerr << "Error handling message: " << e.what() << std::endl;
            }
        });

        // Start the server event loop
        server.run();

    } catch (websocketpp::exception const &e) {
        std::cerr << "WebSocket++ exception: " << e.what() << std::endl;
        return 1;
    }

    return 0;
}