send（）或recv（）不同步

Question

I've been doing the Winsock tutorials and following it exactly. I can't seem to get either send() or recv() to function properly. I have a basic Server and Client program set up, and the connection is being made, but the Server isn't sending a message to the Client. Using the Telnet client also doesn't receive a response from the server. I'm not really sure what's happening, and all the questions I looked at were not basic or had stuff I couldn't really understand what they were doing.

Server.cpp

#include<WinSock2.h>
#include <io.h>
#include <stdio.h>

#pragma comment(lib, "ws2_32.lib")  //winsock library

int main(int argc, char *argv[])
{
    WSADATA wsa;
    SOCKET s, new_socket;
    sockaddr_in server, client;
    int c;
    char *message;

    printf("\nInitialising Winsock...");
    if (WSAStartup(MAKEWORD(2,2), &wsa) != 0)
    {
        printf("Failed. Error Code : %d", WSAGetLastError());
        return 1;
    }
    else
        printf("Initialised.\n");

    //create a socket
    if((s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == INVALID_SOCKET)
    {
        printf("Could not create socket : %d", WSAGetLastError());
        return 2;
    }
    else
        printf("Socket created.\n");

    //prepare the sockaddr_in structure
    server.sin_family = AF_INET;
    server.sin_addr.S_un.S_addr = INADDR_ANY;
    server.sin_port = htons(8888);

    //bind the socket
    if (bind(s, (sockaddr *)&server, sizeof(server)) == SOCKET_ERROR)
    {
        printf("Bind failed with error code : &d", WSAGetLastError());
        return 3;
    }
    else
        puts("Bind done");

    //listen
    listen(s, 3);

    //accept an incoming connection
    puts("Waiting for incoming connections...");

    c = sizeof(sockaddr_in);

    while (new_socket = accept(s, (sockaddr *)&client, &c) != INVALID_SOCKET)
    {
        printf("Connect successful...\n");

        //reply to the client
        message = "Hello Client, I have recieved your connection, but I have to go now, bye!\n";
        send(new_socket, message, strlen(message), 0);

        puts("Message sent.\n");
    }

    if (new_socket == INVALID_SOCKET)
    {
        printf("accept() failed with error code : %d", WSAGetLastError());
        return 4;
    }

    //close the socket
    closesocket(s);
    WSACleanup();

    return 0;
}

Client.cpp

#ifndef WIN32_LEAN_AND_MEAN
#define WIN32_LEAN_AND_MEAN
#endif

#include <Windows.h>
#include <WinSock2.h>
#include <WS2tcpip.h>
#include <IPHlpApi.h>
#include <stdio.h>

#pragma comment(lib, "Ws2_32.lib")

int main(int argc, char *argv[])
{
    //intialize variables
    WSADATA wsa;
    char ip[100] = "192.168.1.117";
    SOCKET s;
    sockaddr_in server;
    char *message, server_reply[75];
    int recv_size;

    //initialize Winsock
    printf("\nInitialising Winsock...\n");
    if(WSAStartup(MAKEWORD(2,2), &wsa) != 0)
    {
        printf("Failed. Error Code : %d", WSAGetLastError());
        return 1;
    }
    printf("Initialised.\n");

    //create the socket
    if((s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == INVALID_SOCKET)
    {
        printf("Could not create socket : %d", WSAGetLastError());
        return 3;
    }
    printf("Socket created.\n");

    server.sin_addr.S_un.S_addr = inet_addr(ip);
    server.sin_family = AF_INET;
    server.sin_port = htons(8888);

    //connect to the server
    if (connect(s, (sockaddr *)&server, sizeof(server)) < 0)
    {
        puts("connect error");
        return 4;
    }
    else
    {
        printf("Connect successful");

        recv_size = recv(s, server_reply, 75, 0);
    }

    if (recv_size <= 0)
    {
        puts("recv() failed\n");
    }
    else
    {
        //add a NULL terminating character to make it a proper string before printing
        server_reply[recv_size] = '\0';
        puts(server_reply);
    }

    getchar();

    //close the socket
    closesocket(s);
    WSACleanup();

    return 0;
}

The client also fails to print the "recv() failed" line; it's like it's stuck at the recv() call.

Answer 1

On the server side:

• you are not checking the return value of listen() for error.

• you are not resetting c on each call to accept() , and you are not calling closesocket() on each client that is accepted.

• you are not checking the return value of send() for error.

Try this instead:

#include <WinSock2.h>
#include <stdio.h>

#pragma comment(lib, "ws2_32.lib")  //winsock library

int main(int argc, char *argv[])
{
    WSADATA wsa;
    SOCKET s, new_socket;
    sockaddr_in server, client;
    int c, res, messagelen;
    const char *message;

    printf("\nInitializing Winsock...");
    res = WSAStartup(MAKEWORD(2,2), &wsa);
    if (res != 0)
    {
        printf("Failed. Error: %d\n", res);
        return 1;
    }
    printf("Initialized.\n");

    //create a socket
    if((s = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP)) == INVALID_SOCKET)
    {
        printf("Could not create socket. Error: %d\n", WSAGetLastError());
        return 2;
    }
    printf("Socket created.\n");

    //prepare the sockaddr_in structure
    server.sin_family = AF_INET;
    server.sin_addr.S_un.S_addr = INADDR_ANY;
    server.sin_port = htons(8888);

    //bind the socket
    if (bind(s, (sockaddr *)&server, sizeof(server)) == SOCKET_ERROR)
    {
        printf("Bind failed. Error: &d", WSAGetLastError());
        return 3;
    }
    printf("Bind done.\n");

    //listen
    if (listen(s, 3) == SOCKET_ERROR)
    {
        printf("Listen failed. Error: &d", WSAGetLastError());
        return 4;
    }
    printf("Listening.\n");

    //accept incoming connections
    printf("Waiting for incoming connections...\n");

    do
    {
        c = sizeof(sockaddr_in);

        new_socket = accept(s, (sockaddr *)&client, &c);
        if (new_socket == INVALID_SOCKET)
        {
            printf("Failed to accept a client. Error: %d\n", WSAGetLastError());
            return 5;
        }

        printf("Client connected...\n");

        //reply to the client
        message = "Hello Client, I have received your connection, but I have to go now, bye!\n";
        messagelen = strlen(message);

        do
        {
            res = send(new_socket, message, messagelen, 0);
            if (res == SOCKET_ERROR)
            {
                printf("Failed to send message. Error: %d\n", WSAGetLastError());
                break;
            }

            message += res;
            messagelen -= res;
        }
        while (messagelen > 0);

        if (messagelen == 0)
            printf("Message sent.\n");

        //close the client socket
        closesocket(new_socket);
    }
    while (true);

    //close the server socket
    closesocket(s);
    WSACleanup();

    return 0;
}

On the client side, the only real problem I see is your recv() call has a potential buffer overflow waiting to happen, since you ask it to read 75 bytes, and that is the exact size of your buffer. It just happens that your server is only sending 74 bytes max, but if it ever sent more, you could overflow the buffer when appending the '\\0' terminator to it.

So, either:

• call recv() with sizeof(server_reply)-1 as the buffer size, to give yourself room for the added terminator:

   recv_size = recv(s, server_reply, sizeof(server_reply)-1, 0); 

• use printf() instead of puts() so you don't need to null-terminate the buffer at all when printing it to the console. You can pass recv_size as a parameter to limit the amount of text being printed:

   //server_reply[recv_size] = '\\0'; //puts(server_reply); printf("%.*s", recv_size, server_reply); 

Answer 2

From the MSDN documentation on closesocket():

Note To assure that all data is sent and received on a connection, an application should call shutdown before calling closesocket (see Graceful shutdown, linger options, and socket closure for more information). Also note, an FD_CLOSE network event is not posted after closesocket is called.

Basically the data you have sent was still pending when you closed the socket.