Client and server can not exchange data because both get “stuck”

See more:

C++

TCP/IP

sockets

WinAPI

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INTRODUCTION:

I have studied the MSDN examples for blocking TCP server[^] and blocking TCP client[^].

I wanted to try something simple, in view of modifying those examples to create simple chat application.

I have tried to implement the following, for a start:

• send message from server
• receive and display that message on client
• send response from client
• receive and display the response from client

RELEVANT INFORMATION

I apologize in advance for the lengthy code, but i strongly believe it is relevant for me to submit SSCCE[^] for both client and the server, in order for community to stand a chance for solving the problem.

I have tried to keep the code as minimal as possible, but did not want to omit basic error checking.

You can copy/paste both in single .cpp file, and they should compile and run without problem:

Server code:

#undef UNICODE

#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <string>

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

#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "27015"

int __cdecl main(void) 
{
    WSADATA wsaData;
    SOCKET ListenSocket = INVALID_SOCKET;
    SOCKET ClientSocket = INVALID_SOCKET;

    struct addrinfo *result = NULL;
    struct addrinfo hints;

    int iResult;
    char recvbuf[DEFAULT_BUFLEN] = "";
    int recvbuflen = DEFAULT_BUFLEN;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) {
        printf("WSAStartup failed with error: %d\n", iResult);
        return 1;
    }

    ZeroMemory(&hints, sizeof(hints));
    hints.ai_family = AF_INET;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;
    hints.ai_flags = AI_PASSIVE;

    // Resolve the server address and port
    iResult = getaddrinfo(NULL, DEFAULT_PORT, &hints, &result);
    if ( iResult != 0 ) {
        printf("getaddrinfo failed with error: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    // Create a SOCKET for connecting to server
    ListenSocket = socket(result->ai_family, result->ai_socktype, result->ai_protocol);
    if (ListenSocket == INVALID_SOCKET) {
        printf("socket failed with error: %ld\n", WSAGetLastError());
        freeaddrinfo(result);
        WSACleanup();
        return 1;
    }

    // Setup the TCP listening socket
    iResult = bind( ListenSocket, result->ai_addr, (int)result->ai_addrlen);
    if (iResult == SOCKET_ERROR) {
        printf("bind failed with error: %d\n", WSAGetLastError());
        freeaddrinfo(result);
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    freeaddrinfo(result);

    iResult = listen(ListenSocket, SOMAXCONN);
    if (iResult == SOCKET_ERROR) {
        printf("listen failed with error: %d\n", WSAGetLastError());
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    // Accept a client socket
    ClientSocket = accept(ListenSocket, NULL, NULL);
    if (ClientSocket == INVALID_SOCKET) {
        printf("accept failed with error: %d\n", WSAGetLastError());
        closesocket(ListenSocket);
        WSACleanup();
        return 1;
    }

    // No longer need server socket, 
    // because I want to accept only 1 client
    closesocket(ListenSocket);

    // ===================== let us try to send a message...
    std::string message = "Test message from server !!!";
    int total = message.size();
    const int messageLength = message.size();

    while (iResult = send( ClientSocket,
        // send only the missing part of the string, if send failed to deliver entire packet:
        // we move the start of the string forward by messageLength - total
        // while we send remaining number of bytes, which is held in total
        message.substr(messageLength - total, total).c_str(), total, 0),
        iResult > 0)
    {
        total -= iResult;
    }

    if (iResult == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }

/*  // adding this, seems to solve the problem ???
    iResult = shutdown(ClientSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed with error: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }
*/
    // receive response from client...
    while (iResult = recv(ClientSocket, recvbuf, recvbuflen, 0), iResult > 0)
    {
        printf("%s", recvbuf);
        memset(recvbuf, '\0', sizeof(recvbuf));
    }

    if(iResult < 0)
    {
        printf("recv failed with error: %d\n", WSAGetLastError());
        closesocket(ClientSocket);
        WSACleanup();
        return 1;
    }

    // cleanup
    closesocket(ClientSocket);
    WSACleanup();

    getchar();  // so I can stop the console from immediately closing...
    return 0;
}

Client code:

#define WIN32_LEAN_AND_MEAN

#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
#include <string>

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


#define DEFAULT_BUFLEN 512
#define DEFAULT_PORT "27015"

int __cdecl main(int argc, char **argv) 
{
    WSADATA wsaData;
    SOCKET ConnectSocket = INVALID_SOCKET;
    struct addrinfo *result = NULL,
                    *ptr = NULL,
                    hints;

    char recvbuf[DEFAULT_BUFLEN] = "";
    int iResult;
    int recvbuflen = DEFAULT_BUFLEN;

    // Initialize Winsock
    iResult = WSAStartup(MAKEWORD(2,2), &wsaData);
    if (iResult != 0) 
    {
        printf("WSAStartup failed with error: %d\n", iResult);
        return 1;
    }

    ZeroMemory( &hints, sizeof(hints) );
    hints.ai_family = AF_UNSPEC;
    hints.ai_socktype = SOCK_STREAM;
    hints.ai_protocol = IPPROTO_TCP;

    // Resolve the server address and port
    iResult = getaddrinfo("127.0.0.1", DEFAULT_PORT, &hints, &result);
    if ( iResult != 0 ) 
    {
        printf("getaddrinfo failed with error: %d\n", iResult);
        WSACleanup();
        return 1;
    }

    // Attempt to connect to an address until one succeeds
    for(ptr=result; ptr != NULL ;ptr=ptr->ai_next) 
    {
        // Create a SOCKET for connecting to server
        ConnectSocket = socket(ptr->ai_family, ptr->ai_socktype, ptr->ai_protocol);
        if (ConnectSocket == INVALID_SOCKET) 
        {
            printf("socket failed with error: %ld\n", WSAGetLastError());
            WSACleanup();
            return 1;
        }

        // Connect to server.
        iResult = connect( ConnectSocket, ptr->ai_addr, (int)ptr->ai_addrlen);
        if (iResult == SOCKET_ERROR) 
        {
            closesocket(ConnectSocket);
            ConnectSocket = INVALID_SOCKET;
            continue;
        }
        break;
    }

    freeaddrinfo(result);

    if (ConnectSocket == INVALID_SOCKET) 
    {
        printf("Unable to connect to server!\n");
        WSACleanup();
        return 1;
    }

    // receive message from server...
    while (iResult = recv(ConnectSocket, recvbuf, recvbuflen, 0), iResult > 0)
    {
        printf("%s", recvbuf);
        memset(recvbuf, '\0', sizeof(recvbuf));
    }

    if(iResult < 0)
    {
        printf("recv failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // ===================== let us try to send a message...
    std::string message = "Client response...";
    int total = message.size();
    const int messageLength = message.size();

    while (iResult = send( ConnectSocket,
        // send only the missing part of the string, if send failed to deliver entire packet:
        // we move the start of the string forward by messageLength - total
        // while we send remaining number of bytes, which is held in total
        message.substr(messageLength - total, total).c_str(), total, 0),
        iResult > 0)
    {
        total -= iResult;
    }

    if (iResult == SOCKET_ERROR) {
        printf("send failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // shutdown the connection since no more data will be sent
    iResult = shutdown(ConnectSocket, SD_SEND);
    if (iResult == SOCKET_ERROR) {
        printf("shutdown failed with error: %d\n", WSAGetLastError());
        closesocket(ConnectSocket);
        WSACleanup();
        return 1;
    }

    // cleanup
    closesocket(ConnectSocket);
    WSACleanup();

    getchar();  // so I can stop the console from immediately closing...
    return 0;
}

PROBLEM:

I have implemented the solution, but did not get the expected result.

Server sends the message, client successfully receives and displays it, but then client gets stuck infinitely, instead of sending it's response to the server, which blocks the server infinitely as well.

What I have tried:

First try:

Using the Debugger, I have placed breakpoint after client's receive block only to determine client never gets there after it receives first message.

I believe while loop should call recv again, which should return 0, thus forcing the loop to end.

Debugger doesn't even continue to show the content of client's receive buffer after I hit Continue, instead it exhibits behavior I can not describe at this moment since I am not a native English speaker.

Second try:

I have also tried to put receiving loop from server into thread, using CreateThread, but that did not help either.

I have also tried to put receiving loop from the client into thread, but that failed too.

I have tried to put both client and server receiving loops into thread, but that failed too.

Third try:

Finally, I have added the call to shutdown( ClientSocket, SD_SEND) in the server code, you shall find it at the lower part of the code, it is commented out.

This seems to fix the problem, but i am not sure if this is the right solution since i am just starting with Winsock.