Question

Ive written and compiled a server program that is meant to echo onto the the screen whatever the client application types in, the code is as below.

/*Server */
#define _WIN32_WINNT 0x501
#include <iostream>
#include <windows.h>
#include <winsock2.h>
#include <ws2tcpip.h>
using namespace std;

const int winsock_version = 2;
#define PORT "3490"
#define MAX_NUM_CONNECTIONS 10

int main(void){

    WSADATA wsadata;

    if (WSAStartup(MAKEWORD(winsock_version,0),&wsadata) == 0){
        cout<<"-WSAStartup Initialized." << endl;

        struct addrinfo hints,*res;
        int sock_fd;

        memset(&hints,0,sizeof hints);
        hints.ai_family = AF_UNSPEC;
        hints.ai_socktype = SOCK_STREAM;
        hints.ai_flags = AI_PASSIVE;

        if (getaddrinfo(NULL,PORT,&hints,&res) != 0){
            cout<<"-Call to getaddress was unsucceful." << endl;
        }

        if( (sock_fd = socket(res->ai_family,res->ai_socktype,res->ai_protocol)) == -1){
            cout<<"-Unable to Create socket." << endl;
        }

        if ( (bind(sock_fd, res->ai_addr, res->ai_addrlen)) != -1 ){
            cout<<"-binding successful." << endl;
        }

        if ( (listen(sock_fd,MAX_NUM_CONNECTIONS)) != -1){
            cout<<"-Listening for incoming connections." << endl;
        }
        //-------------------------------------------------------------

        struct sockaddr_storage incming_info;
        socklen_t sin_size;
        sin_size = sizeof incming_info;

        int new_fd;

        new_fd = accept(sock_fd, (struct sockaddr*)&incming_info,&sin_size);
        if (new_fd == -1){
            cout<<"-Accepting error." << endl;
        }
        if(new_fd == INVALID_SOCKET){
            cout<<"-INVALID SOCKET ERROR." << endl;
        }
        closesocket(new_fd);
        //-------------------------------------------------------------

        cout<<"Connected?" << endl;

        while(true){
            int ret_val;
            char buffer[128];
            ret_val = recv(new_fd,buffer,sizeof(buffer),0);
            if(ret_val == -1){
                cout<<"Receiving Error." << endl;
                break;
            }else if(ret_val == 0){
                cout<<"Connection has been closed!." << endl;
                break;
            }else{
                cout<<"---Server Echoeing what was recieved from client---" << endl;
                cout<<"Server: " << buffer << endl;

            }
            closesocket(new_fd);
        }
        cout<<"-Closing connection" << endl;

    }else{
        cout<<"-WSAStartup Initialization failed." << endl;
        if(WSACleanup()!=0){
            cout<<"-WSACleanup Successful." << endl;
        }else{
            cout<<"-WSACleanup Failed." << endl;
        }
    }
    return 0;
}

The whole thing compiles and runs, however when I try to use putty and connect to localhost on port 3490 the server dies and prints out “Receiving Error”. what is causing this problem?

Answer 1

Immediately after you accept the new socket into new_fd, you call closesocket on it (below). You should not do that until you are done receiving. If you want to close the listener socket at this point, you should be calling closesocket(sock_fd) instead.

  int new_fd;

    new_fd = accept(sock_fd, (struct sockaddr*)&incming_info,&sin_size);
    if (new_fd == -1){
        cout<<"-Accepting error." << endl;
    }
    if(new_fd == INVALID_SOCKET){
        cout<<"-INVALID SOCKET ERROR." << endl;
    }
    closesocket(new_fd);