/******************************************************************************\
* simples.c - Simple TCP/UDP server using Winsock 1.1
*       This is a part of the Microsoft Source Code Samples.
*       Copyright 1996-1997 Microsoft Corporation.
*       All rights reserved.
*       This source code is only intended as a supplement to
*       Microsoft Development Tools and/or WinHelp documentation.
*       See these sources for detailed information regarding the
*       Microsoft samples programs.
\******************************************************************************/

#define WIN32_LEAN_AND_MEAN
#include <winsock2.h>
#include <stdlib.h>
#include <stdio.h>
#include <string.h>

#define DEFAULT_PORT 5001
#define DEFAULT_PROTO SOCK_STREAM // TCP

void Usage(char *progname) {
    fprintf(stderr,"Usage\n%s -p [protocol] -e [endpoint] -i [interface]\n",
        progname);
    fprintf(stderr,"Where:\n\tprotocol is one of TCP or UDP\n");
    fprintf(stderr,"\tendpoint is the port to listen on\n");
    fprintf(stderr,"\tinterface is the ipaddr (in dotted decimal notation)");
    fprintf(stderr," to bind to\n");
    fprintf(stderr,"Defaults are TCP,5001 and INADDR_ANY\n");
    WSACleanup();
    exit(1);
}
int main(int argc, char **argv) {

    char Buffer[128];
    char *interface= NULL;
    unsigned short port=DEFAULT_PORT;
    int retval;
    int fromlen;
    int i;
    int socket_type = DEFAULT_PROTO;
    struct sockaddr_in local, from;
    WSADATA wsaData;
    SOCKET listen_socket, msgsock;

    /* Parse arguments */
    if (argc >1) {
        for(i=1;i <argc;i++) {
            if ( (argv[i][0] == '-') || (argv[i][0] == '/') ) {
                switch(tolower(argv[i][1])) {
                    case 'p':
                        if (!stricmp(argv[i+1], "TCP") )
                            socket_type = SOCK_STREAM;
                        else if (!stricmp(argv[i+1], "UDP") )
                            socket_type = SOCK_DGRAM;
                        else
                            Usage(argv[0]);
                        i++;
                        break;

                    case 'i':
                        interface = argv[++i];
                        break;
                    case 'e':
                        port = atoi(argv[++i]);
                        break;
                    default:
                        Usage(argv[0]);
                        break;
                }
            }
            else
                Usage(argv[0]);
        }
    }
    
    if (WSAStartup(0x202,&wsaData) == SOCKET_ERROR) {
        fprintf(stderr,"WSAStartup failed with error %d\n",WSAGetLastError());
        WSACleanup();
        return -1;
    }
    
    if (port == 0){
        Usage(argv[0]);
    }

    local.sin_family = AF_INET;
    local.sin_addr.s_addr = (!interface)?INADDR_ANY:inet_addr(interface); 

    /* 
     * Port MUST be in Network Byte Order
     */
    local.sin_port = htons(port);

    listen_socket = socket(AF_INET, socket_type,0); // TCP socket
    
    if (listen_socket == INVALID_SOCKET){
        fprintf(stderr,"socket() failed with error %d\n",WSAGetLastError());
        WSACleanup();
        return -1;
    }
    //
    // bind() associates a local address and port combination with the
    // socket just created. This is most useful when the application is a 
    // server that has a well-known port that clients know about in advance.
    //

    if (bind(listen_socket,(struct sockaddr*)&local,sizeof(local) ) 
        == SOCKET_ERROR) {
        fprintf(stderr,"bind() failed with error %d\n",WSAGetLastError());
        WSACleanup();
        return -1;
    }

    //
    // So far, everything we did was applicable to TCP as well as UDP.
    // However, there are certain steps that do not work when the server is
    // using UDP.
    //

    // We cannot listen() on a UDP socket.

    if (socket_type != SOCK_DGRAM) {
        if (listen(listen_socket,5) == SOCKET_ERROR) {
            fprintf(stderr,"listen() failed with error %d\n",WSAGetLastError());
            WSACleanup();
            return -1;
        }
    }
    printf("%s: 'Listening' on port %d, protocol %s\n",argv[0],port,
        (socket_type == SOCK_STREAM)?"TCP":"UDP");
    while(1) {
        fromlen =sizeof(from);
        //
        // accept() doesn't make sense on UDP, since we do not listen()
        //
        if (socket_type != SOCK_DGRAM) {
            msgsock = accept(listen_socket,(struct sockaddr*)&from, &fromlen);
            if (msgsock == INVALID_SOCKET) {
                fprintf(stderr,"accept() error %d\n",WSAGetLastError());
                WSACleanup();
                return -1;
            }
            printf("accepted connection from %s, port %d\n", 
                        inet_ntoa(from.sin_addr),
                        htons(from.sin_port)) ;
            
        }
        else
            msgsock = listen_socket;

        //
        // In the case of SOCK_STREAM, the server can do recv() and 
        // send() on the accepted socket and then close it.

        // However, for SOCK_DGRAM (UDP), the server will do
        // recvfrom() and sendto()  in a loop.

        if (socket_type != SOCK_DGRAM)
            retval = recv(msgsock,Buffer,sizeof (Buffer),0 );
        else {
            retval = recvfrom(msgsock,Buffer,sizeof (Buffer),0,
                (struct sockaddr *)&from,&fromlen);
            printf("Received datagram from %s\n",inet_ntoa(from.sin_addr));
        }
            
        if (retval == SOCKET_ERROR) {
            fprintf(stderr,"recv() failed: error %d\n",WSAGetLastError());
            closesocket(msgsock);
            continue;
        }
        if (retval == 0) {
            printf("Client closed connection\n");
            closesocket(msgsock);
            continue;
        }
        printf("Received %d bytes, data [%s] from client\n",retval,Buffer);

        printf("Echoing same data back to client\n");
        if (socket_type != SOCK_DGRAM)
            retval = send(msgsock,Buffer,sizeof(Buffer),0);
        else
            retval = sendto(msgsock,Buffer,sizeof (Buffer),0,
                (struct sockaddr *)&from,fromlen);
        if (retval == SOCKET_ERROR) {
            fprintf(stderr,"send() failed: error %d\n",WSAGetLastError());
        }
        if (socket_type != SOCK_DGRAM){
            printf("Terminating connection\n");
            closesocket(msgsock);
        }
        else 
            printf("UDP server looping back for more requests\n");
        continue;
    }
}

ROP>/******************************************************************************\
ROP>* simples.c - Simple TCP/UDP server using Winsock 1.1

ROP>    if (WSAStartup(0x202,&wsaData) == SOCKET_ERROR) {

The WSAStartup function returns zero if successful. Otherwise, it returns one of the error codes listed in the following.

ROP>        fprintf(stderr,"WSAStartup failed with error %d\n",WSAGetLastError());
ROP>        WSACleanup();

/*
    Не плохо бы было ещё local нулями забить, перед тем как заполнять и передавать в bind.
*/
ROP>    if (bind(listen_socket,(struct sockaddr*)&local,sizeof(local) ) 
ROP>

...
Все приведенные в статье сервера работали по алгоритму,
Слушающий порт, принимает коннект, выдает аксепт, принимает данные, ретранслирует их, обрывает соединение и заново открывает порт на прослушку.
То есть фактически соединение создается только для передачи одного пакета.

Мне необходимо создать несколько постоянных подключений, по которым на сервер будут слаться запросы и понимже возвращаться данные. Я уже решал подобную задачу, но не имея достаточных знаний по технологии программирования сетей использовал алгоритм:
Основной порт на прослушке, коннект, аксепт, отсылка клиенту номера свободного динамически выделяемого порта сервера, клиент соединяется устанавливае соединение с выделенным портом и работает по нему.
...

ROP>как советуют некоторые товарищи

An application can call listen more than once on the same socket. This has the effect of updating the current backlog for the listening socket. Should there be more pending connections than the new backlog value, the excess pending connections will be reset and dropped.