http://msdn2.microsoft.com/en-us/library/ms737524.aspx

#include <stdio.h>
#include "winsock2.h"
#include "mswsock.h"

void main() {
//----------------------------------------
// Declare and initialize variables
WSADATA wsaData;
HANDLE hCompPort;
LPFN_ACCEPTEX lpfnAcceptEx = NULL;
GUID GuidAcceptEx = WSAID_ACCEPTEX;
WSAOVERLAPPED olOverlap;

SOCKET ListenSocket, AcceptSocket;
sockaddr_in service;
char lpOutputBuf[1024];
int outBufLen = 1024;
DWORD dwBytes;

//----------------------------------------
// Initialize Winsock
int iResult = WSAStartup( MAKEWORD(2,2), &wsaData );
if( iResult != NO_ERROR )
printf("Error at WSAStartup\n");

//----------------------------------------
// Create a handle for the completion port
hCompPort = CreateIoCompletionPort( INVALID_HANDLE_VALUE, NULL, (u_long)0, 0 );

//----------------------------------------
// Create a listening socket
ListenSocket = socket( AF_INET, SOCK_STREAM, IPPROTO_TCP );
if (ListenSocket == INVALID_SOCKET) {
printf("Error at socket(): ListenSocket\n");
WSACleanup();
return;
}

//----------------------------------------
// Associate the listening socket with the completion port
CreateIoCompletionPort((HANDLE)ListenSocket, hCompPort, (u_long)0, 0);

//----------------------------------------
// Bind the listening socket to the local IP address
// and port 27015
hostent* thisHost;
char* ip;
u_short port;
port = 27015;
thisHost = gethostbyname("");
ip = inet_ntoa (*(struct in_addr *)*thisHost->h_addr_list);

service.sin_family = AF_INET;
service.sin_addr.s_addr = inet_addr(ip); service.sin_port = htons(port);

if ( bind( ListenSocket,(SOCKADDR*) &service, sizeof(service) ) == SOCKET_ERROR ) {
printf("bind failed\n");
closesocket(ListenSocket);
return;
}

//----------------------------------------
// Start listening on the listening socket
if (listen( ListenSocket, 100 ) == SOCKET_ERROR) {
printf("error listening\n");
}
printf("Listening on address: %s:%d\n", ip, port);

//----------------------------------------
// Load the AcceptEx function into memory using WSAIoctl.
// The WSAIoctl function is an extension of the ioctlsocket()
// function that can use overlapped I/O. The function's 3rd
// through 6th parameters are input and output buffers where
// we pass the pointer to our AcceptEx function. This is used
// so that we can call the AcceptEx function directly, rather
// than refer to the Mswsock.lib library.
WSAIoctl(ListenSocket,
SIO_GET_EXTENSION_FUNCTION_POINTER,
&GuidAcceptEx,
sizeof(GuidAcceptEx),
&lpfnAcceptEx,
sizeof(lpfnAcceptEx),
&dwBytes,
NULL,
NULL);

//----------------------------------------
// Create an accepting socket
AcceptSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
if (AcceptSocket == INVALID_SOCKET) {
printf("Error creating accept socket.\n");
WSACleanup();
return;
}

//----------------------------------------
// Empty our overlapped structure and accept connections.
memset(&olOverlap, 0, sizeof(olOverlap));

lpfnAcceptEx(ListenSocket,
AcceptSocket,
lpOutputBuf,
outBufLen - ((sizeof(sockaddr_in) + 16) * 2),
sizeof(sockaddr_in) + 16,
sizeof(sockaddr_in) + 16,
&dwBytes,
&olOverlap);

//----------------------------------------
// Associate the accept socket with the completion port
CreateIoCompletionPort((HANDLE)AcceptSocket, hCompPort, (u_long)0, 0);

//----------------------------------------
// Continue on to use send, recv, TransmitFile(), etc.,.
...

}