Winsock2 - How to use IOCP on client side

I’ve recently started learning IOCP on Windows and been reading the following article:

http://www.codeproject.com/Tips/95363/Another-TCP-echo-server-using-IOCP

You can download the sample for the article from:

http://dl.dropbox.com/u/281215/documentation/iocp-1.00.html

The sample contains two simple applications – iocp_echo_server and TcpEchoClient.

I understand that IOCP is usually used on the server side of the client/server model but I’d like to create a client using IOCP.

I’ve so far tried modifying the client sample above so that whenever the server sends a response to the client, it gets picked up automatically, however it doesn’t work.

I’ve left iocp_echo_server.c as is. My modified version of TcpEchoClient.c looks like:

//TcpEchoClient.c - a minimalistic echo client
// -----------------------------------------------------------------------------

// C language includes
#include <stdio.h>
#include <winsock2.h>
#include "mswsock.h"  // for AcceptEx
#include <stdlib.h> // exit
#include <string.h>

// Windows includes
#include <windows.h>

#pragma warning(disable: 4996) // sprintf

// -----------------------------------------------------------------------------

// configuration
enum
{
    BUFLEN = 1000,
    SERVICE_PORT = 4000,
    SERVER_ADDRESS = INADDR_LOOPBACK
};

enum // socket operations
{
    OP_NONE,
    OP_ACCEPT,
    OP_READ,
    OP_WRITE
};

typedef struct _SocketState // socket state & control
{
    char operation;
    SOCKET socket;
    DWORD length;
    char buf[1024];
} SocketState;

// variables
static HANDLE cpl_port;

static SOCKET sock;
static SocketState sock_state;
static WSAOVERLAPPED sock_ovl;

static LPFN_ACCEPTEX pfAcceptEx;
static GUID GuidAcceptEx = WSAID_ACCEPTEX;

static int msgNumber;
static char msgBuf[BUFLEN];
static struct sockaddr_in sin;

// prototypes
static void createConnection(void);
static void createSocket(void);
static void init(void);
static void initWinsock(void);
static void prepareEndpoint(void);
static void recvBuffer(void);
static void run(void);
static void sendBuffer(void);

static SOCKET create_accepting_socket(void);
static void create_io_completion_port(void);
static BOOL get_completion_status(DWORD*, SocketState**,WSAOVERLAPPED**);

// -----------------------------------------------------------------------------

void main(void)
{
    init();
    run();
}

// -----------------------------------------------------------------------------

static void createConnection(void)
{
    printf("* connecting\n");
    if (WSAConnect(sock, (LPSOCKADDR)&sin, sizeof(sin), NULL, NULL, NULL, NULL) == SOCKET_ERROR)
    {
        int err = WSAGetLastError();
        printf("* error %d in connect\n", err);
        exit(1);
    }
    printf("* connected\n");
}

// -----------------------------------------------------------------------------

static void createSocket(void)
{
    sock = WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, WSA_FLAG_OVERLAPPED);
    if (sock == INVALID_SOCKET)
    {
        int err = WSAGetLastError();
        printf("* error %d creating socket\n", err);
        exit(1);
    }

    // for use by AcceptEx
    sock_state.socket = 0; // to be updated later
    sock_state.operation = OP_ACCEPT;

    if (CreateIoCompletionPort((HANDLE)sock, cpl_port, (ULONG_PTR)&sock_state, 0) != cpl_port)
    {
        int err = WSAGetLastError();
        printf("* error %d in listener\n", err);
        exit(1);
    }
}

// -----------------------------------------------------------------------------

static void init(void)
{
    initWinsock();
    create_io_completion_port();
    createSocket();
    prepareEndpoint();
    createConnection();
}

// -----------------------------------------------------------------------------

static void initWinsock(void)
{
    WSADATA wsaData;

    if (WSAStartup(0x202, &wsaData) == SOCKET_ERROR)
    {
        int err = WSAGetLastError();
        printf("* error %d in WSAStartup\n", err);
        exit(1);
    }
}
// -----------------------------------------------------------------------------

static void prepareEndpoint(void)
{
    sin.sin_family = AF_INET;
    sin.sin_addr.s_addr = htonl(SERVER_ADDRESS);
    sin.sin_port = htons(SERVICE_PORT);

    // bind_listening_socket()
    {
        //if (bind(sock, (SOCKADDR*)&sin, sizeof(sin)) == SOCKET_ERROR)
        //{
        //    printf("* error in bind!\n");
        //    exit(1);
        //}
    }

    // start_listening()
    {
        //if (listen(sock, 100) == SOCKET_ERROR)
        //{
        //    printf("* error in listen!\n");
        //    exit(1);
        //}
        //printf("* started listening for connection requests...\n");
    }

    // load_accept_ex()
    {
        //DWORD dwBytes;

        // black magic for me!!!
        // You do not need to call in your code WSAIoctl. You can directly use AcceptEx and adds Mswsock.lib.
        //WSAIoctl(sock, SIO_GET_EXTENSION_FUNCTION_POINTER, &GuidAcceptEx, sizeof(GuidAcceptEx), &pfAcceptEx, sizeof(pfAcceptEx), &dwBytes, NULL, NULL);
    }

    // start_accepting()
    {
        //SOCKET acceptor = create_accepting_socket();
        //DWORD expected = sizeof(struct sockaddr_in) + 16;

        //printf("* started accepting connections...\n");

        // uses listener's completion key and overlapped structure
        //sock_state.socket = acceptor;
        //memset(&sock_ovl, 0, sizeof(WSAOVERLAPPED));

        // starts asynchronous accept
        //if (!pfAcceptEx(sock, acceptor, sock_state.buf, 0 /* no recv */, expected, expected, NULL, &sock_ovl))
        //{
        //    int err = WSAGetLastError();
        //    if (err != ERROR_IO_PENDING)
        //    {
        //        printf("* error %d in AcceptEx\n", err);
        //        exit(1);
        //    }
        //}
    }
}

// -----------------------------------------------------------------------------

static void recvBuffer(void)
{
    char* buf = msgBuf;
    int pendingLen = BUFLEN;

    printf("* receiving reply\n");

    while (pendingLen > 0)
    {
        int partialLen = recv(sock, buf, pendingLen, 0);

        if (partialLen > 0)
        {
            pendingLen -= partialLen;
            buf += partialLen;
            continue;
        }

        // ------

        if (partialLen == 0)
        {
            printf("* connection closed by the server\n");
        }
        else // partialLen < 0
        {
            int err = WSAGetLastError();
            printf("* error %d in recv\n", err);
        }

        exit(1);
    }
}

// -----------------------------------------------------------------------------

static void run(void)
{
    DWORD length;
    BOOL resultOk;
    WSAOVERLAPPED* ovl_res;
    SocketState* socketState;

    for (;;)
    {
        sendBuffer();

        resultOk = get_completion_status(&length, &socketState, &ovl_res);

        recvBuffer();
    }
}

// -----------------------------------------------------------------------------

static void sendBuffer(void)
{
    char* buf = msgBuf;
    int pendingLen = BUFLEN;

    printf("* sending message\n");
    sprintf(msgBuf, "%05 *****", msgNumber++);

    while (pendingLen > 0)
    {
        int partialLen = send(sock, buf, pendingLen, 0);

        if (partialLen > 0)
        {
            pendingLen -= partialLen;
            buf += partialLen;
            continue;
        }

        // -----------

        if (partialLen == 0)
        {
            printf("* connection closed by the server\n");
        }
        else // partialLen < 0
        {
            int err = WSAGetLastError();
            printf("* error %d in send\n", err);
        }

        exit(1);
    }
}

// -----------------------------------------------------------------------------

static SOCKET create_accepting_socket(void)
{
    SOCKET acceptor = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
    if (acceptor == INVALID_SOCKET)
    {
        printf("* error creating accept socket!\n");
        exit(1);
    }
    return acceptor;
}

// -----------------------------------------------------------------------------

static void create_io_completion_port(void)
{
    cpl_port = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
    if (!cpl_port)
    {
        int err = WSAGetLastError();
        printf("* error %d in line %d CreateIoCompletionPort\n", err, __LINE__);
        exit(1);
    }
}

// -----------------------------------------------------------------------------

static BOOL get_completion_status(DWORD* length, SocketState** socketState, WSAOVERLAPPED** ovl_res)
{
    BOOL resultOk;
    *ovl_res = NULL;
    *socketState = NULL;

    resultOk = GetQueuedCompletionStatus(cpl_port, length, (PULONG_PTR)socketState, ovl_res, INFINITE);

    if (!resultOk)
    {
        DWORD err = GetLastError();
        printf("* error %d getting completion port status!!!\n", err);
    }

    if (!*socketState || !*ovl_res)
    {
        printf("* don't know what to do, aborting!!!\n");
        exit(1);
    }

    return resultOk;
}

// -----------------------------------------------------------------------------
// the end

When the server send a response by calling:

WSASend(socketState->socket, &wsabuf, 1, NULL, 0, ovl, NULL)

I’d expect it to be picked up by the client on this line:

resultOk = get_completion_status(&length, &socketState, &ovl_res);

But it doesn’t…

Would anybody be able to tell me what I’m doing wrong?

Edit:

I’ve taken the following points:

On the client side, you use WSAConnect() to create an outbound connection.
Call WSARecv() and WSASend() to start reading/writing operations when needed
you have to use WSASend/WSARecv if you want to use I/O completion ports.

and attempted to create a simple IOCP based client:

#include <iostream>
#include <winsock2.h>
#pragma comment(lib,"ws2_32.lib")

static DWORD WINAPI ClientWorkerThread(LPVOID lpParameter);

int main(void)
{
    WSADATA WsaDat;
    if (WSAStartup(MAKEWORD(2, 2), &WsaDat) != NO_ERROR)
        return 0;

    // Step 1 - Create an I/O completion port.
    HANDLE hCompletionPort = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
    if (!hCompletionPort)
        return 0;

    // Step 2 - Find how many processors.
    SYSTEM_INFO systemInfo;
    GetSystemInfo(&systemInfo);
    const int nNumberOfProcessors = systemInfo.dwNumberOfProcessors;

    // Step 3 - Create worker threads.
    for (int i = 0; i < nNumberOfProcessors; i++)
    {
        HANDLE hThread = CreateThread(NULL, 0, ClientWorkerThread, hCompletionPort, 0, NULL);
        CloseHandle(hThread);
    }

    // Step 4 - Create a socket.
    SOCKET Socket = WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, WSA_FLAG_OVERLAPPED);
    if (Socket == INVALID_SOCKET)
        return 0;

    struct hostent *host;
    if ((host = gethostbyname("localhost")) == NULL)
        return 0;

    SOCKADDR_IN SockAddr;
    SockAddr.sin_family = AF_INET;
    SockAddr.sin_addr.s_addr = *((unsigned long*)host->h_addr);
    SockAddr.sin_port = htons(8888);

    // Step 5 - Associate the socket with the I/O completion port.
    CreateIoCompletionPort((HANDLE)Socket, hCompletionPort, (ULONG_PTR)0, 0);

    if (WSAConnect(Socket, (SOCKADDR*)(&SockAddr), sizeof(SockAddr), NULL, NULL, NULL, NULL) == SOCKET_ERROR)
        return 0;

    char buffer[1000];
    memset(buffer, 0, 999);
    WSABUF wsaBuf = {strlen(buffer), buffer};
    DWORD dwSendBytes = 0;
    DWORD dwReceivedBytes = 0;
    DWORD dwFlags = 0;
    WSAOVERLAPPED wsaOverlapped;
    SecureZeroMemory((PVOID)&wsaOverlapped, sizeof(wsaOverlapped));
    wsaOverlapped.hEvent = WSACreateEvent();

    for(;;)
    {
        WSARecv(Socket, &wsaBuf, 1, &dwReceivedBytes, &dwFlags, &wsaOverlapped, NULL);
        std::cout << wsaBuf.buf;

        //WSASend(Socket, &wsaBuf, 1, &dwSendBytes, 0, &wsaOverlapped, NULL);

        int nError = WSAGetLastError();
        if(nError != WSAEWOULDBLOCK&&nError != 0)
        {
            std::cout << "Winsock error code: " << nError << "\r\n";
            std::cout << "Server disconnected!\r\n";
            shutdown(Socket, SD_SEND);
            closesocket(Socket);

            break;
        }
        Sleep(1000);
    }

    WSACleanup();
    system("PAUSE");
    return 0;
}

static DWORD WINAPI ClientWorkerThread(LPVOID lpParameter)
{
    HANDLE hCompletionPort = (HANDLE)lpParameter;
    DWORD dwBytesTransferred = 0;

    while (TRUE)
    {
        BOOL bRet = GetQueuedCompletionStatus(hCompletionPort, &dwBytesTransferred, (LPDWORD)0, (LPOVERLAPPED*)0, INFINITE);
    }

    return 0;
}

I know there are several things I’m doing wrong but I don’t know what they are.

Could somebody take a look at my code and give me some hints please?

Many thanks

Edit 2:

Sorry this post is getting too long.

I've had another go trying to implement an IOCP based client after reading Remy's comments below but I'm still not sure if I'm on the right track.

I'd really appreciate it if somebody could take a look at my new code (compiles fine under VS2010 & error checking omitted) below and give me some feedback.

NonBlockingClient:

#include <iostream>
#include <winsock2.h>
#pragma comment(lib, "ws2_32.lib")

static DWORD WINAPI ClientWorkerThread(LPVOID lpParameter);

typedef struct _PER_HANDLE_DATA 
{
    SOCKET Socket;
} PER_HANDLE_DATA, * LPPER_HANDLE_DATA;

typedef struct
{
    WSAOVERLAPPED wsaOverlapped;
    WSABUF wsaBuf;
    int OperationType;
} PER_IO_DATA, * LPPER_IO_DATA;

int main(void)
{
    WSADATA WsaDat;
    WSAStartup(MAKEWORD(2, 2), &WsaDat);

    // Step 1 - Create an I/O completion port.
    HANDLE hCompletionPort = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);

    // Step 2 - Find how many processors.
    SYSTEM_INFO systemInfo;
    GetSystemInfo(&systemInfo);

    // Step 3 - Create worker threads.
    for (int i = 0; i < (int)systemInfo.dwNumberOfProcessors; i++)
    {
        HANDLE hThread = CreateThread(NULL, 0, ClientWorkerThread, hCompletionPort, 0, NULL);
        CloseHandle(hThread);
    }

    // Step 4 - Create a socket.
    SOCKET Socket = WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, WSA_FLAG_OVERLAPPED);

    PER_HANDLE_DATA *pPerHandleData = new PER_HANDLE_DATA;
    pPerHandleData->Socket = Socket;

    struct hostent *host;
    host = gethostbyname("localhost");

    SOCKADDR_IN SockAddr;
    SockAddr.sin_family = AF_INET;
    SockAddr.sin_addr.s_addr = *((unsigned long*)host->h_addr);
    SockAddr.sin_port = htons(8888);

    // Step 5 - Associate the socket with the I/O completion port.
    CreateIoCompletionPort((HANDLE)Socket, hCompletionPort, (DWORD)pPerHandleData, 0);

    WSAConnect(Socket, (SOCKADDR*)(&SockAddr), sizeof(SockAddr), NULL, NULL, NULL, NULL);

    static char buffer[1000];
    memset(buffer, 0, 999);

    PER_IO_DATA *pPerIoData = new PER_IO_DATA;

    pPerIoData->wsaBuf.buf = buffer;
    pPerIoData->wsaBuf.len = sizeof(buffer);

    DWORD dwSendBytes = 0;
    DWORD dwReceivedBytes = 0;
    DWORD dwFlags = 0;

    SecureZeroMemory((PVOID)&pPerIoData->wsaOverlapped, sizeof(pPerIoData->wsaOverlapped));
    pPerIoData->wsaOverlapped.hEvent = WSACreateEvent();

    WSARecv(Socket, &pPerIoData->wsaBuf, 1, &dwReceivedBytes, &dwFlags, &pPerIoData->wsaOverlapped, NULL);
    std::cout << pPerIoData->wsaBuf.buf;

    for (;;)
    {
        int nError = WSAGetLastError();
        if (nError != WSAEWOULDBLOCK&&nError != 0)
        {
            std::cout << "Winsock error code: " << nError << "\r\n";
            std::cout << "Server disconnected!\r\n";
            shutdown(Socket, SD_SEND);
            closesocket(Socket);

            break;
        }

        Sleep(1000);
    }

    delete pPerHandleData;
    delete pPerIoData;
    WSACleanup();
    return 0;
}

static DWORD WINAPI ClientWorkerThread(LPVOID lpParameter)
{
    HANDLE hCompletionPort = (HANDLE)lpParameter;
    DWORD bytesCopied = 0;
    OVERLAPPED *overlapped = 0;
    LPPER_HANDLE_DATA PerHandleData;
    LPPER_IO_DATA PerIoData;
    DWORD SendBytes, RecvBytes;
    DWORD Flags;
    BOOL bRet;

    while (TRUE)
    {
        bRet = GetQueuedCompletionStatus(hCompletionPort, &bytesCopied, (LPDWORD)&PerHandleData, (LPOVERLAPPED*)&PerIoData, INFINITE);

        if (bytesCopied == 0)
        {
            break;
        }
        else
        {
            Flags = 0;
            ZeroMemory(&(PerIoData->wsaOverlapped), sizeof(WSAOVERLAPPED));

            PerIoData->wsaBuf.len = 1000;
            WSARecv(PerHandleData->Socket, &(PerIoData->wsaBuf), 1, &RecvBytes, &Flags, &(PerIoData->wsaOverlapped), NULL);
        }
    }

    return 0;
}

NonBlockingServer:

#include <iostream>
#include <winsock2.h>
#pragma comment(lib,"ws2_32.lib")

int main()
{
    WSADATA WsaDat;
    WSAStartup(MAKEWORD(2,2), &WsaDat);

    SOCKET listenSocket = WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, WSA_FLAG_OVERLAPPED);

    SOCKADDR_IN server;
    server.sin_family = AF_INET;
    server.sin_addr.s_addr = INADDR_ANY;
    server.sin_port = htons(8888);

    bind(listenSocket, (SOCKADDR*)(&server), sizeof(server));

    listen(listenSocket, 1);

    SOCKET acceptSocket = SOCKET_ERROR;
    sockaddr_in saClient;
    int nClientSize = sizeof(saClient);
    while (acceptSocket == SOCKET_ERROR)
    {
        std::cout << "Waiting for incoming connections...\r\n";
        acceptSocket = WSAAccept(listenSocket, (SOCKADDR*)&saClient, &nClientSize, NULL, NULL);
    }

    std::cout << "Client connected!\r\n\r\n";

    char *szMessage = "Welcome to the server!\r\n";
    WSAOVERLAPPED SendOverlapped;
    DWORD SendBytes;

    WSABUF DataBuf;
    DataBuf.len = 1000;
    DataBuf.buf = szMessage;

    SecureZeroMemory((PVOID)&SendOverlapped, sizeof(WSAOVERLAPPED));
    SendOverlapped.hEvent = WSACreateEvent();

    for (;;)
    {
        WSASend(acceptSocket, &DataBuf, 1, &SendBytes, 0, &SendOverlapped, NULL);

        int nError = WSAGetLastError();
        if (nError != WSAEWOULDBLOCK && nError != 0)
        {
            std::cout << "Winsock error code: " << nError << "\r\n";
            std::cout << "Client disconnected!\r\n";

            shutdown(acceptSocket, SD_SEND);
            closesocket(acceptSocket);

            break;
        }

        Sleep(1000);
    }

    WSACleanup();
    return 0;
}

Thanks again!

Remy Lebeau

Try something like this:

Client:

#include <iostream>
#include <string>
#include <winsock2.h> 
#pragma comment(lib, "ws2_32.lib") 

typedef struct 
{ 
    WSAOVERLAPPED Overlapped; 
    SOCKET Socket; 
    WSABUF wsaBuf; 
    char Buffer[1024];
    DWORD Flags;
} PER_IO_DATA, * LPPER_IO_DATA; 

static DWORD WINAPI ClientWorkerThread(LPVOID lpParameter) 
{ 
    HANDLE hCompletionPort = (HANDLE)lpParameter; 
    DWORD NumBytesRecv = 0; 
    ULONG CompletionKey; 
    LPPER_IO_DATA PerIoData; 

    while (GetQueuedCompletionStatus(hCompletionPort, &NumBytesRecv, &CompletionKey, (LPOVERLAPPED*)&PerIoData, INFINITE))
    {
        if (!PerIoData)
            continue;

        if (NumBytesRecv == 0) 
        {
            std::cout << "Server disconnected!\r\n\r\n";  
        }
        else
        {
            // use PerIoData->Buffer as needed...
            std::cout << std::string(PerIoData->Buffer, NumBytesRecv);

            PerIoData->wsaBuf.len = sizeof(PerIoData->Buffer); 
            PerIoData->Flags = 0; 

            if (WSARecv(PerIoData->Socket, &(PerIoData->wsaBuf), 1, &NumBytesRecv, &(PerIoData->Flags), &(PerIoData->Overlapped), NULL) == 0)
                continue;

            if (WSAGetLastError() == WSA_IO_PENDING)
                continue;
        }

        closesocket(PerIoData->Socket);
        delete PerIoData;
    } 

    return 0; 
} 

int main(void) 
{ 
    WSADATA WsaDat; 
    if (WSAStartup(MAKEWORD(2, 2), &WsaDat) != 0)
        return 0; 

    HANDLE hCompletionPort = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0); 
    if (!hCompletionPort)
        return 0;

    SYSTEM_INFO systemInfo; 
    GetSystemInfo(&systemInfo); 

    for (DWORD i = 0; i < systemInfo.dwNumberOfProcessors; ++i) 
    { 
        HANDLE hThread = CreateThread(NULL, 0, ClientWorkerThread, hCompletionPort, 0, NULL); 
        CloseHandle(hThread); 
    } 

    SOCKET Socket = WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, WSA_FLAG_OVERLAPPED); 
    if (Socket == INVALID_SOCKET)
        return 0;

    SOCKADDR_IN SockAddr; 
    SockAddr.sin_family = AF_INET; 
    SockAddr.sin_addr.s_addr = inet_addr("127.0.0.1"); 
    SockAddr.sin_port = htons(8888); 

    CreateIoCompletionPort((HANDLE)Socket, hCompletionPort, 0, 0); 

    if (WSAConnect(Socket, (SOCKADDR*)(&SockAddr), sizeof(SockAddr), NULL, NULL, NULL, NULL) == SOCKET_ERROR)
        return 0;

    PER_IO_DATA *pPerIoData = new PER_IO_DATA;
    ZeroMemory(pPerIoData, sizeof(PER_IO_DATA)); 

    pPerIoData->Socket = Socket; 
    pPerIoData->Overlapped.hEvent = WSACreateEvent(); 
    pPerIoData->wsaBuf.buf = pPerIoData->Buffer; 
    pPerIoData->wsaBuf.len = sizeof(pPerIoData->Buffer); 

    DWORD dwNumRecv;
    if (WSARecv(Socket, &(pPerIoData->wsaBuf), 1, &dwNumRecv, &(pPerIoData->Flags), &(pPerIoData->Overlapped), NULL) == SOCKET_ERROR)
    {
        if (WSAGetLastError() != WSA_IO_PENDING)
        {
            delete pPerIoData;
            return 0;
        }
    } 

    while (TRUE) 
        Sleep(1000); 

    shutdown(Socket, SD_BOTH); 
    closesocket(Socket); 

    WSACleanup(); 
    return 0; 
}

Server:

#include <iostream>  
#include <winsock2.h>  
#pragma comment(lib,"ws2_32.lib")  

typedef struct
{
    WSAOVERLAPPED Overlapped;
    SOCKET Socket;
    WSABUF wsaBuf;
    char Buffer[1024];
    DWORD BytesSent;
    DWORD BytesToSend;
} PER_IO_DATA, * LPPER_IO_DATA; 


static DWORD WINAPI ServerWorkerThread(LPVOID lpParameter)
{
    HANDLE hCompletionPort = (HANDLE)lpParameter;
    DWORD NumBytesSent = 0;
    ULONG CompletionKey;
    LPPER_IO_DATA PerIoData;

    while (GetQueuedCompletionStatus(hCompletionPort, &NumBytesSent, &CompletionKey, (LPOVERLAPPED*)&PerIoData, INFINITE))    
    {
        if (!PerIoData)
            continue;

        if (NumBytesSent == 0)
        {
            std::cout << "Client disconnected!\r\n\r\n";
        }
        else
        {
            PerIoData->BytesSent += NumBytesSent;
            if (PerIoData->BytesSent < PerIoData->BytesToSend)
            {
                PerIoData->wsaBuf.buf = &(PerIoData->Buffer[PerIoData->BytesSent]);
                PerIoData->wsaBuf.len = (PerIoData->BytesToSend - PerIoData->BytesSent);
            }
            else
            {
                PerIoData->wsaBuf.buf = PerIoData->Buffer;
                PerIoData->wsaBuf.len = strlen(PerIoData->Buffer);
                PerIoData->BytesSent = 0;
                PerIoData->BytesToSend = PerIoData->wsaBuf.len;
            }

            if (WSASend(PerIoData->Socket, &(PerIoData->wsaBuf), 1, &NumBytesSent, 0, &(PerIoData->Overlapped), NULL) == 0)
                continue;

            if (WSAGetLastError() == WSA_IO_PENDING)
                continue;
        }

        closesocket(PerIoData->Socket);
        delete PerIoData;
    }

    return 0;
} 

int main()  
{  
    WSADATA WsaDat;  
    if (WSAStartup(MAKEWORD(2,2), &WsaDat) != 0)
        return 0;  

    HANDLE hCompletionPort = CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0);
    if (!hCompletionPort)
        return 0;

    SYSTEM_INFO systemInfo;
    GetSystemInfo(&systemInfo);

    for (DWORD i = 0; i < systemInfo.dwNumberOfProcessors; ++i)
    {
        HANDLE hThread = CreateThread(NULL, 0, ServerWorkerThread, hCompletionPort, 0, NULL);
        CloseHandle(hThread);
    } 

    SOCKET listenSocket = WSASocket(AF_INET, SOCK_STREAM, IPPROTO_TCP, NULL, 0, WSA_FLAG_OVERLAPPED);  
    if (listenSocket == INVALID_SOCKET)
        return 0;  

    SOCKADDR_IN server;
    ZeroMemory(&server, sizeof(server));
    server.sin_family = AF_INET;  
    server.sin_addr.s_addr = INADDR_ANY;  
    server.sin_port = htons(8888);  

    if (bind(listenSocket, (SOCKADDR*)(&server), sizeof(server)) != 0)
        return 0;  

    if (listen(listenSocket, 1) != 0)
        return 0;  

    std::cout << "Waiting for incoming connection...\r\n";  

    SOCKET acceptSocket;  
    do  
    {  
        sockaddr_in saClient;  
        int nClientSize = sizeof(saClient);  
        acceptSocket = WSAAccept(listenSocket, (SOCKADDR*)&saClient, &nClientSize, NULL, NULL);  
    }
    while (acceptSocket == INVALID_SOCKET);

    std::cout << "Client connected!\r\n\r\n";  

    CreateIoCompletionPort((HANDLE)acceptSocket, hCompletionPort, 0, 0); 

    LPPER_IO_DATA pPerIoData = new PER_IO_DATA;
    ZeroMemory(pPerIoData, sizeof(PER_IO_DATA));

    strcpy(pPerIoData->Buffer, "Welcome to the server!\r\n");  

    pPerIoData->Overlapped.hEvent = WSACreateEvent(); 
    pPerIoData->Socket = acceptSocket; 
    pPerIoData->wsaBuf.buf = pPerIoData->Buffer;  
    pPerIoData->wsaBuf.len = strlen(pPerIoData->Buffer);  
    pPerIoData->BytesToSend = pPerIoData->wsaBuf.len;  

    DWORD dwNumSent;
    if (WSASend(acceptSocket, &(pPerIoData->wsaBuf), 1, &dwNumSent, 0, &(pPerIoData->Overlapped), NULL) == SOCKET_ERROR)
    {
        if (WSAGetLastError() != WSA_IO_PENDING)
        {
            delete pPerIoData;
            return 0;
        }
    }  

    while (TRUE)  
        Sleep(1000);  

    shutdown(acceptSocket, SD_BOTH);  
    closesocket(acceptSocket);  

    WSACleanup();  
    return 0;  
}

Have you had a look at the example in the MSDN documentation for WSARecv?

Essentially, you have to start the asynchronous WSARecv operation first and then get notified via the completion port of its completion.

Or to put it another way: Windows I/O completion ports are using a proactor model (in contrast to the reactor model of Linux/FreeBSD/NetBSD).

来源：https://stackoverflow.com/questions/10998504/winsock2-how-to-use-iocp-on-client-side

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