A simple application using I/O Completion Ports and WinSock

• Download one-to-one client and server - 2.52 Kb
• Download multithreaded server - 2.89 Kb
• Download multithreaded server using IOCP - 10.7 Kb
• Download IOCP client - 7.1 Kb
• Download IOCP client and server executables - 67.9 Kb

Introduction

The primary objective of this code submission is to provide source code which will demonstrate the use of IOCP using WinSock. This code submission tries to highlight the use of IOCP using a very easy to understand source code: the client and the server perform very simple operations; basically, they send and receive simple string messages. The IOCP client will allow you to stress test the server. You can send a message and provide input on how many times the message should be sent to the server.

Additionally, I have included socket1.zip, this code submission contains a one-to-one client and server implementation, and doesn't use IOCP. Similarly, socket2.zip contains a multi-threaded socket based server that creates a thread for every client connection, and is not a good design. I have included these additional codes so that the reader can compare these implementations with the IOCP implementation. This will provide additional insights to the understanding of the use of IOCP using WinSock.

Background

I was considering the use of IOCP in my current project. I found IOCP using WinSock to be a very useful, robust, and scalable mechanism. IOCP allows an application to use a pool of threads that are created to process asynchronous I/O requests. This prevents the application from creating one thread per client which can have severe performance issues (socket2.zip contains a one thread per client implementation).

Using the code

I have provided four zip files:

• Socket1.zip contains a simple one-to-one client and server implementation using WinSock. This is a pretty straightforward implementation; I won't be discussing this code.
• Socket2.zip contains a multi-threaded server, one thread per client; the client code remains the same and is not discussed.
• ServerIOCP.zip contains a multi-threaded server that uses IOCP, and is the focus of this article.
• ClientIOCP.zip contains the IOCP client; the IOCP client can stress-test the IOCP server.
• IOCPExecutables.zip contains the IOCP client and server executables.

All of the code submissions are console based applications.

Multi-threaded server without IOCP

The implementation of this server can be found in socket2.zip.

Once a listening socket is created, a call is made to the AcceptConnections() function with the listening socket as the input parameter:

//Make the socket a listening socket

if (SOCKET_ERROR == listen(ListenSocket,SOMAXCONN))
{
    closesocket(ListenSocket);
    printf("\nError occurred while listening.");
    goto error;
}
else
{
    printf("\nlisten() successful.");
}
 
//This function will take are of multiple clients using threads

AcceptConnections(ListenSocket);

The AcceptConnections() function will accept incoming client connections, and will spawn a thread for every new client connection:

//This function will loop on while creating

//a new thread for each client connection

void AcceptConnections(SOCKET ListenSocket)
{
     sockaddr_in ClientAddress;
     int nClientLength = sizeof(ClientAddress);
     
     //Infinite, no graceful shutdown of server implemented, 

     //preferably server should be implemented as a service

     //Events can also be used for graceful shutdown

     while (1)
     {
          //Accept remote connection attempt from the client

          SOCKET Socket = accept(ListenSocket, 
                 (sockaddr*)&ClientAddress, &nClientLength);
          
          if (INVALID_SOCKET == Socket)
          {
               printf("\nError occurred while accepting" 
                      " socket: %ld.", WSAGetLastError());
          }
          
          //Display Client's IP

          printf("\nClient connected from: %s", 
                 inet_ntoa(ClientAddress.sin_addr)); 
          
          DWORD nThreadID;
          
          //Spawn one thread for each client 

          //connection, not a wise idea.

          //One should limit the number of threads

          //or use I/O completion port

          CreateThread(0, 0, AcceptHandler, 
                 (void*)Socket, 0, &nThreadID);
     }
}

The AcceptHandler() function is a thread function. It will take an accepted socket as the input, and will perform client related I/O operations on it:

//Thread procedure one thread will be created for each client.

DWORD WINAPI AcceptHandler(void* Socket) 
{
     SOCKET RemoteSocket = (SOCKET)Socket;
     
     char szBuffer[256];
     
     //Cleanup and Init with 0 the szBuffer

     ZeroMemory(szBuffer,256);
     
     int nBytesSent;
     int nBytesRecv;
     
     //Receive data from a connected or bound socket

     nBytesRecv = recv(RemoteSocket, szBuffer, 255, 0 );
     
     if (SOCKET_ERROR == nBytesRecv)
     {
          closesocket(RemoteSocket);
          printf("\nError occurred while receiving from socket.");
          return 1; //error

     }
     else
     {
          printf("\nrecv() successful.");
     }
     
     //Display the message received on console

     printf("\nThe following message was received: %s", szBuffer);
     
     //Send data on a connected socket to the client

     nBytesSent = send(RemoteSocket, ACK_MESG_RECV , 
                                        strlen(ACK_MESG_RECV), 0);
     
     if (SOCKET_ERROR == nBytesSent) 
     {
          closesocket(RemoteSocket);
          printf("\nError occurred while writing to socket.");
          return 1; //error

     }
     else
     {
          printf("\nsend() successful.");
     }
     
     return 0; //success

}

This design is not scalable, and can have severe performance issues.

Multi-threaded server using IOCP

The implementation of this server can be found in ServerIOCP.zip.

The following set of APIs is used in the implementation of IOCP. A quick read or review on MSDN will help in a quick grasping of the following code:

• CreateIoCompletionPort()
• GetQueuedCompletionStatus()
• PostQueuedCompletionStatus()

The InitializeIOCP() function will initialize IOCP, and create a worker thread pool that will process the IOCP requests. Generally, two threads are created per processor. Many programs will find out the number of processors on the host, and will create <Number of Processors * 2> number of worker threads. This can be created as a configurable parameter. This will allow the user of the application to configure the number of threads in an attempt to fine-tune the application. In my code, two threads will be created per processor on the host.

///Function to Initialize IOCPbool InitializeIOCP()

{
     //Create I/O completion port

     g_hIOCompletionPort = 
       CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0 );
     
     if ( NULL == g_hIOCompletionPort)
     {
          printf("\nError occurred while creating IOCP: %d.", 
                 WSAGetLastError());
          return false;
     }
     
     DWORD nThreadID;
     
     //Create worker threads

     for (int ii = 0; ii < g_nThreads; ii++)
     {
          g_phWorkerThreads[ii] = CreateThread(0, 0, 
              WorkerThread, (void *)(ii+1), 0, &nThreadID);
     }
     
     return true;
}

IOCP is to be used with overlapped I/O. The following code shows how to create an overlapped socket:

//Overlapped I/O follows the model established

//in Windows and can be performed only on 

//sockets created through the WSASocket function 

ListenSocket = WSASocket(AF_INET, SOCK_STREAM, 0, 
                       NULL, 0, WSA_FLAG_OVERLAPPED);
if (INVALID_SOCKET == ListenSocket) 
{
     printf("\nError occurred while opening socket: %ld.", 
                                       WSAGetLastError());
     goto error;
}
else
{
     printf("\nWSASocket() successful.");
}

To have a graceful shutdown of this server, I have used WSAEventSelect(). We will process an Accept event, rather than blocking on the accept() call. The creation of WSAEVENT is demonstrated below:

g_hAcceptEvent = WSACreateEvent();
 
     if (WSA_INVALID_EVENT == g_hAcceptEvent)
     {
          printf("\nError occurred while WSACreateEvent().");
          goto error;
     }
    
     if (SOCKET_ERROR == WSAEventSelect(ListenSocket, 
                        g_hAcceptEvent, FD_ACCEPT))
     {
          printf("\nError occurred while WSAEventSelect().");
          WSACloseEvent(g_hAcceptEvent);
          goto error;
     }

The AcceptThread() will keep looking for the Accept event.

//This thread will look for accept event

DWORD WINAPI AcceptThread(LPVOID lParam)
{
     SOCKET ListenSocket = (SOCKET)lParam;
     
     WSANETWORKEVENTS WSAEvents;
     
     //Accept thread will be around to look for 

     //accept event, until a Shutdown event is not Signaled.

     while(WAIT_OBJECT_0 != 
           WaitForSingleObject(g_hShutdownEvent, 0))
     {
          if (WSA_WAIT_TIMEOUT != WSAWaitForMultipleEvents(1, 
              &g_hAcceptEvent, FALSE, WAIT_TIMEOUT_INTERVAL, FALSE))
          {
               WSAEnumNetworkEvents(ListenSocket, 
                     g_hAcceptEvent, &WSAEvents);
               if ((WSAEvents.lNetworkEvents & FD_ACCEPT) && 
                   (0 == WSAEvents.iErrorCode[FD_ACCEPT_BIT]))
               {
                    //Process it.

                    AcceptConnection(ListenSocket);
               }
          }
     }
     
     return 0;
}

The AcceptConnection() will process the Accept event. It will also associate the socket to IOCP, and will post a WSARecv() on the socket to receive the incoming client data.

//This function will process the accept event

void AcceptConnection(SOCKET ListenSocket)
{
     sockaddr_in ClientAddress;
     int nClientLength = sizeof(ClientAddress);
     
     //Accept remote connection attempt from the client

     SOCKET Socket = accept(ListenSocket, 
           (sockaddr*)&ClientAddress, &nClientLength);
     
     if (INVALID_SOCKET == Socket)
     {
          WriteToConsole("\nError occurred while " + 
                         "accepting socket: %ld.", 
                         WSAGetLastError());
     }
     
     //Display Client's IP

     WriteToConsole("\nClient connected from: %s", 
                    inet_ntoa(ClientAddress.sin_addr));
     
     //Create a new ClientContext for this newly accepted client

     CClientContext   *pClientContext  = new CClientContext;
     
     pClientContext->SetOpCode(OP_READ);
     pClientContext->SetSocket(Socket);
     
     //Store this object

     AddToClientList(pClientContext);
     
     if (true == AssociateWithIOCP(pClientContext))
     {
          //Once the data is successfully received, we will print it.

          pClientContext->SetOpCode(OP_WRITE);
          
          WSABUF *p_wbuf = pClientContext->GetWSABUFPtr();
          OVERLAPPED *p_ol = pClientContext->GetOVERLAPPEDPtr();
          
          //Get data.

          DWORD dwFlags = 0;
          DWORD dwBytes = 0;
          
          //Post initial Recv

          //This is a right place to post a initial Recv

          //Posting a initial Recv in WorkerThread

          //will create scalability issues.

          int nBytesRecv = WSARecv(pClientContext->GetSocket(), 
                           p_wbuf, 1, &dwBytes, &dwFlags, p_ol, NULL);
          
          if ((SOCKET_ERROR == nBytesRecv) && 
              (WSA_IO_PENDING != WSAGetLastError()))
          {
               WriteToConsole("\nError in Initial Post.");
          }
     }
} 

The AssociateWithIOCP() will associate the accepted socket to IOCP.

bool AssociateWithIOCP(CClientContext   *pClientContext)
{
    //Associate the socket with IOCP

    HANDLE hTemp = CreateIoCompletionPort((HANDLE)pClientContext->GetSocket(), 
        g_hIOCompletionPort, (DWORD)pClientContext, 0);

    if (NULL == hTemp)
    {
        WriteToConsole("\nError occurred while" + 
            " executing CreateIoCompletionPort().");

        //Let's not work with this client

        RemoveFromClientListAndFreeMemory(pClientContext);

        return false;
    }

    return true;
}

The class CClientContext is used to store client related information, like the client socket, and it has a buffer that will be dedicated to overlapped I/O. We need to ensure that the buffer that is used in overlapped I/O is not updated when the overlapped I/O is underway.

class CClientContext
//To store and manage client related information

{
private:
     
     OVERLAPPED        *m_pol;
     WSABUF            *m_pwbuf;
     
     int               m_nTotalBytes;
     int               m_nSentBytes;
     
     //accepted socket

     SOCKET            m_Socket;
     //will be used by the worker thread 

     //to decide what operation to perform

     int               m_nOpCode;
     char              m_szBuffer[MAX_BUFFER_LEN];
     
public:
     
     //Get/Set calls

     void SetOpCode(int n)
     {
          m_nOpCode = n;
     }
     
     int GetOpCode()
     {
          return m_nOpCode;
     }
     
     void SetTotalBytes(int n)
     {
          m_nTotalBytes = n;
     }
     
     int GetTotalBytes()
     {
          return m_nTotalBytes;
     }
     
     void SetSentBytes(int n)
     {
          m_nSentBytes = n;
     }
     
     void IncrSentBytes(int n)
     {
          m_nSentBytes += n;
     }
     
     int GetSentBytes()
     {
          return m_nSentBytes;
     }
     
     void SetSocket(SOCKET s)
     {
          m_Socket = s;
     }
     
     SOCKET GetSocket()
     {
          return m_Socket;
     }
     
     void SetBuffer(char *szBuffer)
     {
          strcpy(m_szBuffer, szBuffer);
     }
     
     void GetBuffer(char *szBuffer)
     {
          strcpy(szBuffer, m_szBuffer);
     }
     
     void ZeroBuffer()
     {
          ZeroMemory(m_szBuffer, MAX_BUFFER_LEN);
     }
     
     void SetWSABUFLength(int nLength)
     {
          m_pwbuf->len = nLength;
     }
     
     int GetWSABUFLength()
     {
          return m_pwbuf->len;
     }
     
     WSABUF* GetWSABUFPtr()
     {
          return m_pwbuf;
     }
     
     OVERLAPPED* GetOVERLAPPEDPtr()
     {
          return m_pol;
     }
     
     void ResetWSABUF()
     {
          ZeroBuffer();
          m_pwbuf->buf = m_szBuffer;
          m_pwbuf->len = MAX_BUFFER_LEN;
     }
     
     //Constructor

     CClientContext()
     {
          m_pol = new OVERLAPPED;
          m_pwbuf = new WSABUF;
          
          ZeroMemory(m_pol, sizeof(OVERLAPPED));
          
          m_Socket =  SOCKET_ERROR;
          
          ZeroMemory(m_szBuffer, MAX_BUFFER_LEN);
          
          m_pwbuf->buf = m_szBuffer;
          m_pwbuf->len = MAX_BUFFER_LEN;
          
          m_nOpCode = 0;
          m_nTotalBytes = 0;
          m_nSentBytes = 0;
     }
     
     //destructor

     ~CClientContext()
     {
          //Wait for the pending operations to complete

          while (!HasOverlappedIoCompleted(m_pol)) 
          {
               Sleep(0);
          }
          
          closesocket(m_Socket);
          
          //Cleanup

          delete m_pol;
          delete m_pwbuf;
     }
};

Next is the worker thread function, WorkerThread(). This function will wait for requests from IOCP, and it will process them. Depending on the operation code supplied, it will perform the appropriate operation. The WorkerThread(), in turn, will make operation requests to IOCP by setting the appropriate operation code of CClientContext. These requests will be routed to one of the worker threads, including the requesting worker thread.

//Worker thread will service IOCP requests

DWORD WINAPI WorkerThread(LPVOID lpParam)
{    
    int nThreadNo = (int)lpParam;

    void *lpContext = NULL;
    OVERLAPPED       *pOverlapped = NULL;
    CClientContext   *pClientContext = NULL;
    DWORD            dwBytesTransfered = 0;
    int nBytesRecv = 0;
    int nBytesSent = 0;
    DWORD             dwBytes = 0, dwFlags = 0;

    //Worker thread will be around to process requests,

    //until a Shutdown event is not Signaled.

    while (WAIT_OBJECT_0 != WaitForSingleObject(g_hShutdownEvent, 0))
    {
        BOOL bReturn = GetQueuedCompletionStatus(
            g_hIOCompletionPort,
            &dwBytesTransfered,
            (LPDWORD)&lpContext,
            &pOverlapped,
            INFINITE);

        if (NULL == lpContext)
        {
            //We are shutting down

            break;
        }

        //Get the client context

        pClientContext = (CClientContext *)lpContext;

        if ((FALSE == bReturn) || ((TRUE == bReturn) && 
        (0 == dwBytesTransfered)))
        {
            //Client connection gone, remove it.

            RemoveFromClientListAndFreeMemory(pClientContext);
            continue;
        }

        WSABUF *p_wbuf = pClientContext->GetWSABUFPtr();
        OVERLAPPED *p_ol = pClientContext->GetOVERLAPPEDPtr();

        switch (pClientContext->GetOpCode())
        {
        case OP_READ:

            pClientContext->IncrSentBytes(dwBytesTransfered);

            //Write operation was finished, see if all the data was sent.

            //Else post another write.

            if(pClientContext->GetSentBytes() < 
            pClientContext->GetTotalBytes())
            {
                pClientContext->SetOpCode(OP_READ);

                p_wbuf->buf += pClientContext->GetSentBytes();
                p_wbuf->len = pClientContext->GetTotalBytes() - 
                    pClientContext->GetSentBytes();

                dwFlags = 0;

                //Overlapped send

                nBytesSent = WSASend(pClientContext->GetSocket(), 
                    p_wbuf, 1, &dwBytes, dwFlags, p_ol, NULL);

                if ((SOCKET_ERROR == nBytesSent) && 
                (WSA_IO_PENDING != WSAGetLastError()))
                {
                    //Let's not work with this client

                    RemoveFromClientListAndFreeMemory(pClientContext);
                }
            }
            else
            {
                //Once the data is successfully received, we will print it.

                pClientContext->SetOpCode(OP_WRITE);
                pClientContext->ResetWSABUF();

                dwFlags = 0;

                //Get the data.

                nBytesRecv = WSARecv(pClientContext->GetSocket(), p_wbuf, 1, 
                    &dwBytes, &dwFlags, p_ol, NULL);

                if ((SOCKET_ERROR == nBytesRecv) && 
                (WSA_IO_PENDING != WSAGetLastError()))
                {
                    WriteToConsole("\nThread %d: Error occurred" 
                    " while executing WSARecv().", nThreadNo);

                    //Let's not work with this client

                    RemoveFromClientListAndFreeMemory(pClientContext);
                }
            }

            break;

        case OP_WRITE:

            char szBuffer[MAX_BUFFER_LEN];

            //Display the message we recevied

            pClientContext->GetBuffer(szBuffer);

            WriteToConsole("\nThread %d: The following message" 
            " was received: %s", nThreadNo, szBuffer);

            //Send the message back to the client.

            pClientContext->SetOpCode(OP_READ);


            pClientContext->SetTotalBytes(dwBytesTransfered);
            pClientContext->SetSentBytes(0);

            p_wbuf->len  = dwBytesTransfered;

            dwFlags = 0;

            //Overlapped send

            nBytesSent = WSASend(pClientContext->GetSocket(), p_wbuf, 1, 
                &dwBytes, dwFlags, p_ol, NULL);

            if ((SOCKET_ERROR == nBytesSent) && 
            (WSA_IO_PENDING != WSAGetLastError()))
            {
                WriteToConsole("\nThread %d: Error " 
                "occurred while executing WSASend().", nThreadNo);

                //Let's not work with this client

                RemoveFromClientListAndFreeMemory(pClientContext);
            }

            break;

        default:
            //We should never be reaching here, under normal circumstances.

            break;
        } // switch

    } // while


    return 0;
}

Some of the functions used in initialization and cleanup are shown below. Notice that PostQueuedCompletionStatus() is used in CleanUp() to help WorkerThread() get out of blocking calls to GetQueuedCompletionStatus().

bool Initialize()
{
     //Find out number of processors and threads

     g_nThreads = WORKER_THREADS_PER_PROCESSOR * GetNoOfProcessors();
     
     printf("\nNumber of processors on host: %d", GetNoOfProcessors());
     
     printf("\nThe following number of worker threads" + 
            " will be created: %d", g_nThreads);
     
     //Allocate memory to store thread handless

     g_phWorkerThreads = new HANDLE[g_nThreads];
     
     //Initialize the Console Critical Section

     InitializeCriticalSection(&g_csConsole);
     
     //Initialize the Client List Critical Section

     InitializeCriticalSection(&g_csClientList);
     
     //Create shutdown event

     g_hShutdownEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
     
     // Initialize Winsock

     WSADATA wsaData;
     
     int nResult;
     nResult = WSAStartup(MAKEWORD(2,2), &wsaData);
     
     if (NO_ERROR != nResult)
     {
          printf("\nError occurred while executing WSAStartup().");
          return false; //error

     }
     else
     {
          printf("\nWSAStartup() successful.");
     }
     
     if (false == InitializeIOCP())
     {
          printf("\nError occurred while initializing IOCP");
          return false;
     }
     else
     {
          printf("\nIOCP initialization successful.");
     }
     
     return true;
}
 
//Function to Initialize IOCP

bool InitializeIOCP()
{
     //Create I/O completion port

     g_hIOCompletionPort = 
        CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0, 0 );
     
     if ( NULL == g_hIOCompletionPort)
     {
          printf("\nError occurred while creating IOCP: %d.", 
                                          WSAGetLastError());
          return false;
     }
     
     DWORD nThreadID;
     
     //Create worker threads

     for (int ii = 0; ii < g_nThreads; ii++)
     {
          g_phWorkerThreads[ii] = CreateThread(0, 0, WorkerThread, 
                                 (void *)(ii+1), 0, &nThreadID);
     }
     
     return true;
}
 
void CleanUp()
{
     //Ask all threads to start shutting down

     SetEvent(g_hShutdownEvent);
     
     //Let Accept thread go down

     WaitForSingleObject(g_hAcceptThread, INFINITE);
     
     for (int i = 0; i < g_nThreads; i++)
     {
          //Help threads get out of blocking - GetQueuedCompletionStatus()

          PostQueuedCompletionStatus(g_hIOCompletionPort, 0, 
                                    (DWORD) NULL, NULL);
     }
     
     //Let Worker Threads shutdown

     WaitForMultipleObjects(g_nThreads, g_phWorkerThreads, TRUE, INFINITE);
     
     //We are done with this event

     WSACloseEvent(g_hAcceptEvent);
     
     //Cleanup dynamic memory allocations, if there are any.

     CleanClientList();
}
 
void DeInitialize()
{
     //Delete the Console Critical Section.

     DeleteCriticalSection(&g_csConsole);
     
     //Delete the Client List Critical Section.

     DeleteCriticalSection(&g_csClientList);
     
     //Cleanup IOCP.

     CloseHandle(g_hIOCompletionPort);
     
     //Clean up the event.

     CloseHandle(g_hShutdownEvent);
     
     //Clean up memory allocated for the storage of thread handles

     delete[] g_phWorkerThreads;
     
     //Cleanup Winsock

     WSACleanup();
}

To store client information, I am using a vector; and to manage the list, I am using the following set of calls:

//Store client related information in a vector

void AddToClientList(CClientContext   *pClientContext)
{
     EnterCriticalSection(&g_csClientList);
     
     //Store these structures in vectors

     g_ClientContext.push_back(pClientContext);
     
     LeaveCriticalSection(&g_csClientList);
}
 
//This function will allow to remove one single client out of the list

void RemoveFromClientListAndFreeMemory(CClientContext   *pClientContext)
{
     EnterCriticalSection(&g_csClientList);
     
     std::vector <cclientcontext>::iterator IterClientContext;
     
     //Remove the supplied ClientContext 

     //from the list and release the memory

     for (IterClientContext = g_ClientContext.begin(); 
          IterClientContext != g_ClientContext.end(); 
          IterClientContext++)
     {
          if (pClientContext == *IterClientContext)
          {
               g_ClientContext.erase(IterClientContext);
               
               //i/o will be cancelled and socket 

               //will be closed by destructor.

               delete pClientContext;
               break;
          }
     }
 
     LeaveCriticalSection(&g_csClientList);
}
 
//Clean up the list, this function 

//will be executed at the time of shutdown

void CleanClientList()
{
     EnterCriticalSection(&g_csClientList);
     
     std::vector <cclientcontext>::iterator IterClientContext;
     
     for (IterClientContext = g_ClientContext.begin(); 
          IterClientContext != g_ClientContext.end( ); 
          IterClientContext++)
     {
          //i/o will be cancelled and socket 

          //will be closed by destructor.

          delete *IterClientContext;
     }
     
     g_ClientContext.clear();
     
     LeaveCriticalSection(&g_csClientList);
}

I have created and used a WriteToConsole() function which will synchronize the console output that is sent by the worker threads; there will be a race condition for console, as I am using printf() to write to the console. It uses a critical section for synchronization:

//Function to synchronize console output

//Threads need to be synchronized while they write to console.

//WriteConsole() API can be used, it is thread-safe, I think.

//I have created my own function.

void WriteToConsole(char *szFormat, ...)
{
     EnterCriticalSection(&g_csConsole);
     
     va_list args;
     va_start(args, szFormat);
     
     vprintf(szFormat, args );
     
     va_end(args);
     
     LeaveCriticalSection(&g_csConsole);
}

Finally, the code of GetNoOfProcessors(), a function that will get us the number of processors on the host:

//The use of static variable will ensure that

//we will make a call to GetSystemInfo() 

//to find out number of processors, 

//only if we don't have the information already.

//Repeated use of this function will be efficient.

int GetNoOfProcessors()
{
     static int nProcessors = 0;
     
     if (0 == nProcessors)
     {
          SYSTEM_INFO si;
          
          GetSystemInfo(&si);
          
          nProcessors = si.dwNumberOfProcessors;
     }
     
     return nProcessors;
}

IOCP client

The following is an IOCP client; it will let us stress-test the server. It uses traditional socket calls. I have used threads to put additional stress on the server.

#include "stdafx.h"

 
#include <stdio.h>

#include <stdlib.h>

#include <conio.h>

#include <string.h>

#include <winsock2.h>

 
#include "ClientIOCP.h"

 
int main(int argc, char* argv[])
{
     //Validate the input

     if (argc < 3) 
     {
          printf("\nUsage: %s hostname port.", argv[0]);
          return 1; //error

     }
     
     //Initialize Winsock

     WSADATA wsaData;
     
     int nResult = WSAStartup(MAKEWORD(2,2), &wsaData);
     
     if (NO_ERROR != nResult)
     {
          printf("\nError occurred while executing WSAStartup().");
          return 1; //error

     }
     
     //Initialize the Console Critical Section

     InitializeCriticalSection(&g_csConsole);
     
     int nPortNo = atoi(argv[2]);
     
     char szBuffer[MAX_BUFFER_LEN];
     int nNoOfThreads = 0;
     int nNoOfSends = 0;
     
     printf("\nPlease enter message to be sent to the server: ");
     
     //Read the message from server

     gets(szBuffer);
     
     printf("\nPlease enter number of threads to be created: ");
     
     //No. of times we will send the message to the server

     scanf("%d", &nNoOfThreads);
     
     printf("\nPlease enter number of times the" + 
            " messages needs to be sent: ");
     
     //No. of times we will send the message to the server

     scanf("%d", &nNoOfSends);
     
     HANDLE *p_hThreads = new HANDLE[nNoOfThreads];
     ThreadInfo *pThreadInfo = new ThreadInfo[nNoOfThreads];
     
     bool bConnectedSocketCreated = false;
     
     DWORD nThreadID;
     
     for (int ii = 0; ii < nNoOfThreads; ii++)
     {
          bConnectedSocketCreated = 
            CreateConnectedSocket(&(pThreadInfo[ii].m_Socket), 
                                  argv[1], nPortNo);
          
          if (!bConnectedSocketCreated)
          {
               //Clean up memory

               delete[] p_hThreads;
               delete[] pThreadInfo;
               
               //failed in creating of connected socket, error out.

               return 1;
          }
          
          //Populate ThreadInfo

          pThreadInfo[ii].m_nNoOfSends = nNoOfSends;
          pThreadInfo[ii].m_nThreadNo = ii+1;
          sprintf(pThreadInfo[ii].m_szBuffer, 
                  "Thread %d - %s", ii+1, szBuffer);
          
          //Create thread and start banging the server

          p_hThreads[ii] = CreateThread(0, 0, WorkerThread, 
                           (void *)(&pThreadInfo[ii]), 0, &nThreadID);
     }
     
     //Let Worker Threads shutdown

     WaitForMultipleObjects(nNoOfThreads, p_hThreads, TRUE, INFINITE);
     
     //Close the sockets here

     for (ii = 0; ii < nNoOfThreads; ii++)
     {
          closesocket(pThreadInfo[ii].m_Socket);
     }
     
     //Clean up memory

     delete[] p_hThreads;
     delete[] pThreadInfo;
     
     //Delete the Console Critical Section.

     DeleteCriticalSection(&g_csConsole);
     
     //Cleanup Winsock

     WSACleanup();
     return 0;
}
 
//vprintf() is not thread safe

void WriteToConsole(char *szFormat, ...)
{
     EnterCriticalSection(&g_csConsole);
     
     va_list args;
     va_start(args, szFormat);
     
     vprintf(szFormat, args );
     
     va_end(args);
     
     LeaveCriticalSection(&g_csConsole);
}
 
bool CreateConnectedSocket(SOCKET *pSocket, char *szHost, int nPortNo)
{
     struct sockaddr_in ServerAddress;
     struct hostent *Server;
     
     //Create a socket

     *pSocket = socket(AF_INET, SOCK_STREAM, IPPROTO_TCP);
     
     if (INVALID_SOCKET == *pSocket) 
     {
          WriteToConsole("\nError occurred while" + 
                         " opening socket: %d.", WSAGetLastError());
          return false; //error

     }
     
     //Server name will be supplied as a commandline argument

     //Get the server details

     Server = gethostbyname(szHost);
     
     if (Server == NULL) 
     {
          closesocket(*pSocket);
          WriteToConsole("\nError occurred no such host.");
          return false; //error

     }
     
     //Cleanup and Init with 0 the ServerAddress

     ZeroMemory((char *) &ServerAddress, sizeof(ServerAddress));
     
     ServerAddress.sin_family = AF_INET;
     
     //Assign the information received from gethostbyname()

     CopyMemory((char *)&ServerAddress.sin_addr.s_addr, 
          (char *)Server->h_addr,
          Server->h_length);
     
     ServerAddress.sin_port = htons(nPortNo);
     
     //Establish connection with the server

     if (SOCKET_ERROR == connect(*pSocket, 
         reinterpret_cast<const>(&ServerAddress), 
         sizeof(ServerAddress))) 
     {
          closesocket(*pSocket);
          WriteToConsole("\nError occurred while connecting.");
          return false; //error

     }
     
     return true;
}
 
DWORD WINAPI WorkerThread(LPVOID lpParam)
{
     ThreadInfo *pThreadInfo = (ThreadInfo*)lpParam;
     
     char szTemp[MAX_BUFFER_LEN];
     
     int nBytesSent = 0;
     int nBytesRecv = 0;
     
     for (int ii = 0; ii < pThreadInfo->m_nNoOfSends; ii++)
     {
          sprintf(szTemp, "%d. %s", ii+1, pThreadInfo->m_szBuffer);
          
          //Send the message to the server, include the NULL as well

          nBytesSent = send(pThreadInfo->m_Socket, szTemp, strlen(szTemp), 0);
          
          if (SOCKET_ERROR == nBytesSent) 
          {
               WriteToConsole("\nError occurred while " + 
                              "writing to socket %ld.", 
                              WSAGetLastError());
               return 1; //error

          }
          
          //Get the message from the server

          nBytesRecv = recv(pThreadInfo->m_Socket, szTemp, 255, 0);
          
          if (SOCKET_ERROR == nBytesRecv) 
          {
               WriteToConsole("\nError occurred while reading " + 
                              "from socket %ld.", WSAGetLastError());
               return 1; //error

          }
          
          //Display the server message

          WriteToConsole("\nServer sent the following message: %s", szTemp);
     }
     
     return 0; //success

}

Points of interest

I/O completion port is a very powerful mechanism to create highly scalable and robust server applications, and needs to be used with overlapped I/O. It should be used with WinSock in socket based server applications serving multiple clients. IOCP will add a lot of value to the design.

History

• 24^th March, 2006 � Implemented using overlapped I/O.
• April, 2006 � Spent time studying overlapped I/O. Made updates related to overlapped I/O.
• 29^th May, 2006 � Changed the IOCP implementation, and implemented a graceful shutdown of the server.
• 9^th June, 2006 � Added comments for easy understanding of the code, and minor code updates.
• 19^th August, 2006 � Updated client and server for stress-testing.
• 22^nd August, 2006 � Client is multi-threaded now. The client gets additional pounding power.
• 24^th September, 2006 � The number of worker threads created on the IOCP server will be proportional to the number of processors on the host. Also, both client and server use new improved WriteToConsole().
• 28^th September, 2006 � Minor updates.
• 3^rd February, 2007 - Fixed a defect as per input by xircon (CodeProject) and Visual C++ 2005 migration updates.
• March, 2007 - Cut down CPU usage by updating WSAWaitForMultipleEvents().