使用overlapped I/O并搭配event对象-----，会产生两个基础性问题。

第一个问题是，使用WaitForMultipleObjects()，你只能等待最多达MAXIMUM_WAIT_OBJECTS个对象，在Windows NT中此值最大为64。

第二个问题是，你必须不断根据“那一个handle被激发”而计算如何反应。你必须有一个分派表格(dispatch table)和WaitForMultipleObjects()的handles数组结合起来。

      这两个问题可以靠一个所谓的异步过程调用(asynchronous Procedure Calls, APCs)解决，只要使用“Ex”版的ReadFile()和WriteFile()，你就可以调用这个机制。这两个函数允许你指定一个额外的参数，是一个callback函数，这个callback函数被称为I/O completion routine，因为系统是在某一个特别的overlapped I/O操作完成之后调用它。

异步过程调用事例:

 

/* * IoByAPC.c * * Sample code for Multithreading Applications in Win32 * This is from Chapter 6, Listing 6-3 * * Demonstrates how to use APC's (asynchronous * procedure calls) instead of signaled objects * to service multiple outstanding overlapped * operations on a file. */#define WIN32_LEAN_AND_MEAN#include 
    
     #include 
     
      #include 
      
       #include "MtVerify.h"//// Constants//#define MAX_REQUESTS    5#define READ_SIZE       512#define MAX_TRY_COUNT   5//// Function prototypes//void CheckOsVersion();int QueueRequest(int nIndex, DWORD dwLocation, DWORD dwAmount);//// Global variables//// Need a single event object so we know when all I/O is finishedHANDLE  ghEvent;// Keep track of each individual I/O operationOVERLAPPED gOverlapped[MAX_REQUESTS];// Handle to the file of interest.HANDLE ghFile;// Need a place to put all this datachar gBuffers[MAX_REQUESTS][READ_SIZE];int nCompletionCount;/* * I/O Completion routine gets called * when app is alertable (in WaitForSingleObjectEx) * and an overlapped I/O operation has completed. */VOID WINAPI FileIOCompletionRoutine(    DWORD dwErrorCode,  // completion code     DWORD dwNumberOfBytesTransfered,    // number of bytes transferred     LPOVERLAPPED lpOverlapped   // pointer to structure with I/O information     ){    // The event handle is really the user defined data    int nIndex = (int)(lpOverlapped->hEvent);    printf("Read #%d returned %d. %d bytes were read.\n",        nIndex,        dwErrorCode,        dwNumberOfBytesTransfered);    if (++nCompletionCount == MAX_REQUESTS)        SetEvent(ghEvent);  // Cause the wait to terminate}int main(){    int i;    char szPath[MAX_PATH];    CheckOsVersion();    // Need to know when to stop    MTVERIFY(        ghEvent = CreateEvent(                     NULL,    // No security                     TRUE,    // Manual reset - extremely important!                     FALSE,   // Initially set Event to non-signaled state                     NULL     // No name                    )    );    GetWindowsDirectory(szPath, sizeof(szPath));    strcat(szPath, "\\WINHLP32.EXE");    // Open the file for overlapped reads    ghFile = CreateFile( szPath,                    GENERIC_READ,                    FILE_SHARE_READ|FILE_SHARE_WRITE,                    NULL,                    OPEN_EXISTING,                    FILE_FLAG_OVERLAPPED,                    NULL                );    if (ghFile == INVALID_HANDLE_VALUE)    {        printf("Could not open %s\n", szPath);        return -1;    }    // Queue up a few requests    for (i=0; i