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Windows via C/C++: Synchronous and Asynchronous Device I/O

Receiving Completed I/O Request Notifications

At this point, you know how to queue an asynchronous device I/O request, but I haven’t discussed how the device driver notifies you after the I/O request has completed.

Windows offers four different methods (briefly described in Table 10-9) for receiving I/O completion notifications, and this chapter covers all of them. The methods are shown in order of complexity, from the easiest to understand and implement (signaling a device kernel object) to the hardest to understand and implement (I/O completion ports).

Table 10-9 Methods for Receiving I/O Completion Notifications



Signaling a device kernel object

Not useful for performing multiple simultaneous I/O requests against a single device. Allows one thread to issue an I/O request and another thread to process it.

Signaling an event kernel object

Allows multiple simultaneous I/O requests against a single device. Allows one thread to issue an I/O request and another thread to process it.

Using alertable I/O

Allows multiple simultaneous I/O requests against a single device. The thread that issued an I/O request must also process it.

Using I/O completion ports

Allows multiple simultaneous I/O requests against a single device. Allows one thread to issue an I/O request and another thread to process it. This technique is highly scalable and has the most flexibility.

As stated at the beginning of this chapter, the I/O completion port is the hands-down best method of the four for receiving I/O completion notifications. By studying all four, you’ll learn why Microsoft added the I/O completion port to Windows and how the I/O completion port solves all the problems that exist for the other methods.

Signaling a Device Kernel Object

Once a thread issues an asynchronous I/O request, the thread continues executing, doing useful work. Eventually, the thread needs to synchronize with the completion of the I/O operation. In other words, you’ll hit a point in your thread’s code at which the thread can’t continue to execute unless the data from the device is fully loaded into the buffer.

In Windows, a device kernel object can be used for thread synchronization, so the object can either be in a signaled or nonsignaled state. The ReadFile and WriteFile functions set the device kernel object to the nonsignaled state just before queuing the I/O request. When the device driver completes the request, the driver sets the device kernel object to the signaled state.

A thread can determine whether an asynchronous I/O request has completed by calling either WaitForSingleObject or WaitForMultipleObjects. Here is a simple example:

HANDLE hFile = CreateFile(..., FILE_FLAG_OVERLAPPED, ...);
BYTE bBuffer[100];
OVERLAPPED o = { 0 };
o.Offset = 345;

BOOL bReadDone = ReadFile(hFile, bBuffer, 100, NULL, &o);
DWORD dwError = GetLastError();

if (!bReadDone && (dwError == ERROR_IO_PENDING)) {
   // The I/O is being performed asynchronously; wait for it to complete
   WaitForSingleObject(hFile, INFINITE);
   bReadDone = TRUE;

if (bReadDone) {
   // o.Internal contains the I/O error
   // o.InternalHigh contains the number of bytes transferred
   // bBuffer contains the read data
} else {
   // An error occurred; see dwError

This code issues an asynchronous I/O request and then immediately waits for the request to finish, defeating the purpose of asynchronous I/O! Obviously, you would never actually write code similar to this, but the code does demonstrate important concepts, which I’ll summarize here:

  • The device must be opened for asynchronous I/O by using the FILE_FLAG_OVERLAPPED flag.

  • The OVERLAPPED structure must have its Offset, OffsetHigh, and hEvent members initialized. In the code example, I set them all to 0 except for Offset, which I set to 345 so that ReadFile reads data from the file starting at byte 346.

  • ReadFile’s return value is saved in bReadDone, which indicates whether the I/O request was performed synchronously.

  • If the I/O request was not performed synchronously, I check to see whether an error occurred or whether the I/O is being performed asynchronously. Comparing the result of GetLastError with ERROR_IO_PENDING gives me this information.

  • To wait for the data, I call WaitForSingleObject, passing the handle of the device kernel object. As you saw in Chapter 9, calling this function suspends the thread until the kernel object becomes signaled. The device driver signals the object when it completes the I/O. After WaitForSingleObject returns, the I/O is complete and I set bReadDone to TRUE.

  • After the read completes, you can examine the data in bBuffer, the error code in the OVERLAPPED structure’s Internal member, and the number of bytes transferred in the OVERLAPPED structure’s InternalHigh member.

  • If a true error occurred, dwError contains the error code giving more information.

Signaling an Event Kernel Object

The method for receiving I/O completion notifications just described is very simple and straightforward, but it turns out not to be all that useful because it does not handle multiple I/O requests well. For example, suppose you were trying to carry out multiple asynchronous operations on a single file at the same time. Say that you wanted to read 10 bytes from the file and write 10 bytes to the file simultaneously. The code might look like this:

HANDLE hFile = CreateFile(..., FILE_FLAG_OVERLAPPED, ...);

BYTE bReadBuffer[10];
OVERLAPPED oRead = { 0 };
oRead.Offset = 0;
ReadFile(hFile, bReadBuffer, 10, NULL, &oRead);

BYTE bWriteBuffer[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
OVERLAPPED oWrite = { 0 };
oWrite.Offset = 10;
WriteFile(hFile, bWriteBuffer, _countof(bWriteBuffer), NULL, &oWrite);
WaitForSingleObject(hFile, INFINITE);

// We don't know what completed: Read? Write? Both?

You can’t synchronize your thread by waiting for the device to become signaled because the object becomes signaled as soon as either of the operations completes. If you call WaitForSingleObject, passing it the device handle, you will be unsure whether the function returned because the read operation completed, the write operation completed, or both operations completed. Clearly, there needs to be a better way to perform multiple, simultaneous asynchronous I/O requests so that you don’t run into this predicament–fortunately, there is.

The last member of the OVERLAPPED structure, hEvent, identifies an event kernel object. You must create this event object by calling CreateEvent. When an asynchronous I/O request completes, the device driver checks to see whether the hEvent member of the OVERLAPPED structure is NULL. If hEvent is not NULL, the driver signals the event by calling SetEvent. The driver also sets the device object to the signaled state just as it did before. However, if you are using events to determine when a device operation has completed, you shouldn’t wait for the device object to become signaled–wait for the event instead.

If you want to perform multiple asynchronous device I/O requests simultaneously, you must create a separate event object for each request, initialize the hEvent member in each request’s OVERLAPPED structure, and then call ReadFile or WriteFile. When you reach the point in your code at which you need to synchronize with the completion of the I/O request, simply call WaitForMultipleObjects, passing in the event handles associated with each outstanding I/O request’s OVERLAPPED structures. With this scheme, you can easily and reliably perform multiple asynchronous device I/O operations simultaneously and use the same device object. The following code demonstrates this approach:

HANDLE hFile = CreateFile(..., FILE_FLAG_OVERLAPPED, ...);

BYTE bReadBuffer[10];
OVERLAPPED oRead = { 0 };
oRead.Offset = 0;
oRead.hEvent = CreateEvent(...);
ReadFile(hFile, bReadBuffer, 10, NULL, &oRead);

BYTE bWriteBuffer[10] = { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };
OVERLAPPED oWrite = { 0 };
oWrite.Offset = 10;
oWrite.hEvent = CreateEvent(...);
WriteFile(hFile, bWriteBuffer, _countof(bWriteBuffer), NULL, &oWrite);

HANDLE h[2];
h[0] = oRead.hEvent;
h[1] = oWrite.hEvent;
DWORD dw = WaitForMultipleObjects(2, h, FALSE, INFINITE);
switch (dw – WAIT_OBJECT_0) {
   case 0:   // Read completed

   case 1:   // Write completed

This code is somewhat contrived and is not exactly what you’d do in a real-life application, but it does illustrate my point. Typically, a real-life application has a loop that waits for I/O requests to complete. As each request completes, the thread performs the desired task, queues another asynchronous I/O request, and loops back around, waiting for more I/O requests to complete.


Recall that originally Microsoft was not going to document the OVERLAPPED structure’s Internal and InternalHigh members, which meant it needed to provide another way for you to know how many bytes were transferred during the I/O processing and get the I/O’s error code. To make this information available to you, Microsoft created the GetOverlappedResult function:

BOOL GetOverlappedResult(
   HANDLE      hFile,
   OVERLAPPED* pOverlapped,
   PDWORD      pdwNumBytes,
   BOOL        bWait);

Microsoft now documents the Internal and InternalHigh members, so the GetOverlappedResult function is not very useful. However, when I was first learning asynchronous I/O, I decided to reverse engineer the function to help solidify concepts in my head. The following code shows how GetOverlappedResult is implemented internally:

BOOL GetOverlappedResult(
   HANDLE hFile,
   PDWORD pdwNumBytes,
   BOOL bWait) {

   if (po->Internal == STATUS_PENDING) {
      DWORD dwWaitRet = WAIT_TIMEOUT;
      if (bWait) {
         // Wait for the I/O to complete
         dwWaitRet = WaitForSingleObject(
            (po->hEvent != NULL) ? po->hEvent : hFile, INFINITE);

      if (dwWaitRet == WAIT_TIMEOUT) {
         // I/O not complete and we're not supposed to wait

      if (dwWaitRet != WAIT_OBJECT_0) {
         // Error calling WaitForSingleObject

   // I/O is complete; return number of bytes transferred
   *pdwNumBytes = po->InternalHigh;

   if (SUCCEEDED(po->Internal)) {
      return(TRUE);   // No I/O error

   // Set last error to I/O error

Alertable I/O

The third method available to you for receiving I/O completion notifications is called alertable I/O. At first, Microsoft touted alertable I/O as the absolute best mechanism for developers who wanted to create high-performance, scalable applications. But as developers started using alertable I/O, they soon realized that it was not going to live up to the promise.

I have worked with alertable I/O quite a bit, and I’ll be the first to tell you that alertable I/O is horrible and should be avoided. However, to make alertable I/O work, Microsoft added some infrastructure into the operating system that I have found to be extremely useful and valuable. As you read this section, concentrate on the infrastructure that is in place and don’t get bogged down in the I/O aspects.

Whenever a thread is created, the system also creates a queue that is associated with the thread. This queue is called the asynchronous procedure call (APC) queue. When issuing an I/O request, you can tell the device driver to append an entry to the calling thread’s APC queue. To have completed I/O notifications queued to your thread’s APC queue, you call the ReadFileEx and WriteFileEx functions:

BOOL ReadFileEx(
   HANDLE      hFile,
   PVOID       pvBuffer,
   DWORD       nNumBytesToRead,
   OVERLAPPED* pOverlapped,

BOOL WriteFileEx(
   HANDLE      hFile,
   CONST VOID  *pvBuffer,
   DWORD       nNumBytesToWrite,
   OVERLAPPED* pOverlapped,

Like ReadFile and WriteFile, ReadFileEx and WriteFileEx issue I/O requests to a device driver, and the functions return immediately. The ReadFileEx and WriteFileEx functions have the same parameters as the ReadFile and WriteFile functions, with two exceptions. First, the *Ex functions are not passed a pointer to a DWORD that gets filled with the number of bytes transferred; this information can be retrieved only by the callback function. Second, the *Ex functions require that you pass the address of a callback function, called a completion routine. This routine must have the following prototype:

VOID WINAPI CompletionRoutine(
   DWORD       dwError,
   DWORD       dwNumBytes,

When you issue an asynchronous I/O request with ReadFileEx and WriteFileEx, the functions pass the address of this function to the device driver. When the device driver has completed the I/O request, it appends an entry in the issuing thread’s APC queue. This entry contains the address of the completion routine function and the address of the OVERLAPPED structure used to initiate the I/O request.

When the thread is in an alertable state (discussed shortly), the system examines its APC queue and, for every entry in the queue, the system calls the completion function, passing it the I/O error code, the number of bytes transferred, and the address of the OVERLAPPED structure. Note that the error code and number of bytes transferred can also be found in the OVERLAPPED structure’s Internal and InternalHigh members. (As I mentioned earlier, Microsoft originally didn’t want to document these, so it passed them as parameters to the function.)

I’ll get back to this completion routine function shortly. First let’s look at how the system handles the asynchronous I/O requests. The following code queues three different asynchronous operations:

hFile = CreateFile(..., FILE_FLAG_OVERLAPPED, ...);

ReadFileEx(hFile, ...);    // Perform first ReadFileEx
WriteFileEx(hFile, ...);   // Perform first WriteFileEx
ReadFileEx(hFile, ...);    // Perform second ReadFileEx


If the call to SomeFunc takes some time to execute, the system completes the three operations before SomeFunc returns. While the thread is executing the SomeFunc function, the device driver is appending completed I/O entries to the thread’s APC queue. The APC queue might look something like this:

first WriteFileEx completed
second ReadFileEx completed
first ReadFileEx completed

The APC queue is maintained internally by the system. You’ll also notice from the list that the system can execute your queued I/O requests in any order, and that the I/O requests that you issue last might be completed first and vice versa. Each entry in your thread’s APC queue contains the address of a callback function and a value that is passed to the function.

As I/O requests complete, they are simply queued to your thread’s APC queue–the callback routine is not immediately called because your thread might be busy doing something else and cannot be interrupted. To process entries in your thread’s APC queue, the thread must put itself in an alertable state. This simply means that your thread has reached a position in its execution where it can handle being interrupted. Windows offers six functions that can place a thread in an alertable state:

DWORD SleepEx(
   DWORD dwMilliseconds,
   BOOL  bAlertable);

DWORD WaitForSingleObjectEx(
   HANDLE hObject,
   DWORD  dwMilliseconds,
   BOOL   bAlertable);

DWORD WaitForMultipleObjectsEx(
   DWORD   cObjects,
   CONST HANDLE* phObjects,
   BOOL    bWaitAll,
   DWORD   dwMilliseconds,
   BOOL    bAlertable);

BOOL SignalObjectAndWait(
   HANDLE hObjectToSignal,
   HANDLE hObjectToWaitOn,
   DWORD  dwMilliseconds,
   BOOL   bAlertable);

BOOL GetQueuedCompletionStatusEx(
   HANDLE hCompPort,
   ULONG ulCount,
   PULONG pulNumEntriesRemoved,
   DWORD dwMilliseconds,
   BOOL bAlertable);

DWORD MsgWaitForMultipleObjectsEx(
   DWORD   nCount,
   CONST HANDLE* pHandles,
   DWORD   dwMilliseconds,
   DWORD   dwWakeMask,
   DWORD   dwFlags);

The last argument to the first five functions is a Boolean value indicating whether the calling thread should place itself in an alertable state. For MsgWaitForMultipleObjectsEx, you must use the MWMO_ALERTABLE flag to have the thread enter an alertable state. If you’re familiar with the Sleep, WaitForSingleObject, and WaitForMultipleObjects functions, you shouldn’t be surprised to learn that, internally, these non-Ex functions call their Ex counterparts, always passing FALSE for the bAlertable parameter.

When you call one of the six functions just mentioned and place your thread in an alertable state, the system first checks your thread’s APC queue. If at least one entry is in the queue, the system does not put your thread to sleep. Instead, the system pulls the entry from the APC queue and your thread calls the callback routine, passing the routine the completed I/O request’s error code, number of bytes transferred, and address of the OVERLAPPED structure. When the callback routine returns to the system, the system checks for more entries in the APC queue. If more entries exist, they are processed. However, if no more entries exist, your call to the alertable function returns. Something to keep in mind is that if any entries are in your thread’s APC queue when you call any of these functions, your thread never sleeps!

The only time these functions suspend your thread is when no entries are in your thread’s APC queue at the time you call the function. While your thread is suspended, the thread will wake up if the kernel object (or objects) that you’re waiting on becomes signaled or if an APC entry appears in your thread’s queue. Because your thread is in an alertable state, as soon as an APC entry appears, the system wakes your thread and empties the queue (by calling the callback routines). Then the functions immediately return to the caller–your thread does not go back to sleep waiting for kernel objects to become signaled.

The return value from these six functions indicates why they have returned. If they return WAIT_IO_COMPLETION (or if GetLastError returns WAIT_IO_COMPLETION), you know that the thread is continuing to execute because at least one entry was processed from the thread’s APC queue. If the methods return for any other reason, the thread woke up because the sleep period expired, the specified kernel object or objects became signaled, or a mutex was abandoned.

The Bad and the Good of Alertable I/O

At this point, we’ve discussed the mechanics of performing alertable I/O. Now you need to know about the two issues that make alertable I/O a horrible method for doing device I/O:

  • Callback functions. Alertable I/O requires that you create callback functions, which makes implementing your code much more difficult. These callback functions typically don’t have enough contextual information about a particular problem to guide you, so you end up placing a lot of information in global variables. Fortunately, these global variables don’t need to be synchronized because the thread calling one of the six alterable functions is the same thread executing the callback functions. A single thread can’t be in two places at one time, so the variables are safe.

  • Threading issues. The real big problem with alertable I/O is this: The thread issuing the I/O request must also handle the completion notification. If a thread issues several requests, that thread must respond to each request’s completion notification, even if other threads are sitting completely idle. Because there is no load balancing, the application doesn’t scale well.

Both of these problems are pretty severe, so I strongly discourage the use of alertable I/O for device I/O. I’m sure you guessed by now that the I/O completion port mechanism, discussed in the next section, solves both of the problems I just described. I promised to tell you some good stuff about the alertable I/O infrastructure, so before I move on to the I/O completion port, I’ll do that.

Windows offers a function that allows you to manually queue an entry to a thread’s APC queue:

   HANDLE    hThread,
   ULONG_PTR dwData);

The first parameter is a pointer to an APC function that must have the following prototype:


The second parameter is the handle of the thread for which you want to queue the entry. Note that this thread can be any thread in the system. If hThread identifies a thread in a different process’ address space, pfnAPC must specify the memory address of a function that is in the address space of the target thread’s process. The last parameter to QueueUserAPC, dwData, is a value that simply gets passed to the callback function.

Even though QueueUserAPC is prototyped as returning a DWORD, the function actually returns a BOOL indicating success or failure. You can use QueueUserAPC to perform extremely efficient interthread communication, even across process boundaries. Unfortunately, however, you can pass only a single value.

QueueUserAPC can also be used to force a thread out of a wait state. Suppose you have a thread calling WaitForSingleObject, waiting for a kernel object to become signaled. While the thread is waiting, the user wants to terminate the application. You know that threads should cleanly destroy themselves, but how do you force the thread waiting on the kernel object to wake up and kill itself? QueueUserAPC is the answer.

The following code demonstrates how to force a thread out of a wait state so that the thread can exit cleanly. The main function spawns a new thread, passing it the handle of some kernel object. While the secondary thread is running, the primary thread is also running. The secondary thread (executing the ThreadFunc function) calls WaitForSingleObjectEx, which suspends the thread, placing it in an alertable state. Now, say that the user tells the primary thread to terminate the application. Sure, the primary thread could just exit, and the system would kill the whole process. However, this approach is not very clean, and in many scenarios, you’ll just want to kill an operation without terminating the whole process.

So the primary thread calls QueueUserAPC, which places an APC entry in the secondary thread’s APC queue. Because the secondary thread is in an alertable state, it now wakes and empties its APC queue by calling the APCFunc function. This function does absolutely nothing and just returns. Because the APC queue is now empty, the thread returns from its call to WaitForSingleObjectEx with a return value of WAIT_IO_COMPLETION. The ThreadFunc function checks specifically for this return value, knowing that it received an APC entry indicating that the thread should exit.

// The APC callback function has nothing to do
   // Nothing to do in here

UINT WINAPI ThreadFunc(PVOID pvParam) {
   HANDLE hEvent = (HANDLE) pvParam;   // Handle is passed to this thread

   // Wait in an alertable state so that we can be forced to exit cleanly
   DWORD dw = WaitForSingleObjectEx(hEvent, INFINITE, TRUE);
   if (dw == WAIT_OBJECT_0) {
      // Object became signaled
   if (dw == WAIT_IO_COMPLETION) {
      // QueueUserAPC forced us out of a wait state
      return(0);   // Thread dies cleanly
void main() {
   HANDLE hEvent = CreateEvent(...);
   HANDLE hThread = (HANDLE) _beginthreadex(NULL, 0,
      ThreadFunc, (PVOID) hEvent, 0, NULL);

   // Force the secondary thread to exit cleanly
   QueueUserAPC(APCFunc, hThread, NULL);
   WaitForSingleObject(hThread, INFINITE);

I know that some of you are thinking that this problem could have been solved by replacing the call to WaitForSingleObjectEx with a call to WaitForMultipleObjects and by creating another event kernel object to signal the secondary thread to terminate. For my simple example, your solution would work. However, if my secondary thread called WaitForMultipleObjects to wait until all objects became signaled, QueueUserAPC would be the only way to force the thread out of a wait state.

I/O Completion Ports

Windows is designed to be a secure, robust operating system running applications that service literally thousands of users. Historically, you’ve been able to architect a service application by following one of two models:

  • Serial model A single thread waits for a client to make a request (usually over the network). When the request comes in, the thread wakes and handles the client’s request.

  • Concurrent model A single thread waits for a client request and then creates a new thread to handle the request. While the new thread is handling the client’s request, the original thread loops back around and waits for another client request. When the thread that is handling the client’s request is completely processed, the thread dies.

The problem with the serial model is that it does not handle multiple, simultaneous requests well. If two clients make requests at the same time, only one can be processed at a time; the second request must wait for the first request to finish processing. A service that is designed using the serial approach cannot take advantage of multiprocessor machines. Obviously, the serial model is good only for the simplest of server applications, in which few client requests are made and requests can be handled very quickly. A Ping server is a good example of a serial server.

Because of the limitations in the serial model, the concurrent model is extremely popular. In the concurrent model, a thread is created to handle each client request. The advantage is that the thread waiting for incoming requests has very little work to do. Most of the time, this thread is sleeping. When a client request comes in, the thread wakes, creates a new thread to handle the request, and then waits for another client request. This means that incoming client requests are handled expediently. Also, because each client request gets its own thread, the server application scales well and can easily take advantage of multiprocessor machines. So if you are using the concurrent model and upgrade the hardware (add another CPU), the performance of the server application improves.

Service applications using the concurrent model were implemented using Windows. The Windows team noticed that application performance was not as high as desired. In particular, the team noticed that handling many simultaneous client requests meant that many threads were running in the system concurrently. Because all these threads were runnable (not suspended and waiting for something to happen), Microsoft realized that the Windows kernel spent too much time context switching between the running threads, and the threads were not getting as much CPU time to do their work. To make Windows an awesome server environment, Microsoft needed to address this problem. The result is the I/O completion port kernel object.

Creating an I/O Completion Port

The theory behind the I/O completion port is that the number of threads running concurrently must have an upper bound–that is, 500 simultaneous client requests cannot allow 500 runnable threads to exist. What, then, is the proper number of concurrent, runnable threads? Well, if you think about this question for a moment, you’ll come to the realization that if a machine has two CPUs, having more than two runnable threads–one for each processor–really doesn’t make sense. As soon as you have more runnable threads than CPUs available, the system has to spend time performing thread context switches, which wastes precious CPU cycles–a potential deficiency of the concurrent model.

Another deficiency of the concurrent model is that a new thread is created for each client request. Creating a thread is cheap when compared to creating a new process with its own virtual address space, but creating threads is far from free. The service application’s performance can be improved if a pool of threads is created when the application initializes, and these threads hang around for the duration of the application. I/O completion ports were designed to work with a pool of threads.

An I/O completion port is probably the most complex kernel object. To create an I/O completion port, you call CreateIoCompletionPort:

HANDLE CreateIoCompletionPort(
   HANDLE    hFile,
   HANDLE    hExistingCompletionPort,
   ULONG_PTR CompletionKey,
   DWORD     dwNumberOfConcurrentThreads);

This function performs two different tasks: it creates an I/O completion port, and it associates a device with an I/O completion port. This function is overly complex, and in my opinion, Microsoft should have split it into two separate functions. When I work with I/O completion ports, I separate these two capabilities by creating two tiny functions that abstract the call to CreateIoCompletionPort. The first function I write is called CreateNewCompletionPort, and I implement it as follows:

HANDLE CreateNewCompletionPort(DWORD dwNumberOfConcurrentThreads) {

   return(CreateIoCompletionPort(INVALID_HANDLE_VALUE, NULL, 0,

This function takes a single argument, dwNumberOfConcurrentThreads, and then calls the Windows CreateIoCompletionPort function, passing in hard-coded values for the first three parameters and dwNumberOfConcurrentThreads for the last parameter. You see, the first three parameters to CreateIoCompletionPort are used only when you are associating a device with a completion port. (I’ll talk about this shortly.) To create just a completion port, I pass INVALID_HANDLE_VALUE, NULL, and 0, respectively, to CreateIoCompletionPort’s first three parameters.

The dwNumberOfConcurrentThreads parameter tells the I/O completion port the maximum number of threads that should be runnable at the same time. If you pass 0 for the dwNumberOfConcurrentThreads parameter, the completion port defaults to allowing as many concurrent threads as there are CPUs on the host machine. This is usually exactly what you want so that extra context switching is avoided. You might want to increase this value if the processing of a client request requires a lengthy computation that rarely blocks, but increasing this value is strongly discouraged. You might experiment with the dwNumberOfConcurrentThreads parameter by trying different values and comparing your application’s performance on your target hardware.

You’ll notice that CreateIoCompletionPort is about the only Windows function that creates a kernel object but does not have a parameter that allows you to pass the address of a SECURITY_ATTRIBUTES structure. This is because completion ports are intended for use within a single process only. The reason will be clear to you when I explain how to use completion ports.

Associating a Device with an I/O Completion Port

When you create an I/O completion port, the kernel actually creates five different data structures, as shown in Figure 10-1. You should refer to this figure as you continue reading.

The first data structure is a device list indicating the device or devices associated with the port. You associate a device with the port by calling CreateIoCompletionPort. Again, I created my own function, AssociateDeviceWithCompletionPort, which abstracts the call to CreateIoCompletionPort:

BOOL AssociateDeviceWithCompletionPort(
   HANDLE hCompletionPort, HANDLE hDevice, DWORD dwCompletionKey) {

   HANDLE h = CreateIoCompletionPort(hDevice, hCompletionPort, dwCompletionKey, 0);
   return(h == hCompletionPort);

AssociateDeviceWithCompletionPort appends an entry to an existing completion port’s device list. You pass to the function the handle of an existing completion port (returned by a previous call to CreateNewCompletionPort), the handle of the device (this can be a file, a socket, a mailslot, a pipe, and so on), and a completion key (a value that has meaning to you; the operating system doesn’t care what you pass here). Each time you associate a device with the port, the system appends this information to the completion port’s device list.

Figure 10-1

Figure 10-1 The internal workings of an I/O completion port

The second data structure is an I/O completion queue. When an asynchronous I/O request for a device completes, the system checks to see whether the device is associated with a completion port and, if it is, the system appends the completed I/O request entry to the end of the completion port’s I/O completion queue. Each entry in this queue indicates the number of bytes transferred, the completion key value that was set when the device was associated with the port, the pointer to the I/O request’s OVERLAPPED structure, and an error code. I’ll discuss how entries are removed from this queue shortly.

Architecting Around an I/O Completion Port

When your service application initializes, it should create the I/O completion port by calling a function such as CreateNewCompletionPort. The application should then create a pool of threads to handle client requests. The question you ask now is, “How many threads should be in the pool?” This is a tough question to answer, and I will address it in more detail later in “How Many Threads in the Pool?” on page 328. For now, a standard rule of thumb is to take the number of CPUs on the host machine and multiply it by 2. So on a dual-processor machine, you should create a pool of four threads.

All the threads in the pool should execute the same function. Typically, this thread function performs some sort of initialization and then enters a loop that should terminate when the service process is instructed to stop. Inside the loop, the thread puts itself to sleep waiting for device I/O requests to complete to the completion port. Calling GetQueuedCompletionStatus does this:

BOOL GetQueuedCompletionStatus(
   HANDLE       hCompletionPort,
   PDWORD       pdwNumberOfBytesTransferred,
   PULONG_PTR   pCompletionKey,
   OVERLAPPED** ppOverlapped,
   DWORD        dwMilliseconds);

The first parameter, hCompletionPort, indicates which completion port the thread is interested in monitoring. Many service applications use a single I/O completion port and have all I/O request notifications complete to this one port. Basically, the job of GetQueuedCompletionStatus is to put the calling thread to sleep until an entry appears in the specified completion port’s I/O completion queue or until the specified time-out occurs (as specified in the dwMilliseconds parameter).

The third data structure associated with an I/O completion port is the waiting thread queue. As each thread in the thread pool calls GetQueuedCompletionStatus, the ID of the calling thread is placed in this waiting thread queue, enabling the I/O completion port kernel object to always know which threads are currently waiting to handle completed I/O requests. When an entry appears in the port’s I/O completion queue, the completion port wakes one of the threads in the waiting thread queue. This thread gets the pieces of information that make up a completed I/O entry: the number of bytes transferred, the completion key, and the address of the OVERLAPPED structure. This information is returned to the thread via the pdwNumberOfBytesTransferred, pCompletionKey, and ppOverlapped parameters passed to GetQueuedCompletionStatus.

Determining the reason that GetQueuedCompletionStatus returned is somewhat difficult. The following code demonstrates the proper way to do it:

DWORD dwNumBytes;
ULONG_PTR CompletionKey;
OVERLAPPED* pOverlapped;

// hIOCP is initialized somewhere else in the program
BOOL bOk = GetQueuedCompletionStatus(hIOCP,
   &dwNumBytes, &CompletionKey, &pOverlapped, 1000);
DWORD dwError = GetLastError();

if (bOk) {
   // Process a successfully completed I/O request
} else {
   if (pOverlapped != NULL) {
      // Process a failed completed I/O request
      // dwError contains the reason for failure
    } else {
      if (dwError == WAIT_TIMEOUT) {
         // Time-out while waiting for completed I/O entry
      } else {
         // Bad call to GetQueuedCompletionStatus
         // dwError contains the reason for the bad call

As you would expect, entries are removed from the I/O completion queue in a first-in first-out fashion. However, as you might not expect, threads that call GetQueuedCompletionStatus are awakened in a last-in first-out (LIFO) fashion. The reason for this is again to improve performance. For example, say that four threads are waiting in the waiting thread queue. If a single completed I/O entry appears, the last thread to call GetQueuedCompletionStatus wakes up to process the entry. When this last thread is finished processing the entry, the thread again calls GetQueuedCompletionStatus to enter the waiting thread queue. Now if another I/O completion entry appears, the same thread that processed the first entry is awakened to process the new entry.

As long as I/O requests complete so slowly that a single thread can handle them, the system just keeps waking the one thread, and the other three threads continue to sleep. By using this LIFO algorithm, threads that don’t get scheduled can have their memory resources (such as stack space) swapped out to the disk and flushed from a processor’s cache. This means having many threads waiting on a completion port isn’t bad. If you do have several threads waiting but few I/O requests completing, the extra threads have most of their resources swapped out of the system anyway.

In Windows Vista, if you expect a large number of I/O requests to be constantly submitted, instead of multiplying the number of threads to wait on the completion port and incurring the increasing cost of the corresponding context switches, you can retrieve the result of several I/O requests at the same time by calling the following function:

BOOL GetQueuedCompletionStatusEx(
  HANDLE hCompletionPort,
  LPOVERLAPPED_ENTRY pCompletionPortEntries,
  ULONG ulCount,
  PULONG pulNumEntriesRemoved,
  DWORD dwMilliseconds,
  BOOL bAlertable);

The first parameter, hCompletionPort, indicates which completion port the thread is interested in monitoring. The entries present in the specified completion port’s I/O completion queue when this function is called are retrieved, and their description is copied into the pCompletionPortEntries array parameter. The ulCount parameter indicates how many entries can be copied in this array, and the long value pointed to by pulNumEntriesRemoved receives the exact number of I/O requests that were extracted from the completion queue.

Each element of the pCompletionPortEntries array is an OVERLAPPED_ENTRY that stores the pieces of information that make up a completed I/O entry: the completion key, the address of the OVERLAPPED structure, the result code (error) of the I/O request, and the number of bytes transferred.

typedef struct _OVERLAPPED_ENTRY {
   ULONG_PTR lpCompletionKey;
   LPOVERLAPPED lpOverlapped;
   ULONG_PTR Internal;
   DWORD dwNumberOfBytesTransferred;

The Internal field is opaque and should not be used.

If the last bAlertable parameter is set to FALSE, the function waits for a completed I/O request to be queued on the completion port until the specified time-out occurs (as specified in the dwMilliseconds parameter). If the bAlertable parameter is set to TRUE and there is no completed I/O request in the queue, the thread enters an alertable state as explained earlier in this chapter.

How the I/O Completion Port Manages the Thread Pool

Now it’s time to discuss why I/O completion ports are so useful. First, when you create the I/O completion port, you specify the number of threads that can run concurrently. As I said, you usually set this value to the number of CPUs on the host machine. As completed I/O entries are queued, the I/O completion port wants to wake up waiting threads. However, the completion port wakes up only as many threads as you have specified. So if four I/O requests complete and four threads are waiting in a call to GetQueuedCompletionStatus, the I/O completion port will allow only two threads to wake up; the other two threads continue to sleep. As each thread processes a completed I/O entry, the thread again calls GetQueuedCompletionStatus. The system sees that more entries are queued and wakes the same threads to process the remaining entries.

If you’re thinking about this carefully, you should notice that something just doesn’t make a lot of sense: if the completion port only ever allows the specified number of threads to wake up concurrently, why have more threads waiting in the thread pool? For example, suppose I’m running on a machine with two CPUs and I create the I/O completion port, telling it to allow no more than two threads to process entries concurrently. But I create four threads (twice the number of CPUs) in the thread pool. It seems as though I am creating two additional threads that will never be awakened to process anything.

But I/O completion ports are very smart. When a completion port wakes a thread, the completion port places the thread’s ID in the fourth data structure associated with the completion port, a released thread list. (See Figure 10-1.) This allows the completion port to remember which threads it awakened and to monitor the execution of these threads. If a released thread calls any function that places the thread in a wait state, the completion port detects this and updates its internal data structures by moving the thread’s ID from the released thread list to the paused thread list (the fifth and final data structure that is part of an I/O completion port).

The goal of the completion port is to keep as many entries in the released thread list as are specified by the concurrent number of threads value used when creating the completion port. If a released thread enters a wait state for any reason, the released thread list shrinks and the completion port releases another waiting thread. If a paused thread wakes, it leaves the paused thread list and reenters the released thread list. This means that the released thread list can now have more entries in it than are allowed by the maximum concurrency value.

Let’s tie all of this together now. Say that we are again running on a machine with two CPUs. We create a completion port that allows no more than two threads to wake concurrently, and we create four threads that are waiting for completed I/O requests. If three completed I/O requests get queued to the port, only two threads are awakened to process the requests, reducing the number of runnable threads and saving context-switching time. Now if one of the running threads calls Sleep, WaitForSingleObject, WaitForMultipleObjects, SignalObjectAndWait, a synchronous I/O call, or any function that would cause the thread not to be runnable, the I/O completion port would detect this and wake a third thread immediately. The goal of the completion port is to keep the CPUs saturated with work.

Eventually, the first thread will become runnable again. When this happens, the number of runnable threads will be higher than the number of CPUs in the system. However, the completion port again is aware of this and will not allow any additional threads to wake up until the number of threads drops below the number of CPUs. The I/O completion port architecture presumes that the number of runnable threads will stay above the maximum for only a short time and will die down quickly as the threads loop around and again call GetQueuedCompletionStatus. This explains why the thread pool should contain more threads than the concurrent thread count set in the completion port.

How Many Threads in the Pool?

Now is a good time to discuss how many threads should be in the thread pool. Consider two issues. First, when the service application initializes, you want to create a minimum set of threads so that you don’t have to create and destroy threads on a regular basis. Remember that creating and destroying threads wastes CPU time, so you’re better off minimizing this process as much as possible. Second, you want to set a maximum number of threads because creating too many threads wastes system resources. Even if most of these resources can be swapped out of RAM, minimizing the use of system resources and not wasting even paging file space is to your advantage, if you can manage it.

You will probably want to experiment with different numbers of threads. Most services (including Microsoft Internet Information Services) use heuristic algorithms to manage their thread pools. I recommend that you do the same. For example, you can create the following variables to manage the thread pool:

LONG g_nThreadsMin;    // Minimum number of threads in pool
LONG g_nThreadsMax;    // Maximum number of threads in pool
LONG g_nThreadsCrnt;   // Current number of threads in pool
LONG g_nThreadsBusy;   // Number of busy threads in pool

When your application initializes, you can create the g_nThreadsMin number of threads, all executing the same thread pool function. The following pseudocode shows how this thread function might look:

DWORD WINAPI ThreadPoolFunc(PVOID pv) {

   // Thread is entering pool

   for (BOOL bStayInPool = TRUE; bStayInPool;) {

       // Thread stops executing and waits for something to do
       BOOL bOk = GetQueuedCompletionStatus(...);
       DWORD dwIOError = GetLastError();

       // Thread has something to do, so it’s busy
       int nThreadsBusy = InterlockedIncrement(&m_nThreadsBusy);

       // Should we add another thread to the pool?
       if (nThreadsBusy == m_nThreadsCrnt) {     // All threads are busy
          if (nThreadsBusy < m_nThreadsMax) {    // The pool isn’t full
             if (GetCPUUsage() < 75) {    // CPU usage is below 75%

                // Add thread to pool

       if (!bOk && (dwIOError == WAIT_TIMEOUT)) {    // Thread timed out
          // There isn’t much for the server to do, and this thread
          // can die even if it still has outstanding I/O requests
          bStayInPool = FALSE;

       if (bOk || (po != NULL)) {
          // Thread woke to process something; process it

          if (GetCPUUsage() > 90) {        // CPU usage is above 90%
             if (g_nThreadsCrnt > g_nThreadsMin)) { // Pool above min
                bStayInPool = FALSE;    // Remove thread from pool
   // Thread is leaving pool

This pseudocode shows how creative you can get when using an I/O completion port. The GetCPUUsage function is not part of the Windows API. If you want its behavior, you’ll have to implement the function yourself. In addition, you must make sure that the thread pool always contains at least one thread in it, or clients will never get tended to. Use my pseudocode as a guide, but your particular service might perform better if structured differently.

Many services offer a management tool that allows an administrator to have some control over the thread pool’s behavior–for example, to set the minimum and maximum number of threads, the CPU time usage thresholds, and also the maximum concurrency value used when creating the I/O completion port.

Simulating Completed I/O Requests

I/O completion ports do not have to be used with device I/O at all. This chapter is also about interthread communication techniques, and the I/O completion port kernel object is an awesome mechanism to use to help with this. In “Alertable I/O” on page 315, I presented the QueueUserAPC function, which allows a thread to post an APC entry to another thread. I/O completion ports have an analogous function, PostQueuedCompletionStatus:

BOOL PostQueuedCompletionStatus(
   HANDLE      hCompletionPort,
   DWORD       dwNumBytes,
   ULONG_PTR   CompletionKey,
   OVERLAPPED* pOverlapped);

This function appends a completed I/O notification to an I/O completion port’s queue. The first parameter, hCompletionPort, identifies the completion port that you want to queue the entry for. The remaining three parameters–dwNumBytes, CompletionKey, and pOverlapped–indicate the values that should be returned by a thread’s call to GetQueuedCompletionStatus. When a thread pulls a simulated entry from the I/O completion queue, GetQueuedCompletionStatus returns TRUE, indicating a successfully executed I/O request.

The PostQueuedCompletionStatus function is incredibly useful–it gives you a way to communicate with all the threads in your pool. For example, when the user terminates a service application, you want all the threads to exit cleanly. But if the threads are waiting on the completion port and no I/O requests are coming in, the threads can’t wake up. By calling PostQueuedCompletionStatus once for each thread in the pool, each thread can wake up, examine the values returned from GetQueuedCompletionStatus, see that the application is terminating, and clean up and exit appropriately.

You must be careful when using a thread termination technique like the one I just described. My example works because the threads in the pool are dying and not calling GetQueuedCompletionStatus again. However, if you want to notify each of the pool’s threads of something and have them loop back around to call GetQueuedCompletionStatus again, you will have a problem because the threads wake up in a LIFO order. So you will have to employ some additional thread synchronization in your application to ensure that each pool thread gets the opportunity to see its simulated I/O entry. Without this additional thread synchronization, one thread might see the same notification several times.

The FileCopy Sample Application

The FileCopy sample application (10-FileCopy.exe), shown at the end of this chapter, demonstrates the use of I/O completion ports. The source code and resource files for the application are in the 10-FileCopy directory on the companion content Web page. The program simply copies a file specified by the user to a new file called FileCopy.cpy. When the user executes FileCopy, the dialog box shown in Figure 10-2 appears.

Figure 10-2

Figure 10-2 The dialog box for the FileCopy sample application

The user clicks the Pathname button to select the file to be copied, and the Pathname and File Size fields are updated. When the user clicks the Copy button, the program calls the FileCopy function, which does all the hard work. Let’s concentrate our discussion on the FileCopy function.

When preparing to copy, FileCopy opens the source file and retrieves its size, in bytes. I want the file copy to execute as blindingly fast as possible, so the file is opened using the FILE_FLAG_NO_BUFFERING flag. Opening the file with the FILE_FLAG_NO_BUFFERING flag allows me to access the file directly, bypassing the additional memory copy overhead incurred when allowing the system’s cache to "help" access the file. Of course, accessing the file directly means slightly more work for me: I must always access the file using offsets that are multiples of the disk volume’s sector size, and I must read and write data that is a multiple of the sector’s size as well. I chose to transfer the file’s data in BUFFSIZE (64 KB) chunks, which is guaranteed to be a multiple of the sector size. This is why I round up the source file’s size to a multiple of BUFFSIZE. You’ll also notice that the source file is opened with the FILE_FLAG_OVERLAPPED flag so that I/O requests against the file are performed asynchronously.

The destination file is opened similarly: both the FILE_FLAG_NO_BUFFERING and FILE_FLAG_OVERLAPPED flags are specified. I also pass the handle of the source file as CreateFile’s hFileTemplate parameter when creating the destination file, causing the destination file to have the same attributes as the source.

Now that the files are opened and ready to be processed, FileCopy creates an I/O completion port. To make working with I/O completion ports easier, I created a small C++ class, CIOCP, that is a very simple wrapper around the I/O completion port functions. This class can be found in the IOCP.h file discussed in Appendix A “The Build Environment.” FileCopy creates an I/O completion port by creating an instance (named iocp) of my CIOCP class.

The source file and destination file are associated with the completion port by calling the CIOCP’s AssociateDevice member function. When associated with the completion port, each device is assigned a completion key. When an I/O request completes against the source file, the completion key is CK_READ, indicating that a read operation must have completed. Likewise, when an I/O request completes against the destination file, the completion key is CK_WRITE, indicating that a write operation must have completed.

Now we’re ready to initialize a set of I/O requests (OVERLAPPED structures) and their memory buffers. The FileCopy function keeps four (MAX_PENDING_IO_REQS) I/O requests outstanding at any one time. For applications of your own, you might prefer to allow the number of I/O requests to dynamically grow or shrink as necessary. In the FileCopy program, the CIOReq class encapsulates a single I/O request. As you can see, this C++ class is derived from an OVERLAPPED structure but contains some additional context information. FileCopy allocates an array of CIOReq objects and calls the AllocBuffer method to associate a BUFFSIZE-sized data buffer with each I/O request object. The data buffer is allocated using the VirtualAlloc function. Using VirtualAlloc ensures that the block begins on an even allocation-granularity boundary, which satisfies the requirement of the FILE_FLAG_NO_BUFFERING flag: the buffer must begin on an address that is evenly divisible by the volume’s sector size.

To issue the initial read requests against the source file, I perform a little trick: I post four CK_WRITE I/O completion notifications to the I/O completion port. When the main loop runs, the thread waits on the port and wakes immediately, thinking that a write operation has completed. This causes the thread to issue a read request against the source file, which really starts the file copy.

The main loop terminates when there are no outstanding I/O requests. As long as I/O requests are outstanding, the interior of the loop waits on the completion port by calling CIOCP’s GetStatus method (which calls GetQueuedCompletionStatus internally). This call puts the thread to sleep until an I/O request completes to the completion port. When GetQueuedCompletionStatus returns, the returned completion key, CompletionKey, is checked. If CompletionKey is CK_READ, an I/O request against the source file is completed. I then call the CIOReq’s Write method to issue a write I/O request against the destination file. If CompletionKey is CK_WRITE, an I/O request against the destination file is completed. If I haven’t read beyond the end of the source file, I call CIOReq’s Read method to continue reading the source file.

When there are no more outstanding I/O requests, the loop terminates and cleans up by closing the source and destination file handles. Before FileCopy returns, it must do one more task: it must fix the size of the destination file so that it is the same size as the source file. To do this, I reopen the destination file without specifying the FILE_FLAG_NO_BUFFERING flag. Because I am not using this flag, file operations do not have to be performed on sector boundaries. This allows me to shrink the size of the destination file to the same size as the source file.

Module:  FileCopy.cpp
Notices: Copyright (c) 2008 Jeffrey Richter & Christophe Nasarre

#include "stdafx.h"
#include "Resource.h"


// Each I/O request needs an OVERLAPPED structure and a data buffer
class CIOReq : public OVERLAPPED {
   CIOReq() {
      Internal = InternalHigh = 0;
      Offset = OffsetHigh = 0;
      hEvent = NULL;
      m_nBuffSize = 0;
      m_pvData = NULL;

   ~CIOReq() {
      if (m_pvData != NULL)
         VirtualFree(m_pvData, 0, MEM_RELEASE);

   BOOL AllocBuffer(SIZE_T nBuffSize) {
      m_nBuffSize = nBuffSize;
      m_pvData = VirtualAlloc(NULL, m_nBuffSize, MEM_COMMIT, PAGE_READWRITE);
      return(m_pvData != NULL);
   BOOL Read(HANDLE hDevice, PLARGE_INTEGER pliOffset = NULL) {
      if (pliOffset != NULL) {
         Offset     = pliOffset->LowPart;
         OffsetHigh = pliOffset->HighPart;
      return(::ReadFile(hDevice, m_pvData, m_nBuffSize, NULL, this));

   BOOL Write(HANDLE hDevice, PLARGE_INTEGER pliOffset = NULL) {
      if (pliOffset != NULL) {
         Offset     = pliOffset->LowPart;
         OffsetHigh = pliOffset->HighPart;
      return(::WriteFile(hDevice, m_pvData, m_nBuffSize, NULL, this));

   SIZE_T m_nBuffSize;
   PVOID  m_pvData;


#define BUFFSIZE              (64 * 1024) // The size of an I/O buffer
#define MAX_PENDING_IO_REQS   4           // The maximum # of I/Os

// The completion key values indicate the type of completed I/O.
#define CK_READ  1
#define CK_WRITE 2


BOOL FileCopy(PCTSTR pszFileSrc, PCTSTR pszFileDst) {

   BOOL fOk = FALSE;    // Assume file copy fails
   LARGE_INTEGER liFileSizeSrc = { 0 }, liFileSizeDst;

   try {
      // Open the source file without buffering & get its size
      CEnsureCloseFile hFileSrc = CreateFile(pszFileSrc, GENERIC_READ,
      if (hFileSrc.IsInvalid()) goto leave;

      // Get the file's size
      GetFileSizeEx(hFileSrc, &liFileSizeSrc);
      // Nonbuffered I/O requires sector-sized transfers.
      // I'll use buffer-size transfers since it's easier to calculate.
      liFileSizeDst.QuadPart = chROUNDUP(liFileSizeSrc.QuadPart, BUFFSIZE);

      // Open the destination file without buffering & set its size
      CEnsureCloseFile hFileDst = CreateFile(pszFileDst, GENERIC_WRITE,
         0, NULL, CREATE_ALWAYS,
      if (hFileDst.IsInvalid()) goto leave;

      // File systems extend files synchronously. Extend the destination file
      // now so that I/Os execute asynchronously improving performance.
      SetFilePointerEx(hFileDst, liFileSizeDst, NULL, FILE_BEGIN);

      // Create an I/O completion port and associate the files with it.
      CIOCP iocp(0);
      iocp.AssociateDevice(hFileSrc, CK_READ);  // Read from source file
      iocp.AssociateDevice(hFileDst, CK_WRITE); // Write to destination file

      // Initialize record-keeping variables
      LARGE_INTEGER liNextReadOffset = { 0 };
      int nReadsInProgress  = 0;
      int nWritesInProgress = 0;

      // Prime the file copy engine by simulating that writes have completed.
      // This causes read operations to be issued.
      for (int nIOReq = 0; nIOReq < _countof(ior); nIOReq++) {

         // Each I/O request requires a data buffer for transfers
         iocp.PostStatus(CK_WRITE, 0, &ior[nIOReq]);

      // Loop while outstanding I/O requests still exist
      while ((nReadsInProgress > 0) || (nWritesInProgress > 0)) {

         // Suspend the thread until an I/O completes
         ULONG_PTR CompletionKey;
         DWORD dwNumBytes;
         CIOReq* pior;
         iocp.GetStatus(&CompletionKey, &dwNumBytes, (OVERLAPPED**) &pior, INFINITE);

         switch (CompletionKey) {
         case CK_READ:  // Read completed, write to destination
            pior->Write(hFileDst);  // Write to same offset read from source
         case CK_WRITE: // Write completed, read from source
            if (liNextReadOffset.QuadPart < liFileSizeDst.QuadPart) {
               // Not EOF, read the next block of data from the source file.
               pior->Read(hFileSrc, &liNextReadOffset);
               liNextReadOffset.QuadPart += BUFFSIZE; // Advance source offset
      fOk = TRUE;
   catch (...) {

   if (fOk) {
      // The destination file size is a multiple of the page size. Open the
      // file WITH buffering to shrink its size to the source file's size.
      CEnsureCloseFile hFileDst = CreateFile(pszFileDst, GENERIC_WRITE,
         0, NULL, OPEN_EXISTING, 0, NULL);
      if (hFileDst.IsValid()) {

         SetFilePointerEx(hFileDst, liFileSizeSrc, NULL, FILE_BEGIN);



BOOL Dlg_OnInitDialog(HWND hWnd, HWND hWndFocus, LPARAM lParam) {


   // Disable Copy button since no file is selected yet.
   EnableWindow(GetDlgItem(hWnd, IDOK), FALSE);


void Dlg_OnCommand(HWND hWnd, int id, HWND hWndCtl, UINT codeNotify) {

   TCHAR szPathname[_MAX_PATH];
   switch (id) {
   case IDCANCEL:
      EndDialog(hWnd, id);

   case IDOK:
      // Copy the source file to the destination file.
      Static_GetText(GetDlgItem(hWnd, IDC_SRCFILE),
         szPathname, sizeof(szPathname));
      SetCursor(LoadCursor(NULL, IDC_WAIT));
      chMB(FileCopy(szPathname, TEXT("FileCopy.cpy"))
         ? "File Copy Successful" : "File Copy Failed");

      ofn.hwndOwner = hWnd;
      ofn.lpstrFilter = TEXT("*.*\0");
      lstrcpy(szPathname, TEXT("*.*"));
      ofn.lpstrFile = szPathname;
      ofn.nMaxFile = _countof(szPathname);
      ofn.lpstrTitle = TEXT("Select file to copy");
      BOOL fOk = GetOpenFileName(&ofn);
      if (fOk) {
         // Show user the source file's size
         Static_SetText(GetDlgItem(hWnd, IDC_SRCFILE), szPathname);
         CEnsureCloseFile hFile = CreateFile(szPathname, 0, 0, NULL,
            OPEN_EXISTING, 0, NULL);
         if (hFile.IsValid()) {
            LARGE_INTEGER liFileSize;
            GetFileSizeEx(hFile, &liFileSize);
            // NOTE: Only shows bottom 32 bits of size
            SetDlgItemInt(hWnd, IDC_SRCFILESIZE, liFileSize.LowPart, FALSE);
      EnableWindow(GetDlgItem(hWnd, IDOK), fOk);


INT_PTR WINAPI Dlg_Proc(HWND hWnd, UINT uMsg, WPARAM wParam, LPARAM lParam) {

   switch (uMsg) {
   chHANDLE_DLGMSG(hWnd, WM_INITDIALOG, Dlg_OnInitDialog);
   chHANDLE_DLGMSG(hWnd, WM_COMMAND,    Dlg_OnCommand);


int WINAPI _tWinMain(HINSTANCE hInstExe, HINSTANCE, PTSTR pszCmdLine, int) {


//////////////////////////////// End of File //////////////////////////////////