Asynchronous I/O with Threads in VS .NET

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Asynchronous I/O with Threads
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Overlapped and extended I/O achieve asynchronous I/O within a single thread, although the OS creates its own threads to support the functionality These techniques are common, in one form or another, in many older OSs for supporting limited forms of asynchronous operation in single-threaded systems Windows, however, supports threads, so the same effect is possible by performing synchronous I/O operations in multiple, separate threads The multithreaded servers and 7's grepMT have already illustrated this Threads also provide a uniform and, arguably, much simpler way to perform asynchronous I/O An alternative to Program 14-1 and 14-2 is to give each thread its own handle to the file and each thread could synchronously process every fourth record The atouMTc program, not listed here but included on the book's Web site, illustrates how to use threads in this way Not only does atouMT work on all Windows versions, but it is also simpler than the two asynchronous I/O programs because the bookkeeping is less complex Each thread simply maintains its own buffers on its own stack and performs the read, convert, and write sequence synchronously in a loop The performance is also competitive Note: The atouMTc program on the Web site contains some comments about several pitfalls that can occur when a single file is accessed concurrently from several threads In particular, the distinct file handles should all be created with CreateFile rather than with DuplicateHandle My personal preference is to use threads rather than asynchronous I/O for file processing Threads are easier to program, and they provide the best performance in most cases There are two exceptions to this generalization The first exception, as shown earlier in this chapter, is a situation in which there is only a single outstanding operation and the file handle can be used for synchronization The second, and more important, exception occurs with asynchronous I/O completion ports, as will be described at the end of this chapter
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Windows NT supports waitable timers, a type of waitable kernel object You can always create your own timing signal by creating a timing thread that sets an event after waking from a Sleep call serverNP (Program 11-3) also uses a timing thread to broadcast its pipe name periodically Therefore, waitable timers are a redundant but useful way to perform tasks periodically or at specified times In particular, a waitable timer can be set to signal at a specified absolute time A waitable timer can be either a synchronization timer or a manual-reset notification timer A synchronization timer is associated with a callback function, similar to an extended I/O completion routine, whereas a wait function is used to synchronize on a manual-reset notification timer The first step is to create a timer handle with CreateWaitableTimer HANDLE CreateWaitableTimer ( LPSECURITY_ATTRIBUTES lpTimerAttributes, BOOL bManualReset, LPCTSTR lpTimerName);
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The second parameter, bManualReset, determines whether the timer is a synchronization timer or a manual-reset notification timer Program 14-3 uses a synchronization timer, but you can change the comment and the parameter setting to obtain a notification timer Notice that there is also an OpenWaitableTimer function that can use the optional name supplied in the third argument The timer is initially inactive, but SetWaitableTimer activates it and specifies the initial signal time and the time between periodic signals BOOL SetWaitableTimer ( HANDLE hTimer, const LARGE_INTEGER *pDueTime, LONG lPeriod, PTIMERAPCROUTINE pfnCompletionRoutine, LPVOID lpArgToCompletionRoutine, BOOL fResume);
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hTimer is a valid timer handle created using CreateWaitableTimer The second parameter, pointed to by pDueTime, is either a positive absolute time or a negative relative time and is actually expressed as a FILETIME with a resolution of 100 nanoseconds FILETIME variables were introduced in 3 and were used in 6's timep (Program 6-2) The interval between signals is specified in the third parameter, but in millisecond units If this value is 0, the timer is signaled only once A positive value indicates that the timer is a periodic timer and continues signaling periodically until you call CancelWaitableTimer Negative interval values are not allowed
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pfnCompletionRoutine, the fourth parameter, is appropriate when using a synchronization timer and specifies the time-out function (completion routine) to be called when the timer is signaled and the thread enters an alertable wait state The routine is called with the pointer specified in the fifth parameter, plArgToCompletionRoutine, as an argument Having set a synchronization timer, you can now call SleepEx to enter an alertable wait state so the completion routine can be called In the case of a manual-reset notification timer, wait on the timer handle The handle will remain signaled until another call to SetWaitableTimer The complete version of Program 14-3 on the book's Web site allows you to experiment with using the four combinations of the two timer types and with choosing between using a completion routine or waiting on the timer handle The final parameter, fResume, is concerned with power conservation See the documentation for more information Use CancelWaitableTimer to cancel the last effect of a previous SetWaitableTimer, although it will not change the signaled state of the timer Use another SetWaitableTimer call to do that
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