Extended I/O with Completion Routines in Visual Studio .NET

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There is an alternative to the use of synchronization objects Rather than requiring a thread to wait for a completion signal on an event or handle, the system can invoke a user-specified completion routine when an I/O operation completes The completion routine can then start the next I/O operation and perform any other bookkeeping The completion or callback routine is similar to 10's asynchronous procedure call and requires alertable wait states How can the program specify the completion routine There are no remaining ReadFile or WriteFile parameters or data structures to hold the routine's address There is, however, a family of extended I/O functions, identified by the Ex suffix and containing an extra parameter for the completion routine address The read and write functions are ReadFileEx and WriteFileEx, respectively It is also necessary to use one of five alertable wait functions:
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Extended I/O is sometimes called alertable I/O The following sections show how to use the extended functions Note: Extended I/O will not work with disk files or communications ports under Windows 9x Windows 9x extended I/O, however, will work with named pipes, mailslots, sockets, and sequential devices ReadFileEx, WriteFileEx,
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The extended read and write functions can be used with open file, named pipe, and mailslot handles if FILE_FLAG_OVERLAPPED was used at open (create) time Notice that the flag sets a handle attribute and, while overlapped I/O and extended I/O are distinguished, a single overlapped flag is used to enable both types of asynchronous I/O on a handle Overlapped sockets ( 12) can be used with ReadFileEx and WriteFileEx in all Windows versions BOOL ReadFileEx ( HANDLE hFile, LPVOID lpBuffer, DWORD nNumberOfBytesToRead, LPOVERLAPPED lpOverlapped, LPOVERLAPPED_COMPLETION_ROUTINE lpcr) BOOL WriteFileEx ( HANDLE hFile, LPVOID lpBuffer, DWORD nNumberOfBytesToWrite, LPOVERLAPPED lpOverlapped, LPOVERLAPPED_COMPLETION_ROUTINE lpcr)
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The two functions are familiar but have an extra parameter to specify the completion routine
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The overlapped structures must be supplied, but there is no need to specify the hEvent member; the system ignores it It turns out, however, that this member is useful for carrying information, such as a sequence number, to identify the I/O operation, as shown in Program 14-2 In comparison to ReadFile and WriteFile, notice that the extended functions do not require the parameters for the number of bytes transferred That information is conveyed to the completion routine, which must be included in the program The completion routine has parameters for the byte count, an error code, and the overlapped structure The last parameter is required so that the completion routine can determine which of several outstanding operations has completed Notice that the same cautions regarding reuse or destruction of overlapped structures apply here as they did for overlapped I/O VOID WINAPI FileIOCompletionRoutine ( DWORD dwError, DWORD cbTransferred, LPOVERLAPPED lpo)
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As was the case with CreateThread, which also specified a function name, FileIOCompletionRoutine is a placeholder and not an actual function name dwError is limited to 0 (success) and ERROR_HANDLE_EOF (when a read tries to go past the end of file) The overlapped structure is the one used by the completed ReadFileEx or WriteFileEx call Two things must happen before the completion routine is invoked by the system 1 The I/O operation must complete 2 The calling thread must be in an alertable wait state, notifying the system that it should execute any queued completion routines How does a thread get into an alertable wait state It must make an explicit call to one of the alertable wait functions described in the next section In this way, the thread can ensure that the completion routine does not execute prematurely A thread can be in an alertable wait state only while it is calling an alertable wait function; on return, the thread is no longer in this state Once these two conditions have been met, completion routines that have been queued as the result of I/ O completion are executed Completion routines are executed in the same thread that made the original I/O call and is in the alertable wait state Therefore, the thread should enter an alertable wait state only when it is safe for completion routines to execute
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