Overlapped I/O States in Visual Studio .NET

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Overlapped I/O States
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An overlapped ReadFile or WriteFile operationor, for that matter, one of the two named pipe operationsreturns immediately In most cases, the I/O will not be complete, and the read or write returns FALSE GetLastError returns ERROR_IO_PENDING After waiting on a synchronization object (an event or, perhaps, the file handle) for the operation to complete, you need to determine how many bytes were transferred This is the primary purpose of GetOverlappedResult BOOL GetOverlappedResult ( HANDLE hFile, LPOVERLAPPED lpOverlapped, LPWORD lpcbTransfer, BOOL bWait)
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The handle and overlapped structure combine to indicate the specific I/O operation bWait, if trUE, specifies that GetOverlappedResult will wait until the specified operation is complete; otherwise, it returns immediately In either case, the function returns trUE only if the operation has completed successfully GetLastError returns ERROR_IO_INCOMPLETE in case of a FALSE return from GetOverlappedResult, so it is possible to poll for I/O completion with this function The number of bytes transferred is in *lpcbTransfer Be certain that the overlapped structure is unchanged from when it was used with the overlapped I/O operation
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Canceling Overlapped I/O Operations
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The Boolean function CancelIO cancels outstanding overlapped I/O operations on the specified handle (there is just one parameter) All operations issued by the calling thread using the handle are canceled Operations initiated by other threads are not affected The canceled operations will complete with ERROR_OPERATION_ABORTED
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Example: Synchronizing on a File Handle
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Overlapped I/O can be useful and relatively simple when there is only one outstanding operation The program can synchronize on the file handle rather than on an event The following code fragment shows how a program can initiate a read operation to read a portion of a file, continue to perform other processing, and then wait on the handle OVERLAPPED ov = { 0, 0, 0, 0, NULL /* No event */ }; HANDLE hF; DWORD nRead; BYTE Buffer [BUF_SIZE]; hF = CreateFile ( , FILE_FLAG_OVERLAPPED, ); ReadFile (hF, Buffer, sizeof (Buffer), &nRead, &ov); /* Perform other processing nRead is not valid */ /* Wait for the read to complete */ WaitForSingleObject (hF, INFINITE); GetOverlappedResult (hF, &ov, &nRead, FALSE);
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Example: File Conversion with Overlapped I/O and Multiple Buffers
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Program 2-4 (atou) converted an ASCII file to Unicode, processing the file sequentially, and 5 showed how to perform the same sequential file processing with memory-mapped files Program 14-1 (atouOV) performs the same task using overlapped I/O and multiple buffers holding fixed-size records Figure 14-1 shows the program organization with four fixed-size buffers The program is implemented so that the number of buffers is defined in a preprocessor variable, but the following discussion assumes four buffers
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Figure 14-1 An Asynchronous File Update Model
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First, the program initializes all the overlapped structures with events and file positions There is a separate overlapped structure for each input and each output buffer Next, an overlapped read is issued for each of the four input buffers The program then uses WaitForMultipleObjects to wait for a single event, indicating either a read or a write completed When a read is completed, the buffer is copied and converted into the corresponding output buffer and the write is initiated When a write completes, the next read is initiated Notice that the events associated with the input and output buffers are arranged in a single array to be used as an argument to WaitForMultipleObjects
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Program 14-1 atouOV: File Conversion with Overlapped I/O
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/* 14 atouOV OVERLAPPED I/O ASCII to Unicode file conversion Windows NT only */ #include "EvryThngh" #define MAX_OVRLP 4 /* Number of overlapped I/O operations */ #define REC_SIZE 0x8000 /* 32K: Minimum size for good performance */ #define UREC_SIZE 2 * REC_SIZE int _tmain (int argc, LPTSTR argv []) { HANDLE hInputFile, hOutputFile; /* There is a copy of each of the following variables and */ /* structures for each outstanding overlapped I/O operation */ DWORD nin [MAX_OVRLP], nout [MAX_OVRLP], ic, i; OVERLAPPED OverLapIn [MAX_OVRLP], OverLapOut [MAX_OVRLP]; /* The first event index is 0 for read, 1 for write */ /* WaitForMultipleObjects requires a contiguous array */ HANDLE hEvents [2] [MAX_OVRLP]; /* The first index on these two buffers is the I/O operation */ CHAR AsRec [MAX_OVRLP] [REC_SIZE]; WCHAR UnRec [MAX_OVRLP] [REC_SIZE]; LARGE_INTEGER CurPosIn, CurPosOut, FileSize; LONGLONG nRecord, iWaits; hInputFile = CreateFile (argv [1], GENERIC_READ, 0, NULL, OPEN_EXISTING, FILE_FLAG_OVERLAPPED, NULL); hOutputFile = CreateFile (argv [2], GENERIC_WRITE, 0, NULL, CREATE_ALWAYS, FILE_FLAG_OVERLAPPED, NULL); /* Total number of records to process based on input file size */ /* There may be a partial record at the end */ FileSizeLowPart = GetFileSize (hInputFile, &FileSizeHighPart); nRecord = FileSizeQuadPart / REC_SIZE; if ((FileSizeQuadPart % REC_SIZE) != 0) nRecord++; CurPosInQuadPart = 0; for (ic = 0; ic < MAX_OVRLP; ic++) { /* Create read and write events for each overlapped struct */ hEvents [0] [ic] = OverLapIn [ic]hEvent /* Read event/struct*/ = CreateEvent (NULL, TRUE, FALSE, NULL); hEvents [1] [ic] = OverLapOut [ic]hEvent /* Write */ = CreateEvent (NULL, TRUE, FALSE, NULL); /* Initial file positions for each overlapped structure */ OverLapIn [ic]Offset = CurPosInLowPart; OverLapIn [ic]OffsetHigh = CurPosInHighPart; /* Initiate an overlapped read for this overlapped struct */ if (CurPosInQuadPart < FileSizeQuadPart) ReadFile (hInputFile, AsRec [ic], REC_SIZE, &nin [ic], &OverLapIn [ic]); CurPosInQuadPart += (LONGLONG) REC_SIZE; } /* All read operations are running Wait for an event to complete and reset it immediately Read and write events are stored contiguously in the event array */ iWaits = 0; /* Number of I/O operations completed so far */ while (iWaits < 2 * nRecord) { ic = WaitForMultipleObjects (2 * MAX_OVRLP, hEvents [0], FALSE, INFINITE) - WAIT_OBJECT_0;
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iWaits++; /* Increment # of complete I/O operations */ ResetEvent (hEvents [ic / MAX_OVRLP] [ic % MAX_OVRLP]); if (ic < MAX_OVRLP) { /* A read completed */ GetOverlappedResult (hInputFile, &OverLapIn [ic], &nin [ic], FALSE); /* Process the record and initiate the write */ CurPosInLowPart = OverLapIn [ic]Offset; CurPosInHighPart = OverLapIn [ic]OffsetHigh; CurPosOutQuadPart = (CurPosInQuadPart / REC_SIZE) * UREC_SIZE; OverLapOut [ic]Offset = CurPosOutLowPart; OverLapOut [ic]OffsetHigh = CurPosOutHighPart; /* Convert an ASCII record to Unicode */ for (i = 0; i < REC_SIZE; i++) UnRec [ic] [i] = AsRec [ic] [i]; WriteFile (hOutputFile, UnRec [ic], nin [ic] * 2, &nout [ic], &OverLapOut [ic]); /* Prepare for the next read, which will be initiated after the write, issued above, completes */ CurPosInQuadPart += REC_SIZE * (LONGLONG) (MAX_OVRLP); OverLapIn [ic]Offset = CurPosInLowPart; OverLapIn [ic]OffsetHigh = CurPosInHighPart; } else if (ic < 2 * MAX_OVRLP) { /* A write completed */ /* Start the read */ ic -= MAX_OVRLP; /* Set the output buffer index */ if (!GetOverlappedResult (hOutputFile, &OverLapOut [ic], &nout [ic], FALSE)) ReportError (_T ("Read failed"), 0, TRUE); CurPosInLowPart = OverLapIn [ic]Offset; CurPosInHighPart = OverLapIn [ic]OffsetHigh; if (CurPosInQuadPart < FileSizeQuadPart) { /* Start a new read */ ReadFile (hInputFile, AsRec [ic], REC_SIZE, &nin [ic], &OverLapIn [ic]); } } } /* Close all events */ for (ic = 0; ic < MAX_OVRLP; ic++) { CloseHandle (hEvents [0] [ic]); CloseHandle (hEvents [1] [ic]); } CloseHandle (hInputFile); CloseHandle (hOutputFile); return 0; }
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Program 14-1 works only under Windows NT Windows 9x asynchronous I/O cannot use disk files Appendix C shows and comments on atouOV's relatively poor performance results Experiments show that the buffer should be at least 32KB for good performance, but, even then, normal synchronous I/O is faster Furthermore, the program does not benefit from SMP, because the CPU is not the bottleneck in this example, which processes just two files
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