파일의 끝에 대한 테스트
ReadFile 함수는 동기 및 비동기 읽기 작업에 대해 EOF(파일 끝 조건)를 다르게 확인합니다. 동기 읽기 작업이 파일 끝에 도달하면 ReadFile은 TRUE를 반환하고 lpNumberOfBytesRead 매개 변수가 가리키는 변수를 0으로 설정합니다. 비동기 읽기 작업은 ReadFile 호출을 시작하는 동안 또는 파일 포인터가 프로그래밍 방식으로 파일 끝을 넘어간 경우 후속 비동기 작업 중에 파일 끝을 만날 수 있습니다.
다음 C++ 예제에서는 동기 읽기 작업 중에 파일 끝을 테스트하는 방법을 보여줍니다.
// Attempt a synchronous read operation.
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead, NULL);
// Check for eof.
if (bResult && nBytesRead == 0)
{
// at the end of the file
}
비동기 읽기 작업 중 파일 끝 테스트는 유사한 동기 읽기 작업보다 약간 더 많이 연관됩니다. 비동기 읽기 작업의 파일 끝 표시기는 GetOverlappedResult가 FALSE를 반환하고 GetLastError가 ERROR_HANDLE_EOF를 반환하는 경우입니다.
다음 C++ 예제에서는 동기 읽기 작업 중에 파일 끝을 테스트하는 방법을 보여줍니다.
#include <windows.h>
#include <tchar.h>
#include <stdio.h>
#define BUF_SIZE (61)
LPCTSTR ErrorMessage( DWORD error )
// Routine Description:
// Retrieve the system error message for the last-error code
{
LPVOID lpMsgBuf;
FormatMessage(
FORMAT_MESSAGE_ALLOCATE_BUFFER |
FORMAT_MESSAGE_FROM_SYSTEM |
FORMAT_MESSAGE_IGNORE_INSERTS,
NULL,
error,
MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
(LPTSTR) &lpMsgBuf,
0, NULL );
return((LPCTSTR)lpMsgBuf);
}
void GoDoSomethingElse(void)
// Routine Description:
// Placeholder to demo when async I/O might want to do
// other processing.
{
printf("Inside GoDoSomethingElse()\n");
}
DWORD AsyncTestForEnd( HANDLE hEvent, HANDLE hFile )
// Routine Description:
// Demonstrate async ReadFile operations that can catch
// End-of-file conditions. Unless the operation completes
// synchronously or the file size happens to be an exact
// multiple of BUF_SIZE, this routine will eventually force
// an EOF condition on any file.
// Parameters:
// hEvent - pre-made manual-reset event.
//
// hFile - pre-opened file handle, overlapped.
//
// inBuffer - the buffer to read in the data to.
//
// nBytesToRead - how much to read (usually the buffer size).
// Return Value:
// Number of bytes read.
{
char inBuffer[BUF_SIZE];
DWORD nBytesToRead = BUF_SIZE;
DWORD dwBytesRead = 0;
DWORD dwFileSize = GetFileSize(hFile, NULL);
OVERLAPPED stOverlapped = {0};
DWORD dwError = 0;
LPCTSTR errMsg = NULL;
BOOL bResult = FALSE;
BOOL bContinue = TRUE;
// Set up overlapped structure event. Other members are already
// initialized to zero.
stOverlapped.hEvent = hEvent;
// This is an intentionally brute-force loop to force the EOF trigger.
// A properly designed loop for this simple file read would use the
// GetFileSize API to regulate execution. However, the purpose here
// is to demonstrate how to trigger the EOF error and handle it.
while(bContinue)
{
// Default to ending the loop.
bContinue = FALSE;
// Attempt an asynchronous read operation.
bResult = ReadFile(hFile,
inBuffer,
nBytesToRead,
&dwBytesRead,
&stOverlapped);
dwError = GetLastError();
// Check for a problem or pending operation.
if (!bResult)
{
switch (dwError)
{
case ERROR_HANDLE_EOF:
{
printf("\nReadFile returned FALSE and EOF condition, async EOF not triggered.\n");
break;
}
case ERROR_IO_PENDING:
{
BOOL bPending=TRUE;
// Loop until the I/O is complete, that is: the overlapped
// event is signaled.
while( bPending )
{
bPending = FALSE;
// Pending asynchronous I/O, do something else
// and re-check overlapped structure.
printf("\nReadFile operation is pending\n");
// Do something else then come back to check.
GoDoSomethingElse();
// Check the result of the asynchronous read
// without waiting (forth parameter FALSE).
bResult = GetOverlappedResult(hFile,
&stOverlapped,
&dwBytesRead,
FALSE) ;
if (!bResult)
{
switch (dwError = GetLastError())
{
case ERROR_HANDLE_EOF:
{
// Handle an end of file
printf("GetOverlappedResult found EOF\n");
break;
}
case ERROR_IO_INCOMPLETE:
{
// Operation is still pending, allow while loop
// to loop again after printing a little progress.
printf("GetOverlappedResult I/O Incomplete\n");
bPending = TRUE;
bContinue = TRUE;
break;
}
default:
{
// Decode any other errors codes.
errMsg = ErrorMessage(dwError);
_tprintf(TEXT("GetOverlappedResult failed (%d): %s\n"),
dwError, errMsg);
LocalFree((LPVOID)errMsg);
}
}
}
else
{
printf("ReadFile operation completed\n");
// Manual-reset event should be reset since it is now signaled.
ResetEvent(stOverlapped.hEvent);
}
}
break;
}
default:
{
// Decode any other errors codes.
errMsg = ErrorMessage(dwError);
printf("ReadFile GLE unhandled (%d): %s\n", dwError, errMsg);
LocalFree((LPVOID)errMsg);
break;
}
}
}
else
{
// EOF demo did not trigger for the given file.
// Note that system caching may cause this condition on most files
// after the first read. CreateFile can be called using the
// FILE_FLAG_NOBUFFERING parameter but it would require reads are
// always aligned to the volume's sector boundary. This is beyond
// the scope of this example. See comments in the main() function.
printf("ReadFile completed synchronously\n");
}
// The following operation assumes the file is not extremely large, otherwise
// logic would need to be included to adequately account for very large
// files and manipulate the OffsetHigh member of the OVERLAPPED structure.
stOverlapped.Offset += dwBytesRead;
if ( stOverlapped.Offset < dwFileSize )
bContinue = TRUE;
}
return stOverlapped.Offset;
}
int __cdecl _tmain(int argc, TCHAR *argv[])
// To force an EOF condition, execute this application specifying a
// zero-length file. This is because the offset (file pointer) must be
// at or beyond the end-of-file marker when ReadFile is called. For
// more information, see the comments for the AsyncTestForEnd routine.
{
HANDLE hEvent;
HANDLE hFile;
DWORD dwReturnValue;
printf("\n");
if( argc != 2 )
{
printf("ERROR:\tIncorrect number of arguments\n\n");
printf("%s <file_name>\n", argv[0]);
return;
}
hFile = CreateFile(argv[1], // file to open
GENERIC_READ, // open for reading
FILE_SHARE_READ, // share for reading
NULL, // default security
OPEN_EXISTING, // existing file only
FILE_FLAG_OVERLAPPED, // overlapped operation
NULL); // no attr. template
if (hFile == INVALID_HANDLE_VALUE)
{
DWORD dwError = GetLastError();
LPCTSTR errMsg = ErrorMessage(dwError);
printf("Could not open file (%d): %s\n", dwError, errMsg);
LocalFree((LPVOID)errMsg);
return;
}
hEvent = CreateEvent(NULL, TRUE, FALSE, NULL);
if (hEvent == NULL)
{
DWORD dwError = GetLastError();
LPCTSTR errMsg = ErrorMessage(dwError);
printf("Could not CreateEvent: %d %s\n", dwError, errMsg);
LocalFree((LPVOID)errMsg);
return;
}
dwReturnValue = AsyncTestForEnd(hEvent, hFile);
printf( "\nRead complete. Bytes read: %d\n", dwReturnValue);
CloseHandle(hFile);
CloseHandle(hEvent);
}
이 샘플 코드의 출력은 다음과 같습니다.
ReadFile operation is pending
Inside GoDoSomethingElse()
GetOverlappedResult I/O Incomplete
ReadFile operation is pending
Inside GoDoSomethingElse()
ReadFile operation completed
Complete. Bytes read: 541
피드백
출시 예정: 2024년 내내 콘텐츠에 대한 피드백 메커니즘으로 GitHub 문제를 단계적으로 폐지하고 이를 새로운 피드백 시스템으로 바꿀 예정입니다. 자세한 내용은 다음을 참조하세요. https://aka.ms/ContentUserFeedback
다음에 대한 사용자 의견 제출 및 보기