Pruebas para el final de un archivo
La función ReadFile comprueba la condición de fin de archivo (EOF) de forma diferente para las operaciones de lectura sincrónicas y asincrónicas. Cuando una operación de lectura sincrónica llega al final de un archivo, ReadFile devuelve TRUE y establece la variable a la que apunta el parámetro lpNumberOfBytesRead en cero. Una operación de lectura asincrónica puede encontrar el final de un archivo durante la llamada inicial a ReadFile o durante las operaciones asincrónicas posteriores si el puntero de archivo está avanzado mediante programación más allá del final del archivo.
En el siguiente ejemplo de C++ se muestra cómo probar el final de un archivo durante una operación de lectura sincrónica.
// 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
}
La prueba para el final del archivo durante una operación de lectura asincrónica está ligeramente más implicada que para una operación de lectura sincrónica similar. El indicador de fin de archivo para las operaciones de lectura asincrónicas es cuando GetOverlappedResult devuelve FALSE y GetLastError devuelve ERROR_HANDLE_EOF.
En el siguiente ejemplo de C++ se muestra cómo probar el final del archivo durante una operación de lectura asincrónica.
#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);
}
La salida de este código de ejemplo es la siguiente.
ReadFile operation is pending
Inside GoDoSomethingElse()
GetOverlappedResult I/O Incomplete
ReadFile operation is pending
Inside GoDoSomethingElse()
ReadFile operation completed
Complete. Bytes read: 541
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