Testing for the End of the File - Win32 Programmer's Reference

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Testing for the End of the File

The ReadFile function checks for the end-of-file condition (eof) differently for synchronous and asynchronous read operations. When a synchronous read operation reaches the end of a file, ReadFile returns TRUE and sets the variable pointed to by lpNumberOfBytesRead to zero. An asynchronous read operation can encounter the end of a file during the initiating call to ReadFile or during subsequent asynchronous operation.

The test for end-of-file during a synchronous read operation is simple, as shown in the following example:

// attempt a synchronous read operation
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead, NULL);
// check for eof
if (bResult && nBytesRead == 0, ) {
// we're at the end of the file
}

The test for end-of-file during an asynchronous read operation is more difficult. There are three end-of-file indicators for asynchronous read operations:

ReadFile returns FALSE and GetLastError returns ERROR_HANDLE_EOF.
ReadFile returns FALSE and GetLastError returns ERROR_IO_PENDING.
GetOverlappedResult returns FALSE and GetLastError returns ERROR_HANDLE_EOF.

The following example shows how to test for an end-of-file during an asynchronous read operation:

// Attempt to initiate an asynchronous read operation.
bResult = ReadFile(hFile, &inBuffer, nBytesToRead, &nBytesRead, NULL);

// Check if there was a problem.
if (!bResult) {
switch (dwError = GetLastError())

{
case ERROR_HANDLE_EOF:

// At the end of the file.

break;
case ERROR_IO_PENDING:

// I/O pending.

break;
}
}
.
.
.
// Check on an asynchronous read operation.
bResult = GetOverlappedResult(hFile, &gOverlapped, &nBytesRead, TRUE);

// Check if there was a problem.
if (!bResult) {
switch (dwError = GetLastError())

{
case ERROR_HANDLE_EOF:
// At the end of the file
}
}

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