If the SetFilePointerfunction succeeds and lpDistanceToMoveHighis set to NULL, the return value is the low-order DWORDof the new file pointer. If lpDistanceToMoveHighis not set to NULL, the function returns the low-order DWORDof the new file pointer, and puts the high-order DWORDof the new file pointer into the LONGpointed to by that parameter.

If the function fails and lpDistanceToMoveHighis set to NULL, the return value is 0xFFFFFFFF. To get extended error information, call GetLastError.

If the function fails, and lpDistanceToMoveHighis not set to NULL, the return value is 0xFFFFFFFF. However, because 0xFFFFFFFF is a valid value for the low-order DWORDof the new file pointer, check GetLastErrorto determine whether an error occurred. If an error occurred, GetLastErrorreturns a value other than NO_ERROR.

If the new file pointer would have been a negative value, the function fails, the file pointer is not moved, and GetLastErrorreturns ERROR_NEGATIVE_SEEK. The following code example shows what is returned:

	Copy Code
if (lDistanceToMove == 6) return 6;

An FSD exports this function if it supports the SetFilePointerfunction. All FSD functions can be called on re-entry. Therefore, take this into account when developing an FSD.

FSDMGR is a DLL that manages all OS interaction with installable files systems. Each installable file system requires an FSD, which is a DLL that supports an installable file system. The name of the DLL and the names of the functions it exports start with the name of the associated installable file system. For example, if the name of file system is MyFSD, its DLL is MyFSD.dll, and its exported functions are prefaced with MyFSD_*.

FSDMGR provides services to FSDs. The FSDMGR_RegisterVolume, the FSDMGR_CreateFileHandle, and the FSDMGR_CreateSearchHandlefunctions record a DWORDof volume-specific data that an FSD associates with a volume. This volume-specific data is passed as the first parameter of these three functions.

Applications that access an installable file system use standard Win32 functions. For example, when an application creates a folder on a device that contains an installable file system, it calls the CreateDirectoryfunction. FSDMGR recognizes that the path is to a device containing an installable file system and calls the appropriate function, which in the case of the FAT file system is FATFSD_CreateDirectoryW. That is, the application calls CreateDirectory, causing FSDMGR to call FATFSD_CreateDirectoryW.

You cannot use the SetFilePointerfunction with a handle to a nonseeking device, such as a pipe or a communications device.

Be careful when setting the file pointer in a multithreaded application. For example, an application whose threads share a file handle, update the file pointer, and read from the file must protect this sequence by using a critical section object or mutex object.

If the hFilefile handle was opened with the FILE_FLAG_NO_BUFFERING flag set, an application can move the file pointer only to sector-aligned positions. A sector-aligned position is a position that is a whole number multiple of the volume's sector size. An application can obtain a volume's sector size with the GetDiskFreeSpaceExfunction. If an application calls SetFilePointerwith distance-to-move values that result in a position that is not sector-aligned and a handle that was opened with FILE_FLAG_NO_BUFFERING, this function fails, and GetLastErrorreturns ERROR_INVALID_PARAMETER.

It is not an error to set the file pointer to a position beyond the end of the file. The size of the file does not increase until you call the SetEndOfFileor the WriteFilefunction. A write operation increases the size of the file to the file pointer position in addition to the size of the buffer written, leaving the intervening bytes uninitialized.

If the return value is 0xFFFFFFFF and lpDistanceToMoveHighis not set to NULL, an application must call GetLastErrorto determine whether the function has succeeded or failed. The following code example shows this:

Copy Code

// // Case One: calling the function with lpDistanceToMoveHigh == NULL. // Try to move hFile's file pointer some distance. dwPtr = SetFilePointer (hFile, lDistance, NULL, FILE_BEGIN); if (dwPtr == 0xFFFFFFFF) // Test for failure { // Obtain the error code. dwError = GetLastError(); // Resolve the failure. } // End of error handler. // Case Two: calling the function with lpDistanceToMoveHigh != NULL. // Try to move hFile's file pointer some large distance. dwPtrLow = SetFilePointer (hFile, lDistLow, & lDistHigh, FILE_BEGIN); // Test for failure. if (dwPtrLow == 0xFFFFFFFF && (dwError = GetLastError()) != NO_ERROR ) { // Resolve the failure. } // End of error handler.

The lpDistanceToMoveHighparameter is used to manipulate large files. If it is set to NULL, lDistanceToMovehas a maximum value of 2 to the 31st power minus 2, or 2 GB minus two because all file pointer values are signed values. Therefore, if there is even a small chance that the file will grow to that size, handle the file as a large file, and work with 64-bit file pointers. With file compression on an NTFS file system and sparse files, it is possible to have files that large, even if the underlying volume is not that large.

If lpDistanceToMoveHigh is not set to NULL, lpDistanceToMoveHigh and lDistanceToMove form a single 64-bit signed value. The lDistanceToMove parameter is handled as the low-order 32 bits of the value, and lpDistanceToMoveHigh as the upper 32 bits. Thus, lpDistanceToMoveHigh is a sign extension of lDistanceToMove .

To move the file pointer from zero to 2 GB, lpDistanceToMoveHigh can be set either NULL or a sign extension of lDistanceToMove .To move the pointer more than 2 GB, use lpDistanceToMoveHigh and lDistanceToMove as a single 64-bit quantity. For example, to move in the range from 2 GB to 4 GB, set the contents of lpDistanceToMoveHigh to zero, or to –1 for a negative sign extension of lDistanceToMove .

You can use SetFilePointerto determine the length of a file. To do this, use FILE_END for dwMoveMethod, and seek to location zero. The file offset returned is the length of the file. However, this practice can have unintended effects, such as failure to save the current file pointer so that the program can return to that location. It is simpler and safer to use tje GetFileSizefunction.

You can also use the SetFilePointerfunction to query the current file pointer position. To do this, specify a move method of FILE_CURRENT and a distance of zero.

It is conceptually simpler and better design to use a function to hide the interface to SetFilePointer. The following code example shows how to implement the function:

	Copy Code
__int64 myFileSeek (HANDLE hf, __int64 distance, DWORD MoveMethod) { LARGE_INTEGER li; li.QuadPart = distance; li.LowPart = SetFilePointer (hf, li.LowPart, &li.HighPart, MoveMethod); if (li.LowPart == 0xFFFFFFFF && GetLastError() != NO_ERROR) { li.QuadPart = –1; } return li.QuadPart; }