SetFilePointerEx

The SetFilePointerEx function moves the file pointer of an open file.

BOOL SetFilePointerEx(
  HANDLE hFile,                    // handle to file
  LARGE_INTEGER liDistanceToMove,  // bytes to move pointer
  PLARGE_INTEGER lpNewFilePointer, // new file pointer
  DWORD dwMoveMethod               // starting point
);

Parameters

hFile

[in] Handle to the file whose file pointer is to be moved. The file handle must have been created with GENERIC_READ or GENERIC_WRITE access to the file.

liDistanceToMove

[in] Specifies the number of bytes to move the file pointer. A positive value moves the pointer forward in the file and a negative value moves the file pointer backward.

lpNewFilePointer

[in] Pointer to a variable that receives the new file pointer. If this parameter is NULL, the new file pointer is not returned.

dwMoveMethod

[in] Specifies the starting point for the file pointer move. This parameter can be one of the following values.

Value	Meaning
FILE_BEGIN	The starting point is zero or the beginning of the file. If this flag is specified, then the liDistanceToMove parameter is interpreted as an unsigned value.
FILE_CURRENT	The start point is the current value of the file pointer.
FILE_END	The starting point is the current end-of-file position.

Return Values

If the function succeeds, the return value is nonzero.

If the function fails, the return value is zero. To get extended error information, call GetLastError.

Remarks

You cannot use the SetFilePointerEx function with a handle to a nonseeking device such as a pipe or a communications device. To determine the file type for hFile, use the GetFileType function.

Use caution when setting the file pointer in a multithreaded application. You must synchronize access to shared resources. 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 a mutex object. For more information about these objects, see Critical Section Objects and Mutex Objects.

If the hFile 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 by calling the GetDiskFreeSpaceEx function. If an application calls SetFilePointerEx with 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, the function fails, and GetLastError returns ERROR_INVALID_PARAMETER.

Note that 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 SetEndOfFile, WriteFile, or WriteFileEx function. A write operation increases the size of the file to the file pointer position plus the size of the buffer written, leaving the intervening bytes uninitialized.

You can use SetFilePointerEx to 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 side 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 the GetFileSizeEx function instead.

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

Requirements

  Windows NT/2000: Requires Windows 2000.
  Windows 95/98: Unsupported.
  Header: Declared in Winbase.h; include Windows.h.
  Library: Use Kernel32.lib.

See Also

File I/O Overview, File I/O Functions, GetDiskFreeSpaceEx, GetFileSizeEx, GetFileType, SetEndOfFile, WriteFile, WriteFileEx