The function dio_seek() is used to change the file position of the file with descriptor fd. The parameter whence specifies how the position pos should be interpreted:
SEEK_SET - specifies that pos is specified from the beginning of the file.
SEEK_CUR - Specifies that pos is a count of characters from the current file position. This count may be positive or negative.
SEEK_END - Specifies that pos is a count of characters from the end of the file. A negative count specifies a position within the current extent of the file; a positive count specifies a position past the current end. If you set the position past the current end, and actually write data, you will extend the file with zeros up to that position.