说明
bool 
dio_truncate ( resource fd, int offset )
   dio_truncate() truncates a file to at most 
   offset bytes in size.
  
   If the file previously was larger than this size, the extra data is 
   lost. If the file previously was shorter, it is unspecified whether the
   file is left unchanged or is extended. In the latter case the extended 
   part reads as zero bytes.