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.
本函数未在 Windows 平台下实现。
The file descriptor returned by dio_open().
The offset in bytes.