问:O_SYNC与fsync系统调用的区别与联系

neilhappy

      O_SYNC是每次write系统调用后都等待实际的物理I/O完成。而fsync也是会等待物理I/O完成后才返回。apue上是这样写的，但我没有明白:Compare fsync and fdatasync,which update a file's contents when we say so,with the O_SYNC flag,which updates a file's contents every time we write to the file. 我不明白这里的"fsync which update a file's contents when we say so"到底是什么意思。O_SYNC和fsync之间有什么样的区别和联系?这样的问题我没有办法去写程序实践。 小弟想请教大家，非常感谢!

linux_c_py_php

1.        POSIX  provides  for three different variants of synchronized I/O, corresponding to the flags O_SYNC, O_DSYNC, and O_RSYNC.  Currently (2.6.31),
   
2.        Linux only implements O_SYNC, but glibc maps O_DSYNC and O_RSYNC to the same numerical value as O_SYNC.  Most Linux file systems don’t  actually
   
3.        implement  the  POSIX  O_SYNC  semantics,  which  require  all metadata updates of a write to be on disk on returning to userspace, but only the
   
4.        O_DSYNC semantics, which require only actual file data and metadata necessary to retrieve it to be on disk by the time the system call  returns.

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1.        Calling fsync() does not necessarily ensure that the entry in the directory containing the file has also reached disk.   For  that  an  explicit
   
2.        fsync() on a file descriptor for the directory is also needed.

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neilhappy

回复 2# linux_c_py_php
您好，我无法理解这两句话的因果关系“Calling fsync() does not necessarily ensure that the entry in the directory containing the file has also reached disk.   For  that  an  explicit
       fsync() on a file descriptor for the directory is also needed.”请问，为什么"无法确保该条目是否写入磁盘就会导致需要对目录来调用fsync"?或者是我的翻译有误?


   

酷了个cool

一个是原语，一个不是。我猜的。别喷。