fflush和fsync的一些总结

int fflush(FILE *stream);
If stream points to an output stream or an update stream in which the most recent operation was not input, the fflush function causes any unwritten data for that stream to be delivered to the host environment to be written to the file; otherwise, the behavior is undefined.
如果 stream 指向输出流或者更新流（update stream），并且这个更新流最近执行的操作不是输入，那么 fflush 函数将把这个流中任何待写数据传送至宿主环境（host environment）写入文件。否则，它的行为是未定义的。
(宿主环境可以理解为操作系统或内核).

所以如果 stream 指向输入流（如 stdin），那么 fflush 函数的行为是不确定的。故而使用fflush(stdin)是不正确的。

The  function  fflush  forces a write of all user-space buffered data for the given output or update stream via  the stream underlying write function.  The open status of the stream is unaffected. If the stream argument is NULL, fflush flushes all open output streams.
Note  that  fflush() only flushes the user space buffers provided by the C library.  To ensure that the data is physically stored on disk the kernel buffers must be flushed too, e.g. with sync(2) or fsync(2).

int fsync(int fd);
fsync copies all in-core parts of a file to disk, and waits until the device reports that all parts are on stable storage.  It also updates metadata stat information. It does not necessarily ensure that the entry  in  the directory  containing the file has also reached disk.  For that an explicit fsync on the file descriptor of the directory is also needed.

fsync() transfers ("flushes") all modified in-core data of (i.e., modified buffer cache pages for) the file referred to by the file descriptor fd to the disk device (or other permanent storage device) where that file resides. The call blocks until the device reports that the transfer has completed. It also flushes metadata information associated with the file (see stat(2)).

1.提供者fflush是libc.a中提供的方法，fsync是系统提供的系统调用。
2.原形fflush接受一个参数FILE *.fflush(FILE *);fsync接受的时一个Int型的文件描述符。fsync(int fd);
3.功能fflush:是把C库中的缓冲调用write函数写到磁盘[其实是写到内核的缓冲区]。fsync：是把内核缓冲刷到磁盘上。
4.fsync 将文件相关的所有更改都发送到disk device。 这个调用是阻塞的，直到disk通知此函数传输完成。此函数也会将该文件的文件信息flush到disk。
5.fsync最终将缓冲的数据更新到文件里。

所以可以看出fflush和fsync的调用顺序应该是：
c库缓冲-----fflush---------〉内核缓冲--------fsync-----〉磁盘
 
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.

fdatasync() is similar to fsync(), but does not flush modified metadata unless that metadata is needed in order to allow a subsequent data retrieval to be correctly handled. For example, changes to st_atime or st_mtime (respectively, time of last access and time of last modification; see stat(2)) do not not require flushing because they are not necessary for a subsequent data read to be handled correctly. On the other hand, a change to the file size (st_size, as made by say ftruncate(2)), would require a metadata flush.

The aim of fdatasync(2) is to reduce disk activity for applications that do not require all metadata to be synchronised with the disk.

fdatasync与fsync的区别在于fdatasync不会flush文件的metadata信息。这个是为了减少对磁盘的操作

Notes
If the underlying hard disk has write caching enabled , then the data may not really be on permanent storage when fsync() / fdatasync() return.