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标准库的I/O缓冲区-----fflush---------〉内核缓冲区--------fsync-----〉磁盘缓冲区
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.
C标准库的I/O缓冲区有三种类型:全缓冲、行缓冲和无缓冲。当用户程序调用库函数做写操作时, 不同类型的缓冲区具有不同特性。
全缓冲
如果缓冲区写满了就写回内核。常规文件通常是全缓冲的。
行缓冲
如果用户程序写的数据中有换行符就把这一行写回内核,或者如果缓冲区写满了就写回内 核。标准输入和标准输出对应终端设备时通常是行缓冲的。
无缓冲
用户程序每次调库函数做写操作都要通过系统调用写回内核。标准错误输出通常是无缓冲的,这样用户程序产生的错误信息可以尽快输出到设备。
除了写满缓冲区、写入换行符之外,行缓冲还有两种情况会自动做Flush操作。如果:
用户程序调用库函数从无缓冲的文件中读取
或者从行缓冲的文件中读取,并且这次读操作会引发系统调用从内核读取数据
如果用户程序不想完全依赖于自动的Flush操作,可以调fflush函数手动做Flush操作。
#include <stdio.h>
int fflush(FILE *stream);
返回值:成功返回0,出错返回EOF并设置errno
fflush函数用于确保数据写回了内核,以免进程异常终止时丢失数据,如fflush(stdout); 作为一个特例,调 用fflush(NULL)可以对所有打开文件的I/O缓冲区做Flush操作。
参考：https://www.cnblogs.com/alantu2018/p/8472718.html