Difference between fflush and fsync

I could say that for simplicity:

use fsync() with not streaming files (integer file descriptors)

use fflush() with file streams.

Also here is the help from man:

int fflush(FILE *stream); // flush a stream, FILE* type

int fsync(int fd); // synchronize a file's in-core state with storage device
                    // int type

fflush() and fsync() can be used to try and ensure data is written to the storage media (but it is not always be possible):

1. first use fflush(fp) on the output stream (fp being a FILE * obtained from fopen or one of the standard streams stdout or stderr) to write the contents of the buffer associated with the stream to the OS.
2. then use fsync(fileno(fp)) to tell the OS to write its own buffers to the storage media.

Note however that fileno() and fsync() are POSIX functions that might not be available on all systems, notably Microsoft legacy systems where alternatives may be named _fileno(), _fsync() or _commit()...

The standard C function fflush() and the POSIX system call fsync() are conceptually somewhat similar. fflush() operates on C file streams (FILE objects), and is therefore portable. fsync() operate on POSIX file descriptors. Both cause buffered data to be sent to a destination.

On a POSIX system, each C file stream has an associated file descriptor, and all the operations on a C file stream will be implemented by delegating, when necessary, to POSIX system calls that operate on the file descriptor.

One might think that a call to fflush on a POSIX system would cause a write of any data in the buffer of the file stream, followed by a call of fsync() for the file descriptor of that file stream. So on a POSIX system there would be no need to follow a call to fflush with a call to fsync(fileno(fp)). But is that the case: is there a call to fsync from fflush?

No, calling fflush on a POSIX system does not imply that fsync will be called.

The C standard for fflush says (emphasis added) it

causes any unwritten data for [the] stream to be delivered to the host environment to be written to the file

Saying that the data is to be written, rather than that is is written implies that further buffering by the host environment is permitted. That buffering by the "host environment" could include, for a POSIX environment, the internal buffering that fsync flushes. So a close reading of the C standard suggests that the standard does not require the POSIX implementation to call fsync.

The POSIX standard description of fflush does not declare, as an extension of the C semantics, that fsync is called.

fflush() works on FILE*, it just flushes the internal buffers in the FILE* of your application out to the OS.

fsync works on a lower level, it tells the OS to flush its buffers to the physical media.

OSs heavily cache data you write to a file. If the OS enforced every write to hit the drive, things would be very slow. fsync (among other things) allows you to control when the data should hit the drive.

Furthermore, fsync/commit works on a file descriptor. It has no knowledge of a FILE* and can't flush its buffers. FILE* lives in your application, file descriptors live in the OS kernel, typically.