Difference between fflush and fsync

Question

I thought fsync() does fflush() internally, so using fsync() on a stream is OK. But I am getting an unexpected result when executed under network I/O.

My code snippet:

FILE* fp = fopen(file, "wb");
/* multiple fputs() calls like: */
fputs(buf, fp);
...
...
fputs(buf.c_str(), fp);
/* get fd of the FILE pointer */
fd = fileno(fp);
#ifndef WIN32
ret = fsync(fd);
#else
ret = _commit(fd);
fclose(fp);

But it seems _commit() is not flushing the data (I tried on Windows and the data was written on a Linux exported filesystem).

When I changed the code to be:

FILE* fp = fopen(file, "wb");
/* multiple fputs() calls like: */
fputs(buf, fp);   
...   
...
fputs(buf.c_str(), fp);
/* fflush the data */
fflush(fp);
fclose(fp);

it flushes the data.

I am wondering if _commit() does the same thing as fflush(). Any inputs?

Answer 1

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.

Answer 2

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.

Answer 3

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

Answer 4

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()...

Answer 5

To force the commitment of recent changes to disk, use the sync() or fsync() functions.

fsync() will synchronize all of the given file's data and metadata with the permanent storage device. It should be called just before the corresponding file has been closed.

sync() will commit all modified files to disk.

Answer 6

I think below document from python (https://docs.python.org/2/library/os.html) clarifies it very well.

os.fsync(fd) Force write of file with filedescriptor fd to disk. On Unix, this calls the native fsync() function; on Windows, the MS _commit() function.

If you’re starting with a Python file object f, first do f.flush(), and then do os.fsync(f.fileno()), to ensure that all internal buffers associated with f are written to disk.

Availability: Unix, and Windows starting in 2.2.3.