What is fflush exactly and what does it do?

Question

I was reading http://www.cplusplus.com/reference/cstdio/fflush/ and I was curious about what it means. According to the website it says:

If the given stream was open for writing (or if it was open for updating and the last i/o operation was an output operation) any unwritten data in its output buffer is written to the file.

What does the output buffer to file mean?

It's not clear what you don't understand. Can you ask a more specific question? — David Schwartz, May 22 '14 at 22:48
What's actually unclear about the statements there? BTW, here's the [better reference](http://en.cppreference.com/w/cpp/io/c/fflush) IMHO. — πάντα ῥεῖ, May 22 '14 at 22:49
The data isn't immediately written to the physical file, but buffered up until a certain amount of data is available to store in the output device. That's done because the physical storing may be a costly operation, you don't want to do for every single byte written. — πάντα ῥεῖ, May 22 '14 at 22:56
@Storm Yes, I gave you this link http://en.cppreference.com/w/cpp/io/c/fflush, it's still there?!? — πάντα ῥεῖ, May 22 '14 at 23:05
'What's actually unclear about the statements there? BTW, here's the better reference IMHO' — Storm, May 22 '14 at 23:14
@Storm Sorry, I still don't get it? What link are you talking about? All links I gave on this question where in my 1st comment, and are still reachable from there! — πάντα ῥεῖ, May 22 '14 at 23:15
@Storm: You are not making any sense whatsoever. Please speak in complete sentences. — Benjamin Lindley, May 22 '14 at 23:16
@πάνταῥεῖ, There is no link in your first comment. I cant see one — Storm, May 22 '14 at 23:17
@Storm Hover your mouse pointer over [**better reference**](http://en.cppreference.com/w/cpp/io/c/fflush)! Facepalm ... — πάντα ῥεῖ, May 22 '14 at 23:18
@πάνταῥεῖ, omg, sorry, didnt know u changed link to hyperlink XD — Storm, May 22 '14 at 23:21
possible duplicate of [Understanding the need of fflush() and problems associated with it](http://stackoverflow.com/questions/16780908/understanding-the-need-of-fflush-and-problems-associated-with-it) — manlio, May 29 '14 at 08:36

Remy Lebeau · Accepted Answer · 2014-05-22T23:25:21.960

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Some streams buffer output data and do not write to the device immediately. fflush forces the contents of the stream's buffer, if there is one, to be written to the device and the buffer cleared.

edited May 22 '14 at 23:25

answered May 22 '14 at 22:53

Remy Lebeau

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Might be worth mentioning that by default, `stdout` is line buffered (will flush after newlines), and `stderr` is unbuffered. – Kerrek SB May 22 '14 at 23:10

score 2 · Answer 2 · answered May 22 '14 at 22:53

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Generally, when data is intended to be written to a file, it is stored in a construct known as a "buffer". Once the buffer has reached a certain threshold, all the data stored in the buffer will be written out to the file at once.

Fflush empties the buffer and forces all changes to be written to the file. This is particularly useful when you are done writing to the file, since it is good practice to flush the buffer before closing the file (thereby making sure that all data has been successfully written to the file).

This goes for other types of filestreams too.

answered May 22 '14 at 22:53

ControlAltDeplete

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`fclose` flushes the buffer; calling `fflush` yourself before closing is redundant. I don't really think its a 'best practice'. – Colonel Thirty Two May 22 '14 at 22:55
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I got to agree with @ColonelThirtyTwo, it irks me when people do it, the same with checking for `NULL` before calling delete on a pointer... – AliciaBytes May 22 '14 at 22:59

What is fflush exactly and what does it do?

2 Answers2