---
name: "fflush"
section: "3"
platform: "freebsd"
package: "base"
version: "16.0-CURRENT-20261027-61c70dc16a79"
release: "FreeBSD 16.0-CURRENT (2026-10-27)"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/fflush"
---

# fflush(3)

flush a stream

## NAME

**fflush**, **fflush\_unlocked**, **fpurge** – flush a stream

## LIBRARY

Standard C Library (libc, -lc)

## SYNOPSIS

**#include \<stdio.h\>**

*int*\

**fflush**(*FILE \*stream*);

*int*\

**fflush\_unlocked**(*FILE \*stream*);

*int*\

**fpurge**(*FILE \*stream*);

## DESCRIPTION

The function **fflush**() forces a write of all buffered data for the given output or update *stream* via the stream's underlying write function. The open status of the stream is unaffected.

If the *stream* argument is NULL, **fflush**() flushes *all* open output streams.

The **fflush\_unlocked**() function is equivalent to **fflush**(), except that the caller is responsible for locking the stream with [flockfile(3)](/flockfile/3.md) before calling it. This function may be used to avoid the overhead of locking the stream and to prevent races when multiple threads are operating on the same stream.

The function **fpurge**() erases any input or output buffered in the given *stream*. For output streams this discards any unwritten output. For input streams this discards any input read from the underlying object but not yet obtained via [getc(3)](/getc/3.md); this includes any text pushed back via [ungetc(3)](/ungetc/3.md).

## RETURN VALUES

Upon successful completion 0 is returned. Otherwise, EOF is returned and the global variable *errno* is set to indicate the error.

## ERRORS

- \[EBADF\]

  The *stream* argument is not an open stream.

The function **fflush**() may also fail and set *errno* for any of the errors specified for the routine [write(2)](/write/2.md), except that in case of *stream* being a read-only descriptor, **fflush**() returns 0.

## SEE ALSO

[write(2)](/write/2.md), [fclose(3)](/fclose/3.md), [fopen(3)](/fopen/3.md), [setbuf(3)](/setbuf/3.md)

## STANDARDS

The **fflush**() function conforms to ISO/IEC 9899:1990 (“ISO C90”).

## HISTORY

The **fflush**() function first appeared in Version 4 AT&T UNIX. The **fpurge**() function first appeared in 4.4BSD.