---
name: "fflush"
section: "3"
platform: "linux"
package: "man-pages"
version: "6.19-1"
release: "Arch Linux 2026-11-01"
platforms: ["linux", "freebsd", "macos"]
url: "https://new.carta.tech/fflush"
---

# fflush(3)

flush a stream

## NAME

fflush - flush a stream

## LIBRARY

Standard C library (*libc*, *-lc*)

## SYNOPSIS

```
#include <stdio.h>
```

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

## DESCRIPTION

For output streams, **fflush**() forces a write of all user-space buffered data for the given output or update *stream* via the stream's underlying write function.

For input streams associated with seekable files (e.g., disk files, but not pipes or terminals), **fflush**() discards any buffered data that has been fetched from the underlying file, but has not been consumed by the application.

The open status of the stream is unaffected.

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

For a nonlocking counterpart, see [**unlocked\_stdio**(3)](/unlocked_stdio/3.md).

## RETURN VALUE

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

## ERRORS

- **EBADF**

  *stream* is not an open stream, or is not open for writing.

The function **fflush**() may also fail and set *errno* for any of the errors specified for [**write**(2)](/write/2.md).

## ATTRIBUTES

For an explanation of the terms used in this section, see [**attributes**(7)](/attributes/7.md).

| | | |
| --- | --- | --- |
| **Interface** | **Attribute** | **Value** |
| **fflush**() | Thread safety | MT-Safe |

## STANDARDS

C11, POSIX.1-2008.

## HISTORY

C89, POSIX.1-2001, POSIX.1-2008.

POSIX.1-2001 did not specify the behavior for flushing of input streams, but the behavior is specified in POSIX.1-2008.

## NOTES

Note that **fflush**() flushes only 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, for example, with [**sync**(2)](/sync/2.md) or [**fsync**(2)](/fsync/2.md).

## SEE ALSO

[**fsync**(2)](/fsync/2.md), [**sync**(2)](/sync/2.md), [**write**(2)](/write/2.md), [**fclose**(3)](/fclose/3.md), [**fileno**(3)](/fileno/3.md), [**fopen**(3)](/fopen/3.md), [**fpurge**(3)](/fpurge/3.md), [**setbuf**(3)](/setbuf/3.md), [**unlocked\_stdio**(3)](/unlocked_stdio/3.md)