fflush(3) - Linux manual page (original) (raw)

fflush(3) Library Functions Manual fflush(3)

NAME top

   fflush - flush a stream

LIBRARY top

   Standard C library (_libc_, _-lc_)

SYNOPSIS top

   **#include <stdio.h>**

   **int fflush(FILE *_Nullable** _stream_**);**

DESCRIPTION top

   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)](../man3/unlocked%5Fstdio.3.html).

RETURN VALUE top

   Upon successful completion 0 is returned.  Otherwise, **EOF** is
   returned and _[errno](../man3/errno.3.html)_ is set to indicate the error.

ERRORS top

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

   The function **fflush**() may also fail and set _[errno](../man3/errno.3.html)_ for any of the
   errors specified for [write(2)](../man2/write.2.html).

ATTRIBUTES top

   For an explanation of the terms used in this section, see
   [attributes(7)](../man7/attributes.7.html).
   ┌──────────────────────────────────────┬───────────────┬─────────┐
   │ **Interface** │ **Attribute** │ **Value** │
   ├──────────────────────────────────────┼───────────────┼─────────┤
   │ **fflush**()                             │ Thread safety │ MT-Safe │
   └──────────────────────────────────────┴───────────────┴─────────┘

STANDARDS top

   C11, POSIX.1-2008.

HISTORY top

   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 top

   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)](../man2/sync.2.html) or [fsync(2)](../man2/fsync.2.html).

SEE ALSO top

   [fsync(2)](../man2/fsync.2.html), [sync(2)](../man2/sync.2.html), [write(2)](../man2/write.2.html), [fclose(3)](../man3/fclose.3.html), [fileno(3)](../man3/fileno.3.html), [fopen(3)](../man3/fopen.3.html),
   [fpurge(3)](../man3/fpurge.3.html), [setbuf(3)](../man3/setbuf.3.html), [unlocked_stdio(3)](../man3/unlocked%5Fstdio.3.html)

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Linux man-pages 6.10 2024-07-23 fflush(3)

Pages that refer to this page:fsync(2), fclose(3), fcloseall(3), fmemopen(3), fopen(3), fpurge(3), fseek(3), open_memstream(3), popen(3), setbuf(3), stdin(3), stdio(3), xdr(3)