man_3_fflush.txt

FFLUSH(3)                          Linux Programmer's Manual                         FFLUSH(3)

NAME
       fflush - flush a stream

SYNOPSIS
       #include <stdio.h>

       int fflush(FILE *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 underly‐
       ing 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).

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).

ATTRIBUTES
       For an explanation of the terms used in this section, see attributes(7).

       ┌──────────┬───────────────┬─────────┐
       │Interface │ Attribute     │ Value   │
       ├──────────┼───────────────┼─────────┤
       │fflush()  │ Thread safety │ MT-Safe │
       └──────────┴───────────────┴─────────┘
CONFORMING TO
       C89, C99, POSIX.1-2001, POSIX.1-2008.

       POSIX.1-2001 did not specify the behavior for flushing of input streams, but the behav‐
       ior 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) or fsync(2).

SEE ALSO
       fsync(2),   sync(2),   write(2),   fclose(3),   fileno(3),   fopen(3),  setbuf(3),  un‐
       locked_stdio(3)

COLOPHON
       This page is part of release 5.05 of the Linux man-pages project.  A description of the
       project,  information about reporting bugs, and the latest version of this page, can be
       found at https://www.kernel.org/doc/man-pages/.

GNU                                       2017-09-15                                 FFLUSH(3)