fflush(3) Library Functions Manual fflush(3)
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).
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 │
└────────────────────────────────────────────┴───────────────┴─────────┘
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 ker-
nel 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), fpurge(3),
setbuf(3), unlocked_stdio(3)
Linux man-pages 6.9.1 2024-05-02 fflush(3)
Generated by dwww version 1.16 on Sun Dec 7 05:22:02 CET 2025.