FFLUSH(3) Linux Programmer's Manual FFLUSH(3)
fflush - flush a stream
int fflush(FILE *stream);
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,
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).
Upon successful completion 0 is returned. Otherwise, EOF is returned and errno is set to
indicate the error.
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
Multithreading (see pthreads(7))
The fflush() function is thread-safe.
C89, C99, POSIX.1-2001, POSIX.1-2008.
The standards do not specify the behavior for input streams. Most other implementations
behave the same as Linux.
Note that fflush() only flushes 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).
fsync(2), sync(2), write(2), fclose(3), fopen(3), setbuf(3), unlocked_stdio(3)
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GNU 2013-07-15 FFLUSH(3)