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 con-
sumed by the application. The open status of the stream is unaffected.
If the stream argument is NULL, fflush() flushes all open output
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 write(2).
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), sync(2), write(2), fclose(3), fopen(3), setbuf(3),
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GNU 2013-07-15 FFLUSH(3)