Flushing output on a buffered stream means transmitting all
accumulated characters to the file. There are many circumstances when
buffered output on a stream is flushed automatically:
When you try to do output and the output buffer is full.
When a newline is written, if the stream is line buffered.
Whenever an input operation on any stream actually reads data
from its file.
If you want to flush the buffered output at another time, call
fflush, which is declared in the header file stdio.h.
— Function: int fflush (FILE *stream)
This function causes any buffered output on stream to be delivered
to the file. If stream is a null pointer, then
fflush causes buffered output on all open output streams
to be flushed.
This function returns EOF if a write error occurs, or zero
otherwise.
— Function: int fflush_unlocked (FILE *stream)
The fflush_unlocked function is equivalent to the fflush
function except that it does not implicitly lock the stream.
The fflush function can be used to flush all streams currently
opened. While this is useful in some situations it does often more than
necessary since it might be done in situations when terminal input is
required and the program wants to be sure that all output is visible on
the terminal. But this means that only line buffered streams have to be
flushed. Solaris introduced a function especially for this. It was
always available in the GNU C library in some form but never officially
exported.
— Function: void _flushlbf (void)
The _flushlbf function flushes all line buffered streams
currently opened.
This function is declared in the stdio_ext.h header.
Compatibility Note: Some brain-damaged operating systems have
been known to be so thoroughly fixated on line-oriented input and output
that flushing a line buffered stream causes a newline to be written!
Fortunately, this “feature” seems to be becoming less common. You do
not need to worry about this in the GNU system.
In some situations it might be useful to not flush the output pending
for a stream but instead simply forget it. If transmission is costly
and the output is not needed anymore this is valid reasoning. In this
situation a non-standard function introduced in Solaris and available in
the GNU C library can be used.
— Function: void __fpurge (FILE *stream)
The __fpurge function causes the buffer of the stream
stream to be emptied. If the stream is currently in read mode all
input in the buffer is lost. If the stream is in output mode the
buffered output is not written to the device (or whatever other
underlying storage) and the buffer the cleared.
This function is declared in stdio_ext.h.
Published under the terms of the GNU General Public License
