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:
exit. See Normal Termination.
If you want to flush the buffered output at another time, call
fflush, which is declared in the header file stdio.h.
This function causes any buffered output on stream to be delivered to the file. If stream is a null pointer, then
fflushcauses buffered output on all open output streams to be flushed.
This function returns
EOFif a write error occurs, or zero otherwise.
fflush_unlockedfunction is equivalent to the
fflushfunction except that it does not implicitly lock the stream.
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
_flushlbffunction 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.
__fpurgefunction 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.