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<TITLE>Implicit Synchronization Using the unitbuf Format Flag</TITLE>
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<H2>35.3 Implicit Synchronization Using the unitbuf Format Flag</H2>
<A NAME="idx877"><!></A>
<A NAME="idx878"><!></A>
<P>You can achieve a kind of automatic synchronization for output files by using the format flag <SAMP>ios_base::unitbuf</SAMP>. It causes an output stream to flush its buffer after each output operation as follows:</P>
<UL><PRE>
std::ofstream ostr("/tmp/fil");
std::ifstream istr("/tmp/fil");
ostr &lt;&lt; std::ios_base::unitbuf; //1
while (some_condition) {
ostr &lt;&lt; "... some output ..."; //2
// process the output
istr &gt;&gt; s;
// ...
}
</PRE></UL>
<TABLE CELLPADDING="3">
<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>Set the <SAMP>unitbuf</SAMP> format flag.
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>After each insertion into the file <SAMP>/tmp/fil,</SAMP> the buffer is automatically flushed, and the output is available to other streams that read from the same file.
</TABLE>
<A NAME="idx879"><!></A>
<P>Since it is not overly efficient to flush after every single token that is inserted, you might consider switching off the <SAMP>unitbuf</SAMP> flag for a lengthy output that is not supposed to be read partially.</P>
<UL><PRE>
ostr.unsetf(std::ios_base::unitbuf); //1
ostr &lt;&lt; "... some lengthy and complicated output ...";
ostr.flush().setf(std::ios_base::unitbuf); //2
</PRE></UL>
<TABLE CELLPADDING="3">
<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>Switch off the <SAMP>unitbuf</SAMP> flag. Alternatively, using manipulators, you can use <SAMP>ostr &lt;&lt; std::ios_base::nounitbuf;</SAMP>
<TR VALIGN="top"><TD><SAMP>//2</SAMP></TD><TD>Flush the buffer and switch on the <SAMP>unitbuf</SAMP> flag again. Alternatively, you can use the <SAMP>std::ios_base::flush</SAMP> and <SAMP>std::ios::base::unitbuf</SAMP> manipulators.
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