81 lines
4.3 KiB
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81 lines
4.3 KiB
HTML
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<TITLE>Implicit Synchronization Using the unitbuf Format Flag</TITLE>
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<A HREF="35-2.html"><IMG SRC="images/bprev.gif" WIDTH=20 HEIGHT=21 ALT="Previous file" BORDER=O></A><A HREF="noframes.html"><IMG SRC="images/btop.gif" WIDTH=56 HEIGHT=21 ALT="Top of Document" BORDER=O></A><A HREF="booktoc.html"><IMG SRC="images/btoc.gif" WIDTH=56 HEIGHT=21 ALT="Contents" BORDER=O></A><A HREF="tindex.html"><IMG SRC="images/bindex.gif" WIDTH=56 HEIGHT=21 ALT="Index page" BORDER=O></A><A HREF="35-4.html"><IMG SRC="images/bnext.gif" WIDTH=25 HEIGHT=21 ALT="Next file" BORDER=O></A><DIV CLASS="DOCUMENTNAME"><B>Apache C++ Standard Library User's Guide</B></DIV>
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<H2>35.3 Implicit Synchronization Using the unitbuf Format Flag</H2>
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<A NAME="idx877"><!></A>
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<A NAME="idx878"><!></A>
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<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>
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<UL><PRE>
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std::ofstream ostr("/tmp/fil");
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std::ifstream istr("/tmp/fil");
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ostr << std::ios_base::unitbuf; //1
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while (some_condition) {
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ostr << "... some output ..."; //2
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// process the output
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istr >> s;
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// ...
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}
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</PRE></UL>
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<TABLE CELLPADDING="3">
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<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>Set the <SAMP>unitbuf</SAMP> format flag.
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<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.
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</TABLE>
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<A NAME="idx879"><!></A>
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<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>
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<UL><PRE>
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ostr.unsetf(std::ios_base::unitbuf); //1
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ostr << "... some lengthy and complicated output ...";
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ostr.flush().setf(std::ios_base::unitbuf); //2
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</PRE></UL>
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<TABLE CELLPADDING="3">
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<TR VALIGN="top"><TD><SAMP>//1</SAMP></TD><TD>Switch off the <SAMP>unitbuf</SAMP> flag. Alternatively, using manipulators, you can use <SAMP>ostr << std::ios_base::nounitbuf;</SAMP>
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<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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