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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 << std::ios_base::unitbuf; //1 |
| |
| while (some_condition) { |
| ostr << "... some output ..."; //2 |
| // process the output |
| istr >> 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 << "... 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 << 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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