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POCopying one std stream to another efficiently
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  1. POCopying one std stream to another efficiently
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    copied!<p>Ok, Here's some code that outlines what I'm trying to do.</p> <pre><code>#include &lt;sys/types.h&gt; #include &lt;sys/stat.h&gt; #include &lt;sys/fcntl.h&gt; #include &lt;iostream&gt; #include &lt;sstream&gt; int main( int c, char *v[] ) { int fd = open( "data.out", O_RDONLY | O_NONBLOCK ); std::cout &lt;&lt; "fd = " &lt;&lt; fd &lt;&lt; std::endl; char buffer[ 1024000 ]; ssize_t nread; std::stringstream ss; while( true ) { if ( (nread = read( fd, buffer, sizeof( buffer ) - 1 )) &lt; 0 ) break; ss.write( buffer, nread ); while( true ) { std::stringstream s2; std::cout &lt;&lt; "pre-get : " &lt;&lt; (((ss.rdstate() &amp; std::ios::badbit) == std::ios::badbit) ? "bad" : "") &lt;&lt; " " &lt;&lt; (((ss.rdstate() &amp; std::ios::eofbit) == std::ios::eofbit) ? "eof" : "") &lt;&lt; " " &lt;&lt; (((ss.rdstate() &amp; std::ios::failbit) == std::ios::failbit) ? "fail" : "" ) &lt;&lt; " " &lt;&lt; std::endl; ss.get( *s2.rdbuf() ); std::cout &lt;&lt; "post-get : " &lt;&lt; (((ss.rdstate() &amp; std::ios::badbit) == std::ios::badbit) ? "bad" : "") &lt;&lt; " " &lt;&lt; (((ss.rdstate() &amp; std::ios::eofbit) == std::ios::eofbit) ? "eof" : "") &lt;&lt; " " &lt;&lt; (((ss.rdstate() &amp; std::ios::failbit) == std::ios::failbit) ? "fail" : "" ) &lt;&lt; " " &lt;&lt; std::endl; unsigned int linelen = ss.gcount() - 1; if ( ss.eof() ) { ss.str( s2.str() ); break; } else if ( ss.fail() ) { ss.str( "" ); break; } else { std::cout &lt;&lt; s2.str() &lt;&lt; std::endl; } } } } </code></pre> <p>It firstly reads large chunks of data into a data buffer. I know there's better C++ ways of doing this part but in my real application I am handed a char[] buffer and a length.</p> <p>I then write the buffer into a std::stringstream object so I can remove a line at a time from it.</p> <p>I thought I'd use the get( streambuf &amp; ) method on the stringstream to write one line to another stringstream where I can then output it.</p> <p>Ignoring the fact that this may not be the best way to extract a line at a time from the buffer I've read in (although I'd like anyone to offer up a better alternative to the one I post here), as soon as the first <code>ss.get( *s2.rdbuf() )</code> is called the <code>ss</code> is in a fail state and I can't work out why. There's plenty of data in the input file so <code>ss</code> should definately contain more than one line of input.</p> <p>Any ideas?</p>
 

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