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<HTML>
<HEAD>
<META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1">
<META NAME="Author" CONTENT="James CE Johnson">
<TITLE>ACE Tutorial 015</TITLE>
</HEAD>
<BODY TEXT="#000000" BGCOLOR="#FFFFFF" LINK="#000FFF" VLINK="#FF0F0F">
<CENTER><B><FONT SIZE=+2>ACE Tutorial 015</FONT></B></CENTER>
<CENTER><B><FONT SIZE=+2>Building a protocol stream</FONT></B></CENTER>
<P>
<HR WIDTH="100%">
Here we implement the details of our compression. By having both
compression and decompression in one object it's easier to keep track
of implementation details. Splitting Xmit/Recv like I did will make
things more difficult if something has to change in their interaction.
<HR>
<PRE>
<font color=red>// $Id$</font>
<font color=blue>#include</font> "<font color=green>Compressor.h</font>"
<font color=blue>#include</font> "<font color=green>ace/SOCK_Stream.h</font>"
<font color=red>/* Construct our baseclass with the proper thread count. I really
should remove this option...
*/</font>
<font color=#008888>Compressor::Compressor</font>( int _thr_count )
: inherited(_thr_count)
{
;
}
<font color=#008888>Compressor::~Compressor</font>(void)
{
;
}
<font color=red>/* This is where you insert your compression code. Most compressors
want to work on a block of data instead of a byte-stream.
Fortunately the message block has a block that can be compressed.
Take a look at libz for a quick way to add compression to your
apps
*/</font>
int <font color=#008888>Compressor::send</font>(ACE_Message_Block *message, ACE_Time_Value *timeout)
{
ACE_DEBUG ((LM_INFO, "<font color=green>(%P|%t) <font color=#008888>Compressor::send</font>() compressing (%s)\n</font>", message->rd_ptr() ));
<font color=red>// Create a block to hold the compressed data. I belive libz</font>
<font color=red>// recommends a buffer about 10-20% larger than the source.</font>
<font color=red>// Other libraries/algorithms may have their own quirks.</font>
ACE_Message_Block * compressed = new ACE_Message_Block( message->size() );
<font color=red>// Perform a bogus compression algorithm. 'CD' just tells me</font>
<font color=red>// that this is compressed data and when we "<font color=green>decompress</font>" we'll </font>
<font color=red>// look for this signature to validate the data received.</font>
<font color=#008888>ACE_OS::sprintf</font>( compressed->wr_ptr(), "<font color=green>CD:%s</font>", message->rd_ptr() );
compressed->wr_ptr( strlen(compressed->wr_ptr())+1 );
<font color=red>// Send the compressed data down the stream to the next module</font>
this->put_next( compressed );
<font color=red>// We're done here.</font>
message->release();
return( 0 );
}
<font color=red>/* And here's the decompression side. We've written Xmit/Recv so that
we're guaranteed to get an entire block of compressed data. If
we'd used recv() in the Recv object then we might have gotten a
partial block and that may not decompress very nicely.
*/</font>
int <font color=#008888>Compressor::recv</font>(ACE_Message_Block *message, ACE_Time_Value *timeout)
{
ACE_DEBUG ((LM_INFO, "<font color=green>(%P|%t) <font color=#008888>Compress::recv</font>() decompressing (%s)\n</font>", message->rd_ptr() ));
<font color=red>// Room for the decompressed data. In the real world you</font>
<font color=red>// would probably want to send the original (uncompressed)</font>
<font color=red>// data size in the message. You can predict the maximum</font>
<font color=red>// possible decompression size but it's cheap and easy just to </font>
<font color=red>// send that along. Look again at how I do exacly that</font>
<font color=red>// between Xmit and Recv.</font>
ACE_Message_Block * decompressed = new ACE_Message_Block( message->size() );
<font color=red>// Check for our signature. Even when you use a real</font>
<font color=red>// compression algorithm you may want to include your own</font>
<font color=red>// signature so that you can verify the block. It pays to be</font>
<font color=red>// paranoid!</font>
if( <font color=#008888>ACE_OS::strncmp</font>( message->rd_ptr(), "<font color=green>CD:</font>", 3 ) )
{
ACE_DEBUG ((LM_INFO, "<font color=green>(%P|%t) Improperly encompressed data.\n</font>" ));
message->release();
return(-1);
}
<font color=red>// Skip past the signature before going any further.</font>
message->rd_ptr( 3 );
<font color=red>// Perform a bogus decompression algorithm. This is where you </font>
<font color=red>// would feed to libz or your favorite decompressor. (It's</font>
<font color=red>// costly but you could invoke popen() on gzip!)</font>
<font color=#008888>ACE_OS::sprintf</font>( decompressed->wr_ptr(), "<font color=green>%s</font>", message->rd_ptr() );
decompressed->wr_ptr( strlen(decompressed->wr_ptr())+1 );
<font color=red>// Recv the decompressed data down the stream to the next module</font>
this->put_next( decompressed );
<font color=red>// We're done here.</font>
message->release();
return( 0 );
}
</PRE>
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