Author: James CE Johnson

ACE Tutorial 015
Building a protocol stream

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.

    // page19.html,v 1.11 1999/04/03 21:56:31 jcej Exp

    #include "Compressor.h"
    #include "ace/SOCK_Stream.h"

    Compressor::Compressor( void )
            : Protocol_Task()
    {
        ;
    }

    Compressor::~Compressor(void)
    {
        ;
    }

    /* 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
     */
    int Compressor::send(ACE_Message_Block *message, ACE_Time_Value *timeout)
    {
        ACE_DEBUG ((LM_INFO, "(%P|%t) Compressor::send() compressing (%s)\n", message->rd_ptr() ));

            // Create a block to hold the compressed data.  I belive libz
            // recommends a buffer about 10-20% larger than the source.
            // Other libraries/algorithms may have their own quirks.
        ACE_Message_Block * compressed = new ACE_Message_Block( message->size() );

            // Perform a bogus compression algorithm.  'CD' just tells me
            // that this is compressed data and when we "decompress" we'll
            // look for this signature to validate the data received.
        ACE_OS::sprintf( compressed->wr_ptr(), "CD:%s", message->rd_ptr() );
        compressed->wr_ptr( strlen(compressed->wr_ptr())+1 );

            // Send the compressed data down the stream to the next module
        this->put_next( compressed );

            // We're done here.
        message->release();

        return( 0 );
    }

    /* 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.
     */
    int Compressor::recv(ACE_Message_Block *message, ACE_Time_Value *timeout)
    {
        ACE_DEBUG ((LM_INFO, "(%P|%t) Compress::recv() decompressing (%s)\n", message->rd_ptr() ));

            // Room for the decompressed data.  In the real world you
            // would probably want to send the original (uncompressed)
            // data size in the message.  You can predict the maximum
            // possible decompression size but it's cheap and easy just to
            // send that along.  Look again at how I do exacly that
            // between Xmit and Recv.
        ACE_Message_Block * decompressed = new ACE_Message_Block( message->size() );

            // Check for our signature.  Even when you use a real
            // compression algorithm you may want to include your own
            // signature so that you can verify the block.  It pays to be
            // paranoid!
        if( ACE_OS::strncmp( message->rd_ptr(), "CD:", 3  ) )
        {
            ACE_DEBUG ((LM_INFO, "(%P|%t) Improperly encompressed data.\n" ));
            message->release();
            return(-1);
        }

            // Skip past the signature before going any further.
        message->rd_ptr( 3 );

            // Perform a bogus decompression algorithm.  This is where you
            // would feed to libz or your favorite decompressor.  (It's
            // costly but you could invoke popen() on gzip!)
        ACE_OS::sprintf( decompressed->wr_ptr(), "%s", message->rd_ptr() );
        decompressed->wr_ptr( strlen(decompressed->wr_ptr())+1 );

            // Recv the decompressed data down the stream to the next module
        this->put_next( decompressed );

            // We're done here.
        message->release();

        return( 0 );
    }

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