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POZlib uncompress returns Z_DATA_ERROR
 

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  1. POZlib uncompress returns Z_DATA_ERROR
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    <p>I am working on a client server application, where client compresses a 2MB data sends to the server, server receives the data uncompresses it and writes it to a file. </p> <p>For some packets uncompression was failing and I added MD5 sum to both client side and server side code and also debugged using uncompression at the client side after compressing the data. The same parameters that passes for uncompress function in client side is failing with Z_DATA_ERROR in the server side. The data's MD5sum seems same. Am totally clueless what I could do next. </p> <p>Server Side cod looks like this:</p> <pre><code>int ret = uncompress((Bytef*)unCompressedBuffer, &amp;dwUncompressedBytes, (const Bytef*) receivedBuffer+525, dwBlockLength); if (ret == Z_OK) { } else { std::cout &lt;&lt; " Uncompression failed for Block: " &lt;&lt; iBlock &lt;&lt; std::endl; std::cout &lt;&lt; " PacketType: 4" &lt;&lt; " Block Number:" &lt;&lt; iBlock &lt;&lt; " Length:" &lt;&lt; dwBlockLength &lt;&lt; "Error:" &lt;&lt; ret &lt;&lt; std::endl; PrintMD5SumResult((PBYTE)receivedBuffer+525, compressedSize-525); std::cout &lt;&lt; " Uncompressed MD5 Checksum:0"; PrintMD5SumResult((PBYTE)unCompressedBuffer, dwUncompressedBytes); } } </code></pre> <p>Client Code Looks like this:</p> <pre><code>int ret = compress2(l_pCompressData + 4, &amp;destLen, (const Bytef*) pBlockData, dwBlockSize, 6); memcpy(m_pWriteBuffer+525, l_pCompressData, destLen); m_dwWriteBytes = destLen+525; std::cout &lt;&lt; " \n Compressed MD5 Sum:0"; PrintMD5SumResult(m_pWriteBuffer, m_dwWriteBytes); PrintMD5SumResult(m_pWriteBuffer+525, m_dwWriteBytes-525); int ret = uncompress(m_pUnCompressData, &amp;uncomLen, (const Bytef*)m_pWriteBuffer+525, destLen); if(ret != Z_OK) { std::cout &lt;&lt; " Uncompression has failed." &lt;&lt; std::endl; } else { //std::cout &lt;&lt; " UnCompressed MD5 Sum:0"; //PrintMD5SumResult((PBYTE)m_pUnCompressData, md5Output, dwBlockSize); } // Write the 2MB to the network WriteDataOverNetwork(m_NetworkStream, m_pWriteBuffer, m_dwWriteBytes, &amp;dwNumBytes, TRUE); </code></pre> <p>I narrowed down the problem to the following piece of code in zlib - but have a hard time understanding it. In the inflate() function, (ZSWAP32(hold)) != state->check) this statement is failing. Can someone help me out here? MD5sum used here is from Boton C++ library. </p> <pre><code> case CHECK: if (state-&gt;wrap) { NEEDBITS(32); out -= left; strm-&gt;total_out += out; state-&gt;total += out; if (out) strm-&gt;adler = state-&gt;check = UPDATE(state-&gt;check, put - out, out); out = left; if (( #ifdef GUNZIP state-&gt;flags ? hold : #endif ZSWAP32(hold)) != state-&gt;check) { strm-&gt;msg = (char *)"incorrect data check"; state-&gt;mode = BAD; break; } </code></pre>
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    1. USSivaram Kannan
    1. USSivaram Kannan
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      1. POZlib uncompress returns Z_DATA_ERROR
      1. USJustinB
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    1. COThe MD5 "seems" the same? Are the MD5's of the compressed data on the two sides exactly the same or not? In the client code you do the MD5 on m_dwWriteBytes-525 bytes, but uncompress destLen bytes. It is not clear how m_dwWriteBytes is updated with the destLen result from compress().
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      1. POZlib uncompress returns Z_DATA_ERROR
      1. USMark Adler
    2. COSorry for the confusion. Buffer+525 is actually the start of the compressed data. MD5(Buffer) was just to make sure the whole data is received properly. As for the uncompress destLen at the client side, it is the output of compress2 function earlier. In the server side, I receive it in the packet at an offset and read it to dwBlockLength. I stepped in the code, all the values to the functions at server and client seems to be same.
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      1. POZlib uncompress returns Z_DATA_ERROR
      1. USSivaram Kannan
    3. COWhat is the value of m_dwWriteBytes? To get the MD5 of what compress() returned, m_dwWriteBytes would need to be equal to destLen + 525.
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      1. POZlib uncompress returns Z_DATA_ERROR
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