Hello!
I need some help again
I'm trying to decompress gzipped files using the qUncompress. As far I know, the difference between gzip-created file and the qCompressed one consists in:
1. Different headers. On my testing example (the same file that was compressed with gzip and qCompress) the header of qCompressed file = 10 bytes. But according to the source and docs, it must be only 4 bytes with the data length value. I'm confused.
2. Different tails. I know what gzip writes the 8-bytes trail (CRC32 and ISIZE). But what is 4 bytes of qCompressed trail?
Here is my testing code, that produces "Z_DATA_ERROR: Input data is corrupted" error:
Qt Code: 
  1. //.gz file header
  2. typedef struct {
  3.         quint8 id1;
  4.         quint8 id2;
  5.         quint8 cm;
  6.         quint8 flg;
  7.         float mtime;
  8.         quint8 xfl;
  9.         quint8 os;
  10.        } t_gzip_header;
  11.  
  12. //bit flags
  13. #define gzip_hdr_FTEXT 0
  14. #define gzip_hdr_FHCRC 1
  15. #define gzip_hdr_FEXTRA 2
  16. #define gzip_hdr_FNAME 3
  17. #define gzip_hdr_FCOMMENT 4
  18.  
  19.  
  20. void rvln::test()
  21. {
  22.   QFile file ("/home/test/test.cpp.gz");
  23.  
  24.   if (! file.open (QFile::ReadOnly))
  25.      return;
  26.  
  27.   t_gzip_header gzip_header;
  28.  
  29.   QDataStream s_in (&file);
  30.  
  31. //read the header to the struct instance:
  32.   s_in >> gzip_header.id1;
  33.   s_in >> gzip_header.id2;
  34.   s_in >> gzip_header.cm;
  35.   s_in >> gzip_header.flg;
  36.   s_in >> gzip_header.mtime;
  37.   s_in >> gzip_header.xfl;
  38.   s_in >> gzip_header.os;
  39.  
  40. //read some additional chunks if flags are set
  41.  
  42.   if (gzip_header.flg & (1 << gzip_hdr_FEXTRA))
  43.      {
  44.       quint16 size;
  45.       s_in >> size;
  46.       char *data = new char [size];
  47.       s_in.readRawData (data,size);
  48.       delete data;
  49.      }
  50.  
  51.   if (gzip_header.flg & (1 << gzip_hdr_FNAME))
  52.      {
  53.       qint8 c;
  54.       QString original_fname;
  55.       do
  56.         {
  57.          s_in >> c;
  58.          original_fname += static_cast<char> (c);
  59.         }
  60.        while (c != 0);
  61.  
  62.       qDebug() << original_fname;
  63.     }
  64.  
  65.  
  66.     if (gzip_header.flg & (1 << gzip_hdr_FCOMMENT))
  67.       {
  68.        qint8 c;
  69.        QString comment;
  70.        do
  71.          {
  72.           s_in >> c;
  73.           comment += static_cast<char> (c);
  74.          }
  75.        while (c != 0);
  76.  
  77.        qDebug() << comment;
  78.       }
  79.  
  80.     if (gzip_header.flg & (1 << gzip_hdr_FHCRC))
  81.        {
  82.         quint16 crc16;
  83.        s_in >> crc16;
  84.        }
  85.  
  86. //here is the stream reaches to the compressed data
  87. //so I read this data
  88. //and gzip's 8-bytes trail
  89. //to the buffer compressed_data
  90.  
  91.   int len = file.size() - sizeof (t_gzip_header);
  92.   char *compressed_data = new char [len];
  93.   s_in.readRawData (compressed_data, len);
  94.  
  95. //the temporary array for compressed data
  96. //plus the 4-bytes header with the 
  97. //uncompressed data length,
  98. //but how I can know the uncompressed data length
  99. //before uncompression?
  100.  
  102.  
  103. //trying to set the 4-bytes header
  104. //with the approx. uncompressed data length
  105.  
  106.   len *= 10;
  107.  
  108.   ta.resize(4);
  109.  
  110.   ta[0] = (len & 0xff000000) >> 24;
  111.   ta[1] = (len & 0x00ff0000) >> 16;
  112.   ta[2] = (len & 0x0000ff00) >> 8;
  113.   ta[3] = (len & 0x000000ff);
  114.  
  115. //add the compressed data to temp array
  116.  
  117.   ta.append (compressed_data);
  118.  
  119. //remove the 8-byte gzip trail
  120.  
  121.   ta.chop(8);
  122.  
  123. //trying to uncompress
  124.  
  125.   QByteArray aucomp = qUncompress (ta);
  126.  
  127. //oops! Got "Z_DATA_ERROR: Input data is corrupted"
  128. //here
  129.  
  130. //and close the file in a deep sorrow
  131.   file.close();
  132. }
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P.S. all gzip-format related data is taken from http://www.gzip.org/zlib/rfc-gzip.html