Hello, I stumble upon a problem, and can't find a solution. So what I want to do is uncompress data in qt, using qUncompress(QByteArray), send from www in gzip format. I used wireshark to determine that this is valid gzip stream, also tested with zip/rar and both can uncompress it. Code so far, is like this: static const char dat[40] = { 0x1f, 0x8b, 0x08, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x03, 0xaa, 0x2e, 0x2e, 0x49, 0x2c, 0x29, 0x2d, 0xb6, 0x4a, 0x4b, 0xcc, 0x29, 0x4e, 0xad, 0x05, 0x00, 0x00, 0x00, 0xff, 0xff, 0x03, 0x00, 0x2a, 0x63, 0x18, 0xc5, 0x0e, 0x00, 0x00, 0x00 }; //this data contains string: {status:false}, in gzip format QByteArray data; data.append( dat, sizeof(dat) ); unsigned int size = 14; //expected uncompresed size, reconstruct it BigEndianes //prepand expected uncompressed size, last 4 byte in dat 0x0e = 14 QByteArray dataPlusSize; dataPlusSize.append( (unsigned int)((size >> 24) & 0xFF)); dataPlusSize.append( (unsigned int)((size >> 16) & 0xFF)); dataPlusSize.append( (unsigned int)((size >> 8) & 0xFF)); dataPlusSize.append( (unsigned int)((size >> 0) & 0xFF)); QByteArray uncomp = qUncompress( dataPlusSize ); qDebug() << uncomp; And uncompression fails with: qUncompress: Z_DATA_ERROR: Input data is corrupted. AFAIK gzip consist of 10 byte header, DEFLATE peyload, 12 byte trailer ( 8 byte CRC32 + 4 byte ISIZE - uncompresed data size ). Striping header and trailer should leave me with DEFLATE data stream, qUncompress yields same error. I checked with data string compressed in PHP, like this: $stringData = gzcompress( "{status:false}", 1); and qUncompress uncompress that data.(I didn't see and gzip header though i.e. ID1 = 0x1f, ID2 = 0x8b ) I checked above code with debug, and error occurs at: if ( #endif ((BITS(8) << 8) + (hold >> 8)) % 31) { //here is error, WHY? long unsigned int hold = 35615 strm->msg = (char *)"incorrect header check"; state->mode = BAD; break; } inflate.c line 610. I know that qUncompress is simply a wrapper to zlib, so I suppose it should handle gzip without any problem. Any comments are more then welcome. Best regards