[delphizip] Re: Method ExtractStreamToStream
- From: "Roger Aelbrecht" <Aelbrecr@xxxxxxxxxx>
- To: <delphizip@xxxxxxxxxxxxx>
- Date: Wed, 23 Feb 2005 20:14:07 +0100
Roger Aelbrecht wrote:
> Werner Kok wrote:
> > Hi,
> >
> > Please look at what I'm doing below and help my frustrated soul, I
> > will be forever gratefull!
> >
> >
> > procedure TForm1.Button1Click(Sender: TObject);
> > var FileData: TMemoryStream;
> > MyData: TMemoryStream;
> > MyOutData: TZipStream;
> > Buf1: Array[1..6] of Byte;
> >
> > begin
> >
> > FileData := TMemoryStream.Create;
> > FileData.Position := 0;
> > FileData.LoadFromFile('c:\Example.zip');
> >
> > MyData := TMemoryStream.Create;
> > MyData.Position := 0;
> > Buf1[1] := 0;
> > Buf1[2] := 0;
> > Buf1[3] := 9;
> > Buf1[4] := 9;
> > Buf1[5] := 9;
> > Buf1[6] := 9;
> > MyData.Write(Buf1,6);
> > MyData.Position := 6;
> > MyData.CopyFrom(FileData,0);
> > MyData.Position := 0;
> >
> > MyOutData := ZipMaster1.ExtractStreamToStream(MyData, 32768);
> > MyOutData.Position := 0;
> > MyOutData.SaveToFile('c:\Extracted.txt');
> >
> > end;
> >
> >
> >
> > Why oh why is the file 'c:\Extracted.txt' still the compressed zip
> > file?
> >
> >
> >
> >
> > Thank you so much!,
> >
> > Werner Kok
> >
> >
>
>
> As far as I can see in your code the extra bytes you entered are not
> correct.
> You have set it as 'stored' so no decompression is done, because the
> UnzDLL thinks the data in the stream was not compressed.
>
> From the help file :
> <quote>
>
> The first 6 bytes of the input stream are reserved:
>
> - The first two bytes are used as a flag, STORED = 0 or DEFLATED = 8.
>
> Type
> pFlag = ^Smallint;
> pCRC = ^Cardinal;
> Var
> Flag: SmallInt;
> CRC: Cardinal;
>
> Flag := 8;
> (pFlag( ZipMaster1.ZipStream.Memory ))^ := Flag;
>
> </quote>
>
> So for a compressed file the flag should be set to 8.
>
> Then there is the problem of the CRC.
> the last 4 bytes are the calculated CRC, if you do not have this
> value you will receive CRC errors, of course you can ignore them.
> To ignore the CRC errors create an OnCRCError event handler
> and set the DoExtract parameter to true.
>
> HTH
>
> --
> Roger Aelbrecht
> http://home.tiscali.be/driehoeksw
I forgot to mention in my previous post that the statement
MyOutData := ZipMaster1.ExtractStreamToStream(MyData, 32768);
is also dangerous. If your Mydata stream is larger than 32768
there could be a problem.
It is better to use
MyOutData := ZipMaster1.ExtractStreamToStream(MyData, MyData.Size);
--
Roger Aelbrecht
http://home.tiscali.be/driehoeksw
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