compression problem on wince - zlib & memorystream
lazarus / 35-Windows CE

----------------------------------------------------------------------
AlerMeta · 2010-05-21 13:50

Hi,

Pls help me.

Code: [Select]uses zstream....

const
  cZIP_BUFFER_SIZE=1024;

var
  mCsomag: TStrings;
  mMSin,mMSout: TMemoryStream;

  mZipstream: TCompressionStream;
  Buf : PByte;
  I,Count,NewCount : Integer;

begin
  mCsomag:= TStringlist.Create;
  mMSin := TMemoryStream.Create;
  mMSout:= TMemoryStream.Create;
  try
    mCsomag.Append('abcdefghijklmnopqsrt');
    mCsomag.SaveToStream(mMSin);
    Buf:=GetMem(cZIP_BUFFER_SIZE);
    Try
      mZipstream := TCompressionStream.Create(clDefault, mMSout,
true);
      try
        Repeat
          Count:=mMSin.Read(Buf^,cZIP_BUFFER_SIZE);
          NewCount:=Count;
          While (NewCount>0) do
            NewCount:=NewCount-mZipstream.Write(Buf^,NewCount);
        Until (Count=0);

        mMSout.SaveToFile(Application.Location+'out.txt');  // WRONG:
0 byte size !!!!!

      finally
        mZipstream.Free;
      end;
    Finally
      FreeMem(Buf);
    end;
  finally
    mMSin.Free;
    mMSout.Free;
    mCsomag.Free;
  end;

This code working on wince, but out.txt size: 0 byte ! Why ?
I use lazarus v0.9.28.2.

----------------------------------------------------------------------
JuhaManninen · 2010-05-21 18:00

> This code working on wince, but out.txt size: 0 byte ! Why ?

I don't know exactly.
  Count:=mMSin.Read(Buf^,cZIP_BUFFER_SIZE);
returns zero for some reason.

The following code works:

Code: [Select]var
  mCsomag: TStrings;
  mMSout: TFileStream;
  mZipstream: TCompressionStream;
  S: String;
begin
  mCsomag:= TStringlist.Create;
  mMSout:= TFileStream.Create(Application.Location+'out.z', fmCreate);
  mZipstream := TCompressionStream.Create(clDefault, mMSout, true);
  try
    mCsomag.Append('abcdefghijklmnopqsrt');
    mCsomag.Append('1234567890');
    S:=mCsomag.Text;
    mZipstream.Write(Pointer(S)^, Length(S));
  finally
    mZipstream.Free;
    mMSout.Free;
    mCsomag.Free;
  end;

Note that 'out.txt' was a misleading name for output file because it
is compressed.
'out.z' is better.


Regards,
Juha

----------------------------------------------------------------------
AlerMeta · 2010-05-21 20:50

Thx for your reply. But my code only a test. Final goal: Tmemorystream
to Tmemorystream

1. create tmemorystream from stringlist, tbitmap (from camera),
database data (from query), etc.
2. compressing in memory (zlib, etc.)
3. create tmemorystream (compressed data)

4. insert to blobfield (sqlite db)
5. scheduled send compressed data over tcp/ip to server (lnet
components)

------------------------------------------
S:=mCsomag.Text;
mZipstream.Write(Pointer(S)^, Length(S));
------------------------------------------
Not only text content. Photos, and other binary data.

----------------------------------------------------------------------
felipemdc · 2010-05-26 15:35

I think you will have to debug this. You can either use the IDE
debugger or put various DebugLn function calls from unit LCLProc to
Print the state of various variables in various points in the program.
Run it in the PDA and in your desktop and check where the variables
are different for both. Takes time but totally doable.

----------------------------------------------------------------------
Laksen · 2010-05-27 01:51

After you call mCsomag.SaveToStream(mMSin); it's pointing at the last
byte written. So when you try to read it again it won't read anything

You need to call mMSin.Seek(0, soFromBeginning); after
mCsomag.SaveToStream(mMSin);

----------------------------------------------------------------------
JuhaManninen · 2010-05-27 04:39

Quote from: Laksen on May 27, 2010, 03:51:59 amAfter you call
mCsomag.SaveToStream(mMSin); it's pointing at the last byte written.
So when you try to read it again it won't read anything

You need to call mMSin.Seek(0, soFromBeginning); after
mCsomag.SaveToStream(mMSin);

Thanks Laksen for finding the error!
Streams can be little tricky to use.

Juha

----------------------------------------------------------------------
felipemdc · 2010-05-27 12:18

Quote from: JuhaManninen on May 27, 2010, 06:39:06 amThanks Laksen for
finding the error!
Streams can be little tricky to use.

If you speak german, I recommend the Lazarus book. There is a good
explanation about streams there.

----------------------------------------------------------------------
AlerMeta · 2010-07-09 10:46

Sorry for long time.

Thanks for answers. This working.