LZO Compression - How set the destination length?

Question

I'm trying use lzo.dll to compress some file, my code(Delphi) is that:

function lzo2a_999_compress(const Source: Pointer; SourceLength: LongWord; Dest: Pointer; var DestLength: LongWord; WorkMem: Pointer): Integer; cdecl; external 'lzo.dll';
...
function LZO_compress(FileInput, FileOutput: String): Integer;
var
   FInput, FOutput: TMemoryStream;
   WorkMem: Pointer;
   Buffer: TBytes;
   OutputLength: LongWord;
begin
   FInput := TMemoryStream.Create;
   FOutput := TMemoryStream.Create;
   FInput.LoadFromFile(FileInput);
   FInput.Position := 0;
   GetMem(WorkMem, 1000000);
   OutputLength := ??!?!?!;
   SetLength(Buffer, OutputLength);
   try
      lzo2a_999_compress(FInput.Memory, FInput.Size, Buffer, OutputLength, WorkMem);
   finally
      FOutput.CopyFrom(Buffer, Length(Buffer));
   end;
   FOutput.SaveToFile(FileOutput);
   FreeMem(WorkMem, 1000000);
   FInput.Free;
   FOutput.Free;
end;
...

The problem is: how set the "OutputLength"? I can allocate a huge size to prevent problems but the FOutput will be the same size of the Buffer. How can I save only the compressed data on OutputFile? Thanks in advance.

Answer 1

You can not (and need not) know it before the function call. It is a var parameter and will be set by the function at return. You can then use your OutputLength variable to know how many bytes to copy from the buffer:

OutputLength := 0; // initialize only
...
try
  lzo2a_999_compress(FInput.Memory, FInput.Size, Buffer, OutputLength, WorkMem);
finally
  FOutput.CopyFrom(Buffer, OutputLength);