I've got trouble finding out how to fill and retrieve data for a ftVarBytes field in a TBufDataset.

.AsString doesn't seem to work well - if I fill it with the string How do I fill this field?
I get back
How

I tried SetData, which takes a buffer as data, so I tried filling a PChar with the string data. Retrieving it leads to a big dump of memory, so I wonder how you define the end of the buffer in the SetData call.

Hope somebody can point me to some documentation or tell me the proper way to do this. I've looked at Delphi docs, but that dealt mostly with general BLOB fields, don't know if that includes varBytes.

Below some test code and output:
program varbytesproject;
{$mode objfpc}{$H+}
{$APPTYPE CONSOLE}

uses

  {$IFDEF UNIX}{$IFDEF UseCThreads}
  cthreads,
  {$ENDIF}{$ENDIF}
  Classes, SysUtils,
  { you can add units after this }
  DB, BufDataSet;

var
  TestDataset: TBufDataset;
  FieldDef: TFieldDef;
  TestString: String;
  TempPChar: PChar;
begin
  TestDataset := TBufDataset.Create(nil);
  FieldDef := TestDataset.FieldDefs.AddFieldDef;
  FieldDef.Name := 'ftVarBytes';
  FieldDef.DataType := ftVarBytes;
  FieldDef.Size := 2;
  TestDataset.CreateDataSet;
  TestDataset.Open;
  TestDataset.Append;
  writeln('1. How do I fill an ftVarBytes field?');
  Teststring:='How do I fill this field?';
  writeln('2. Like this?');
  TestDataset.FieldByName('ftVarBytes').Asstring := Teststring;
  writeln('ftVarbytes.AsString: ' +
TestDataset.FieldByName('ftVarBytes').AsString);
  writeln('3. Or like this?');
  TempPChar:=PChar(TestString);
  TestDataSet.FieldByName('ftVarBytes').SetData(TempPChar);
  writeln('ftVarbytes.AsString: ' +
TestDataset.FieldByName('ftVarBytes').AsString);
  TestDataset.Post;
  writeln('4. Get data using this:');
  writeln('ftVarbytes.AsString: ' +
TestDataset.FieldByName('ftVarBytes').AsString);
  writeln('5. Or get data using this:');
  TempPchar:='';
  if TestDataset.FieldByName('ftVarBytes').GetData(TempPChar) then
  begin
    writeln('We filled TempPChar with: ' + TempPChar);
  end
  else
  begin
    writeln('The Getdata function didn''t work.');
  end;
  TestDataset.Close;
  TestDataset.Free;
end.

Output:
1. How do I fill an ftVarBytes field?
2. Like this?
ftVarbytes field contains: How
3. Or like this?
ftVarbytes field contains: How
4. Get data using this:
ftVarbytes field contains: How
5. Or get data using this:
We filled TempPChar with:

!The Getdata function didn't work.

TObject♦
<snip more data from memory>

Does anybody have a suggestion on how to use ftVarbytes fields or aren't they supported in FPC?

In our previous episode, Reinier Olislagers said:

Afaik currently mostly ftblob is used for variable binary storage. I'm not entirely sure what the difference is?

This is the way to do it. However:
- FieldDef.Size := 2; You only store 2 characters.
- TCustomBufDataset.GetFieldSize doesn't recognise TBinaryField descendants and fixes their length arbitrary as 10 bytes... Setting fieldsize to more than 10 bytes won't store more than 10 chars. TMemDataset handles this more elegantly by raising a SErrFieldTypeNotSupported.
So the answer is that TBufDataset doesn' support TBinaryField descendants correctly.

For test purposes, limit field size to 10 bytes ;)

Ludo

Oops. Correct.

Thanks for the answer, I will ;)
(I'll have a go at adding this info to the wiki as well)

Reinier