Delphi Prism: Reading a string from binary file returns with wierd characters

Question

I am writing a string into a file and reading it back out using binarywriter and binaryreader. When the string is read back out, it looks all funny and very long. I don't know why it is doing that.

Here is how I write into a file with binarywriter:

TFileHeader = Record
        ID:String;
        version:SmallInt;
end;

method WriteGroups(fname:string; cg:ArrayList);
var
  strm : BinaryWriter;
  i:integer;
  cnt:Integer;
  GroupHeader:TFileHeader;
begin    
  GroupHeader.ID:='GroupFile'; <<<-----I am having problem with this string.
  GroupHeader.version:=6;

  if Environment.OSVersion.Platform = System.PlatformID.Unix then
     fname := baseDir +'/calgroup.dat'
  else
     fname := baseDir +'\calgroup.dat';

  if cg.Count > 0 then
  begin
    strm := new BinaryWriter(File.Create(fname));

    cnt := cg.Count;
    strm.Write(GroupHeader.version);
    strm.Write(GroupHeader.ID);
    strm.Write(cnt);

    for i := 0 to cnt - 1 do
    begin
      TCalGroup(cg[i]).WriteMGroup(strm);
    end;
    strm.Close;
  end;
end;

Here is how I read from a file using BinaryReader:

method ReadGroups(fname:string; cg:ArrayList);
var
  strm : BinaryReader;
  GroupHeader:TFileHeader;
  cnt:SmallInt;
  i:integer;
  cgp:TMagiKalCalGroup;
begin
  GroupHeader.ID :='';
  GroupHeader.version := 0;

  if Environment.OSVersion.Platform = System.PlatformID.Unix then
     fname := baseDir +'/calgroup.dat'
  else
     fname := baseDir +'\calgroup.dat';

  if File.Exists(fname) then
  begin
    ClearGroups(cg);
    strm := new BinaryReader(file.OpenRead(fname));

    GroupHeader.version:=strm.ReadInt32;
    GroupHeader.ID := strm.ReadString; <-----Here is the problem. See the image below..

    if ((GroupHeader.ID='') or (GroupHeader.version>100)) then
    begin
        strm.Close;
        Exit;
    end;

    if (GroupHeader.version<5) then
    begin
      strm.Close;
      exit;
    end;

    cnt := strm.ReadInt16;

    for i := 0 to cnt - 1 do
    begin
      reformat:=false;
      cgp := new TMagiKalCalGroup('New Grp');
      cgp.ReadMGroup(Strm);
      cgp.UpdateDateTime(System.DateTime.Now);
      cg.Add(cgp);
    end;
    //strm.Free;
    strm.Close;
  end;
end;

Here what I see when I debug the code:

As you can see or not see, "GroupHeader.ID" should only contain "Groupfile" not long string with garbage in it.

So, what am I doing wrong? Is this a string format error?

Answer 1

Smallint is a 16-bit value. When reading the file, you are reading that value as a 32-bit value instead of a 16-bit value, so you end up reading some of the bytes that belong to the length of the stored string. When you then read the string, the first 2 bytes of the string characters get interpreted as part of the string length, which is why you end up with garbage.

You have a similar logic bug when reading the groups. Integer is a 32-bit value. When reading the group count, you are reading it as a 16-bit value instead, which means your group reads are going to be off by 2 bytes and will be corrupted as well.

You need to change these lines inside of your ReadGroups() function:

cnt: Smallint;
...
GroupHeader.version:=strm.ReadInt32;
...
cnt := strm.ReadInt16;

To these instead:

cnt: Integer;
...
GroupHeader.version := strm.ReadInt16;
...
cnt := strm.ReadInt32;