Increment Pointer in Delphi / Create stream from pointer

Question

Hi!

Short form: Is it possible to create a stream from a pointer?

I have a pointer which points to file data I need to read. I used WriteBuffer() to transfer data from the pointer to a TFileStream, which works. But now I want to read it block wise to display a progress bar. So I need to read it step by step.

I have to options:

1) Increment the pointer Buff, and use WriteBuffer() as usual. (WriteBuffer always reads from the beginning, therefore I need to increment the pointer)

2) Create a Stream from the pointer. Is this possible?

Code:

var InputStream : TMemoryStream;
    Buff: Pointer;


    Header: TOCHeader;
begin

// Here I get the pointer
Size := GetOverlay(Buff);

// Transfer data to memory stream
InputStream.WriteBuffer(Buff^, SizeOf(Header));

InputStream.Seek(0, soFromBeginning);

// Read the header
InputStream.ReadBuffer(Header, SizeOf(Header));

// Increment the pointer. Doesn't work :-(. Message Ordinal type required
Inc(Buff, SizeOf(TOC));

Answer 1

You could create your own TCustomMemoryStream descendant, which calls SetPointer() in the constructor, using the address and size that you receive from GetOverlay(). Consult the Delphi documentation of TCustomMemoryStream for further information.

Edit:

Well, this has nothing to do with your question, but in a comment you write that you only wish to read from the running executable file via a stream. That's easy enough to do:

procedure TForm1.Button1Click(Sender: TObject);
var
  Str: TStream;
begin
  Str := TFileStream.Create(ParamStr(0), fmOpenRead or fmShareDenyNone);
  try
    Caption := Format('Size of executable: %u bytes', [Str.Size]);
    // start to read the contents of the file ...

  finally
    Str.Free;
  end;
end;

I will leave the original answer, in case somebody really wants to create a (read-only) stream from a pointer to a chunk of memory.

Answer 2

A question comes up my mind when I read your question. You have a pointer to filedata (from disk I presume?) and you are filling a filestream with data from that pointer (although in your question your filling a memorystream instead). Why not use a TFileStream to open the file you are reading?

var
  FileStream: TFileStream;
  Header: TOCHeader;
begin
  FileStream := TFileStream.Create('c:\fileIWantToRead.txt', fmOpenRead);
  FileStream.ReadBuffer(Header, SizeOf(Header));
  <...>
end;

If you want to increment your pointer, then first cast it to a pointer of a type, so the compiler knows how to increase your pointer.

Inc(PByte(Buff), SizeOf(TOC)); //sizeof returns size in bytes, so cast pointer to a bytepointer

Answer 3

I need the reverse THIS FUNCTION....

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function StrToMemoryStream(SourceString : string) : TMemoryStream; begin Result := TMemoryStream.Create; Result.WriteBuffer(Pointer(SourceString)^, Length(SourceString)); Result.Position := 0; SetLength(SourceString, Result.Size); Result.ReadBuffer(Pointer(SourceString)^, Result.Size); end;

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YOUR HELP ME ????