DELPHI内存流问题

[hangyug]

Function Cjt_BmpLoad(ImgBmp:TImage;SourceFile:String):Boolean;
var
Source:TFileStream;
MyFileSize:integer;
begin
Source:=TFileStream.Create(SourceFile,fmOpenRead or fmShareDenyNone);
try
try
Source.Seek(-sizeof(MyFileSize),soFromEnd);
Source.ReadBuffer(MyFileSize,sizeof(MyFileSize));//读出资源
Source.Seek(-MyFileSize,soFromEnd);//定位到资源开始位置
ImgBmp.Picture.Bitmap.LoadFromStream(Source);
finally
Source.Free;
end;
except
Result:=False;
Exit;
end;
Result:=True;
end;
上面-sizeof和-MyFileSize都是是什么意思？为什么前面有个负号？分别和sizeof与Myfilesize有什么区别？请高手指教。。。

[myy]

read help...

[僵哥]

-MyFileSize，即(-1) * MyFileSize