文本文件中的read和readln的简单问题

procedure TForm1.Button4Click(Sender: TObject);
var
F,l:TextFile;
str:string;
begin
try
assignfile(f,'c:\os2.txt');
assignfile(l,'c:\os3.txt');
rewrite(l);
reset(f);
while not eof(f) do
begin
   read(f,str);//为什么这里用read程序就死了,必须得用readln
   write(l,str);
end;
finally
  closefile(f);
  closefile(l);
end;
end;