Hello.

Is storing and using pointers to temporary strings safe? Like in the following program. I think I remember reading on the mailing list that it is not, but I cannot find it. How can I prove one way or the other? I could not get the program to crash, even if I used 20 variables, it didn't show any wrong result. Valgrind and Dr. Memory didn't show any problems.

program project1;
{$mode objfpc}{$H+}
function GetNew: String;
var
  i: Integer;
begin
  Result := '';
  for i := 0 to 10 do
    Result := Result + string(chr(Random(Ord('z')-Ord('a'))+Ord('a')));
end;
var
  p1,p2,p3: PChar;
begin
  Randomize;
  p1 := PChar(GetNew);
  p2 := PChar(GetNew);
  p3 := PChar(GetNew);
  WriteLn(string(p1));
  WriteLn(string(p2));
  WriteLn(string(p3));
end.

cobines wrote

Is storing and using pointers to temporary strings safe?

No.

Like in the following program. I think I remember reading on the mailing list that it is not, but I cannot find it. How can I prove one way or the other?

The only thing you can prove is finding code that crashes. You can't prove that it's safe, because it's impossible to prove a negative (namely that no code sequence exists that will crash).

I could not get the program to crash, even if I used 20 variables, it didn't show any wrong result. Valgrind and Dr. Memory didn't show any problems.

Nevertheless it's not safe. It depends on management of temporary variables of the compiler, which in itself depends on the performed optimizations and compiler version. Just don't do it.

Jonas

OK. Thanks, Jonas. I'll save a link to this message this time.