Re: [Gm2] Problem with urandom

[Gaius Mulley]

Martin Kalbfuß <address@hidden> writes:

> Hi,
>
> I created the following module to produce random integers. But when I
> call RandInt I always get the same value. Maybe I do something wrong
> about The stream. Opening the file with fopen in C an read a value from
> it produces a random number. Do you see anything wrong here? I tried it
> with stream file, too. 
>
> Thanks
>
> IMPLEMENTATION MODULE Dice;
>
> IMPORT SeqFile,
>        WholeIO,
>        IOChan,
>        EXCEPTIONS,
>        SYSTEM;
>
> VAR urandom : SeqFile.ChanId; 
>     res     : SeqFile.OpenResults;
>     source  : EXCEPTIONS.ExceptionSource;
>
> PROCEDURE RandInt() : INTEGER;
> VAR result : INTEGER;
> BEGIN
>       WholeIO.ReadInt(urandom, result);
>       RETURN result;
> END RandInt;
>
> BEGIN
>       EXCEPTIONS.AllocateSource(source);
>
>       SeqFile.OpenRead(urandom, '/dev/urandom', SeqFile.raw, res);
>       IF urandom = IOChan.InvalidChan() THEN
>                               EXCEPTIONS.RAISE(source, 0, 'Error: Could not 
> access /dev/urandom');
>       END;
> FINALLY
>       SeqFile.Close(urandom);
> END Dice.
>
> The calling module:
> MODULE Kniffel;
>
> IMPORT Dice,
>        SWholeIO,
>        STextIO;
> BEGIN
>       SWholeIO.WriteInt(Dice.RandInt(), 0);
>       STextIO.WriteLn;        
>       SWholeIO.WriteInt(Dice.RandInt(), 0);
>       STextIO.WriteLn;
>       SWholeIO.WriteInt(Dice.RandInt(), 0);
>       STextIO.WriteLn;                
> END Kniffel.
>
> The results:
>
> +134653772
> +134653772
> +134653772
>
> Always the same number. All the time. No matter what I do.

Hi Martin,


I've been wanting to get to the heart of this problem for sometime
as:
   od -x /dev/random

does yield a seemingly random sequence of hex bytes.  I think there is
a bug in the code:


> PROCEDURE RandInt() : INTEGER;
> VAR result : INTEGER;
> BEGIN
>       WholeIO.ReadInt(urandom, result);
>       RETURN result;
> END RandInt;


should be:


PROCEDURE RandInt() : INTEGER;
VAR
   result : INTEGER;
   n      : CARDINAL;
BEGIN
   (* WholeIO.ReadInt(urandom, result); *)
   IOChan.RawRead(urandom, SYSTEM.ADR(result), SIZE(result), n) ;
   IF n#SIZE(result)
   THEN
      EXCEPTIONS.RAISE(source, 0, 'Error: Could not read SIZE(INTEGER)
      number of bytes from /dev/urandom');
   END ;
   RETURN result;
END RandInt;

trying to read the raw bytes as a string is likely to hit a problem.
On my machine it gave me zero each time as the first character was
not in the range '-+012345679'.  However with the modifications above
I now get:

-725512388
+517208660
+900127671
address@hidden:~/GM2/ThirdPartyTests/non-free/martin-kalbfuß$ ./a.out 
+2121694642
+1452980218
-1248358007
address@hidden:~/GM2/ThirdPartyTests/non-free/martin-kalbfuß$ ./a.out 
-531434612
-1484797461
+1139867131
address@hidden:~/GM2/ThirdPartyTests/non-free/martin-kalbfuß$ ./a.out 
-403106276
-129135769
+1581116526
address@hidden:~/GM2/ThirdPartyTests/non-free/martin-kalbfuß$ ./a.out 
-865335780
-1686289691
-526479807
address@hidden:~/GM2/ThirdPartyTests/non-free/martin-kalbfuß$ ./a.out 
 +82033948
-861875516
-215214017
address@hidden:~/GM2/ThirdPartyTests/non-free/martin-kalbfuß$ ./a.out 
-622441135
-853786266
+472446955


regards,
Gaius