in reply to Getting srand's seed

There are two possibilities: it is (as you suggest) easy, or it is impractical. Given that the srand documentation doesn't describe a way to do it I'd guess it is impractical.

A workaround that may be good enough for you is:

my $srandSeed = rand (2**31);

srand ($srandSeed);

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which uses the standard technique for seeding, then reseeds using the first "random" number generated. Note however that Perl's built in rand has many problems that may be an issue depending in your application. For serious work you would be much better using Math::Random::MT or one of the other random number generating modules.

Perl reduces RSI - it saves typing

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Re^2: Getting srand's seed
by fangly (Initiate) on Oct 10, 2008 at 01:55 UTC
    The thing is that trying with my own value is really not an optimal way. Perl should really be able to return the value of the seed it uses.

    I tried using my own value with:
    $seed = int(rand(1e10));
    But then some numbers appeared way more often than you would expect.
    Using that worked much better:
    $seed = int(rand(2**31));
    No idea why...

    But since I'm messing around with creating my own seed, I will probably go with the "recommended" way (in the srand function documentation):
    $seed = time ^ ($$ + ($$ << 15));

    I looked at the Math::Random::* option. It seems like Math::Random has a random_get_seed function, which is really what I want. But do I want to add this module dependency, just to retrieve a seed value??

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      There are a number of really important things that you haven't told us. The most important pertains to the specific build of Perl that you are using because that influences the number of bits that rand uses for its seed. For standard Windows builds of Perl 2**15 values are all there are!

      I suggested $seed = int(rand(2**31)); because most current Perl builds handle 32 bit integers without trouble and that would encompass the majority of seed sizes you are likely to encounter with standard builds of Perl. It also has the large advantage over using something like $seed = time ^ ($$ + ($$ << 15)); that the full entropy used to generate the "system" seed is retained.

      We can't tell what you really want unless you tell us the nature of your application. If all you are doing is generating mazes for net hack then there really isn't an issue.

      Perl reduces RSI - it saves typing
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        My Perl is "perl v5.8.8 built for i486-linux-gnu-thread-multi". The application I am writing does not pertain to security or cryptography, it is for a scientific application. I'd rather keep it as portable as possible.

        I'm not overly worried about the quality of the stock Perl rand() or about exhausting the numbers of random numbers it can produce. I don't think I would need more than 100,000 numbers.

        I originally only wanted access to the seed, but not being able to do so using stock Perl has led me to deal with things I am not very familiar with like how to generate a seed, etc...

        What method would you recommend me to do? I'm unclear about some of the differences.

      • Just use the stock srand() with a home-made seed like you explained earlier
      • Or maybe using the Math::Random module. It seems nice because it has the option to create and report a seed. But is it a higher quality random number generator. Is it slower than the stock one?
      • Or maybe Math::Random::RT? It seems to be higher quality but is a bit slower. But would have to be seeded manually?

        Thanks for the help GrandFather!

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      Perl should really be able to return the value of the seed it uses.

      No. It's a feature that it doesn't return its seed somehow. The random number generator is there to generate random data, to the extend that this is practical on a deterministic machine.

      So it's a feature that the sequence which rand() returns isn't easily reproducible - unless you've taken care to make it so, by first calling srand.

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