Sets and returns the random number seed for the rand operator.
The point of the function is to "seed" the rand function so that rand can produce a different sequence each time you run your program. When called with a parameter, srand uses that for the seed; otherwise it (semi-)randomly chooses a seed. In either case, starting with Perl 5.14, it returns the seed. To signal that your code will work only on Perls of a recent vintage:
- use 5.014; # so srand returns the seed
If srand is not called explicitly, it is called
implicitly without a parameter at the first use of the
rand operator. However, there are a few situations
where programs are likely to want to call srand. One
is for generating predictable results, generally for testing or
debugging. There, you use
srand($seed), with the same
time. Another case is that you may want to call srand
after a fork to avoid child processes sharing the same seed
value as the parent (and consequently each other).
Do not call
srand() (i.e., without an argument) more than once per
process. The internal state of the random number generator should
contain more entropy than can be provided by any seed, so calling
srand again actually loses randomness.
Most implementations of srand take an integer and will
truncate decimal numbers. This means
srand(42) will usually
produce the same results as
srand(42.1). To be safe, always pass
srand an integer.
A typical use of the returned seed is for a test program which has too many combinations to test comprehensively in the time available to it each run. It can test a random subset each time, and should there be a failure, log the seed used for that run so that it can later be used to reproduce the same results.
rand is not cryptographically secure. You should not rely on it in security-sensitive situations. As of this writing, a number of third-party CPAN modules offer random number generators intended by their authors to be cryptographically secure, including: Data::Entropy, Crypt::Random, Math::Random::Secure, and Math::TrulyRandom.