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srand EXPR

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 (see below). 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 v5.14;	# 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 $seed each 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 silently 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.

If the PERL_RAND_SEED environment variable is set to a non-negative integer during process startup then calls to srand() with no arguments will initialize the perl random number generator with a consistent seed each time it is called, whether called explicitly with no arguments or implicitly via use of rand(). The exact seeding that a given PERL_RAND_SEED will produce is deliberately unspecified, but using different values for PERL_RAND_SEED should produce different results. This is intended for debugging and performance analysis and is only guaranteed to produce consistent results between invocations of the same perl executable running the same code when all other factors are equal. The environment variable is read only once during process startup, and changing it during the program flow will not affect the currently running process. See perlrun for more details.

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.