Perl 5 version 8.7 documentation
- srand EXPR
Sets the random number seed for the
If srand() is not called explicitly, it is called implicitly at the first use of the
randoperator. However, this was not the case in versions of Perl before 5.004, so if your script will run under older Perl versions, it should call
Most programs won't even call srand() at all, except those that need a cryptographically-strong starting point rather than the generally acceptable default, which is based on time of day, process ID, and memory allocation, or the /dev/urandom device, if available.
You can call srand($seed) with the same $seed to reproduce the same sequence from rand(), but this is usually reserved for generating predictable results for testing or debugging. Otherwise, don't call srand() more than once in your program.
Do not call srand() (i.e. without an argument) more than once in a script. 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
srandtake 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
In versions of Perl prior to 5.004 the default seed was just the current
time. This isn't a particularly good seed, so many old programs supply their own seed value (often
time ^ $$or
time ^ ($$ + ($$ << 15))), but that isn't necessary any more.
Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:
If you're particularly concerned with this, see the
Math::TrulyRandommodule in CPAN.
Frequently called programs (like CGI scripts) that simply use
- time ^ $$
for a seed can fall prey to the mathematical property that
- a^b == (a+1)^(b+1)
one-third of the time. So don't do that.