srand
time. This isn't a particularly good seed, so many old programs supply their own seed value (often
time ^ $$ or
time ^ ($$ + )), but that isn't necessary any more.
In fact, it's usually not necessary to call srand at all,
because if it is not called explicitly, it is called implicitly at the
first use of the rand operator. However, this was not the case in version of Perl before 5.004,
so if your script will run under older Perl versions, it should call
srand.
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:
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see the Math::TrulyRandom module in CPAN.
Do not call srand multiple times in your program unless you know
exactly what you're doing and why you're doing it. The point of the
function is to ``seed'' the rand function so that
rand can produce a different sequence each time you run your
program. Just do it once at the top of your program, or you won't get random numbers out of rand!
Frequently called programs (like CGI scripts) that simply use
time ^ $$
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.