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srand - seed the random number generator |
srand - seed the random number generator
srand EXPR
srand
Sets the random number seed for the rand operator. If
EXPR is
omitted, uses a semi-random value supplied by the kernel (if it supports
the /dev/urandom device) or based on the current time and process
ID, among other things. 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 ^ $$ or
time ^ ($$ + ($$ << 15))), but that isn't necessary any more.
In fact, it's usually not necessary to call srand at all, because if
it is not called explicitly, it is called implicitly at the first use of
the rand operator. However, this was not the case in version of Perl
before 5.004, so if your script will run under older Perl versions, it
should call srand.
Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:
srand (time ^ $$ ^ 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.
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srand - seed the random number generator |