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 ^ $$ or
time ^ ($$ + ($$ << 15))), but that isn't necessary any more.
In fact, it's usually not necessary to call
srand at all, because if it is not called explicitly, it is called implicitly at the first use of the
rand operator. However, this was not the case in version of Perl before 5.004, so if your script will run under older Perl versions, it should call
Note that you need something much more random than the default seed for cryptographic purposes. Checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:
srand (time ^ $$ ^ 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
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.