Programming Perl, 4th Edition

srand

srand EXPR
srand

This function sets the random number seed for the rand operator. If EXPR is omitted, it uses a semirandom 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 either case, starting with v5.14, it returns the seed. 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 true in versions of Perl before v5.004 (1997), so if your script needs to run under older Perl versions, it should call srand.

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. Use this instead:

srand( time() ^ ($$ + ($$ << 15)) );

You’ll need something much more random than the default seed for cryptographic purposes. On some systems, the /dev/random device is suitable. Otherwise, checksumming the compressed output of one or more rapidly changing operating system status programs is the usual method. For example:

srand (time ^ $$ ^ unpack "%32L*", `ps wwaxl | 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 ...