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Next: stat FILEHANDLE
Sets the random number seed for the rand operator. If
EXPR is omitted, uses a semi-random value 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 ^ $$ or
time ^ ($$ + ($$ << 15))), but that isn't necessary any more.
In fact, it's usually not necessary to call
srand() at all,
because if it is not called explicitly, it is called implicitly at the
first use of the rand operator. However, this was not the case in version of Perl before 5.004,
so if your script will run under older Perl versions, it should call
Note that you need something much more random than the default seed for
cryptographic purposes. Checksumming the compressed output of one or more
rapidly changing operating system status programs is the usual method. For
srand (time ^ $$ ^ unpack "%L*", `ps axww | gzip`);
If you're particularly concerned with this, see the Math::TrulyRandom module in
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.
Source: Perl builtin functions
Copyright: Larry Wall, et al.
Previous: sqrt EXPR
(Corrections, notes, and links courtesy of RocketAware.com)
Up to: Random Numbers
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