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Re: random() function produces wrong range

From: Malcolm Beattie <mbeattie(at)sable(dot)ox(dot)ac(dot)uk>
To: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
Cc: Thomas Swan <tswan(at)olemiss(dot)edu>, Thomas Lockhart <lockhart(at)alumni(dot)caltech(dot)edu>, pgsql-hackers(at)postgresql(dot)org, pgsql-general(at)postgresql(dot)org
Subject: Re: random() function produces wrong range
Date: 2000-08-03 15:27:56
Message-ID: 20000803162756.B715@sable.ox.ac.uk (view raw or flat)
Thread:
Lists: pgsql-generalpgsql-hackers
Tom Lane writes:
> I thought about that last night.  We could do a configure test.  Since
> it'd be probing random() results there'd be a small probability of
> failure, but if we wire in an assumption that the max value must be
> 2^(15 + n*16)-1 for some n, ten or so probes would give us a failure
> probability on the order of 2^-160, which ought to satisfy anyone.
> 
> However, in the absence of any hard evidence that there are platforms
> where the value is different from 2^31-1, it's probably just a waste of
> configuration cycles at the moment.
> 
> I suggest we add a config.h constant like
> 
> /* The local random() function yields values 0 .. MAX_RANDOM_VALUE */
> #define MAX_RANDOM_VALUE  <2^31-1>
> 
> and use that in the code.  Then, if we ever find a platform where
> random() does actually produce 64-bit results, it'll be time enough
> to crank up a real configure test to set the value.
> 
> Comments?

If any platform *does* produced 64-bit results, it won't be compliant
with SUSv2 which states explicitly that the resulting range is up to
2^31-1. Since most portability problems are with older platforms which
haven't caught up, I'd be hopeful that any new 64-bit-int platforms
would get it right from the outset. Maybe I'm being over-optimistic :-)

--Malcolm

-- 
Malcolm Beattie <mbeattie(at)sable(dot)ox(dot)ac(dot)uk>  I am looking for a Linux (and
Unix Systems Programmer                  maybe Apache/mod_perl) job/contract
Oxford University Computing Services   http://users.ox.ac.uk/~mbeattie/cv.html

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