Date: Sun, 22 May 2016 13:10:09 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Andrey Chernov <ache@freebsd.org> Cc: Bruce Evans <brde@optusnet.com.au>, Pedro Giffuni <pfg@freebsd.org>, cem@freebsd.org, src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org Subject: Re: svn commit: r300377 - head/sys/compat/ndis Message-ID: <20160522124923.I1697@besplex.bde.org> In-Reply-To: <8bd5428f-6955-72c3-0294-72f7ef936e16@freebsd.org> References: <201605211752.u4LHqiHQ031457@repo.freebsd.org> <CAG6CVpXjU3tHdar7d=xyr%2BTmffg0NrQu3q7SD=b6%2BjF=yvVr-Q@mail.gmail.com> <a88c14ea-ee78-54de-6142-08a561a49d98@FreeBSD.org> <CAG6CVpV_3%2B%2BWqg2X23=RM942zaDkyL6fxH2YN0TXUqpPjneCOw@mail.gmail.com> <262938a6-50bd-b6f4-24c9-895b837a368e@FreeBSD.org> <20160522091455.A1014@besplex.bde.org> <8bd5428f-6955-72c3-0294-72f7ef936e16@freebsd.org>
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On Sun, 22 May 2016, Andrey Chernov wrote: > On 22.05.2016 3:06, Bruce Evans wrote: >> ... >> FreeBSD still has the ACM version in libkern, and has a fixed copy >> of that in libc, with the bad old version under an ifdef. >> >> The libc version now adjusts the range from [0, 0x7fffffff] to >> 0, 0x7ffffffd] and reduces RAND_MAX by 2 to match. The claimed uniformity >> for the larger range is very wrong, since the ACM algorithm can only >> produce numbers in the range [1(or is it 0?), 0x7ffffffe] starting from a >> seed in the range [1, 0x7ffffffe(or is it 1 higher?)]. There are problems >> at both extremities, and it isn't clear if the new or old adjustments to >> avoid them preserve uniformity. It is clear that the range was at least >> 1 too high, since the ACM algorithm does a modulo by 0x7fffffff. > > libc version does range adjustment for better uniformity only for > rand(3), not for random(3). There is no RAND_MAX constant in the > random(3) API. POSIX require that random(3) should stay in [0, 2^31-1]. The one in the kernel is random(9) being used to implement rand(9). This doesn't have to conform to POSIX or be very good. random(9) is actually really rand(9) under a different name. It uses a LCG with a small period, but random(3) is specified to be the BSD one, with wording too similar to that in the FreeBSD man page -- it must use a nonlinear method with a default period of 16 times larger than 2**31-1, and must give a period larger than 2**69 with a state table size of 256. The default should have been changed to much more than 16 times larger long before POSIX standardized the 1980's default. Bruce
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