Date: Wed, 3 Jul 2013 14:43:56 +1000 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Andrey Chernov <ache@FreeBSD.org> Cc: svn-src-head@FreeBSD.org, svn-src-all@FreeBSD.org, "Pedro F. Giffuni" <pfg@FreeBSD.org>, src-committers@FreeBSD.org, Bruce Evans <brde@optusnet.com.au> Subject: Re: RAND_MAX broken Message-ID: <20130703144126.L1498@besplex.bde.org> In-Reply-To: <51D33552.1030208@freebsd.org> References: <201307012143.r61Lhemi067176@svn.freebsd.org> <20130702130818.V865@besplex.bde.org> <20130702165642.X1571@besplex.bde.org> <51D2F571.8050108@freebsd.org> <20130703020550.E8632@besplex.bde.org> <51D33552.1030208@freebsd.org>
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On Wed, 3 Jul 2013, Andrey Chernov wrote: > On 02.07.2013 20:33, Bruce Evans wrote: >> I checked the values returned by rand(). The ACM part works as >> intended, so it never returns RAND_MAX. It also never returns 0. So >> the distribution of values in the documented range [0, RAND_MAX] is >> very non-uniform. It is uniform in [1, RAND_MAX - 1]. To use this >> algorithm for rand(), 1 should have been subtracted, giving a range >> of [0, 0x7ffffffe]. > > Do you mean [0, 0x7ffffffd] (assuming 1 subtracted)? Yes. > See attached patch. > I don't see compatibility problems at least from POSIX specs point of > view - they don't say something specific about RAND_MAX. I like the patch. It is binary compatibility problems and maybe applications assuming that (RAND_MAX + 1) is 0x80000000 so as to do magic bit operations with it that may be problems. Bruce
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