Date: Sun, 6 May 2007 14:31:02 +0200 From: Erik Trulsson <ertr1013@student.uu.se> To: Diomidis Spinellis <dds@aueb.gr> Cc: Garance A Drosehn <gad@freebsd.org>, arch@freebsd.org Subject: Re: Accounting changes Message-ID: <20070506123102.GA56344@owl.midgard.homeip.net> In-Reply-To: <463DBF7A.8070200@aueb.gr> References: <19235.1178303887@critter.freebsd.dk> <463BB88F.4020804@aueb.gr> <p06240809c262e5d7ac79@[128.113.24.47]> <463D9A7A.1080800@aueb.gr> <20070506101020.GL825@turion.vk2pj.dyndns.org> <463DBF7A.8070200@aueb.gr>
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On Sun, May 06, 2007 at 02:43:54PM +0300, Diomidis Spinellis wrote: > Peter Jeremy wrote: > >On 2007-May-06 12:06:02 +0300, Diomidis Spinellis <dds@aueb.gr> wrote: > >>Garance A Drosehn wrote: > >>>Does this mean the new accounting record will be using the > >>>native-hardware format for floating point numbers? Does that mean > >>>the records produced will be different for different hardware? > >>My intention is to use the standard (IEEE 754-1985 / IEEE 854-1987 / IEC > >>60559) 32-bit float format. This is the C "float" type on all the > >>architectures we support. I could add a typedef clarifying this, but I > >>doubt we'll ever support an architecture (VAX?) where float is a > >>different format. > > > >IEEE-754 etc define how to interpret a 32-bit object as a floating > >point number. AFAIK, it does not define how that object is laid > >out in memory so that a float written on SPARC (big-endian) will > >be different to that written on an i386 (little-endian). > > IEEE-754 defines the order of bits in a number. The intention is to > allow lexicographical comparison of (valid) floating point numbers, > using the normal byte compare instructions. Not quite. They are defined in such a manner as to allow them to be lexicographically compared just as if they were integers. And just like integers the byte order can vary between different architectures. > If you write a file with a > float on a SPARC you can read it back correctly on an i386. No you can't. I just tested this to be certain. On a SPARC a 32-bit float with the value 3.14 is stored as the bytes (in hexadecimal notation): 40 48 f5 c3 On i386 the order is c3 f5 48 40 -- <Insert your favourite quote here.> Erik Trulsson ertr1013@student.uu.se
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