From: "Russ Cox" <rsc@swtch.com>
To: 9fans@cse.psu.edu
Subject: Re: [9fans] fun and scary evil C code
Date: Thu, 20 Dec 2007 11:29:55 -0500 [thread overview]
Message-ID: <20071220162924.124251E8C1C@holo.morphisms.net> (raw)
> Does C99 or any other C mandate the actual memory layout
> of floats and doubles or the exact conversion of constant
> representations?
> I'm fairly sure they somehow mandate IEEE 754 properties,
> but do they actually say that floats and doubles have to be stored
> exactly that way in 4 or 8 bytes?
>
> Even if we assume sizeof(double) == 8,
> what if my implementation is perverse and interleaves the exponent
> bits amongst the mantissa bits?
> Where is this disallowed in the standard(s)?
It doesn't matter what the standard says; it matters what implementations do.
C implementations are going to provide what the underlying
hardware does, and almost all hardware does IEEE 754.
> as long as the conversion you're after has an exact ieee representation,
> i can't see how two compliant implementations could come up with
> differing representations. (two different numbers can't have the same
> ieee representation, except -0 and +0.) the conversion process doesn't
> need any floating point itself and the only interpolation comes when
> numbers don't have exact representations.
Historically, it has not always been true that decimal <-> binary
conversion of doubles has been precise enough to replicate a
specific bit pattern. While conversion doesn't require floating
point, it is often done in floating point anyway for convenience.
Russ
next reply other threads:[~2007-12-20 16:29 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2007-12-20 16:29 Russ Cox [this message]
-- strict thread matches above, loose matches on Subject: below --
2007-12-19 15:15 David Leimbach
2007-12-19 20:37 ` Russ Cox
2007-12-19 22:32 ` Robert William Fuller
2007-12-19 22:38 ` erik quanstrom
2007-12-19 22:51 ` Bakul Shah
2007-12-20 0:18 ` dave.l
2007-12-20 0:33 ` dave.l
2007-12-20 0:51 ` erik quanstrom
2007-12-20 9:29 ` roger peppe
2007-12-20 13:02 ` erik quanstrom
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