NetNews Offline 2/NetNews Offline Volume 2.iso/news/comp/std/c/369

Internet Message Format | 1996-08-06 | 2KB

Path: theory.lcs.mit.edu!wald
From: [e-mail] (David Wald)
Newsgroups: comp.std.c
Subject: sizeof(char) ~= sizeof(float)
Date: 24 Feb 1996 18:15:32 GMT
Organization: Theory of Computation, LCS, MIT
Message-ID: <[e-mail]>
NNTP-Posting-Host: woodpecker.lcs.mit.edu

During a recent recurrence of the perennial "how big is an int"
discussion on comp.lang.c.moderated, I described a freestanding C
implementation I'd dealt with in which, as in some other word-based
implementations, sizeof(char) == sizeof(long) == sizeof(float).
However, one quirk of this architecture was that not all
interpretations of a memory word could see all bits.  In particular,
though integral types and floats each took one word, a float used 8
more bits than the integral types.  When a word was used in integral
operations, the lower 8 bits were invisible and harmless.  Thus you
could have two sections of memory which could be distinguished when
viewed through float*'s but not when viewed through char*'s.

Can such an implementation be ANSI-conformant?  The basic argument
against, presented by Tanmoy Bhattacharya, is that memory compare, and
possibly memory copy functions, can't be implemented in a portable
fashion if two dissimilar regions of memory can't be distinguished by
comparing them char-wise.  (I'm unsure about the memory copy, since
I've forgotten whether the implementation actually preserved the
hidden 8 bits when you copied a char.)  Does this violate a constraint
of the standard?  At Tanmoy's suggestion, I'm putting the question
before the collected wisdom comp.std.c.

-David


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David Wald      http://theory.lcs.mit.edu/~wald/     [e-mail]
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