Re: A one's complement sanity check, please

Dan Henry wrote:

I need a sanity check. The following is an exchange on
comp.arch.embedded with CBFalconer in a rather long MISRA related
thread. Since my little section of that thread and area of interest
was never really about MISRA, but rather the one's complement
representation of integer constant 0 and now how to zero memory
portably, I am bringing the topic to this group. Try to ignore CBF's
errors in the 1's complement -1 and sign magnitude -1 representations.
My questions are about 1's complement representation of the integer
constant 0. The exchange:

[exchange]
Dan Henry wrote:
CBFalconer <cbfalconer@xxxxxxxxx> wrote:
Dan Henry wrote:
CBFalconer <cbfalconer@xxxxxxxxx> wrote:
Dan Henry wrote:
CBFalconer <cbfalconer@xxxxxxxxx> wrote:

... snip ...

Wrong. C representation may be sign/magnitude or 1's complement.

2's comp: -1 ---> -0x0001 ---> 0xffff
~ 1 ---> ~0x0001 ---> 0xffff (note NOT sign)
~ 0 ---> ~0x0000 ---> 0xffff "
1's comp: -1 ---> -0x0001 ---> 0xffff
~ 1 ---> ~0x0001 ---> 0xfffe (note NOT sign)
~ 0 ---> ~0x0000 ---> 0xffff "
or ~ 0 ---> ~0xffff ---> 0x0000 "
sign mag: -1 ---> -0x0001 ---> 0xffff
~ 1 ---> ~0x0001 ---> 0xfffe (note NOT sign)
~ 0 ---> ~0x0000 ---> 0xffff "

because of the rules for bringing unsigned into range.

~1? I read Marcin to be asking whether ~0 won't set all 1s.

See the additions above. Only -1 always returns 0xffff.

Isn't the integer constant 0 (lacking the unary - operator) in a
1's complement system positive zero, not negative zero? Doesn't
your second ~0 example actually represent ~-0?

In 1's complement -0 is equal to +0.

Right, but I am wondering about the predictability of the
underlying bit patterns.

Are you saying that in 1's complement the integer constant 0 could
have the bit pattern for +0 (all 0's) *or* the bit pattern for -0
(all 1's) and that using the unary - operator on the integer
constant 0 can yield either all 1's or all 0's -- that my
memset(&object, 0, sizeof object) is equally likely to zero all
bits as set all bits of the memory that object occupies?

Exactly.
[/exchange]

The implied conclusion is that memset(&object, 0, sizeof object)
may not zero a chunk of memory on a one's complement system. I'm
having an extremely difficult time accepting that. I don't
interpret 6.2.6.2p2 to allow for (normal) 0 to have any nonzero
bits in its one's complement representation. I have never seen
purportedly portable code that does anything special with memset's
second argument to zero a chunk of memory. When using integer
constant 0 as the second argument to memset, I don't see how any
of the operators listed in 6.2.6.2p3 could possibly come into play
to have changed normal 0 into negative 0. If integer constant 0
does in fact yield a nonzero bit pattern, then the one portable
way I can see to pass an all-zeroes bit pattern as memset's second
argument is:

From N869:

7.21.6.1 The memset function

Synopsis

[#1]

#include <string.h>
void *memset(void *s, int c, size_t n);

Description

[#2] The memset function copies the value of c (converted to
an unsigned char) into each of the first n characters of the
object pointed to by s.

Returns

[#3] The memset function returns the value of s.


#define ZERO (0^0)
memset(&object, ZERO, sizeof object);

But I don't see that in purportedly portable code. Or could my (0^0)
also be flawed as the above quoted exchange implies, because the first
0 in ZERO's expansion could be represented by all zeroes and the
second 0 in the expansion could be represented by all ones (or vice
versa). Wouldn't the translator stick to one representation or the
other and not flip willy-nilly? Or is it that the XOR operator being
in 6.2.6.2p3 means that (0^0) could yield negative zero?

Apart from the above I am going to stay out of this on this group.
At least for now. :-)

--
<http://www.cs.auckland.ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfocus.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
<http://kadaitcha.cx/vista/dogsbreakfast/index.html>
cbfalconer at maineline dot net


--
Posted via a free Usenet account from http://www.teranews.com

.

Relevant Pages

  • A ones complement sanity check, please
    ... representation of integer constant 0 and now how to zero memory ... errors in the 1's complement -1 and sign magnitude -1 representations. ... My questions are about 1's complement representation of the integer ...
    (comp.lang.c)
  • Re: shame on MISRA
    ... 1's complement system positive zero, ... In 1's complement -0 is equal to +0. ... Are you saying that in 1's complement the integer constant 0 could ... memset(&object, 0, sizeof object) is equally likely to zero all ...
    (comp.arch.embedded)
  • Re: shame on MISRA
    ... 1's complement system positive zero, ... In 1's complement -0 is equal to +0. ... Are you saying that in 1's complement the integer constant 0 could ... memory that object occupies? ...
    (comp.arch.embedded)
  • Re: Working with negative numbers
    ... One's complement: -128 through +127 ... Yes, except that implementations ... zero in particular!. ... an integer object with the representation for negative zero, ...
    (comp.lang.c)
  • Re: Why the Range of Int is less by one at positive side?.........
    ... i think this is coorect because of 2 zeros in 1's compliment. ... taking 2's compliment for representation of -ve numbers and that consumes ... > The C language permits a 1's complement or sign-magnitude representation ... > 2 separate representations for the number zero. ...
    (comp.lang.c)