Re: !!, what is it?

Tim Woodall wrote: 
On Mon, 12 Sep 2005 09:36:01 +0200,
    Michael Mair <Michael.Mair@xxxxxxxxxxxxxxx> wrote:

As an aside:
I recall seeing some platform specific headers which went for the
all-bits-one representation of "true" -- but the C implementations
gave 1 for !!TRUE as well...


int is_it_seven(int x)
{
    return x==7;
}

if(is_it_seven(7) == TRUE)
    printf("7 is seven\n");
else
    printf("7 is not seven\n");

While I would never write the explicit test for TRUE [1], I would be
horrified at any header that defined TRUE such that this code didn't
behave as expected.

[1] Other peoples coding standards excepted. 

Sorry, without C99's _Bool and _True, this argument is bogus.
Apart from the possible range of return values one could expect from
is_it_seven() and the possible mismatch with, say isalnum() == TRUE
or strcmp() != TRUE, there is no benefit in that.
IIRC, this "TRUE" was intended for bitwise operations and conveniently
fulfilled !TRUE == FALSE. However, this would not have worked on a
1s complement platform.

One other explanation for this misunderstanding is that you may have been using
1 bit bitfields: it is implementation defined whether these have values 0 and 1
or 0 and -1.  As far as I recall, C99 did not change that.


Naive question after having a look at C99, 6.7.2.1:
Does this ambiguity only hold for "int" bitfields or also for
"signed int" bitfields?


If you are using sign-magnitude or ones complement encoding then the
possible values of a signed int bitfield will be +0 and -0 or 0 and a
trap representation.

(I hope there isn't something somewhere that says that bitfields must be
twos complement. I can't see it but I'm sure if there is someone here
will be along quickly to correct me. ;-) )


Sorry, I probably did not phrase it carefully enough.

This was not my question. I am well aware of s-m and 1s complement,
my question more or less is whether it is possible that a signed
int bitfield could be treated as if it was an unsigned int
bitfield?

Here some context; see especially the footnote:

"
6.7.2.1:

4 A bit-field shall have a type that is a qualified or unqualified
version of _Bool, signed int, unsigned int, or some other implementation-defined type.

Semantics
[...]
8 A member of a structure or union may have any object type other
than a variably modified type.102)
In addition, a member may be declared to consist of a specified
number of bits (including a sign bit, if any). Such a member is
called a bit-field;103) its width is preceded by a colon.
9 A bit-field is interpreted as a signed or unsigned integer type
consisting of the specified number of bits.104) If the value 0 or
1 is stored into a nonzero-width bit-field of type _Bool, the
value of the bit-field shall compare equal to the value stored.
____
104) As specified in 6.7.2 above, if the actual type specifier
used is int or a typedef-name defined as int, then it is
implementation-defined whether the bit-field is signed or unsigned.
"

Cheers
 Michael
--
E-Mail: Mine is an   /at/ gmx /dot/ de   address.
.

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