Annoying compiler warning

[Richard Heathfield]

James Kuyper said:

Section 6.2.6.2p1 of the C99 standard is quite clear about this; if N is
the number of value bits in an unsigned type, the corresponding maximum
is 2^N-1, which is inherently never a negative number.

ITYM "never an even number".

But of course, it is easy to show that this is wrong. If N is, say,
15.000044+ (i.e. if there are 15.000044+ value bits in the unsigned type),
then the maximum is 32768, which is an even number.

 

[James Kuyper]

James Kuyper said:


ITYM "never an even number".

But of course, it is easy to show that this is wrong. If N is, say,
15.000044+ (i.e. if there are 15.000044+ value bits in the unsigned type),
then the maximum is 32768, which is an even number.

I suppose you're correct; the standard never specifically requires that
there be an integer number of value bits.

 

[Eric Sosman]

James Kuyper said:


ITYM "never an even number".

But of course, it is easy to show that this is wrong. If N is, say,
15.000044+ (i.e. if there are 15.000044+ value bits in the unsigned type),
then the maximum is 32768, which is an even number.

The Standard requires a "pure binary representation," but
the representation you suggest can only be called "impure."

 

[santosh]

I suppose you're correct; the standard never specifically requires
that there be an integer number of value bits.

The standard never specifically requires C to be restricted to computer
programming alone.

 

[Chris Dollin]

Richard Heathfield wrote:

The Standard requires a "pure binary representation," but
the representation you suggest can only be called "impure."

Rubbish. "Bloody stupid" would be a reasonable alternative.

 

[CBFalconer]

.... snip ...

The Standard requires a "pure binary representation," but
the representation you suggest can only be called "impure."

Ahh, now we are getting to something of general interest.

 

[Walter Roberson]

The Standard requires a "pure binary representation," but
the representation you suggest can only be called "impure."

Is that the C99 Standard? Is there a corresponding C89 clause?

I've been reviewing C89 but I have not yet found in there any
requirement of binary representation, beyond that implied by the definition
of "bit" and "byte". I also have not found any requirement in C89
that the complete possible span of binary be usable as values.

If there is anything in C89 that would prevent UINT_MAX from being
2^N-2 with 2^N-1 reserved as a trap value, could someone point me
to the relevant C89 clauses?

 

[Ben Pfaff]

Is that the C99 Standard? Is there a corresponding C89 clause?

From C89 6.1.2.5 "Types": "The representations of integral types
shall defines values by use of a pure binary numeration system."

 

[Walter Roberson]

(e-mail address removed)-cnrc.gc.ca (Walter Roberson) writes:

From C89 6.1.2.5 "Types": "The representations of integral types
shall defines values by use of a pure binary numeration system."

Thanks. It appears the C89 clause is 3.1.2.5; "6.1.2.5" must be the C90
numbering.

I do see the quoted sentance in C89 now, but even with all of the
discussions there about subranges and representations, it is not
yet clear to me that in C89, UINT_MAX could not be 2^N-2 with 2^N-1
reserved. (It would require more than 16 bits for int, but that's not
difficult at all these days.)

 

[jameskuyper]

Walter Roberson wrote:
....

If there is anything in C89 that would prevent UINT_MAX from being
2^N-2 with 2^N-1 reserved as a trap value, could someone point me
to the relevant C89 clauses?

Well, the fact that the standard didn't officially acknowledge the
possibility of trap values until C99 would be a bit of a barrier to
that possibility.

 

[Army1987]

Ian Collins wrote:
[About infinite loops]

How about:

for( u = 1; u&1; u += 2 )

It's the reversed form of the Italian humorous threat "Ti prendo a
schiaffi a due a due finché non diventano dispari", "I'm giving you slaps
two by two until they become odd".
BTW, what about
#define HELL_TEMPER 1.416785e32 /* actually, Planck temperature */
#define FREEZING_TEMPER 273.15
#define HELL_FREEZES_OVER (HELL_TEMPER < FREEZING_TEMPER)

while (!HELL_FREEZES_OVER) { ... }
?