A byte can be greater than 8 bits?

[borophyll]

As I read it, C99 states that a byte is an:

"addressabl e unit of data storage large enough to hold any member of
the basic character
set of the execution environment" (3.6)

and that a byte must be at least 8 bits:

"The values given below shall be replaced by constant expressions
suitable for use in #if
preprocessing directives. Moreover, except for CHAR_BIT and
MB_LEN_MAX, the
following shall be replaced by expressions that have the same type as
would an
expression that is an object of the corresponding type converted
according to the integer
promotions. Their implementation-defined values shall be equal or
greater in magnitude
(absolute value) to those shown, with the same sign."

number of bits for smallest object that is not a bit-field (byte)
CHAR_BIT 8 (5.2.4.2.1)

Does this mean that a byte can be larger than 8 bits (ie CHAR_BIT >
8)? I have gotten the impression that a byte, or unsigned char, was
always 8 bits, but perhaps I was wrong. If I am not, is there
somewhere in the standard that defines a byte as always being 8 bits?

Regards,
B.

[Richard]

Philip Potter <pg*@see.sig.in validwrites:

jacob navia wrote:

>Philip Potter wrote:

[snip]

The fact that Philippe could not bring a single example
means probably that there isn't any machines where
all those terrible conditions apply!

I'm flattered that you think so! But my knowledge of particular quirks
of different C implementations is much more restricted than you
think. I have only coded for three implementations : windows, linux,
and microblaze. I don't know the ins-and-outs of these
implementations , for reasons I've already given.

I'm quite certain that others, if they wanted, could provide details
of some implementations with some of the requested features.

Besides, I did provide a single example:

>A) Systems where a byte is more than 8 bits :

DSPs are a common example here.

No, you didn't. You mentioned a range of HW where it may or may not be
true :)

[Eric Sosman]

jacob navia wrote On 10/01/07 09:42,:

Eric Sosman wrote:

>>jacob navia wrote:

>>>Eric Sosman wrote:

jacob navia wrote:

>Philip Potter wrote:
>
>>Give me an example of code which relies on -1 being all-bits-one.
>>
>
>#include <stdio.h>
>int maskNLowerBits( unsigned d,int n)
>{
return d & (-1 << n);
>}

Maybe Philip should have specified "code that doesn't
invoke undefined behavior." After the fix s/1/1u/ the
corrected code no longer relies on -1 being all-bits-one.
??
Is it legal to make -1u??
Unary minus applied to unsigned operand?

Yes: unary minus takes an "arithmetic type" (6.5.3.3).

Anyway, that relies still in that all bits not dropping
of the left side are all 1. And that did not change with this minor
cosmetic change!

6.5.7p4 defines the value of the result of `-1u << n'
once UINT_MAX and n are known (and if n is in range).
6.2.6.2p4 specifies that this value is represented by n
bits zero and the rest one. That is, the fixed code relies
on the guarantees of the Standard and does not rely on the
representation of `-1'.

6.5.7p4 also says that `-1 << n' gives undefined behavior.

The difference between undefined and defined seems to me
more than "minor" or "cosmetic."

--
Er*********@sun .com

[Philip Potter]

Richard wrote:

Philip Potter <pg*@see.sig.in validwrites:

>Besides, I did provide a single example:

>>>>A) Systems where a byte is more than 8 bits :
DSPs are a common example here.

No, you didn't. You mentioned a range of HW where it may or may not be
true :)

Well you wanted the examples, you go find them! :P

--
Philip Potter pgp <atdoc.ic.ac. uk

[Richard]

Philip Potter <pg*@see.sig.in validwrites:

Richard wrote:

>Philip Potter <pg*@see.sig.in validwrites:

>>Besides, I did provide a single example:

>A) Systems where a byte is more than 8 bits :
DSPs are a common example here.

No, you didn't. You mentioned a range of HW where it may or may not be
true :)

Well you wanted the examples, you go find them! :P

Er, I was. Here. From the people who keep telling us about them all.

[Richard Heathfield]

Philip Potter said:

<snip>

[...] Null pointers can always be set by:
foo *x = (foo *)0;
(I'm not sure if the cast is necessary)

(No, it isn't.)

even if the null pointer representation is not all-bits-zero.

Yes, even if it means the compiler doing some magic to make it all work.

I personally don't know of any non-all-bits-zero null pointer systems,

Several such machines are mentioned in the FAQ.

I can't think why you'd want to [store a function pointer in a void *].

It's not all that unusual a thing to want to do, actually, although in
practice there's always a way around it if you want it badly enough.

As for whether systems exist that lose information when copying a function
pointer value into a void *, the answer is yes, and I've got one right
here.

--
Richard Heathfield <http://www.cpax.org.uk >
Email: -http://www. +rjh@
Google users: <http://www.cpax.org.uk/prg/writings/googly.php>
"Usenet is a strange place" - dmr 29 July 1999

[Philip Potter]

Richard wrote:

Philip Potter <pg*@see.sig.in validwrites:

>Richard wrote:

>>Philip Potter <pg*@see.sig.in validwrites:
Besides, I did provide a single example:

>>A) Systems where a byte is more than 8 bits :
DSPs are a common example here.
No, you didn't. You mentioned a range of HW where it may or may not be
true :)

Well you wanted the examples, you go find them! :P

Er, I was. Here. From the people who keep telling us about them all.

Was my answer not good enough for you? It sounded to me that you had a
benevolent quest to convince people that their assumptions about the
implementation were not necessarily correct. Surely you can put some
effort into finding some? I've already given you a hint for CHAR_BIT != 8.

Meanwhile, I won't make these assumptions, and I won't have to rely on
the implementations I use to comply with these assumptions.

--
Philip Potter pgp <atdoc.ic.ac. uk

[Richard]

Philip Potter <pg*@see.sig.in validwrites:

Richard wrote:

>Philip Potter <pg*@see.sig.in validwrites:

>>Richard wrote:
Philip Potter <pg*@see.sig.in validwrites:
Besides, I did provide a single example:
>
>>>A) Systems where a byte is more than 8 bits :
>DSPs are a common example here.
No, you didn't. You mentioned a range of HW where it may or may not be
true :)
Well you wanted the examples, you go find them! :P

Er, I was. Here. From the people who keep telling us about them all.

Was my answer not good enough for you? It sounded to me that you had a
benevolent quest to convince people that their assumptions about the
implementation were not necessarily correct. Surely you can put some
effort into finding some? I've already given you a hint for CHAR_BIT
!= 8.

Are you joking? I asked for some examples of systems. Not vague "maybe
these". So, no, to be honest your "I think there are some DSPs" is not a
good enough example :-; Sorry. Your example is no more helpful than the
"On some systems a byte is more than 8 bits" type statements. Not
helpful to someone new who might like to see a concrete example. Maybe I
was not clear enough in which case I apologise. But it seems you don't
have any experience of any such platforms - so be it.

As for putting some "effort" in - I am. I am asking *here* where there
are a plethora of people who know a lot about such systems. I am hoping
they will give some examples of where a byte is more than 8 bits or
CHAR_BITS is more than 8. And it is a benevolent quest. I want to see
these systems and look at their specs too. It would be really good to
reference them whenever telling people their code is not standards
compliant.

>
Meanwhile, I won't make these assumptions, and I won't have to rely on
the implementations I use to comply with these assumptions.

But to do that you need to know the standard backwards. Whereas real
examples give real warnings.

[santosh]

Richard wrote:

Philip Potter <pg*@see.sig.in validwrites:

>Richard wrote:

>>Philip Potter <pg*@see.sig.in validwrites:

Richard wrote:
Philip Potter <pg*@see.sig.in validwrites:
>Besides, I did provide a single example:
>>
>>>>A) Systems where a byte is more than 8 bits :
>>DSPs are a common example here.
No, you didn't. You mentioned a range of HW where it may or may not be
true :)
Well you wanted the examples, you go find them! :P

Er, I was. Here. From the people who keep telling us about them all.

Was my answer not good enough for you? It sounded to me that you had a
benevolent quest to convince people that their assumptions about the
implementati on were not necessarily correct. Surely you can put some
effort into finding some? I've already given you a hint for CHAR_BIT
!= 8.

Are you joking? I asked for some examples of systems. Not vague "maybe
these". So, no, to be honest your "I think there are some DSPs" is not a
good enough example :-; Sorry. Your example is no more helpful than the
"On some systems a byte is more than 8 bits" type statements. Not
helpful to someone new who might like to see a concrete example. Maybe I
was not clear enough in which case I apologise. But it seems you don't
have any experience of any such platforms - so be it.

Concrete examples of systems with CHAR_BITS 8 have been mentioned before.
IIRC Jack Klein gave an example a few months ago. I also believe Keith
Thompson said that some Cray Vector machines he was familiar with had this
property.

[Philip Potter]

Richard wrote:

Philip Potter <pg*@see.sig.in validwrites:

>Richard wrote:

>>Philip Potter <pg*@see.sig.in validwrites:

Richard wrote:
Philip Potter <pg*@see.sig.in validwrites:
>Besides, I did provide a single example:
>>
>>>>A) Systems where a byte is more than 8 bits :
>>DSPs are a common example here.
No, you didn't. You mentioned a range of HW where it may or may not be
true :)
Well you wanted the examples, you go find them! :P
Er, I was. Here. From the people who keep telling us about them all.

Was my answer not good enough for you? It sounded to me that you had a
benevolent quest to convince people that their assumptions about the
implementati on were not necessarily correct. Surely you can put some
effort into finding some? I've already given you a hint for CHAR_BIT
!= 8.

Are you joking? I asked for some examples of systems. Not vague "maybe
these". So, no, to be honest your "I think there are some DSPs" is not a
good enough example :-; Sorry. Your example is no more helpful than the
"On some systems a byte is more than 8 bits" type statements.

God helps those who help themselves:
http://groups.google.com/group/comp....rch+this+group

There is your answer, already answered several times in the history of
clc. All I did was put "DSP CHAR_BIT" into a google groups search of clc.

So no, I'm not joking. I honestly thought the information I had given
you would be enough. If I was wrong, I apologise.

Not
helpful to someone new who might like to see a concrete example. Maybe I
was not clear enough in which case I apologise. But it seems you don't
have any experience of any such platforms - so be it.

As for putting some "effort" in - I am. I am asking *here* where there
are a plethora of people who know a lot about such systems.

....and where the question has been answered a plethora of times before.
Have a look through the archives on google groups. And the FAQ (Richard
Heathfield pointed out elsethread there are some examples of
non-all-bits-zero NULLs there).

I am hoping
they will give some examples of where a byte is more than 8 bits or
CHAR_BITS is more than 8. And it is a benevolent quest. I want to see
these systems and look at their specs too. It would be really good to
reference them whenever telling people their code is not standards
compliant.

Yes, it would be very useful. I want to stress that it is not necessary,
that is, even if there is no system where you can't fit a function
pointer into a void *, you still shouldn't do it. But it is good to have
examples of where assumptions are wrong.

>Meanwhile, I won't make these assumptions, and I won't have to rely on
the implementations I use to comply with these assumptions.

But to do that you need to know the standard backwards. Whereas real
examples give real warnings.

Or heed the advice such as in the response to the OP in this thread.

--
Philip Potter pgp <atdoc.ic.ac. uk

[Richard]

santosh <sa*********@gm ail.comwrites:

Richard wrote:

>Philip Potter <pg*@see.sig.in validwrites:

>>Richard wrote:
Philip Potter <pg*@see.sig.in validwrites:

Richard wrote:
>Philip Potter <pg*@see.sig.in validwrites:
>>Besides , I did provide a single example:
>>>
>>>>>A) Systems where a byte is more than 8 bits :
>>>DSPs are a common example here.
>No, you didn't. You mentioned a range of HW where it may or may not be
>true :)
Well you wanted the examples, you go find them! :P

Er, I was. Here. From the people who keep telling us about them all.

Was my answer not good enough for you? It sounded to me that you had a
benevolent quest to convince people that their assumptions about the
implementatio n were not necessarily correct. Surely you can put some
effort into finding some? I've already given you a hint for CHAR_BIT
!= 8.

Are you joking? I asked for some examples of systems. Not vague "maybe
these". So, no, to be honest your "I think there are some DSPs" is not a
good enough example :-; Sorry. Your example is no more helpful than the
"On some systems a byte is more than 8 bits" type statements. Not
helpful to someone new who might like to see a concrete example. Maybe I
was not clear enough in which case I apologise. But it seems you don't
have any experience of any such platforms - so be it.

Concrete examples of systems with CHAR_BITS 8 have been mentioned before.
IIRC Jack Klein gave an example a few months ago. I also believe Keith
Thompson said that some Cray Vector machines he was familiar with had this
property.

Yes, so lets get them with links now so we can keep them. I would gladly
keep a list up to date.

As for Cray Vector machines, lets try and get a list of machines more
available to the common man but they would be a good start if we could
get a link to the specs.