bytes calculation

[seema]

Hi all,

How to calculate upper 4 bytes and lower 4 bytes of a value? Can
somebody please explain??
say for example,
unsigned int scnlen_lo;
int scnlen_hi;
unsigned long long scnlen;

scnlen=888888;
scnlen_hi= ??//how to calculate upper 4 bytes of scnlen?
scnlen_lo =?? //how to calculate lower 4 bytes of scnlen??

Thanks in advance,
Seema Rao

[Kelsey Bjarnason]

[snips]

On Mon, 30 Jul 2007 18:04:40 +0200, Richard wrote:

FWIW, there is nothing wrong with making mistakes. There is something
very wrong, however, when sneering at people and acting like a little
princess and being wrong all in one.

People who should know better have been ragging him repeatedly for giving
correct and helpful answers, even if ones that may require a little noodle
work on their part. Why should he *not* give these people a hearty "Screw
you if you don't want help"?

He should. He has. Such people are contemptible.

[Jack Klein]

On Mon, 30 Jul 2007 16:55:28 +0200, jacob navia
<ja***@jacob.re mcomp.frwrote in comp.lang.c:

Spoon wrote:

Pietro Cerutti wrote:

Assuming that sizeof(unsigned long long) == 64 and sizeof(int) == 32

I think you meant "Assuming CHAR_BIT == 8, sizeof(int) == 4, and
sizeof(unsigned long long) == 8".

Please, are you a lawyer?

Can you tell me of a machine where char_bit != 8 ?

And please, a machine in use 2007 ok?

Sure, the Texas Instruments TMS320F2812. We have it on two different
boards in a safety-cricitcal medical device. I have written quite a
bit of code for it, and will be writing some more later this week.

CHAR_BIT = 16
sizeof (char) is 1
sizeof (short) is 1
sizeof (int) is 1
sizeof (long) is 2
sizeof (long) long is 4
sizeof (void *) is 2

--
Jack Klein
Home: http://JK-Technology.Com
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[somenath]

/* first ensure that a result is possible */
if(sizeof scnlen_lo < 4 || scnlen_hi < 4)
{

is the test "scnlen_hi < 4)" valid ? it may always fail

[Mark Bluemel]

somenath wrote:

> /* first ensure that a result is possible */
if(sizeof scnlen_lo < 4 || scnlen_hi < 4)
{

is the test "scnlen_hi < 4)" valid ? it may always fail

See my corrections to this code. It is (or should be) fairly obvious
what's missing here.

[Army1987]

On Mon, 30 Jul 2007 17:08:53 +0200, jacob navia wrote:

Pietro Cerutti wrote:

>jacob navia wrote:

>>Pietro Cerutti wrote:
Richard Heathfield wrote:
Pietro Cerutti said:
>
>seema wrote:
>>Hi all,
>>>
>>How to calculate upper 4 bytes and lower 4 bytes of a value? Can
>>somebod y please explain??
>>say for example,
>> unsigned int scnlen_lo;
>> int scnlen_hi;
>>unsigne d long long scnlen;
>>>
>> scnlen=888888;
>> scnlen_hi= ??//how to calculate upper 4 bytes of scnlen?
>> scnlen_lo =?? //how to calculate lower 4 bytes of scnlen??
>sclen_hi = scnlen >32;
>sclen_lo = scnlen & 0xFFFFFFFF;
>>
>Assuming that sizeof(unsigned long long) == 64 and sizeof(int) == 32
Under those assumptions and assuming CHAR_BIT is 8, sclen_hi now
actually contains the top 28 bytes of scnlen, which isn't what was
asked.
Yes, sorry. I meant sizeof(unsigned long long) == 8, and sizeof(int) == 4

If you meant those to be bit counts rather than byte counts, your
code invokes undefined behaviour.
Can you explain me this point, please?
At least, is it so under C99?

cat ctemp.c
#include <stdio.h>

int main(void)
{
/**** START ****/
int hi, lo;
unsigned long long full;

full = 0xaaaaaaaabbbbb bbb;

hi = full >32;
lo = full & 0xFFFFFFFF;

printf("full is %llx\n", full);
printf("hi is %x\n", hi);
printf("low is %x\n", lo);

/**** STOP ****/
return(0);
}

gcc -Wall -std=c99 -o ctemp ctemp.c && ./ctemp
full is aaaaaaaabbbbbbb b
hi is aaaaaaaa
low is bbbbbbbb
Please do not believe everything heathfield says...

He is a language lawyer, and those people do not live by the
same rules as the rest of us

:-)

I can't get the point.
Does my code invokes undefined behavior or not?

I do not see any. It works in linux/windows/Mac/and many others.
But heathfield sees something, I do not know what, he is the lawyer,
not me!

Even if int has 32 bits and no padding, 0xaaaaaaaa doesn't fit in
it, and will become a negative value. But the specifier %x is for
unsigned values.
This happens to work on two's complement machines where signed int
overflow silently wrap around (so essentially the only difference
between signed and unsigned is the *meaning*, not the behavior, of
representations with the most significant bit set). But as per the
Standard, signed integer overflow is UB. (This is easily fixed by
declaring hi and lo as unsigned, or better as unsigned long (and
using "%lx") which is required to have at least 32 bits.)
--
Army1987 (Replace "NOSPAM" with "email")
"Never attribute to malice that which can be adequately explained
by stupidity." -- R. J. Hanlon (?)

[CBFalconer]

Pietro Cerutti wrote:

jacob navia wrote:

>Pietro Cerutti wrote:

.... snip ...

>>

>>#include <stdio.h>

int main(void) {
/**** START ****/
int hi, lo;
unsigned long long full;

full = 0xaaaaaaaabbbbb bbb;
hi = full >32;
lo = full & 0xFFFFFFFF;

printf("full is %llx\n", full);
printf("hi is %x\n", hi);
printf("low is %x\n", lo);

/**** STOP ****/
return(0);
}

full is aaaaaaaabbbbbbb b
hi is aaaaaaaa
low is bbbbbbbb

Please do not believe everything heathfield says...

....

>
I can't get the point.
Does my code invokes undefined behavior or not?

Yes. Nothing enforces CHAR_BIT == 8 for one thing. Then you have
unbandied use of ints to hold unsigned values. etc.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
cbfalconer at maineline dot net

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

[CBFalconer]

Mark Bluemel wrote:

>

.... snip ...

>
Here we enter into the realms of philosophy and personal taste - Mr
Heathfield wishes for all C code to be provably correct (loosely
speaking) for a general abstract C implementation which sticks to
the letter of the language specification, while Mr Navia will
accept C code working on the sample of machines he happens to work
with. [Disclaimer - I do not speak for either party here, and my
comments above are my interpreations of their positions]

However, RH's code is much more likely to work on any machine than
JNs. If RH wishes to use machine specific code he can do so, and
isolate it in easily replaced modules. Believe it or not, there
is no extra effort involved in doing this. If does, however,
require knowing the restrictions and limitations placed on C by the
standard.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
cbfalconer at maineline dot net
--
Posted via a free Usenet account from http://www.teranews.com

[CBFalconer]

seema wrote:

>
How to calculate upper 4 bytes and lower 4 bytes of a value? Can
somebody please explain??
say for example,
unsigned int scnlen_lo;
int scnlen_hi;
unsigned long long scnlen;

scnlen=888888;
scnlen_hi= ??//how to calculate upper 4 bytes of scnlen?
scnlen_lo =?? //how to calculate lower 4 bytes of scnlen??

Why do you even conceive that sclen_hi (or lo) contains 4 bytes?

You can usually use the modulo and division instructions to do this
sort of isolation.

--
<http://www.cs.auckland .ac.nz/~pgut001/pubs/vista_cost.txt>
<http://www.securityfoc us.com/columnists/423>
<http://www.aaxnet.com/editor/edit043.html>
cbfalconer at maineline dot net

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

[jacob navia]

CBFalconer wrote:

However, RH's code is much more likely to work on any machine than
JNs.

I am amazed at your powers.

Without ever seeing a line of code from me, you are able to
assert that my code is much more likely to work
in any machine than RH's

http://dictionary.reference.com/browse/prejudice

prejudice
Pronunciation[prej-uh-dis]
–noun
1. an unfavorable opinion or feeling formed beforehand or without
knowledge, thought, or reason.
2. any preconceived opinion or feeling, either favorable or unfavorable.

[santosh]

jacob navia wrote:

CBFalconer wrote:

>However, RH's code is much more likely to work on any machine than
JNs.

I am amazed at your powers.

Without ever seeing a line of code from me, you are able to
assert that my code is much more likely to work
in any machine than RH's

Not your code, Richard H's.

[snip]