Subtracting Two Pointers

**c99** · 02-26-2004

pinko_liberal.

Ah! I note you've quoted straight from ISO/IEC9899 or C99.

Yes, it appears you are right, the condition is not satisfied.

A couple of points to note though, firstly, I doubt the condition
was ever intended to be satisfied, and secondly and most
importantly the code portion initally quoted is more likely an
exercise designed to highlight the sizeof difference between data types.

However, using pointer arithematic on an array shall meet all
those conditions from the section you quoted.

**quzah** · 02-26-2004

One thing you'll both want to note, is that the origional poster said nothing about arrays. Simply pointer arithmetic.

Quzah.

**pinko_liberal** · 02-26-2004

quzah,
You can subtract two pointers only if they point to the elements in the same array object or at most one past the end of the array.
Other pointer subtractions are meaningless.
This is my reading of the standard, I might be wrong but I would like to know why.
regards,
Pinko

**c99** · 02-26-2004

Originally posted by quzah
One thing you'll both want to note, is that the origional poster said nothing about arrays. Simply pointer arithmetic.

Quzah.

And one thing you should note quzah is that I did in fact note that.

Another dick that doesn't read entire posts before he opens his mouth.

Sheesh! You'll try anything for a bite wont you, you dick.

**pinko_liberal** · 02-26-2004

Originally posted by c99
And one thing you should note quzah is that I did in fact note that.

Another dick that doesn't read entire posts before he opens his mouth.

Sheesh! You'll try anything for a bite wont you, you dick.

quzah is a very respected member of this board, please do not use such language against him

**c99** · 02-26-2004

pinky.

Respect is earned, not given!

Language? What language? I have never at any time used
language that was not used by mods or other members.

**Prelude** · 02-26-2004

>Respect is earned, not given!
quzah has earned his. You haven't. And you probably won't in the forseeable future judging from your childish attitude.

>I have never at any time used language that was not used by mods or other members
Mods and other members that you've harrassed have been around longer than you.

**c99** · 02-26-2004

Originally posted by Prelude

quzah has earned his. You haven't.

He may have earnt your respect but not mine. Deal with it.
Stop being such a cry baby.

Mods and other members that you've harrassed have been around longer than you.

Read the above over and over again until you realise the stupidity
of what you wrote.

I have not at any point initiated harrassment against anyone. Check your facts dick.

**XSquared** · 02-26-2004

>>He may have earnt your respect but not mine. Deal with it.
>>Stop being such a cry baby.
Don't mess with Prelude. She will hurt you.

>>I have not at any point initiated harrassment against anyone. Check your facts dick.
"Read the above over and over again until you realise the stupidityof what you wrote."
Take your own advice.

**Thantos** · 02-26-2004

Threads_Locked_Because_Of_c99++;

**XSquared** · 02-26-2004

Pfft. Write more efficient, and more easily read code:

++threadsLockedBecauseOfC99;

**quzah** · 02-26-2004

Originally posted by pinko_liberal
quzah,
You can subtract two pointers only if they point to the elements in the same array object or at most one past the end of the array.
Other pointer subtractions are meaningless.
This is my reading of the standard, I might be wrong but I would like to know why.
regards,
Pinko

I stand correct. Other pointer subtractions are undefined.

Quzah.

**Thantos** · 02-26-2004

Bah! Doing a pre-incrament produces the same code and a post-incrament in this case.
For example outputting the following two samples produces the same asm code using gcc with the -S parameter

Code:

int main(void)
{
  int x=5;
  x++;
  return 0;
}

And

Code:

int main(void)
{
  int x=5;
  ++x;
  return 0;
}

Produces:

Code:

        .file   "test1.c"
        .version        "01.01"
gcc2_compiled.:
.text
        .align 4
.globl main
        .type    main,@function
main:
        pushl %ebp
        movl %esp,%ebp
        subl $24,%esp
        movl $5,-4(%ebp)
        incl -4(%ebp)
        xorl %eax,%eax
        jmp .L2
        .p2align 4,,7
.L2:
        leave
        ret
.Lfe1:
        .size    main,.Lfe1-main
        .ident  "GCC: (GNU) 2.95.4 20011002 (Debian prerelease)"

Ok technically its not the same code since the file name will be different but the code is still the same

***kermi3*** · 02-26-2004

too much trolling to delete....thread closed. If anyone wants to brng this problem back up please use a new thread.

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