void pointers

suchi_s · 10-30-2004, 06:36 AM

Which one of the following is a true statement about non-generic (non-'void*') pointers?
Choice 1
A pointer to one type may not be cast to a pointer to any other type.
Choice 2
For efficiency, pointer values are always stored in machine registers.
Choice 3
Similarly typed pointers may be added to each other.
Choice 4
Similarly typed pointers may be subtracted from each other.
Choice 5
They are always 32-bit values.

Anup Kumar · 10-30-2004, 06:41 AM

Choice 5 is true.

Mara · 10-30-2004, 07:12 AM

Please don't post homework questions. LQ rules clearly state:

Quote:

Do not expect LQ members to do your homework - you will learn much more by doing it yourself.

jlliagre · 10-30-2004, 08:32 AM

Moreover, don't always trust the answers.
Anup Kumar's one looks wrong to me.

Dave Kelly · 10-30-2004, 10:22 AM

I thought pointers were system dependent.

I''ve got an 8 bit machine that would absolutely choke on a 32 bit pointer.

jlliagre · 10-30-2004, 10:30 AM

That's what I said, actually 3 and 4 are both true, but 3's result is useless.

cetialphav · 10-30-2004, 01:41 PM

Actually, 3 is not true. In fact it is specifically illegal.

From K&R2, page 103:

Quote:

The valid pointer operations are assignment of pointers of the same type, adding or subtracting a pointer and an integer, subtracting or comparing two pointers to members of the same array, and assigning or comparing to zero. All other pointer arithmetic is illegal. It is not legal to add two pointers

jlliagre · 10-30-2004, 02:48 PM

Oops, my mistake, you're right it's illegal, and compilers enforce it !

Code:

main()
{
        char *a=(char *)malloc(100);
        char *b=(char *)malloc(100);
        printf("%ld\n",b-a);
        printf("%ld\n",b+a);
}

a.c:6: invalid operands to binary +

I should have tested before ...

cetialphav · 10-30-2004, 06:13 PM

Yeah, its an easy mistake to make because you can add and subtract integers to pointers, but you can only subtract two pointers. It makes sense when you think about it, but it isn't what you would intuitively think.

Not to be too nitpicky, but strictly speaking both of the pointer operations in your test are unsupported. The subtraction of pointers is only meaningful when the pointers are in the same array. Subtraction of pointers in different arrays (as in your test) is undefined. I.e., it won't necessarily be an error but you have no guarantees at all about what you will get.

jlliagre · 11-08-2004, 03:05 PM

You're right my example isn't respecting the same array prerequisite, and was just meant to test the compiler behaviour.
That said, pointers being adresses, substracting two of them gives an offset in the program (possibly virtual) memory addressing space which is not an undefined value, although not terribly useful ...